Patent Publication Number: US-6991693-B2

Title: Screen cloth insertion apparatus and method

Description:
This application is a continuation-in-part of International Application No. PCT/IB00/01716, filed Aug. 23, 2000, which is a continuation-in-part of U.S. patent application Ser. No. 09/379,102 filed Aug. 23, 1999, now U.S. Pat. No. 6,331,223 B1, which is a continuation-in-part of U.S. patent application Ser. No. 08/997,737 filed Dec. 24, 1997, now U.S. Pat. No. 6,279,644 B1. This application claims the benefit of U.S. Provisional Patent Application No. 60/272,334, filed Feb. 28, 2001. 

   FIELD OF THE INVENTION 
   The present invention relates to a screen and frame assembly for windows, doors and the like, and methods and apparatus for fabricating such frame assemblies. 
   BACKGROUND OF THE INVENTION 
   The general purpose of screens is to prevent the ingress of insects, while providing ventilation. A typical screen assembly is made up of screen cloth, fabric, or mesh attached to a screen frame in a manner discussed in more detail below. For brevity, the term “screen” is used herein, and includes such screen cloth, fabric, mesh or similar ventilation material. 
   Screen frames for windows, doors, operable skylights and the like are commonly made of four elongated frame members, called screen bars, of uniform cross section. These bars are typically roll-formed from aluminum or sheet steel, although some may be extruded aluminum. (Plastic and wood are also used, but to a lesser extent.) These screen bars are supplied from the screen bar manufacturer in lineal form and are cut to a final length by the screen assembly manufacturer. Further, these screen bars are held together at the corners with plastic or metal inserts, called corner keys, to form the screen frame. 
   Different style corner keys are available and are designed to match the particular screen bar used. The most popular corner key allows the screen bar to be cut straight at 90° at the ends. These keys typically are made from injection molded plastic and have a square block body to visibly fill the corner area of the frame. Attached to the body are insertion prongs that are pushed into the hollow screen bar profile to create friction fit connections. Corner keys requiring a 45° miter cut on the ends of the screen bar also can be used. These keys, usually metal, are less expensive and entirely hidden inside the screen bar. These keys also provide a friction fit connection. 
   Screen is then affixed to the screen frame, in a manner discussed below, to form a screen and frame assembly. These assemblies are then removably secured to windows, doors (e.g., patio screen doors), operable skylights, and the like. Screen and frame assemblies for such openings are very similar, often differing only in size. Accordingly, for brevity, screen and frame assemblies for windows are described herein. Nevertheless, it will be understood that this discussion applies equally to screen and frame assemblies for doors, operable skylights and the like. 
   It is desirable that the screen be a light-weight fabric or mesh, and stretched taut across the screen frame to avoid unsightly sag and to allow a viewer to see through the screen with minimal visual interference. However, if the screen is tensioned excessively, the screen bars deform inwardly in an hourglass shape. This resultant shape is not only aesthetically undesirable, but also can prevent proper installation in the window opening. Excess screen tension also increases the risk of tearing the screen during manufacture of the screen and frame assembly or while the assembly is in service. 
   Typically, the screen is fiberglass yarn or roving, which is coated, for example, with polyvinyl chloride (PVC), woven and heat fused. The next most popular form of screen is made by weaving drawn aluminum wire, which is subsequently painted. The PVC coated fiberglass screen is the most popular type, by approximately a 4 to 1 ratio (in area). However, both offer the desired attributes of suitable strength and an open weave. 
   To compensate for deformation of the screen frame into the hourglass shape discussed above, generally the screen bars are manufactured with an outward bow, in the plane of the screen, before the screen is installed. After the screen is installed into the screen bar by the manufacturer, its final tension straightens the frame members in the final assembly. This “pre-bow” is set into the screen frame during the extrusion or roll forming process to make the screen bar lineal. 
   Typically, roll-formed bar has approximately 20 millimeters (0.75 inches) of bow over a 3.7 meter (12 feet) length. Additional bow is usually set by hand into the roll-formed bar prior to screen installation when the length of the frame members is greater than 1 meter (approximately 3.5 feet). Pre-bowing is not generally required, however, when the screen bar is sufficiently rigid to resist deformation caused by the resultant screen tension. 
   It is the current practice, essentially industry-wide, to secure screen in open grooves formed along inside edges of the screen frames using a stuffer strip known as “spline” and its associated fastening techniques. The open grooves are known as “spline grooves.” A spline is often a wire-like, extruded rigid plastic or foam material, although some splines are made from metal, especially for use with aluminum screens. A spline is usually round or T-shaped in cross section, but can be U-shaped, for example. 
   U.S. Pat. No. 5,039,246 (the &#39;246 patent) shows a conventional method of securing screen to a frame member using a spline. Using the reference numerals of the &#39;246 patent, the spline 58 is forced into a spline groove or recess 56 in the screen bar 20, with the screen 22 sandwiched between the spline 58 and the spline groove 56. 
   The screen 22 is held by friction between the spline 58 and the spline groove 56 with the resulting interference fit. A lip 50 and a ledge 52, part way down one side of the groove wall, are typically included to help trap and improve the strength in retaining the screen 22. The spline 58 and trapped screen 22 are forced into the groove 56, usually by hand, with the use of a roller device 70, including a roller 72. The term, “hand wiring”, is used to describe the action of securing the screen 22 with the spline 58 into the spline groove 56. Many attempts have been made to automate the installation of spline by machine. However, this automation has proven to be very difficult and machines of this nature have not been widely accepted as a viable option to hand wiring. 
   The conventional procedure for manufacturing and hand wiring a screen and frame assembly is discussed in more detail below. First, the screen bars are cut to length:, accounting for the corner key dimensions. Then, the screen frame is assembled using the cut screen bars and corner keys. As discussed above, when light construction screen bars are used, as is normally the case, a balance between pre-bow tension and screen tension is necessary to ensure straight screen bars and desirable tension in the final assembly. When the screen bar has insufficient pre-bow tension, the bars are deformed by hand a sufficient degree after the corner keys have been inserted. As discussed above, the amount of pre-bow is determined based on experience, but is typically a few millimeters of bow per meter length of the screen bar. 
   The screen frame is then secured to a table using locator (stop) blocks, which prevent shifting and maintain the frame square during screen installation. The table typically has permanent stop blocks for orienting the screen frame. If the screen bar is not constrained, when the spline is inserted into the screen bar, excessive tension may be placed on the frame, causing the frame to hourglass inwards. To avoid hourglassing, removable blocks are located on the inside of the frame segment to limit deflection of the screen bar by the screen tension on assembly. (The spline groove must be facing up and unobstructed by the blocks.) More elaborate tables use removable blocks arranged in grooves cut into the table, with the removable blocks being secured by integral friction clamps. To avoid the need for blocking to prevent hourglassing, some manufacturers use extruded screen bar, instead of roll-formed screen bar, because of the greater strength of a (thicker) extruded section. 
   After the screen frame is secured to the table, the screen is pulled from a roll and positioned to cover the opening formed by the frame. Ideally, no excess screen is used, but this is difficult to achieve in practice. As a result, most manufacturers cut the screen approximately two inches wider than the frame width, so that the screen is pulled past the end of the frame by approximately one inch to ensure that sufficient amount of screen can be rolled into the spline groove along the frame perimeter. In either technique, the screen is positioned over, with edges parallel to, the secured screen frame. 
   The screen and spline are installed into the spline groove by starting in one of the frame corners. The screen is then pulled taut at the next corner with one hand, keeping it straight and parallel to the edge of the mating screen bar. The spline is simultaneously held above the groove in the same manner as the screen, with the same hand. With the other hand, the installation roller is pushed along towards the upcoming corner with a firm downward force to push the spline and trap the screen into the spline groove. This action is repeated on the second and third screen bars. On the last screen bar, most of the tension is set into the screen. On this leg, the screen is pushed into the screen bar with the installer&#39;s finger, just prior to the insertion of the spline. This pre-insertion technique reduces the final tension in the screen to the desired level. The spline is cut at the final corner with a utility knife. 
   After the spline and screen are inserted in all screen bars, excess screen around the edge of the frame is cut away with a utility knife. To do this, the point of the blade is pushed against the screen bar, through the screen, immediately adjacent to the spline groove around the outside edge of the screen bar. Care must be taken to cut the screen close to the spline groove without cutting the screen covering the opening formed by the frame. The finished screen and frame assembly is removed from the table, inspected, and any necessary hardware is attached. 
   The current hand wiring process using spline has several drawbacks, however. 
   Current standards for screen and frame assemblies are established by associations such as the Screen Manufacturers Association (ANSI-SMA SMT 31-1990) in the United States and the General Standards Board in Canada (CAN-CGSB-79.1-M91). These standards cover particular elements of screen and frame assemblies for windows, patio doors and the like. For example, these standards set forth tolerances in terms of the strength of the screen, the strength required to fasten the screen to the screen bar, the amount of sag in the screen, etc. Although these standards generally can be met by using the spline technology discussed above, very close and consistent dimensional tolerances are required between the spline and the spline groove, respectively, in order to achieve the specified fastening strength. These tolerances require close attention and skill with current screen bar roll-forming and extrusion technology and current spline hand wiring techniques. Any out-of-tolerance spline and screen bar produced costs the manufacturer in wasted time, material and goodwill. 
   Further, the amount of force required by an installer to secure the screen with the spline in the spline groove may be high enough to cause repetitive strain injury, e.g., carpal tunnel syndrome, to one who routinely performs this job. This is of major importance, since this type of injury is serious and has recently received heightened public awareness. Further, such an injury to an installer is also costly to the manufacturer in terms of compensation and loss of skilled labor. 
   Also, the hand wiring technique is particularly difficult and time-consuming. Notably, it is difficult to control the wire-like spline material and simultaneously control the screen tension with one hand, while the spline is rolled in with the other hand. This operation requires a high degree of skill and careful attention. This adds to the final manufacturing cost, and, hence, increases the final cost to the consumer. Final product consistency is difficult to maintain. 
   Quality control also has become an issue with current spline techniques. Specifically, installers have learned ways to make their jobs easier, to the detriment of quality control. This is particularly true when using PVC spline. For example, an installer will stretch the PVC spline just prior to insertion, in order to reduce the diameter of the spline. This, of course, makes it easier to install. However, this also reduces the “pull-out” force or attachment strength of the spline and screen. The result is that the screen can be more easily pulled out from the spline groove, which is undesirable. (This, however, is not an issue with polyethylene spline, which does not stretch in the manner of PVC spline.) 
   There are other drawbacks associated with conventional spline techniques. In particular, the use of a separate fastening device, such as a spline, requires separate inventory control and associated costs. Screen manufacturers prefer to minimize inventory. Therefore, it is desirable to eliminate the spline as a separate item. Also, the need to have a strong interference fit in securing the spline necessitates stiff walls on the spline groove. Further, the spline technology makes the design of automatic assembly equipment extremely complex. 
   For the foregoing reasons, a need has arisen to provide a screen and frame assembly that eliminates the requirement of a spline. An additional need has arisen to manufacture screen products more easily. 
   Some attempts have been made in the art to provide screen and frame assemblies without a traditional spline. For example, in U.S. Pat. No. 3,255,810, a continuous strip of fusible material is fused with the screen material and then inserted into the groove in the frame. In U.S. Pat. No. 4,568,455, the bonding of a screen to a thermoplastic frame is accomplished by resistance heating of the screen using an electrical potential of four volts and a current of approximately, 2,200 amps, which is applied for approximately forty-eight seconds, to fuse the thermoplastic. This method, however, requires external tensioning until the thermoplastic cools and solidifies. 
   In another aspect, U.S. Pat. No. 4,968,366 teaches a complex method of manufacturing tension screens using an apparatus that includes a screen tensioning frame and a platform positioned adjacent to the tensioned screen. The platform includes heating elements about the periphery of a sheet heater. The heating elements receive a screen frame which can be lifted into contact with the screen in the tensioning frame. The screen cloth is pre-tensioned by an external frame. The screen frame is heated to thermally expand the screen frame. Then the screen cloth is expanded by heating, by an amount substantially equal to the amount of thermal expansion of the screen frame during the step of heating the screen frame. Next, the expanded and pre-tensioned screen cloth is bonded to the heated screen frame. The screen frame is then cooled by blowing air over the screen frame. The heat of the screen cloth is maintained by shielding the bonded screen cloth from the blowing air and heating the bonded screen cloth concurrently, while cooling the screen frame, so that the screen cloth does not cool faster than the screen frame during cooling of the screen frame. 
   Thus, in the arrangement of U.S. Pat. No. 4,968,366, it is necessary to heat the entire mating surface, while the screen is maintained under high tension, and to match, or compensate for, the different thermal expansions of the frame and screen cloth. This complex technique requires high manufacturing precision, including proper tensioning of the screen and mating of the heating elements and the tensioning frame. Further, this technique is too slow and cumbersome to be considered practical for the manufacture of screen and frame assemblies for windows and the like. 
   Other techniques, in general, are known to fuse screening material to frames. For example, U.S. Pat. No. 4,675,065 (the &#39;065 patent) shows a method for securing a microsieve to a support member. A laser beam is directed against a point on the upper edge of a well which contains the microsieve to melt fusible material in contact with the laser beam. The laser-melted fusible material travels down the well wall, contacts the edge of the microsieve and solidifies to secure the microsieve. Japanese patent document No. 63-137828 (the &#39;828 document) shows a single step method of ultrasonically welding screening net to the bottom of a small, cylindrical container using resin and a single, vibrating tip, which is identical in size to the container bottom. The exotic techniques for the small parts, as described in the &#39;065 patent and the &#39;828 document, are generally limited to their particular applications. 
   Accordingly, a need has arisen for a screen and frame assembly for windows, doors and the like in which the screen is secured to the frame quickly, with reduced manual labor. 
   SUMMARY OF THE INVENTION 
   One aspect of the invention is a method and apparatus for securing ventilation cloth to a screen frame. A screen frame is oriented in an approximately vertical position. The screen frame has a plurality of segments. Each segment has a mounting surface on a face thereof. At least one of said segments has adhesive on the mounting surface thereof. A ventilation cloth is hung across the mounting surface of said one segment. The adhesive in said one of the segments is melted. The ventilation cloth is inserted in the adhesive across a length of said one of the segments. 
   Another aspect of the invention is a method and apparatus for securing a ventilation cloth to a screen bar segment. A screen bar segment is provided having a mounting surface on a face thereof. The segment has adhesive on the mounting surface. The ventilation cloth is spread across the mounting surface of the screen bar segment. The adhesive is melted. The ventilation cloth is inserted into the adhesive with an elongated insertion member substantially across a length of the screen bar segment simultaneously. 
   Still another aspect of the invention is a ventilation cloth insertion apparatus. A fixture clamps a screen frame. The screen frame has a plurality of segments. Each segment has a mounting surface on a face thereof. At least one of the segments have adhesive on the mounting surface thereof. The fixture has a plurality of clamping arms. The clamping arms are positionable so that each clamping arm clamps a respective outside edge of a respective one of the plurality of sides of the screen frame while attaching a ventilation cloth to the screen frame. The outer edges of the screen frame are the edges of the segments that are furthest from a center of the screen frame. Each of the plurality of clamping arms is positioned at a common height with respect to a plane in which the ventilation cloth is positioned. A heater melts the adhesive in the one segment. At least one insertion device inserts a ventilation cloth in the adhesive substantially across a length of said one of the segments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top plan view showing a station including two frame assembly machines according to the present invention. 
       FIG. 2  is a side elevation view of one of the machines of  FIG. 1 , taken along section line  2 — 2  of  FIG. 1 . 
       FIG. 3  is a side elevation view of the machine of  FIG. 2 , in a position for heating the adhesive in the frame bar segment. 
       FIG. 4  is a side elevation view of the machine of  FIG. 2 , in a position for inserting the screen and cooling the adhesive in the frame bar segment. 
       FIG. 5  is an enlarged, partial cutaway perspective view (with the insulation partially removed) of the nozzle section of the machine shown in  FIG. 1 . 
       FIG. 6  is an isometric view of a first exemplary screen bar segment suitable for assembly in the machine shown in  FIG. 1 . 
       FIG. 7  is an isometric view of the screen bar segment of  FIG. 6 , with a portion of screen material attached thereto. 
       FIG. 8  is an isometric view of a second exemplary screen bar segment suitable for assembly in the machine shown in  FIG. 1 . 
       FIG. 9  is an isometric view of the screen bar segment of  FIG. 8 , with a portion of screen material attached thereto. 
       FIG. 10  is an enlarged view of a portion of the screen assembly shown in  FIG. 1 . 
       FIG. 11  is a cross sectional view of the screen bar segment shown in  FIG. 10 , taken along section line  11 — 11  of  FIG. 10 . 
       FIG. 12  is a top plan view showing a second exemplary station including two frame assembly machines according to the present invention arranged in an alternative configuration. 
       FIGS. 13A and 13B  are cross sectional views showing a further exemplary embodiment of the invention, using an adhesive tape. 
       FIG. 14A  shows a method of attaching a screen to a frame using a roller type inserting apparatus. 
       FIGS. 14B–14D  show exemplary methods for cleaning the cutting tool shown in  FIG. 14A . 
       FIGS. 15A and 15B  show a variation of the exemplary method using a shielding tape between the pins and the adhesive. 
       FIG. 16  shows a detail of the apparatus of  FIG. 1 . 
       FIG. 17  shows an alternative embodiment of the inserting pin shown in  FIG. 5   
       FIG. 18  shows a further variation of the embodiment of  FIG. 1 . 
       FIGS. 19A–19C  show still another variation of the embodiment of  FIG. 1 . 
       FIG. 20  is an isometric view of another exemplary embodiment of the invention. 
       FIG. 21  is an isometric view of the clamping subassembly shown in  FIG. 20 . 
       FIG. 22  is an isometric view of the press subassembly shown in  FIG. 20 . 
       FIG. 23  is an isometric view of the yoke shown in  FIG. 21 . 
       FIG. 24  is an isometric view of a yoke for use on the subassembly shown in  FIG. 22 . 
       FIG. 25  is a cross sectional view taken along section line  25 — 25  of  FIG. 22 . 
       FIG. 26  is a cross sectional view taken along section line  26 — 26  of  FIG. 22 . 
       FIG. 27  is a side elevation view of a corner shown in  FIG. 21 . 
       FIG. 28A  is a side elevation view of the apparatus of  FIG. 20 , with the press subassembly raised and the foam platen lowered. 
       FIG. 28B  is a side elevation view of the apparatus of  FIG. 20 , with the press subassembly raised and the foam platen raised. 
       FIG. 28C  is a side elevation view of the apparatus of  FIG. 20 , with the press subassembly lowered and the foam platen raised. 
       FIG. 29  is a block diagram of the control system for the apparatus of  FIG. 20 . 
       FIG. 30  is a plan view of a work cell including the apparatus of  FIG. 20 . 
       FIGS. 31A–31C  show another exemplary apparatus for automatic insertion of screen cloth into screen frames, wherein the frame is held in a vertical position during insertion. 
       FIG. 32  shows an alternate insertion device suitable for use in any of the insertion apparatus described herein. 
       FIG. 33A  is a plan view of the clamping and insertion apparatus of  FIG. 31A , with the arms positioned for clamping a large screen frame. 
       FIG. 33B  is a plan view of the apparatus of  FIG. 33A , with the arms configured for clamping a small screen frame. 
       FIG. 34  is a side elevation view of the apparatus shown in  FIG. 33A . 
       FIG. 35  is a bottom elevation view of the apparatus shown in  FIG. 33A . 
       FIGS. 36–39  are detailed elevation views of the clamping and insertion devices of  FIGS. 34 and 35 . 
       FIG. 40  is an elevation view of a variation of the apparatus shown in  FIG. 31A . 
       FIG. 41  is an isometric view of one of the carts shown in  FIG. 40 . 
       FIG. 42  is a right side elevation view of the oven shown in  FIG. 40 . 
       FIG. 43  is an enlarged detail of  FIG. 42 . 
       FIG. 44  is a top partial plan view of the apparatus of  FIG. 40 . 
       FIG. 45  is an enlarged detail of  FIG. 44 . 
       FIG. 46  shows the “folding finger’ of  FIG. 45  in an alternate position. 
       FIG. 47  is a rear elevation view of the apparatus of  FIG. 33A , showing screen cloth feeding apparatus. 
       FIG. 48  is a side elevation view of the screen cloth feeding apparatus of  FIG. 47 . 
   

   DETAILED DESCRIPTION 
   PCT International Application No. PCT/IB00/01716, filed Aug. 23, 2000, U.S. patent application Ser. No. 09/379,102 filed Aug. 23, 1999, and U.S. patent application Ser. No. 08/997,737 filed Dec. 24, 1997 are all expressly incorporated by reference herein in their entireties. 
   The invention includes a method and apparatus for securing a screen  34  to a frame  30 , or to a screen bar segment  30   a  of the frame  30 . The invention also includes a frame and screen assembly formed by the method, and a screen bar stock used in the assembly. 
   As shown in  FIG. 1 , the exemplary frame  30  includes a plurality of screen bar segments  30   a – 30   d . Each screen bar segment  30   a – 30   d  has a mounting surface  32   a  which may be a bottom of a groove or tensioning step  32  or  32 ′ (best seen in  FIGS. 6–11 ) on a face of the frame  30 . The frame  30  may have a flat face, and the mounting surface may be a portion of the flat surface (not shown), but a groove  32  or tensioning step  32 ′ is preferred, because it enhances removal of slack in the screen upon insertion of the pins. The tensioning step  32 ′ has a bottom  32   a ′ and at least one side  32   b ′ (shown in  FIG. 9 ). Essentially, a groove  32  is a tensioning step that further includes a second side  32   c  (shown in  FIG. 7 ). 
   These structures and their equivalents are collectively referred to as a “mounting surface” or “tensioning step” herein, for ease of discussion. A mounting surface may be flat or may include a tensioning step. It will be understood that, as used herein the term “tensioning step” encompasses both a tensioning step that is part of a groove, and a step that is not part of a groove. This tensioning step is described in more detail below. 
   The screen bar segment  30   a  has adhesive  36  at the bottom  32   a  or side  32   b  of the tensioning step  32 . The adhesive  36  may be pre-installed in each screen bar segment  30   a – 30   d  before the screen bar segments  30   a – 30   d  are assembled to form the frame  30 . 
   The screen  34  is spread across the frame  30 , so that the screen  34  extends over the mounting surface (tensioning step  32 ) of each screen bar segment  30   a – 30   d  ( FIG. 10 ). The screen  34  is secured to the face of the frame  30  with an adhesive  36  at a plurality of positions  37  across a length of the tensioning step  32  of at least one of the screen bar segments  30   a – 30   d.    
   Preferably, forced convection with a heated gas having a temperature above the melting point of the adhesive is used to heat the adhesive. For example, the heated gas may be air heated to about 175 C, blown directly onto the adhesive  36  (as shown in  FIG. 3 ) to melt the adhesive. The screen  34  is inserted with an inserting apparatus  52 , which may include a plurality of pins  54 . Pins  54  embed or suspend the screen  34  in the adhesive  36  intermittently across a length of the screen bar segment  30   a,  until portions of the screen beneath the pins  54  ate inserted in and possibly contact the bottom  32   a  of the mounting surface (as shown in  FIG. 11 ). The pins  54  of the inserting apparatus  52  contact the adhesive  36  during the inserting step. Natural or forced convection may be used in combination with conduction to cool the adhesive  36 . If convection is used, a cool gas having a temperature below the melting temperature of the adhesive  36  is provided. The cool gas may be ambient temperature air, and is blown onto the adhesive  36 , or onto the frame, near the adhesive. Preferably, the plurality of pins are removed after allowing the adhesive to cool below the melting point of the adhesive. 
   The adhesive may be a hot melt adhesive or a thermoplastic resin having a heat resistance temperature of at least about 35° C., preferably between about 100° C. and about 130° C., and a viscosity that is preferably below 5400 poise at about 200° C. For example, the adhesive may be a hot melt adhesive such as polyester, polyamide, polyolefin, polypropylene, polyurethane, butyl or ethylene vinyl acetate based adhesives. 
   Referring again to  FIGS. 1–5 , the apparatus  100  for securing the screen  34  to a screen bar segment  30   a  includes a support surface  101  that holds the screen bar segment. One or more pre-loading blocks  40  ( FIGS. 1–5 ) are provided to hold a pre-bowed frame  30  against the support surface  101 , so that the frame  30  is distorted to a desired camber while the screen  34  is secured. The frame  30  may be held substantially straight, or may be given a reverse camber while attaching the screen, if desired. Preferably, the apparatus  100  includes a plurality of pre-loading blocks  40  arranged outside of the frame to engage all of the screen bar segments  30   a – 30   d  of the frame  30  simultaneously. A heat source applies heat directly to the adhesive  36  to melt the adhesive. The heat source may include a plurality of nozzles  58  (shown in  FIGS. 2  and  5 ) that direct a heated gas onto the adhesive  36 . The nozzles  58  may be located on a movable body  50 . The source of the heated gas may include a hot air plenum  60 . In the exemplary embodiment, the plenum  60  may be located on the movable body  50 . 
   According to an aspect of the invention, the pre-loading blocks  40  may be positioned outside of the frame  30 , without using any stop-blocks inside the frame. The frame  30  may be deformed inward elastically (hourglassed) slightly, so that when the frame is removed from the pre-loading blocks  40 , the frame returns to a substantially straight configuration, with sufficient movement to remove wrinkles from the screen material  34 . (The screen material  34  has a high modulus of elasticity (Young&#39;s modulus) relative to the frame members, so that the frame members are held straight by the screen material. A pair of inside (backstop) blocks may be used to limit the amount of movement when the frame is pre-loaded by the pre-loading blocks  40 . The amount of this pre-bow or pre-tensioning is sufficiently small so that, when the frame  30  is released from the pre-loading blocks  40 , the screen material  34  is substantially wrinkle-free, but has a sufficiently small amount of tension so as not to overly distort the screen bar. 
   A convenient blocking system includes a ferrous table top  101  (e.g., steel) and a plurality of blocks  40  that are strong permanent magnets, such as ceramic-type magnets. A few magnets can provide the desired force to clamp a lightly pre-bowed frame into a straight configuration during screen insertion. For this purpose, a total force on each side of the frame  30  need only be about 9–18 Newtons (2–4 pounds). The magnets can be quickly and easily positioned manually, using a visual inspection to determine when the frame  30  is straight. 
   Alternatively, the configuration may include pre-loading blocks  40  on all four sides of the frame, with backstop blocks inside of the frame on only two sides; the inside blocks may be used on the two sides of the frame into which the screen material is currently being embedded in the adhesive. The two sides into which the screen is currently being inserted are held straight, whereas the remaining two sides are allowed to deflect inward towards the center of the frame, so as to have a reverse camber. 
   The plurality of pins  54  (best seen in  FIG. 5 ) are located on the movable body  50 , proximate to the nozzles  58 . The plurality of pins  54  may be arranged in a straight line segment. An actuator  84  raises and lowers the body  50  (or the table) so that the pins  54  simultaneously push the screen  34  into the adhesive  36 . The pins  54  are capable of being actuated to embed the screen  34  in the adhesive  36 . A release coating (e.g., tetrafluoroethylene (“TEFLON®”) or silicone) may be applied to the plurality of pins  54  before inserting the screen  34  with the pins  54 . The plurality of pins  54  may be spring loaded with springs  56  to accommodate corners. Successive pins  54  may be spaced apart from each other by a distance δ ( FIG. 5 ) of between about 0.6 centimeters (cm) and about 2.5 cm. Preferably, the distance  6  between pins is about 1.25 cm. 
   As shown in  FIG. 4 , the nozzles  58  may also be configured to direct a cool gas directly onto the adhesive  36  when the nozzles  58  are connected to the source of the cool gas. The source of the cool gas may be plenum  70  and may contain ambient air. In the example shown, the nozzles  58  are connectable to either the source of heated gas (hot air plenum  60 ) or a source of a cool gas (cold air plenum  70 ). 
   The pins  54  may have a diameter P ( FIG. 5 ) that is less than a width W ( FIG. 10 ) of the groove  32  of the screen bar segment  30   a  by between about 0.05 centimeter and about 0.1 centimeter. For example, the tensioning step may be a groove  32  having a width W of about 0.35 centimeter. A preferred set of pins  54  corresponding to this width have a diameter between about 0.15 centimeter and about 0.34 centimeter, preferably between about 0.25 centimeter and about 0.3 centimeter. 
   The pins  54  may be arranged to simultaneously insert the fabric into the adhesive on any non-zero number of sides of the frame. Preferably, the fabric is attached to two of the sides at a time. As shown in  FIG. 5 , in an exemplary embodiment of the apparatus, the plurality of pins  54  include a row and a column of pins aligned in an angle-shaped configuration, for inserting the screen  34  into the adhesive  36  on two screen bar segments  30   a  and  30   b  of the frame  30 , simultaneously. The angle may be a right angle as shown in  FIG. 5 , or another angle for a non-rectangular window. Once the screen  34  is attached to two adjacent sides, the frame is rotated by 180 degrees, and the heating, inserting and cooling steps are repeated to insert the screen  34  into the tensioning steps  32  on a third screen bar segment  30   c  and a fourth screen bar segment  30   d  of the frame  30  simultaneously. 
   More generally, for any window having an even number of equal sides 2N (where N is an integer greater than one), the pins may be arranged to insert the screen in two of the sides simultaneously. The window can be rotated N-1 times by (360/N) degrees per rotation, to complete installation of the screen  34  in N inserting steps. 
   Although the apparatus could include pins for all four sides of the frame, such an arrangement would be limited to a specific size of frame (unless at least two of the sides of the apparatus are adjustable, which complicates the apparatus). By including pins on only two sides, a single machine can accommodate a variety of sizes easily, without adjustment. Other arrangements are also contemplated, as described below. 
     FIGS. 6–11  show a segment of a first type of screen bar  30   a  for use in forming a screen and frame assembly.  FIG. 6  is an isometric view of the screen bar segment  30   a  before assembly.  FIG. 7  is an isometric view of the screen bar segment of  FIG. 6 , with a portion of screen material  34  attached thereto. 
     FIG. 10  is a top plan view of the screen bar segment  30   a  and screen material  34  shown in  FIG. 7 . In  FIG. 10 , the segment of screen bar  30   a  includes a tensioning step provided by the bottom  32   a  and one side  32   b  of a groove  32 . Adhesive  36  is applied along the base  32   a  of the tensioning step, in the groove  32  of the screen bar  30   a.  Therefore, as shown in  FIG. 10 , the adhesive is secured to the screen bar  30   a  at the base  32   a  of the groove  32 . Also shown in  FIG. 10  are a plurality of indentations  37  formed in the adhesive  36  by the insertion pins  54 , while embedding the screen material  34  into the adhesive. 
     FIG. 11  is a cross sectional view taken along section line  11 — 11  of  FIG. 10 .  FIG. 11  is not to scale; vertical dimensions are exaggerated to show features of the exemplary assembly. In particular, the screen material  34  may be pushed substantially all of the way to the bottom  32   a  of the groove  32  by pins  54 , forming indentations or openings  37  in the adhesive bead  36  or film, so that the screen substantially contacts the bottom  32   a  (i.e., not more than a microscopically thin film is interposed between the screen material and the bottom of the groove beneath the indentations.) In between the indentations  37 , the screen material  34  is intermittently suspended slightly above a thin layer of adhesive. Thus, the screen material  34  acts to strengthen and reinforce the adhesive  36  in the regions between the indentations  37 . The resulting structure is very strong. 
   Optionally, the mounting surface  32   a  of the tensioning step  32  may have a plurality of features  38 . The features  38  may be dimples, indentations, holes, slots, striations, or the like. The features  38  are intended to provide a better mechanical bonding surface for the adhesive  36 . 
     FIG. 8  shows a cross-sectional view of a segment of a second type of screen bar  30   a ′ for use in forming a screen and frame assembly in which screen can be adhesively secured to the screen bar.  FIG. 8  shows that the segment of screen bar  30   a ′ includes a step, lip or wall (hereafter, called a “step”)  32 ′ along one side thereof. Adhesive  36 ′ applied along the base of the step  32 ′ of the screen bar  30   a ′. In this embodiment, since the base of the step  32 ′ has a relatively sharp angle, the adhesive may be applied against the base of the step  32 ′. Therefore, as shown in  FIG. 8 , the adhesive  36 ′ is secured to the screen bar  30   a ′ along and adjacent to the step  32 ′. 
   In the embodiments shown in  FIG. 6  or  8 , a tensioning step can be provided by a conventional spline groove or the like, or by a step, lip, or wall, for example, as desired. A groove ( FIGS. 6 ,  7 ,  10  and  11 ) is preferred over a step ( FIGS. 8 and 9 ), lip or wall that is not a groove, because the groove allows the homeowner to install a replacement spline to replace the screen, if necessary, and may be more aesthetically pleasing (The adhesive and the edge of the screen can be hidden from view.) A groove  32  also protects the adhesive bond area from weather and ultraviolet radiation from the sun, to some degree. Also, if a groove is not used, greater pre-tensioning of the screen material may be necessary to achieve tension in the screen fabric  34 . 
   Systems according to the present invention use adhesive  36  in the groove  32  or tensioning step of the screen bar  30   a  (or at the bottom of a tensioning step  32 ′, shown in  FIGS. 8 and 9 ) to secure the screen  34  to the screen bar  30   a . The present invention solves problems associated with automated installation of screen material  34  on a frame  30 . It is a tremendous improvement over manual techniques for attaching a frame using adhesives, and over the current spline technology for at least the following reasons: 
   The invention eliminates the need for manually inserting the screen in the frame. This elimination results in: 
   No repetitive strain injury—specifically, a worker is not likely to suffer carpal tunnel syndrome as a result of practicing an assembly technique according to the invention. 
   Much less effort (physical strength) is required to install screen material using the invention. There is less difficulty and manual work to manufacture screen assemblies. 
   Little or no skill is required to operate the assembly equipment. 
   Screen-to-frame retention (bond strength) fabricated by a method according to the invention is three to four times stronger than bonds fabricated using spline technology. Frame and screen assemblies fabricated using apparatus and methods according to the present invention consistently exceed the current standards for pull out strength, whereas spline technology marginally meets these standards. 
   The strength of the fastening is not dependent upon the gauge of the screen bar metal (as is the case with spline technology), thus allowing reduced metal gauge without loss of retention strength performance 
   Reduced part cost 
   The invention provides a two to three-fold increase in assembly throughput, reducing overall cost significantly. 
   An apparatus according to the invention can provide low cost, using simple, low-tech machinery. It is far simpler and far better than any automated screen assembly machine currently available commercially. 
   Can use existing screen bar profiles, connectors, fastening hardware. 
   A frame-screen assembly fabricated according to the invention still allows screen replacement using traditional spline technology by the homeowner. 
   Improved consistency of tensioning over manual methods and control of quality independent of the skill of the operator. 
   Referring again to  FIGS. 1 and 2 , an exemplary work station  110  including two frame-screen assembly systems  100  is shown. Each machine  100  includes a movable block or body  50  which includes heating, inserting and cooling apparatus  52 . The exemplary body  50  has a plurality of spring-loaded pins  54 , an insulated hot air plenum  60 , a cold air plenum  70 , and a plurality of common slot nozzles  58  for heating and cooling. Insulation  66  on the hot air plenum provides a uniform temperature distribution across the plurality of nozzles  58  throughout the heating, inserting and cooling apparatus  52 . The hot air plenum  60  receives the hot air supply via a tube  62 , and the cold air plenum  60  receives the cold air supply via a tube  72 . 
   The plenums  60  and  70  are vessels or containers for gas. The plenums  60  and  70  may be pressurized. Although the drawings show plenums  60  and  70  as being parallelepipeds (boxes), any convenient shape may be used. 
   Although the exemplary apparatus  52  includes a plurality of nozzles  58  ( FIG. 5 ), one of ordinary skill recognizes that a single elongated nozzle (not shown) extending along the length of the body  52  may be used. Alternatively, a plurality of elongated nozzles (not shown) may extend along the length of the body  52 . Hereinafter, reference is only made to a plurality of nozzles, but the description below also applies to single nozzle configurations. 
     FIGS. 2–4  show the nozzles  58  and pins  54  in line with each other within a single row. For example, the nozzles  58  and pins  54  may alternate with each other. In the configuration of  FIGS. 2–4 , the nozzles are directed downward. In a variation (not shown), the nozzles and pins may be arranged in two parallel lines which are proximate to each other. The nozzles of  FIG. 5  may be slightly angled (depending on the relative positions of the nozzles and the adhesive), so the heated gas and cooled gas are obliquely applied to the adhesive or frame members. 
   Although the exemplary apparatus includes a single set of common nozzles that direct either hot air or cold air onto the adhesive, one of ordinary skill could readily configure an apparatus having a plurality of hot air nozzles and a separate and distinct set of cold air nozzles. For example, there may be a row of hot air nozzles and a separate row of cold air nozzles. Alternatively, hot and cold air nozzles may alternate within a single row. 
     FIG. 5  is an enlarged, partial cutaway perspective view (with the insulation  67  partially removed) of the nozzle section of the machine  100 . Insulation  66  and  67  may be provided to surround the hot air plenum  60  and the interior of the common nozzles  58 . The plenum  60  has a plurality of openings  57  which are connected to the common nozzles  58  by respective passages  59 . The insulation  66  and  67  reduces the heat retained in the nozzles when the flow of heated air to nozzles  58  is interrupted, thus reducing the time for the temperature to stabilize upon switching from hot air to cold (Similarly, the insulation reduces time to switch from cold air to hot air.). This insulation may be preferred to minimize cycle time but is not required for the apparatus to function. In an alternate embodiment, if separate hot and cold nozzles are used (not shown), the insulation keeps the hot nozzles hot and the cool nozzles cool. 
     FIGS. 2–4  show the position and orientation of the nozzles  58  and insertion pins  54  in relation to the screen bar  30   a  in the loading/unloading position ( FIG. 2 ), heating position ( FIG. 3 ) and the screen insertion/cooling position ( FIG. 4 ). In the loading/unloading position ( FIG. 2 ), the hot air can be either blowing (preferred for pre-heating the plenum  60 ) or shut off. It may be preferable to have the cold air blower shut off when the apparatus is in the loading/unloading position of  FIG. 2 , to reduce wasted energy. In the heating position ( FIG. 3 ) the only blower that is turned on is the hot air blower, providing air via tube  62 . In  FIG. 3 , the nozzles  58  are directly above the groove  32 . This position and orientation of nozzles  58  is optimized to direct hot air directly into the groove  32  (in a direction perpendicular to the surface of the adhesive) for focused heating of the adhesive  36 , while minimizing the amount of heating applied to the frame substrate  30  which would increase the cooling required. 
   One of ordinary skill can readily place the nozzles  58  in other positions and orientations to direct the air onto the frame substrate  30  to indirectly heat the adhesive through the frame substrate  30 . For example, if the nozzle is not directly over groove  32 , the nozzle may be oriented at an oblique angle. Indeed, this may appear advantageous from the perspective of machine design simplicity, because the nozzles  58  can be further away from the pins  54 . The nozzles  58  could also be below the frame, blowing on the bottom. Nevertheless, directly heating the adhesive  36  (instead of the frame  30 ) has a different advantage: less total heat is required to heat the adhesive  36  to its melting point when the heat is directly applied to the adhesive. This reduces both the heating time to melt the adhesive  36 , and the subsequent cooling time. Cooling time is especially reduced by applying heat to the adhesive instead of the frame. If the frame were heated, residual heat in the frame would be conducted back to the adhesive during cooling, increasing cooling time and possibly remelting cooled adhesive. 
   In the insertion/cooling position ( FIG. 4 ) the only blower that is on is the cool gas blower (not shown), providing gas via tube  72 . Cool gas (for example, room temperature air) from tube  72  passes through the cold air plenum  70  and out through the same (common) nozzles  58  as the hot air. The pins  54  are positioned proximate to the nozzles  58 . The apparatus may be configured with separate hot and cool gas blowers (not shown), or there may be a single blower coupled with appropriate valving to both hot and cool gas plenums for circulating both hot and cool gas. 
   Optionally, the hot air tube  62  and cold air tube  62  may each have a means to limit reverse flow of air. For example, there may be a means for limiting flow of the cool gas into hot air tube  62 , and/or a means for limiting flow of the hot gas into the cool air tube  72 . Each of these limiting means may comprise a lightweight flapper valve (not shown). 
   In another optional variation, a flapper valve (not shown) may be provided in the hot air stream, while allowing a trickle of cold air to flow throughout the heating, inserting, and retracting steps of the fabrication process. This may help reduce heating of the cold gas plenum. 
   As shown in  FIGS. 2–5 , the exemplary actuator  80  includes a linear bearing  87  to maintain the alignment of the support arm  90 , and a pair of actuating cylinders  81  and  84 . In the example, cylinder  84  has a relatively long stroke, and cylinder  81  has a relatively short stroke. Cylinders  81  and  84  may be either hydraulic or pneumatic cylinders. Cylinder  81  has a pressurized input line  82  and an output line  83 . Cylinder  84  has a pressurized input line  85  and an output line  86 . The pressurized lines  82  and  85  are each coupled to one or more raise valve assemblies (not shown). The raise valve assemblies may include conventional position control valves (e.g., spool valves, not shown), and may include check valves (not shown) to prevent backwards flow. 
   To maximize safety, the apparatus may be biased (using springs, for example) to the raised position, and only moved to the lowered position when actuated by the hydraulic pneumatic cylinders. 
   Each raise valve has an input to receive the pressurized gas or fluid from a pump (not shown). Output lines  83  and  86  may be coupled to lower valve assemblies (not shown). The lower valves controllably release the gas or fluid from the cylinders  81  and  84  as desired to lower the support arm  90 . If cylinders  81  and  84  are hydraulic cylinders, then the lower valves return the hydraulic fluid to tank. 
   The pair of cylinders may be operated in at least two optional ways. In a first method, both cylinders are extended in the raised position of  FIG. 2 . The large cylinder  84  is lowered completely to move the insertion assembly  50  into the heating position of  FIG. 3 . Once heating is complete (7–10 seconds at 350° F. for the 6107 adhesive), then the short cylinder  81  is lowered to the position of  FIG. 4 , to perform the actual insertion step. 
   Although the exemplary embodiment shows actuating cylinders, one of ordinary skill recognizes that other conventional mechanical actuators may be used. 
     FIG. 18  shows an alternative design, in which the inserting apparatus is included in a body  1850  that is fixed to the ceiling  1887  or to a rigid overhead support (not shown) fixed to a floor mounted riser (not shown). In this example, the plenums  1860  and  1870 , the nozzles  1857  and the inserting pins  1854  are all fixed relative to the ceiling. The working surface  1801  is mounted on a vertically movable platform  1800 . Rather than raising or lowering the inserting apparatus, the screen and frame materials are raised to meet the inserting apparatus. This may be a simpler configuration, because the components that are connected by hoses and tubes to the air blower(s) are all fixed (to the ceiling). 
     FIGS. 19A–19C  show a further alternative method and apparatus for securing a screen to a screen bar segment. A screen bar segment  1930  has a mounting surface  1932  (in this case, a groove) on its face. The segment  1930  has adhesive  1936  on the mounting surface  1932 . A screen  1934  is spread across the mounting surface  1932  of the screen bar segment  1930 . A plurality of pins  1954  are provided. The adhesive  1936  is heated without heating the pins  1954  ( FIG. 19A ). When the relatively cool pins  1954  are inserted into the melted adhesive  1936  ( FIG. 19B ), heat is conducted out of the adhesive into the pins, helping to cool the adhesive adjacent to the pins more rapidly than the adhesive remote from the pins. The relatively cool pins are important for reducing cycle time. Using cool pins  1954 , the adhesive  1936  adjacent the pins solidifies sufficiently to allow clean extraction of the pins in about eight seconds. (Extracting pins before the adjacent adhesive solidifies may result in formation of “strings” of adhesive). Applying a release coating onto the pins  1954  may further assist in preventing formation of strings of adhesive upon extraction. 
   With the pins  1954  in the raised position ( FIG. 19A ), the adhesive can be melted without heating the pins. This is achieved by blowing a first gas (e.g., heated air) through a nozzle  1957  (having a rotatable orifice  1957   a ) onto either the screen bar segment  1930  or the adhesive  1936 , without blowing the first gas onto the pins  1954 . In the example of  FIGS. 19A–19C , the plurality of pins  1954  are positioned on arms  1902  that are actuable independently of the heated air source. For example, in  FIG. 19A , an arm  1902  is attached to an actuating cylinder  1983  (which may be pneumatic or hydraulic). Raising the cylinder  1983  to the position of  FIG. 19A  raises the plurality of pins  1954  out of the path of the stream of air exiting from the nozzles  1957  while the heated air from plenum  1960  is melting the adhesive. While in the raised position of  FIG. 19A , the pins cool off, mainly through convection. 
     FIG. 19A  also shows a slidable clamp  1940 . When the operator places the frame  1930  on the table  1901 , the operator slides the clamp  1940  to the desired location along the length of the frame. The clamp  1940  is mounted to a slide  1941 , that slides along a fixed rail  1942 . The exemplary clamp has a toggle  1943  to lock the clamp in place, but any type of clamping mechanism may be used. 
   After the adhesive  1936  is melted, the hot air is discontinued, and the cylinder  1983  is lowered, to lower the pins  1954  into the adhesive (as shown in  FIG. 19B ). This operation pushes the screen  1934  with the plurality of pins, so the screen contacts the adhesive across a length of the screen bar segment  1930 . The pins  1954  may be mounted on the cool air plenum  1970 . The plenum  1970  may have nozzles  1971  that continuously direct a second gas (e.g., cool air having a temperature which is below a melting temperature of the adhesive) at the pins  1954  at all times, except optionally when hot air is blowing, and/or nozzles that only direct cool air onto the adhesive  1936  when the pins are in the lowered position shown in  FIG. 19B , blowing cool air onto the screen bar segment or the adhesive. 
     FIG. 19C  shows the configuration in the corner of the apparatus  1900 . To ensure proper insertion of the screen cloth  34  into the groove  1932  at the corner of the frame  1930 , it is important that none of the pins  1954  in the corner (outside of the groove) clamps the cloth against the corner key  1990  and prevents proper insertion of the screen in the groove. Apparatus  1900  includes another means for restraining the corner pin(s) so they do not interfere with insertion. 
   The pins  1954  near the corner (or all of the pins  1954 ) have knobs  1981  at their top ends. A slidable block  1985  has a slot  1989  ( FIG. 19B ) for receiving the shaft of pin  1954 , beneath knob  1981 . The slot  1989  is sized larger than the shaft of pins  1954 , but smaller than the knobs  1981 . The block  1985  slides along a rail  1986 . The corner pins  1954  can be easily lifted, and the block  1985  slides along rail  1986  to capture the knobs  1981  of any desired number of pins  1954 . For a typical frame profile, one or two pins are sufficient. Once the desired number of pins are captured, a clamp  1987  holds the block  1985  in place during subsequent insertion operations. Any conventional clamp may be used for this purpose. With the cylinder  1983  in the lowered insertion position of  FIG. 19C , the corner pin  1954  is secured above the corner key  1990 .  FIG. 19B  shows one of the remaining unconstrained pins  1954  as it appears during insertion at the same time the corner pin of  FIG. 19C  is being restrained, with the cylinder  1983  in the same position as  FIG. 19B . 
   An alternative means of repositioning the pins near the corner is to insert a block (not shown) between the top of bracket  1972  and the bottom of knob  1981 . The block has a slot to receive the shaft of pin  1954 . This block can be inserted manually or automatically. 
   Preferably, a PID controller is used to control the heating of the heating plenum assembly  1960 . When the heating begins, the air supply is diverted from cold air manifold  1970  to the hot air plenum  1960 . There is a blast of air over a coil  1962   a  that has heated up. If the temperature is too cold, it takes longer to melt the adhesive  1936 . If the temperature is too hot, smoking may occur. Thus, it is desirable to control the temperature of the air leaving the nozzle  1957  to plus or minus five degrees C. 
   Although only one of the arms  1902 .is shown in  FIGS. 19A–19C , it is understood that the apparatus includes a plurality of arms, each arm having a plurality of pins mounted thereon. 
     FIGS. 19A and 19B  also show modifications to the hot air plenum assembly  1960  to ensure more uniform heating of the air along the entire length of the plenum, so as to more uniformly provide heated air to melt the adhesive along the length of the screen bar segment  1930 . The exemplary plenum assembly  1960  may have at least one heating element  1962  along the length of the plenum, to heat the air provided to the entire length of the screen bar segment. 
     FIGS. 19A and 19B  show an alternative modification which may be used in addition to, or in place of, the heating element  1962 . A baffle  1961  extends partially along said length of the plenum  1962 , so as to transport heated air from a hotter end of the plenum to a cooler end of the plenum. Cool air flows through the baffle  1961  (at a rate of about 50 cubic feet per minute) and passes through openings  1961   a,  where the air is routed around a triangular heating element  1962 , which includes a coil  1962   a  wrapped around an insulator  1962   b.  The exemplary heating coil produces about five kilowatts of power. The air is forced to travel through the coil  1962   a,  around the insulator  1962   b.  The heated air flows through the nozzle  1957  and through a rotatable orifice  1957  that can be directed towards relatively cool spots (e.g., near a corner) and away from relatively hot spots. 
   For example, because the corner of the frame  30  tends to receive less heated air than the rest of the frame, the corner tends to run cooler than the rest of the frame. One way to partially compensate is to direct the nozzle orifices near the corner to divert air from the side towards the corner. By using separate nozzle orifices  1957   a,  the sizes of the orifices can be used to distribute heat more evenly. For example, by putting smaller orifices near the middle of the frame side, and larger orifices near the corner, more flow of hot air is directed towards the corner. Still another way of evening the distribution of heat is to place the nozzles closer together near the cold spots (e.g., near the corner). If necessary, additional heat can be added using an external heater (not shown) near the corner. 
   In a variation of apparatus  1900  (not shown), it is also possible for the blocking member to be connected to the same arm-mounted sub-assembly  1970  that includes the pins  1954 , so that blocking and insertion of the screen material are performed by a single downward motion of the press assembly (or upward motion of the platen assembly). To prevent the block from catching and pulling the screen cloth  1934 , the screen cloth must be cut to approximately its final size, so that the screen cloth does not hang over the outside edge of screen bar  1930 . In addition, the block has a ramped or curved edge, which prevents the block from crushing or damaging frame  1930 . Because the screen bar is formed with a convex camber, the block bends the frame  1930  inward while descending, and the ramped surface employs a cam action to gradually bend the frame. 
   In this variation, the block travels a longer distance while contacting the screen bar  1930  than the thickness of the adhesive  1936 , so that the frame  1930  is straightened and blocked into place before the pins push the screen cloth into the adhesive  1936 . This is important to ensure that the pins  1954  are properly aligned with the groove  1932 . 
   In this variation, when the pins  1954  reach the bottom of the groove  1932 , the blocking member is in place to block the screen bar segment. When the cylinder  1983  is actuated to raise the cold air plenum  1970  and pins  1954 , the blocking member is raised, to allow easy removal of the completed frame. To make sure that the block does not pull the frame  1930  up when the cooling plenum assembly is raised, block  1940  should have a non-stick coating, such as TFE. A spring may also be used on top of the frame, to push the frame away from the block. 
   Reference is again made to  FIG. 1 .  FIG. 1  shows a preferred configuration of the apparatus  100  in plan view.  FIG. 1  shows an “L” shaped assembly, capable of securing two sides  30   c  and  30   d  of a screen assembly simultaneously. A screen bar frame  30  is shown in position along with pre-loading blocks  40 .  FIGS. 2–4  show an exemplary pneumatic cylinder actuator assembly  80  to position the plenum block  50  in the three different positions as shown in  FIGS. 2–4 . 
   The plurality of nozzles  58  may include nozzles proximate to all four frame members  30   a – 30   d  to heat all four members simultaneously for screen insertion. Of course, the apparatus may also be configured to bond one side at a time. It is preferred, however, to heat and insert only two or three sides simultaneously rather than all four sides, as this simplifies the design of the machine and reduces set-up time for different size screen assemblies. It may be most preferred to heat and insert two sides  30   c  and  30   d  using an “L”-shape nozzle and pin insertion assembly. Once two sides are completed (as described herein) the frame  30  is removed from the machine  100 , rotated 180 degrees and re-inserted into the machine  100  to complete the other two sides of the screen assembly. 
   Similarly, for an octagonal window (not shown), it may be preferred to include nozzles  58  and pins  54  for heating and inserting two contiguous sides simultaneously. Instead of the pins being arranged in a right angle, the pins may be arranged in a 135° angle, so that any arbitrarily sized equilateral octagon is accommodated by one machine. The first two sides are bonded. The frame is rotated 90 degrees, and the next two sides are then bonded. This is repeated a total of four times, so that all eight sides are bonded. 
   Additional configurations for non-rectangular windows may include an “L” shaped apparatus with articulating arms, to accommodate a variety of angles between sides. Alternatively, any polygon can be accommodated by configuring the apparatus to bond only one side at a time. 
   Further, the apparatus may be configured with pins on two opposite sides (not shown). For example, there may be one fixed row of pins and a movable row of pins parallel to the fixed row of pins. The movable row of pins may be moved closer to (or further from) the fixed row of pins, to accommodate two sides of a rectangular or octagonal window simultaneously. 
   Only one degree of freedom, namely up and down motion, is used in this example to position the heating, inserting and cooling apparatus  52 . Although other arrangements may include additional degrees of freedom to position the inserting and cooling apparatus  52 , the single degree of freedom (with three positions) may be preferred to minimize cost and design complexity. 
   Other exemplary arrangements (not shown) may include having separate hot and cold air nozzles, optionally locating the hot and cold nozzles in separate rows with separate angles to direct the air onto the adhesive. If separate rows of hot and cold air nozzles are included, it may be necessary (depending on the location and angle of the nozzles) to either move the frame, or move the apparatus relative to the frame, when switching between hot and cold air. 
     FIG. 1  shows a work station including two machines  100  in service orientation with the operator in between the machines. This may be the preferred arrangement as it allows one machine  100  to be loaded and unloaded while the other machine is performing the automated insertion sequence. This approach is believed to maximize throughput for a single operator. 
   As shown in  FIG. 1 , the heating, inserting and cooling apparatus  52  is located closer to the outside of the “L” members  90 . This arrangement may be more preferred generally, as it is versatile and is preferred for larger screen assemblies where the operator stands between machines  100  as shown in  FIG. 1 . Having the pins  54  and nozzles  58  to the outside of the “L” facilitates viewing and positioning the screen cloth  34  prior to insertion. 
     FIG. 12  shows an alternative for positioning the two machines  100 . The heating, inserting and cooling apparatus  52  may alternatively be located on the outer perimeter of the two-machine configuration, further from the operator. This configuration may be preferred for smaller width machines that are limited to making smaller width screen assemblies. This alternative configuration, being narrow, may be easier for handling (i.e. loading and unloading) smaller screen frames. For larger screens (i.e. greater than approximately 60 cm wide), viewing and positioning the screen cloth  34  becomes difficult with the configuration of  FIG. 12 . 
   In both the configurations ( FIGS. 1 and 12 ) the pins  54  and nozzles  58  are preferably arranged along the side of the apparatus closer to the operator. 
   Insertion Pin Design 
   The pins  54  are used both to insert the screen cloth  34  and remove the slack from the cloth. Essentially, the action of pushing the screen cloth  34  past the tensioning step  32 , (which is preferably a groove), pulls the cloth  34  taut and pulls out small wrinkles. The taut screen  34  thus holds the pre-bowed frame members  30   a – 30   d  straight upon removing the assembly  30  from the pre-loading blocks  40  upon cycle completion. In effect, both the insertion of the pins  54  over the tensioning step  32  and the pre-loading of the frame  30  contribute to consistently setting the desired tension. Thus, it is believed to be most preferred to use both means together. However, tensioning may be achieved by either method, if used alone. 
   The insertion pins are large enough to push the open mesh screen cloth  34  into the molten adhesive  36  without passing through the mesh and missing the strands. If the tensioning step  32  is in a groove, the pins  54  must be sized to fit into the groove. The exemplary pins  54  have an axis of rotational symmetry; they are generally approximately cylindrical in shape. In experiments conducted by the inventor, the preferred pin diameter was greater than 0.15 cm (0.060″) and smaller than 0.34 cm (0.135″) to work effectively with common fiberglass window screen and a screen bar groove of 0.140″. The most preferred diameter observed was 0.25 cm (0.100″) to 0.3 cm (0.120″). Rectangular shaped pins also appear to function well. Rectangular pins may have a cross section with a larger dimension of about 0.3 cm (0.12″) to 1.27 cm (0.5″), big enough so that the pins  54  do not enter the holes in the screen material  34  during insertion. A cross-section of 0.6 cm to 1.27 cm is preferred. One of ordinary skill in the art can readily provide alternate pin cross-sections without any undue experimentation. The larger dimension of the pins may be nearly as wide as the center-to-center spacing between successive pins. 
   The mechanism of insertion using pins  54  is different from the spline insertion mechanism in the prior art. The pins  54  push the screen material  34  into the adhesive substantially without any friction between the screen and the mounting surface. The screen is held in place by the adhesive, not by friction. Because this method does not rely on friction between the screen material and the mounting surface, it is possible to use thinner screen bar material than could be used with conventional spline methods. In contrast, the spline technique relies on friction to hold the screen to the frame; a heavy frame material is needed to absorb the insertion force. 
   The preferred spacing of the pins is between 0.63 cm (0.25 inch) to 2.54 cm (1.0 inch) to achieve a practical design. Pins spaced further apart than 2.54 cm are not as effective at pushing the screen  34  in the molten adhesive  36  between the pins. Pins closer together than 0.63 cm do not improve the insertion and only add cost. The most preferred spacing is approximately 1.27 cm. 
   It is important for the pins  54  to extract cleanly from the adhesive  36  (after it has solidified) without undue forces and without strings of adhesive forming as the pins are extracted. Waiting until the adhesive  36  has fully solidified (forced air cooling helps to reduce the cooling time) avoids formation of strings in the adhesive upon extracting the pins  54  from the adhesive  36 . Preferably, the pins  54  are smooth (preferably polished), or coated with a release coating such as tetrafluouroethylene (TFE) or the like, to prevent the adhesive  36  from bonding to the pins  54 . Exemplary pin materials include aluminum, brass and stainless steel. Stainless steel offer the best durability, corrosion resistance and surface qualities for extraction and is thus believed to be the preferred material. Other materials such as ceramic or high temperature plastic may also be used. Further, pins formed of chrome (or plated with chrome) or TFE are also contemplated. A beryllium material may be preferred for the pins. Beryllium offers high strength and wear resistance and high thermal conductivity for rapid cooling of the adhesive. 
   Spring loaded pins  54  may travel approximately the depth of the groove  32  and allow the screen  34  to be assembled without interference by the pins  54  at the corner key of the frame  30  being assembled. Essentially, the pins  54  are pushed up, compressing the springs  56 , at the corners of the frame  30 . Thus, it is unnecessary to remove pins  54  to accommodate different sized screen frames  30 . This feature may be used where the screen cloth is cut to size, instead of designs (e.g.,  FIG. 17 ) in which the pins are adjusted or removed to accommodate differently sized frames, which increases set up time between fabrication of two screen assemblies having different sizes. In the exemplary embodiment, the springs are intended to be compressed only when there is interference at the corners. Along the sides, the remaining pins inserting the screen typically do not compress their respective springs. 
     FIG. 17  shows a variation of the pins. Each bayonet pin  1754   a  and  1754   b  has a roll pin  1757  mounted perpendicular to the axis of the pin. Bayonet pins  1754   a  and  1754   b  may be easily switched (manually) between two different positions, as an alternative to using spring loaded pins. In  FIG. 17 , the left pin  1754   a  is in the extended position, and the right pin  1754   b  is in the retracted position. A bias spring  1756  biases the bayonet pins  1754  towards the retracted position of the right pin. Spring  1756  is compressed between the roll pin  1757  and a flange  1760 , pulling the pin  1754   b  towards its retracted position. 
   The pins  1754   a  and  1754   b  are sandwiched between a front web  1762  and a rear web (not shown) behind the front web, forming a channel. Roll pins  1757  are longer than the width of this channel, so the pin  1754   b  cannot rotate freely within the channel. The front web  1762  has a horizontal slot  1758  that allows roll pin  1757  to rotate only when the roll pin is positioned at the height of the slot. With roll pin  1757  at the height of the slot  1758 , pin  1754   a  can be (manually) rotated until roll pin  1757  reaches the detent  1758   a.  If pin  1754   a  is released with roll pin  1757  projecting through detent  1758   a,  pin  1754   a  is prevented from inadvertent rotation. Thus, pin  1754   a  is locked in the extended position, as shown. 
   To switch a pin  1754   a  to its retracted position, pin  1754   a  is pulled down, to free roll pin  1757  of detent  1758   a,  and pin  1754  is rotated until the roll pin  1757  is freed from slot  1758 . Pin  1754   a  is then released, and spring  1756  retracts pin  1754   a  to the position of pin  1754   b.    
   Using the pins shown in  FIG. 17 , the pins near the center of the row of pins may be held rigidly in the extended position, while pins over the corner keys of the row are retracted so as to avoid interference with the relative movement between the inserting apparatus and the frame being assembled. 
   Another aspect of the pins  1754   a  and  1754   b  is the use of a tapered end  1761 . The tapered ends assist in ensuring that the adhesive does not stick to the pins with the pins are removed. By including only a few degrees of draft angle, the cleanness of the extraction is significantly improved. 
   Tapered end  1761  also helps assure proper insertion, even if there is a slight misalignment between the pin  1754   a,    1754   b  and the groove or tensioning step of the frame. 
   The tapered pin  1761  may even allow the use of a pin size that approaches the width of the groove, whereas a straight pin would be more likely to catch on the edge of the groove in the event of any slight misalignment. If pins are used that approach the size of the groove, then there would be friction between the screen  34  and the sides of the groove during insertion. This friction will cause greater tension in the cloth during insertion, and could result in localized over-tensioning and visible distortion at the pins. To prevent hourglassing if pins that approach the size of the groove are used, stop blocks should also be used inside the frame. Stop block  41  is a backstop to limit the amount of movement to ensure that the screen bar is held straight when the pre-loading block  40  is pushed against the screen bar frame on two sides. 
   In a further variation of the exemplary embodiments, the pins may be formed of adhesive. Instead of using a pre-installed adhesive, the adhesive pins may be used to insert the cloth. Once the cloth is inserted, the pins may be melted using heated gas or heat from the frame, as described above. The frame and adhesive can then be cooled using cool gas provided from a plenum, as described above. If glue pins are used, the diameter of the pins should be larger than the diameter of the metal pins described above, to insure good contact and wetting between the adhesive and the surfaces of the tensioning step. In this variation, the cloth can optionally be applied to all four sides simultaneously. 
   Methods of Heating 
   Although many different methods of heating would be effective for practicing the exemplary method of  FIGS. 2–4 , forced convection with hot air blowing directly onto the adhesive  36  is believed to be most preferred, because it is simple, fast, consistent and controllable. It is also the most cost-effective approach. Focussing the hot air onto the adhesive  36  (and not onto the surrounding frame substrate  30   a – 30   d ) quickens the melting of the adhesive  36  and avoids warming the substrate excessively. Keeping the frame substrate as cool as possible during the heating cycle reduces the cooling cycle time, because a cooler substrate sinks the heat away from the adhesive  36  more rapidly. Also, increasing the impingement velocity of the air onto the adhesive  36  increases the mass flow rate of air and the convective heat transfer coefficient, and thus increases the rate of heating. The trade-off is increased cost, and increased noise. 
   Other heated gases may be used, including, for example, nitrogen or an inert gas. 
   To achieve a 10 second heating time with an exemplary Henkel 6107 polyamide adhesive, air at 350° F. and 2 standard cubic feet per minute (SCFM) per inch is blown directly onto the adhesive  36 , through the screen  34 . (The 350° F. temperature does not create a hazard). Although faster rates may be achieved by increasing the flow rate, this is a reasonable, effective rate of heating. Increasing the temperature would also increase the heating rate, but may generate undesirable smoke. 
   A 2″×2″ (5 cm×5 cm) plenum having an attached nozzle with an opening of 0.050″(1.27 cm) wide and continuous in length (at least as long as the screen bar) positioned approximately ¼″ (0.63 cm) away from the screen frame  30  was found to be effective (see  FIG. 3 ). To achieve the desired 2 SCFM of 350° F. (177° C.) air per inch for a machine that can secure 2 sides of a 6′×3′ (182 cm×91 cm) screen simultaneously, approximately 200 SCFM total air volume is used. A minimum temperature for the hot air is greater than the melting temperature of the specific adhesive used. 
   Many different methods may be used to supply the hot air to the plenum  60 . The exemplary method is to pass air from a blower (not shown) through an electric heat exchanger (not shown), which is simpler, or indirect gas fired heat exchanger (less expensive). To deliver the hot air using the electric heat exchanger, a Leister ASO blower, model 9K attached to two Leister 10,000S tools, model 8D7 attached to each end of the “L” shaped plenum (see  FIG. 3 ) may be used. Leister ElektroGeratebau is located at 6056 Kagiswil/Switzerland. 
   Although the embodiment of  FIGS. 1–5  includes insulation around the nozzle, as shown in  FIG. 18 , the nozzles  1859  may be surrounded with a high thermal conductivity, high thermal diffusivity material  1889 , such as copper. This allows heat from the nozzles to be rapidly dissipated between the heating step and the cooling step, so that the nozzle does not heat the cool gas that is used to cool the adhesive. 
   Referring again to  FIGS. 1–5 , conductive heating through the aluminum substrate of frame  30 , although potentially faster than hot air (convection), may be difficult to achieve for some screen bar profiles, due to the contours of the profile shape of the screen bar. The frame  30  may be pre-heated by a variety of methods. The heated gas nozzles may be directed onto the frame  30  instead of the adhesive  36 . Alternatively, the frame may be pre-heated in an oven or heating apparatus. If the frame is pre-heated, the maximum temperature of the oven or heating apparatus must be sufficiently low so as not to damage any plastic components (e.g., corners) of the frame. This would also facilitate insertion of four sides simultaneously. 
   Using conductive heating through the pins  54  or other elements directly onto the adhesive  36  would not be effective in heating the adhesive between the pins, and wrinkles in the material would result, unless the pins are very close together. Tensioning and cooling may also be more difficult with this approach. 
   Induction heating may be impractical, if used to heat the entire frame simultaneously and is more costly than hot air (convective) heating. Induction heating is better suited to a continuous feed operation, heating a small area only. 
   Infrared (radiant) heating is not preferred, as the higher temperatures involved may cause undesirable smoke from the screen if the screen is positioned between the emitter and adhesive during operation. Infrared is typically more expensive than convective heating and more cumbersome to integrate into the design. 
   Operation 
   Briefly summarizing, the assembly machine operator loads the machine  100  with screen frames  30 , positions the screen cloth  34 , initiates the automated assemble sequence by activating a control, and unloads the finished screen assemblies when they are completed. 
   Preferably, the screen bar has pre-applied hot melt adhesive in the groove  32  (or at the base of the tensioning step). The assembly sequence is as follows: 
   The pre-assembled screen frame  30  is loaded onto the blocking table  101  where the pre-bow in the screenbar is straightened using blocking on the outside of the frame. Essentially, the pre-bowed screen bar is made into a frame  30 , the frame  30  is then mounted onto the surface  101  of a table, and pre-loading blocks  40  are used to straighten or slightly hourglass the frames  30  (or distort the frame into any desired camber for tensioning the screen). This is called “blocking”. After the screen cloth  34  is installed and the finished screen assembly is removed from the pre-loading blocks  40 , the frame members  30   a – 30   d  attempt to return to their pre-bowed condition due to their inherent elasticity. When this occurs, the screen cloth  34  is put under additional tension beyond that imparted by the tensioning step during the insertion operation, but the frame members  30   a – 30   d  stay straight due to the high modulus of the screen material. Both the tensioning step  32  and pins  54 , and blocking  40 , contribute to create the desired screen tension, which is sufficient to remove wrinkles. 
   The screen (cloth)  34  is positioned with its edges over the groove  32  or tensioning step, and extends past the groove  32  or tensioning step by a small amount to allow the subsequent insertion into the adhesive  36 . As best seen in  FIG. 16 , screen fabric  34  is preferably supported on a surface  39  during the fabrication operation. Preferably, surface  39  has a height that is substantially the same as the height of the tensioning step. This allows the screen fabric  34  to lay flat during fabrication. Thus, the screen material does not sag, and there is less slack in the screen cloth during assembly, which improves consistency. 
   The automated sequence is started by activating a control (which may be, for example, a button, toggle, switch, knob, or the like.) 
   An elongated (tubular plenum) hot air nozzle assembly positioned over the screen bar lowers to blow hot air at approximately 350° F. (177° C.) into the area of the adhesive  36  (i.e. into and/or around the groove  32  or tensioning step where the adhesive  36  is located). 
   Once the adhesive is melted (approximately 7–10 seconds when the air flow is approximately 2 SCFM per linear inch) the flow of hot air is shut off, and the screen insertion pins  54 , positioned in line over the groove  32 , push the screen cloth  34  into the molten hot melt adhesive  36 . The strands of the screen cloth  54  are thus embedded into either a bead (most preferred) of molten adhesive  36  or pushed in contact with a film of molten adhesive. (Note: this adhesive may have been applied previously, preferably at the time of manufacture of the screen bar  30   a – 30   d ). The screen cloth  34  is held in the molten hot melt adhesive  36  by the pins  54  until the adhesive  36  has solidified by cooling. 
   During testing, cooling was observed to take 10 to 15 seconds when the adhesive was allowed to cool naturally in the ambient air. Forced air cooling by blowing room temperature or chilled air onto the adhesive and onto the screen bar speeds up the rate of cooling and is thus preferred. By blowing room temperature air at the adhesive at approximately 2 SCFM per linear inch, the cooling time is decreased to approximately 5 seconds. 
   After the adhesive  36  is solidified, the insertion pins  54  are extracted and the finished screen assembly is removed. Allowing the adhesive to solidify completely before the pins  54  are removed ensures that the pins  54  extract cleanly from the adhesive  36 . Extraction is not a problem when smooth pins  54  are used. A release coating such as TFE may be used on the pins to lower the force of extraction and reduce the possibility of adhesive bonding to the pins and is thus preferred (but not necessary.) 
   Assuming that the apparatus inserts two sides of the screen, and that a four side screen is being inserted, the screen is rotated by 180 degrees, and steps  1 – 6  are repeated. Then insertion of the screen material is completed. 
   Apparatus for Simultaneous Insertion of Four Sides 
     FIG. 20  shows a further exemplary apparatus  2000  according to the present invention. Apparatus  2000  is configured for inserting screen fabric  34  into all four sides  30   a – 30   d  of a frame  30  simultaneously. The exemplary apparatus  2000  is also configured to accommodate a variety of frame sizes, and does not require a priori knowledge of the size of each frame  30  loaded on the apparatus. The operation of the exemplary apparatus  2000  is controlled by a programmable logic controller (PLC)  2900  (shown in  FIG. 29 ), using ladder logic, although other control apparatus may be used. 
   Apparatus  2000  includes three main subassemblies: a clamping subassembly  2100 , a press subassembly  2200  positioned directly above clamping subassembly  2100 , and a screen support  2800  ( FIG. 28 ) that prevents screen  34  from sagging or falling through the frame members during insertion. Clamping subassembly  2100 , press subassembly  2200 , and support  2800  are operated by a process controller  2900 . A frame  30  is pre-heated in an oven (not shown) to melt its adhesive  36 . A pre-cut screen  34  is placed on the frame, and the frame and screen are placed on the clamping subassembly  2100 . Clamping subassembly  2100  positions, straightens, and measures frame  30 . The measurement information is transmitted to the controller  2900 , which configures the press subassembly  2200 , based on the measurements, to accommodate the size of frame  30 . Press subassembly  2200  has a plurality of insertion pins  2222 ,  2242  depending from its lower surfaces. When press subassembly  2200  is lowered, the insertion pins  2222 ,  2242  simultaneously insert the screen  34  into the adhesive  36 . The structure of the clamping subassembly  2100  and press subassembly  2200  are explained below, followed by a detailed description of the operation of apparatus  2000 . 
   The pre-loading function of the apparatus of  FIG. 20  may be contrasted with the pre-bowing of the apparatus  100  of  FIGS. 1–4 . In both instances, the screen bar material begins with a convex camber, bowing outward slightly at the center of each side of the frame. In the apparatus  100 , two sides are clamped and two sides are free during insertion, so there is relatively little tension placed on the screen cloth. Consequently, in apparatus  100 , before inserting the screen, the clamped frame members  30   a,    30   b  are pre-bowed to a slightly hourglassed camber. In contrast, apparatus  2000  of  FIG. 20  inserts the screen cloth all four sides of the frame simultaneously, creating the potential to impart greater tension. Therefore, apparatus  2000  pre-bows the frame to an approximately straight shape before inserting the screen; it is not necessary to pre-bow the frame to an hourglass shape. 
     FIG. 21  is an isometric view of clamping subassembly  2100 . Clamping subassembly  2100  has a first fixed frame support  2101  and second fixed frame support  2102 , on which are placed the first side member  30   a  and second side member  30   b  of the frame  30 , respectively. A movable frame support  2103  is automatically slidable under the third side member  30   c  of the frame  30 . A movable clamping arm  2104  automatically compresses the fourth side member  30   d  towards the second side member  30   b.  The first fixed frame support  2101 , second fixed frame support  2102  and the movable frame support  2103  are coplanar, forming a three-sided support surface on which members  30   a – 30   c  rest during screen insertion. The movable clamping arm  2104  is slidably mounted above the supports  2101 – 2103 , and is immediately adjacent to the fourth frame side member  30   d.  The four arms  2101 – 2104  together provide a clamping structure for the frame  30 . 
   One of the functions of the subassembly  2100  is to register and properly position the frame  30  for screen insertion. Because apparatus  2000  does not require a priori knowledge of the size of frame  30 , a standard positioning convention is used. In most screen frames  30 , regardless of the frame dimensions, the registration distance RD (shown in  FIG. 10 ) between the inside edge (lip) of the screen bar segment  30   a  and the centerline of the spline (insertion) groove  32  is a constant for all screen profiles. The inner edge RE of the screen bar  30   a  ( FIG. 10 ) is used as the reference edge. 
   As shown in  FIG. 21 , three screen bar segments  30   a – 30   c  of the frame  30  rest on respective arm assemblies  2101 – 2103 , which are described in detail below. Screen bar segment  30   a  is positioned on arm assembly  2101  and the inner reference edge RE of segment  30   a  is placed against the registration clamp  2111 . A profile clamp  2113  is actuated to contact and clamp the outer edge OE ( FIG. 10 ) of the screen bar  30   a,  and the profile of screen bar  30   a  may be measured using an LVDT (linear variable differential transformer), which may be combined in a single unit with clamp  2113 . The device measures the distance between the inner and outer edges of the screen bar  30   a . Device  2113  provides the controller  2900  with the profile of the screen bar, for use in positioning the insertion pins  2222 , discussed below. 
   As an alternative to an LVDT, other high precision measuring devices may be used. For example, it is known to provide an actuating cylinder with an integral linear potentiometer. An exemplary device capable of performing the clamping and profile measuring functions is a position-feedback pneumatic cylinder with an integral linear resistive transducer, part No. PFC-091-X, manufactured by the Bimba Manufacturing Co., Monee, Ill., USA. This device has a linear potentiometer with a probe that measures extension of the cylinder. The exemplary device has a 2.7 cm (1 1/16″) bore and a 2.5 cm (1″) stroke, and provides an output of 0–10 Volts. The analog output signal is provided to an analog-to-digital converter, which outputs a digital signal to the controller  2900 . 
   The first arm assembly  2101  of the clamping subassembly  2100  includes a frame member  2110 , registration clamp stops  2111 , one or more outer clamp blocks  2112 , and the clamp/position-feedback cylinder  2113 . The first arm  2110  is fixed in the horizontal plane to provide a reference position. The frame member  2110  may be formed from an aluminum extrusion, for example a 6105-T5 aluminum material. An air cylinder  2924  ( FIG. 29 ) applies a light clamping pressure to the profile clamps  2112  and  2113 , which results in a total clamping force of about 9–13 Newtons (2–3 pounds). A control valve (e.g., a spool valve  2922 ,  FIG. 29 ) controls the flow of air to the cylinder to close the profile clamps. 
   Although  FIG. 21  only shows two centrally located clamping members  2111 , it is desirable to have clamping members  2111  at the ends of frame member  30   a,  as near as possible to the corner key. This provides the clamping force at the ends, so as to avoid excess deformation of the frame members, which could occur if the whole clamping force were applied in the middle of the frame members. Similarly, it is desirable to position clamping members on each of the other three frame members  30   b – 30   d,  as close as possible to the corner keys. 
   As noted above, clamping subassembly  2100  pre-loads frame  30  to a straight condition. Because the corner keys of frame  30  may project in the “Y” direction beyond the outer edge of screen bar segment  30   a,  a single, monolithic clamping member that would contact the corner keys would not be able to remove all of the convex pre-bow that the frame may have; the center of the frame member  30   a  would be bowed out by an amount approximately equal to the distance by which the corner keys protrude beyond the frame member  30   a . Therefore, the clamping is only done by direct contact with the frame members  30   a – 30   d,  and not with the corner keys  30   e.  For this purpose, clamping members  2112 ,  2122  are spaced about every 30–45 cm (12″–18″) apart along each arm assembly  2101 ,  2102 . 
   A second arm assembly  2102  of the clamping subassembly  2100  provides a second registration edge and a second clamping function. The structure and operation of the second arm assembly  2102  is similar to those of the first arm assembly  2101 , except that the second arm assembly does not require an LVDT (assuming that all four of the screen bar segments  30   a – 30   d  of the frame  30  have the same profile, as is typical). The first and second arm assemblies  2101  and  2102  meet to form an “L” shaped support beneath segments  30   a  and  30   b  of frame  30 . The second arm assembly  2102  includes a frame member  2120 , registration clamp stops  2121 , and one or more outer profile clamps  2122  and  2123 . The second arm assembly  2102  is fixed in the horizontal plane to provide a reference position. The inner edge RE of second frame segment  30   b  is registered against registration clamp stops  2121 . An air cylinder  2922  ( FIG. 29 ) applies a light clamping pressure to the profile clamps  2122  and  2123 . The profile clamps  2122  and  2123  may be operated and controlled by the same spool valve that controls the profile clamps of the first arm assembly  2101 , because the profile clamps of the first and second arm assemblies  2101  and  2102  are closed and opened at the same times. 
   A third arm assembly  2103  of the clamping subassembly  2100  is movable. Movable frame support  2103  performs several functions including: supporting screen bar segment  30   c;  clamping the frame  30  in the “Y” direction to steady the frame  30  and remove the convex camber (pre-bow) from screen bar segment  30   c;  measuring the “Y” dimension of frame  30 ; and providing the dimension information to the controller  2900  for positioning a corresponding arm  2203  ( FIG. 22 ) that inserts the screen  34  in the adhesive in screen bar segment  30   c.    
   Movable frame support  2103  is actuated using one or two rodless pneumatic cylinders  2150 . The rodless cylinder  2150  may be a conventional model no. 40 rodless pneumatic cylinder manufactured by Lanamatic AG, of Lengwil/Oberhofen, Switzerland. Only one rodless cylinder  2150  is shown in  FIG. 21 ; two are shown in  FIG. 20 . If two rodless cylinders  2150  are used, as shown in  FIG. 20 , both must be actuated simultaneously in the same direction. If only one rodless cylinder  2150  is used (as shown in  FIG. 21 ), then the end of movable frame support  2103  opposite the rodless cylinder  2150  may be slidably supported on extrusion  2182  by a conventional linear motion flange bearing  2183 , such as those manufactured by 80/20 Inc. of Columbia City, Ind., or those described in U.S. Pat. No. 5,429,438. 
   The arm  2130  is mechanically attached to the yoke  2151  of the rodless cylinder  2150 . The rodless cylinder  2150  includes direct power transmission, from a double-acting cylinder (not shown) inside the tube  2152  of the rodless cylinder. The double-acting cylinder is connected through a slot (not shown) in the tube  2152  to the yoke  2151 . 
   The rodless cylinder  2150  is driven by at least one control valve  2914 ,  2916  ( FIG. 29 ). Preferably, two control valves  2914 ,  2916  are used, to give the rodless cylinder multiple actuating speeds. For example, direction control can be provided using a five-way, three-position, fast-open/stop/fast-close valve  2914  ( FIG. 29 ); a speed control can be provided by a five-way, two-position, fast/slow valve  2916  ( FIG. 29 ) in series with the three-position, direction-control valve. Alternatively, a single three-position valve may be used, operating only at the slow speed, which may increase the length of the cycle whenever the frame size is changed. 
   Reference is again made to  FIG. 21 . One of ordinary skill recognizes that alternative actuating mechanisms may be used. For example, instead of a rodless cylinder, each end of arm  2130  may be connected to a respective timing belt, each timing belt coupled to a timing belt pulley, with the two timing belt pulleys connected to each other to rotate together. A pneumatic or electric drive motor would also be included. 
   Movable frame support  2103  has a plurality of spring loaded back stops  2134 . Back stops  2134  engage the third screen bar segment  30   c,  and clamp the frame in the “Y” direction. Each back stop  2134  has a stop member  2134   a  that is biased by a spring  2134   b  to the raised position shown in  FIG. 21 . Each stop member  2134   a  has two ramped or chamfered corners  2134   c.  When the movable clamping arm assembly  2104  (described below) slides across the movable frame support  2103 , stops  2134  are pushed down by cam action to permit assembly  2104  to pass. 
   A screen frame proximity sensor  2137  on the third arm  2130  detects when the third arm  2130  approaches within a predetermined distance from the third screen bar segment  30   c.  Before the movable frame support  2103  approaches the screen bar segment  30   c,  the assembly  2103  can be moved towards the frame  30  at the fast speed. The proximity sensor  2137  determines when the spool valves controlling the rodless cylinder  2150  are adjusted to reduce the speed of approach. This ensures that the arm assembly  2103  moves in slowly beneath the screen bar segment  30   c  and does not damage the frame  30 . A variety of conventional proximity sensors may be used, such as optical or capacitance type sensors. 
   When the frame  30  is first placed on the apparatus, only two screen bar segments  30   a  and  30   b  are supported from underneath by arm members  2110  and  2120 . Thus, it is possible for the unsupported corner of the frame  30   e  (between segments  30   c  and  30   d ) to sag. Optionally, the third arm  2130  may have a ramp or chamfer on its leading edge (not shown), to scoop up the third screen bar segment  30   c . By moving the third arm assembly  2103  inward slowly, the third screen bar segment  30   c  is lifted up by the ramped or chamfered surface, using a cam action. 
   As best seen in  FIG. 20 , a conventional TTL linear encoder  2190  may be used to accurately measure the “Y” position of the movable frame support  2103 , and provide a digital output signal to the controller  2900 . The sensor of the linear encoder  2190  may be placed on the yoke  2151  of the rodless cylinder  2150 . An exemplary linear encoder suitable for this purpose is model No. LR 005 N D3, Dynapar brand LR/LS inductive linear encoder, manufactured by the Bimba Manufacturing Co., Gurnee, Ill., USA. 
   The movable clamping arm assembly  2104  accommodates the variable position of the movable frame support  2103 . Thus, arm assembly  2104  is positioned above, and parallel to the remaining three arm assemblies  2101 – 2103 . Whereas arm extrusions  2110 ,  2120  and  2130  provide a support surface beneath respective screen bar segments  30   a – 30   c,  the arm  2140  of arm assembly  2104  moves next to the screen bar segment  30   d , and clamp  2146  includes a small ledge  2146   a  ( FIG. 23 ) to provide support from underneath. 
   Arm assembly  2104  performs several functions including: clamping the frame  30  in the “X” direction to steady the frame  30  and remove the convex camber (pre-bow) from screen bar segments  30   b  and  30   d;  measuring the “X” dimension of frame  30 ; and providing the dimension information to the controller  2900  for positioning a corresponding arm  2204  ( FIG. 22 ) that inserts the screen  34  in the adhesive in screen bar segment  30   d.    
   Arm assembly  2104  may be actuated using one or two rodless pneumatic cylinders  2160 , in a manner similar to that described above with reference to arm assembly  2103 . The arm  2140  is mechanically attached to the yoke  2161  of the rodless cylinder  2160 . If one rodless cylinder  2160  is used, then the opposite end of arm  2140  is slidably supported by a bearing, such as a conventional linear motion flange bearing  2181 . 
   Two spool control valves  2922 ,  2924  ( FIG. 29 ) may be provided for actuating the rodless cylinder  2160 , and may be identical to the valves that control rodless cylinder  2150 . The spool valves that control rodless cylinder  2160  are separate from the valves that control rodless cylinder  2150 , because the arms  2103  and  2104  are actuated independently of each other. 
   On the movable clamping arm assembly  2104 , one or more blocking members  2146  are provided to clamp segment  30   d , and remove the convex camber (pre-bow) therefrom during insertion. The blocking members  2146  project slightly inward from the inner edge of the arm  2140 , at least as far as the distance by which the corner key extends in the “X” direction beyond screen bar segment  30   d.  The “Y” coordinate of blocking members  2146  may be manually adjustable, to ensure that the blocking members clamp against the screen bar segment  30   d , and not the corner key  30   e.    
   A screen frame proximity sensor  2147  on the fourth arm  2140  detects when the fourth arm approaches within a predetermined distance from the fourth screen bar segment  30   d . The proximity sensor  2147  determines when the spool valves controlling the rodless cylinder  2160  are adjusted to reduce the speed of approach. Sensor  2147  may be similar to sensor  2137  described above. 
   As best seen in  FIG. 20 , a conventional TTL linear encoder  2191  may be used to accurately measure the “X” position of the clamping arm assembly  2104 , and provide a digital output signal to the controller  2900 . 
   The fourth arm assembly  2104  also has a cooling manifold  2148  described below with reference to  FIG. 23 . 
     FIG. 23  is an enlarged detailed view of the yoke  2170 . The yoke  2170  connects the third and fourth arms  2130  and  2140  in such a manner that either arm can move freely in its respective lateral direction. Also shown in  FIG. 23  is a manifold  2148  that is attached to the inner end of arm  2140 . The manifold  2148  has a plurality of nozzles  2149  that provide cooling air to the screen bar segment  30   d  to solidify the adhesive therein. In the exemplary embodiment, manifold  2148  is advantageously close to screen bar segment  30   d , and provides a means of introducing cooling air to that portion of the screen-frame assembly. 
   Yoke  2170  has an elongated tongue  2171  that slides freely in the “X” direction in a groove  2138  of arm  2130 , when the fourth arm assembly  2104  moves. Yoke  2170  has a bushing  2174  that includes a pair of downwardly projecting low-friction blocks that straddle the manifold  2148 . The bushing  2174  allow yoke  2170  to slide freely in the “Y” direction, when the third arm assembly  2103  moves. The bushing  2174  may be made from nylon, for example. 
   Yoke  2170  has a backstop  2169  with a pair of cams  2172  and  2173  that engage the cam surfaces  2134   c  of the spring loaded back stops  2134  when the fourth arm assembly  2104  moves. Cam  2172  smoothly lowers back stop  2134  when the fourth arm assembly moves in the minus X direction. Cam  2173  smoothly lowers back stop  2134  when the fourth arm assembly moves in the plus X direction. In addition to the cam function, backstop  2169  provides a clamping force as near as possible to the corner key. If all of the clamping force were applied at the center of the screen bar segment  30   c,  deformation of the segment  30   c  could occur. By applying clamping force near the corner key, the clamping force on segment  30   c  is absorbed as a compressive load on segments  30   b  and  30   d , reducing the likelihood of deforming segment  30   c . 
     FIG. 27  is a cross-sectional view of a shield  2185  shown in  FIG. 21 . The shield  2185  facilitates insertion of the screen  34  at the corners. As explained further below, a plurality of spring-loaded pins  2222  insert the screen  34  into the groove  32  of each screen bar segment. Using a typical corner key (not shown), pins  2222   b  (which are located above the corner key and beyond the end of the groove  32 ) could catch and pull the screen cloth  34 , and prevent the remaining pins  2222   a  from properly inserting the screen  34  in the screen bar segment  30   a  near the corner. The shield  2185  blocks the spring loaded pins  2222   b  from contacting the screen  34  over the corner key, so the screen in the corner can be freely pulled into the insertion grooves  32  by the pins  2222   a.  The shield  2185  enables use of retractable pins  2222  without the complexity of the bayonet style pins shown in  FIG. 17 . Similar shields (not shown) cover the corner keys where the first and second screen bar segments  30   a  and  30   b  meet and where second and third screen bar segments  30   b  and  30   c  meet. The yoke shown in  FIG. 23  incorporates the shield function in a shield portion  2170   a,  for the corner key where segments  30   c  and  30   d  meet. 
   Reference is now made to  FIG. 22 , which shows the press subassembly  2200 . The main functions of press subassembly  2200  are: to position the movable arms  2230  and  2240  so that the pins  2222 ,  2242  thereon are properly aligned with the grooves  32  of screen bar segments  30   c  and  30   d;  to simultaneously insert all four sides of the screen  34  into the adhesive  36  with the plurality of pins  2222  and  2242 ; and to provide cooling gas (air) to cool the adhesive on at least some of the screen bar segments  30   a – 30   c . Several items, which are described further below and appear in other figures, are omitted from  FIG. 22 , merely to simplify the drawing. 
   Press subassembly  2200  includes a plurality of frame members that are located above respective frame members in the clamp subassembly  2100 . Specifically, press subassembly  2200  has four arm assemblies,  2201 – 2204  corresponding to assemblies  2101 – 2104 . Arm assemblies  2201  and  2202  are fixed arms, located above fixed arm assemblies  2101  and  2102 . Arms  2201  and  2202  have pins  2222  that are fixedly positioned above the grooves  32  of respective screen bar segments  30   a  and  30   b.  The third arm assembly  2203  is movable in the “Y” direction, and is coplanar with arm assemblies  2201  and  2202 . The fourth arm subassembly  2204  is movable in the “X” direction. Arm assembly  2204  is positioned above and parallel to assemblies  2201 – 2203 , to accommodate the various positions in which the third arm assembly  2203  may be positioned. 
   The movable third arm assembly  2203  requires accurate positioning, so that the pins  2222  are aligned with the groove of the third screen bar segment  30   c . A standard servo linear actuator  2250  has sufficient precision to locate the pins  2222  on arm assembly  2230  within 0.025 centimeters of the centerline of the groove  32  in screen bar segment  30   c , which is adequate for this purpose. Actuator  2250  is commanded to move to a position defined by the process controller  2900 , based on the location of arm assembly  2103  of clamp subassembly  2130 . 
   Similarly, the movable fourth arm assembly  2204  has a servo linear actuator  2260  to locate the pins  2242  on arm assembly  2204  within 0.025 centimeters of the centerline of the groove  32  in screen bar segment  30   d.    
   The exemplary pins  2222  and  2242  are all rectangular in cross section. The larger dimension of the cross section is preferably between about 0.3 centimeters and about 1.27 centimeters. Pins  2222  and  2242  may have an elongated rectangular cross section with a larger dimension of 2 centimeters or more, up to nearly the center-to-center distance between pins, but a dimension of 1.27 cm or less is preferred. The smaller dimension of the cross section need only be large enough to ensure that the pins  2222 ,  2242  do not bend upon insertion, for example, between about 0.08 cm (0.03″) and about 0.36 cm (0.14″), to accommodate the width of the groove  32 . 
     FIG. 25  is a cross sectional view of arm assembly  2201 , taken along section line  25 — 25  of  FIG. 22 . The exemplary arm  2210  is an aluminum extrusion, such as a 6105-T5 aluminum. This configuration provides a low weight with a relatively high area moment of inertia. A cool air manifold  2215  is attached to arm  2210 . The pin assembly  2211  is attached to the manifold  2215 . The pin assembly includes a plurality of pins  2222 . The exemplary pins  2222  are spring loaded, with a spring  2217 . A respective nozzle  2216  is provided near each of the pins. The nozzles  2216  are connected to manifold  2215  for directing cool air onto the adhesive  36 , near each pin  2222 . Because there is a small distance between the nozzles  2216  and the screen bar segments  30   a,  it is effective to provide cooling for the screen bar segments  30   a – 30   c  by blowing air out of the nozzles  2216  in arm assemblies  2201 – 2203 . 
     FIG. 26  is a cross sectional view of arm assembly  2204 , taken along section line  26 — 26  of  FIG. 22 . The exemplary arm  2240  is an aluminum extrusion, such as a 6105-T5 aluminum. The exemplary arm assembly  2204  may not have a separate air manifold for blowing air onto the screen bar segment  30   d . The hollow section  2240   a  of the extrusion  2240  can be used as an air manifold, with a nozzle  2240   b.  Because arm assembly  2204  is positioned above assemblies  2201 – 2203 , it is farther from screen bar segment  30   d , and does not provide an optimal source of cooling air. For this reason, the primary air manifold for screen bar segment is located on the clamping arm assembly  2104 , as described above, with reference to  FIG. 21 . 
   The pin assembly  2241  may be pivotally attached to the extrusion  2240 , for example. The pin assembly  2241  includes a plurality of pins  2242 . The pins  2242  are substantially longer than the pins  2222 , so as to insert screen  34  in the screen bar segment  30   d  simultaneously while the pins  2222  of arm assemblies  2201 – 2203  are inserted in respective screen bar segments  30   a – 30   c.    
   As shown in  FIG. 26 , the fourth arm assembly  2204  has an actuating shaft  2275 . The shaft has bearings (not shown) on both ends and may be driven by an air cylinder and a crank mechanism (not shown). The pins  2242  are all attached to the shaft  2275 . The shaft  2275  can be rotated, to rotate the pins  2242 . Each pin  2242  has a respective torsion spring  2244  that biases the pin to rotate in the counterclockwise direction relative to shaft  2275 . When the shaft  2275  is rotated clockwise ninety degrees, the torsion springs  2244  bias the pins  2242  toward the horizontal position (shown in phantom in  FIG. 26 ). When the shaft  2275  is rotated counter-clockwise ninety degrees, the torsion springs  2244  bias the pins  2242  toward the vertical position, (shown by the solid lines in  FIG. 26 ). Preferably, a stop limits the rotation of the pins  2242 , so they do not rotate past the horizontal position when the shaft  2275  is rotated clockwise, or past the vertical position when the rod is rotated counterclockwise. Each pin  2242  has its own spring block. When the actuating shaft  2275  is positioned so that the pins  2242  are biased to the vertical position, any one of the pins can be independently pushed to the horizontal position by application of a force sufficient to overcome the bias of its torsion spring  2244 . 
   Prior to moving the third arm assembly  2203 , the shaft  2275  is rotated clockwise, to raise the pins  2242  to the horizontal position so they do not interfere with movement of the arms  2230 ,  2240 . After the third and fourth arm assemblies  2203  and  2204  are positioned for screen insertion, the actuating shaft  2275  is rotated counter-clockwise ninety degrees, to position the pins vertically. For the pins  2242  that are directly above the third arm  2230 , when the pins contact the arm  2230 , the pins over the arm  2230  are pushed back up into the horizontal position, overcoming their respective torsion springs. 
   Other means are contemplated for repositioning the pins, so that the pins do not interfere with any of the other arms. For example, the pins may be provided with a linear up-and-down motion, using an actuator or the like. Alternatively, a chain and sprocket arrangement may be provided, with the pins attached to the chain. Advancing the chain can move the pins out of the way of the moving arm. 
   Reference is now made to  FIG. 24 . Press subassembly  2200  also includes a yoke (not shown in  FIG. 22 ) connecting arms  2230  and  2240 .  FIG. 24  is an enlarged detail of the yoke assembly  2270  that connects arms  2230  and  2240 . The yoke  2270  allows the arm members  2230  and  2240  to move freely in the “Y” and “X” directions, respectively. The yoke  2270  has a channel-shaped main body  2271  and a bushing  2272  including two low friction blocks  2274 . Bushing  2272  may be made of nylon, for example. Blocks  2274  slide over arms  2230  and  2240 . 
   Yoke  2270  includes a pin assembly  2276  depending from the bottom of the channel member  2271 . The pin assembly  2276  has a row of corner pins  2277 . As shown in  FIG. 24 , pins  2242  that are within the boundary of yoke  2270  are prevented from rotating to the vertical position by the bottom flange  2273  of the yoke, even when the actuating shaft  2275  is rotated counter-clockwise. Because it is important to have the screen  34  inserted in the adhesive at the corner of the frame  30 , the pin assembly  2276  is provided. Pin assembly  2276  ensures screen insertion all the way to the corner of the frame  30 . 
   To ensure that the screen  34  of the completed assembly does not sag or fall through the opening of the frame  30 , it is important to support the screen cloth from beneath while the screen is being inserted in the frame  30 . A variety of supports may be designed. For example, a flat supporting surface (not shown) may be formed using straps, hoses, tubes, cords or the like. The flat supporting surface may be raised and lowered using a system of pulleys and idler arms (not shown), controlled by a pneumatic cylinder. The flat supporting surface is lowered while the movable arm assemblies  2103 ,  2104  of the clamping subassembly  2100  are moved into the clamping positions. Then, the flat supporting surface is raised up to the height of the lip L ( FIGS. 6 and 7 ) of the screen bar grooves  32  in the frame  30 . In this position, the flat supporting surface would support the screen  34  without sag in the correct position, even if the screen  34  were completely free of attachment to the frame  30 . The flat supporting surface remains in place for the duration of the insertion operation. The flat supporting surface is lowered at the completion of cooling, before the movable arm assemblies  2103 ,  2104  are moved from the clamping position to the open position. Thus, the supporting surface does not interfere with movement of the arms  2130  and  2140 . 
     FIGS. 28A–28C  show an exemplary structure and method for supporting the screen cloth  34  during insertion. In this example, a planar platen  2802  is positioned beneath the clamping subassembly  2100 . The top surface of the platen is covered with a layer  2800  of low density foam, such as a conventional convoluted foam construction, 5 centimeter thick “raised rib” foam commonly referred to as “egg crate.” The tops of the foam ribs  2800  form a flat support surface for supporting the screen  34 . The platen  2802  and egg crate foam  2800  are sized to completely support the largest size frame for which the system  2000  is used, having an “X” dimension that is approximately the distance between arm  2110  and member  2182 , and a “Y” dimension that is approximately the distance between arm  2120  and member  2180 . 
   The platen has a lowered position ( FIG. 28A ) and a raised position ( FIGS. 28B and 28C ), and moves under control of an actuator  2804 . The height at the raised position is selected so that the tops of the foam ribs are at the desired height at which the screen cloth  34  lies on the frame  30  with no sag. This is the height of the top of the lip L ( FIGS. 6 and 7 ) of the screen bar  30   a.    
   The low density foam  2800  provides a simple structure for accommodating multiple frame sizes. Regardless of the positions of arm assemblies  2103  and  2104 , when the platen  2802  is raised, the foam  2800  is compressed beneath the arms  2110 ,  2120  and  2130 , and fills in the space around the arms, without using complex pulley and idler arm systems. Because the arms  2110 ,  2120  and  2130  lie beneath frame members  30   a – 30   c , respectively, the foam  2800  applies little pressure on these frame members. No arm lies beneath screen bar segment  30   d , but it is easy to prevent upward deformation of segment  30   d  by having clamp  2146  ( FIG. 21 ) extend over the top of segment  30   d . Alternatively, a hinged ramp (not shown) with a limited range of rotation or a rigid ramp (not shown) may be attached to arm  2240 , to keep the foam  2800  from contacting the underside of the fourth screen bar segment  30   d  of frame  30 . 
     FIG. 29  is a block diagram of the control system for operating the apparatus  2000 . The system is operated by the programmable logic controller  2900  (PLC). PLC  2900  receives three operator control inputs from the clamp switch  2902  (which is preferably foot operated), the press switch (which is preferably an anti-tie-down switch, and a reset switch  2906 , that is activated when a frame having a new frame size is to be fabricated. 
   PLC  2900  receives several data inputs, including: the X-position of the movable clamp arm assembly  2104  from linear encoder  2191 ; the Y-position of the movable frame support assembly  2103  from linear encoder  2190 ; the detection signal sent from the X proximity sensor  2147  when arm  2140  approaches the frame; the detection signal sent from the Y proximity sensor  2137  when arm  2130  approaches the frame; and the frame profile from position feedback cylinder  2113 . 
   PLC  2900  provides control signals to several devices to operate the apparatus  2000 , including: signals to control when the blower  2926  is turned on and off; signals to control when the profile clamp spool valve  2922  provides air to the profile clamp cylinder  2924 ; signals to operate the X clamp three-position (forward/stop/reverse) spool valve  2914 ; signals to operate the X-clamp two-position (fast/slow) valve  2916 ; signals to operate the Y clamp three-position (forward/stop/reverse) spool valve  2922 ; signals to operate the Y-clamp two-position (fast/slow) valve  2924 ; signals to command the foam platen spool valve  2912  to control air flow the foam platen cylinder  2804 ; signals to control the X servo  2260  for positioning arm  2240 , signals to control the Y servo  2250  for positioning arm  2230 ; and signal for commanding the spool valve  2908  for controlling the press cylinder  2910  to raise and lower the press subassembly  2200 . 
   Other cushioning supports may be used instead of foam. Alternatives include air bags, a helical spring, and the like. 
   Operation 
     FIG. 30  is a plan view of an exemplary work cell including the apparatus  2000 . Frames  30  are assembled on a table  3006  and loaded into an oven  3008 . Once heated, the frames are placed on a table  3004 . A portion of screen cloth  34  is cut from a roll  3002  (or a pre-cut portion of screen cloth may be taken from a table (not shown). The screen  34  is positioned and tacked on the frame  30 . The frame  30  and screen  34  are placed in the press for insertion. After the insertion operation is completed, the screen-frame assembly is moved to a trim table  3010  where excess cloth is cut from around the frame grooves  32 . Finished screen-frame assemblies are stacked on a table or palette  3012 . 
   The operator begins the fabrication procedure by obtaining a heated frame  30  from the oven  3008 . The frames  30  may be manually placed in an oven. Optionally, the frames may pass through an elongated heated enclosure on a conveyer. Alternatively, the frames may be removed from the oven by a pick-and-place robot. The operator places the heated frame  30  on a work surface  3004 . The operator places a pre-cut piece of screen cloth  34  on the frame. Preferably, a small amount of a tacky, pressure-sensitive adhesive is placed on the screen bar segment furthest from the operator, to keep that side of the screen cloth in place. The operator can hold the two nearest corners of the screen cloth  34  in place on the frame  30  with his or her hands. 
     FIGS. 28A–28C  show the operation of the press. The clamping subassembly  2100  is put in an open position. In the exemplary embodiment, the press subassembly  2200  is in its raised position, and the foam platen  2802  is in its lowered position, as shown in  FIG. 28A . The blower is turned off. 
   If this is the first frame or if this frame is of a size different than the last frame, then the movable arm assemblies  2103  and  2104  are spread out as far as possible from respective arm assemblies  2101  and  2102  prior to clamping. If this is not the first frame, and this frame is the same size as the immediately preceding frame, then the movable arm assemblies  2103  and  2104  may open to a ready position about 1.27–2.54 centimeters from the clamped position for this size frame. 
   The operator places the frame  30  and screen  34  on the two fixed arms  2101  and  2102  of the clamping subassembly  2100 , with screen bar segments  30   a  and  30   b  engaging respective clamping blocks  2111  and  2121 . The operator can continue holding the screen  34  in place at the near corners of the frame at this time to prevent sagging. Because the clamps  2112  and  2113  are operated at low pressure, there is no danger to the operator&#39;s safety, even if a hand were placed in the clamp. 
   The operator actuates a control, preferably a foot operated switch  2902  ( FIG. 29 ). In response to activation of the switch, the clamping blocks  2112 ,  2113  clamp screen bar segment  30   a , and blocks  2122  and  2123  clamp segment  30   b . The position feedback cylinder  2113  determines the screen bar profile of frame  30  and sends this information to the process controller  2900 . The valves controlling rodless cylinder  2150  starts the movable frame support  2103  moving at the fast speed. When the proximity sensor  2137  detects that it is approaching frame member  30   c , the valves operating the rodless cylinder  2150  are switched to the slow speed. The movable frame support  2103  moves inward, till spring-loaded back stops  2134  clamp screen bar segment  30   c . The valves controlling rodless cylinder  2160  starts the movable clamping arm assembly  2104  moving at the fast speed. When the proximity sensor  2147  detects frame member  30   d , the valves operating the rodless cylinder  2160  are switched to the slow speed. The clamping arm assembly  2104  moves inward, till stop blocks  2146  clamp screen bar segment  30   d . The operator can visually detect that the arms  2130  and  2140  are in the fully clamped positions. The linear encoders  2190  and  2191  determine the “Y” and “X” coordinates of the arm assemblies  2103 ,  2014 , and send this information to the process controller  2900 . 
   Once the operator determines by visual inspection that the frame  30  is clamped, the operator can release the switch. Upon release of the switch, the screen cloth support surface  2800  rises into position supporting the screen cloth  34  (shown in  FIG. 28B ). The operator can now release his or her hands from the screen cloth without the screen sagging. The blower is started. 
   The process controller  2900  uses the screen bar profile data and the arm position data, and determines the corresponding positions for the arms  2230 ,  2240  of the press subassembly  2200 . For example, look-up tables may be used to determine the positions of arms  2230  and  2240 . The process controller  2900  commands the servo-controlled positioning systems  2250  and  2260  to accurately position the press arms  2230 ,  2240  for screen cloth insertion in frame  30 . The insertion arm assemblies  2203  and  2204  may be continuously repositioned while the clamping arm assemblies  2103  and  2104  are positioning themselves. Alternatively, the arm assemblies  2203  and  2204  may remain in a standby position until the operator releases the switch (signifying that the correct clamping position is reached), and move directly to the final position for insertion when the switch is released. The former approach may decrease cycle time. 
   A separate control is used to either lower the press subassembly  2200  or raise the clamping subassembly  2100  for insertion of the screen  34  into the grooves  32  of the frame  30 . As shown in  FIG. 28C , the exemplary system lowers the clamping subassembly  2200  to insert the screen  34  in the frame  30 . Preferably, a conventional anti-tie-down type dual-control switch is used. Such a switch requires the operator to actuate two separate controls simultaneously, or within a predetermined short period of time from each other, before lowering the press subassembly  2200 . This mechanism ensures that the operator&#39;s hands are safely out of the way of the press. 
   Once the anti-tie-down switch is activated, the cooling air begins to blow through the manifolds  2215  of the three press arm assemblies  2201 – 2203  and the manifold  2148  of the clamping arm assembly  2104 . (Alternatively, the blowing air may begin as soon as the operator releases the foot-activated switch). The press subassembly  2200  is lowered, and remains in the lowered position for the required amount of cooling time (e.g., about 5 to 10 seconds). 
   While the frame  30  is cooling, the operator is free to perform another operation. For example, if a previously fabricated screen-frame assembly is awaiting final trimming, the operator can trim any excess screen cloth from the frame during this time. Alternatively, the operator can fetch the next heated frame  30  from the oven, and place a screen on the next frame. 
   Depending on the length and width of the frame  30  and the width of the groove  32 , thermal expansion may have a significant affect on the ability of the system to maintain the pins  2222 ,  2242  centered within the grooves  32  for the duration of the cooling period. For a relatively long window, the thermal expansion may be of the same order of magnitude as the width of the groove  32 . Thus, to maintain the pins  2222 ,  2242  approximately centered within the grooves, it is desirable to continually sample the positions of arm assemblies  2103  and  2104 . As the frame  30  cools down, the frame shrinks, and application of clamping pressure keeps the frame  30  straight (i.e., removes the bow). The linear encoders  2190 ,  2191  measure the positions of arms  2130  and  2140 , and sends these data to the process controller  2900 . The process controller  2900  commands the servo positioning systems  2250 ,  2260  to move the arm assemblies  2203  and  2204  to the appropriate offsets, to keep the pins  2222  and  2242  centered in the grooves. The greater the sampling and adjustment frequency, the more accurate the positioning of the pins  2222  and  2242 . 
   When the cooling time is completed, the press subassembly  2200  returns to the raised position, the blower stops, and the support foam  2800  drops to its lowered position, as shown in  FIG. 28A . In a typical production run, a plurality of frames of the same size are made in a batch. The bottom clamps  2112 ,  2113 ,  2122 ,  2123 ,  2103  and  2104  jog open slightly—between about 1.27 and 2.54 centimeters. This provides sufficient clearance for the operator to remove the screen and frame assembly. The operator can place the next frame  30  and screen  34  on the clamping subassembly  2100 . The clamping steps for the second and subsequent screen and frame assemblies proceed more quickly, because the clamps have shorter distances to travel. Also, the corner of the frame  30  connecting the third and fourth screen bar segments  30   c  and  30   d  does not droop. If a different size screen frame is to be loaded next, the operator pushes a reset button, and the movable arms  2130 ,  2140 ,  2230  and  2240  return to their completely opened positions. 
   Other variations of the operating method are contemplated. For example, the frame dimensions may be manually input, in which case the process controller  2900  can either: (1) move the clamps to the exact dimensions corresponding to the size that is input; or (2) calculate “rough” clamping locations that correspond to the size that is input, and use the slow clamping speed to perform the final approach between the clamping members and the frame  30 . Further, if a bar code is placed on each frame, indicating the frame size, then the operator can scan the bar code instead of inputting the dimensions manually. Because variations in frame sizes as large as 0.3 centimeters are not uncommon, it is believed that the fully automated method described above provides better placement of the insertion pins  2222 ,  2242  in the grooves  32  than either manually inputting the frame size or scanning in the frame size. 
   Further, the clamping may be done manually, by manually moving the clamping blocks and arms  2112 ,  2113 ,  2122 ,  2123 ,  2130 ,  2140  until the frame  30  is clamped, and the screen bar segments  30   a – 30   d  appear straight by visual inspection. The clamping blocks and members  2112 ,  2113 ,  2122 ,  2123 ,  2130 ,  2140  are locked in place, and the linear encoders  2190 ,  2191  perform position determination for automated placement of the press subassembly arms  2230  and  2240 , as described above. 
   In another variation of the exemplary embodiment, apparatus  2000  may include automatic means for cutting the excess screen cloth from the assembled screen and frame assembly, either during cooling or after it is cooled, but before the press is opened. Essentially, a separate “L”-shaped blade is jammed into each corner to sever any screen cloth that extends beyond the groove  32  on each side of the frame  30 , and a blade is run across each side of the frame. 
   According to this method, four steps are added to the process, after the adhesive has cooled sufficiently to firmly hold the screen  34 . In the first step, the “L” shaped blades are added to each of the four corners of the frame  30 , on the exterior portion of the grooves. In the second step, the screen cloth  34  is pulled out and upward from the frame  30  to tear the corner of the screen cloth outside of the “L” shaped blade. Conventional pneumatic grippers may be used to grip the screen; the grippers may be placed on a pneumatic slide to apply tension. The screen cloth is pulled in a direction that is about 45 degrees from the horizontal. In the third step, the “L” shaped blades are removed from each corner. In the fourth step, a straight blade is run across each side of the screen cloth, along the horizontal surface of the screen bar segment immediately outside of the groove  32 . 
   The operator can manually insert the “L” shaped blades, and manually run the straight blade along the edge of frame to sever the excess cloth. Optionally, the “L” shaped blades can hang down from the bottom of the press subassembly  2200 , and be automatically inserted by an air cylinder when assembly  2200  is lowered. A further option is to mount four straight blades on runners, each of which may be controlled by a pneumatic cylinder, a rodless cylinder, a gear and chain drive, or other linear actuating device. 
   If the trimming is performed while the screen and frame assembly is still on the apparatus  2000 , then there is no need for the trim table  3010 , and the assembly can be moved directly from the apparatus  2000  to the finished screen table or palette  3012 . 
   Vertical Assembly Apparatus 
     FIGS. 31A–31C  show another exemplary apparatus  3100  for fabricating screen assemblies  3101  according to the present invention. Apparatus  3100  includes some of the general concepts of the apparatus of  FIGS. 20–30 . The ventilation cloth insertion apparatus  3100  comprises a fixture  3102  that orients a screen frame  3130   a – 3130   d  in an approximately vertical position, as best seen in the side elevation view of  FIG. 34 . As shown in  FIG. 34 , the slope  3390  of the exemplary machine is about 5°. Preferably, the angle  3390  is between 5 and 10 degrees or slightly less. 
   Preferably, the slope is nearly vertical. The more vertical the machine is, the less likely that the screen cloth  34  will become caught on the frame  30  or a portion of the apparatus  3102  while the cloth is being draped down into position for insertion into the screen cloth. The cloth  34  passes down through a gap  3382  ( FIG. 34 ) between the frame  30  and the arms  3306 – 3309  ( FIG. 33A ) on which the insertion device is mounted. Preferably, the cloth can be draped down into position for insertion in the frame  30  without the cloth touching the frame or the machine while the cloth is moving into position. The closer the apparatus is to vertical, the less floor space is required. Also, the more vertical the apparatus, the easier the cloth handling becomes. And the more vertical the machine is, the easier it is to transfer the frame from the oven to the insertion station  3102 . A small angle allows the frame to be supported from behind, for example by backplate  3302 . Similarly, with a small angle, the frame can be supported by rails while being transported. A perfectly vertical frame would require more support from the transports  4010  and  4040  ( FIGS. 41 and 45 ) that move the frames. 
   Alternatively, a perfectly vertical frame could be used, and the conveyor may have a hook or extension to prevent the frame from falling over. If the frame is stabilized, a completely vertical screen provides the best screen cloth drape. An angle between 0 degrees and about 30 degrees may be used, so long at the angle is sufficiently small so that there is no significant interference with the ability to drape the cloth  34  down over the frame. 
   The apparatus  3100  also provides for automated feeding of the frames  30  through the oven to the insertion apparatus, and automated feed of the screen cloth  34  to a position for insertion in the screen frames  30 . 
   The apparatus includes an insertion fixture  3102  and a heater  3103 , which may be a separate oven  3103 , as shown in  FIG. 31A , or an integrated heating mechanism on the insertion fixture  3102 . Preferably, a conveyor  3104  delivers the screen frames  3130   d  to the oven  3103 , conveys one or more frames being heated through the oven, and delivers heated frames  3130   a  to the insertion apparatus  3102 . The conveyor  3104  is optional. In configurations having the heater (not shown) mounted on the insertion fixture, the conveyor may be significantly shorter (e.g., to accommodate only one frame), or may be omitted. 
   The apparatus  3100  can accommodate a variety of screen frames that may have different sizes. For example, a completed frame-screen assembly  3101  exiting the apparatus (on the left) has a size that is much smaller than the size of the frame  3130   a  entering the screen insertion apparatus  3102 . The mechanisms for accommodating different screen sizes are explained in detail below. Each screen frame  3130   a – 3130   d  has a plurality of segments. Each segment has a mounting surface on a face thereof. At least one (or, preferably, each) of the segments has adhesive on the mounting surface thereof. 
   A hanger hangs a ventilation cloth across the mounting surface of the at least one segment having the mounting surface. Preferably, the hanger hangs the ventilation cloth so that the cloth hangs over each of the mounting surfaces simultaneously. 
   The heater melts the adhesive in said one of the segments. As noted above, this may be accomplished using an oven  3103 , a hot air blower, electric heaters or other heating mechanism (not shown) on the insertion apparatus  3102 . 
   At least one insertion device inserts the ventilation cloth in the adhesive substantially across a length of said one of the segments. A variety of insertion devices may be used. For example, the insertion device may include a plurality of pins  2222  (shown in  FIG. 22 ), or an elongated insertion blade or band  3200  as shown in  FIG. 32 . The insertion blade or band  3200  may extend for all or substantially all of the length of the screen bar segment  30   a – 30   d , so that a respective single blade or band performs the insertion of screen cloth  34  for substantially the whole length of each respective side of the frame, excluding the corner keys. 
     FIGS. 33A–35  are detailed views of the insertion apparatus  3102 . The assembly  3302  has four clamping arms,  3306 – 3309 . The clamping arms  3306 – 3309  are positionable so that each clamping arm clamps a respective outside edge of a respective one of the plurality of sides of the screen frame  30  while attaching a ventilation cloth  34  to the screen frame. (The outer edges of the screen frame  30  are the edges of each frame segment  30   a – 30   d  that are furthest from a center of the screen frame.) The four arms  3306 – 3309  are coplanar. That is, each of the plurality of clamping arms  3306 – 3309  is positioned at a common height with respect to a plane in which the ventilation cloth  34  is positioned. 
   Referring now to  FIG. 33A , in the exemplary fixture  3102 , three of the clamping arms  3307 – 3309  are movable with respect to the remaining arm  3306 . Arm  3307  is slidably mounted to translate in the left-right direction in  FIG. 33 . One end of arm  3307  slides along stationary arm  3306  in a pair of yolks  3323  and  3324 . Each yolk  3323  and  3324  is slidably mounted on a respective rail  3318  and  3316 . 
   Arm  3308  moves both vertically and horizontally. The right end of arm  3308  moves from left to right and right to left along with arm  3307 . In addition, arm  3308  moves up and down with respect to arm  3307 . 
   Arm  3309  only moves up and down, in a direction parallel to its length. Thus, arm  3309  always is positioned at the left end of arm  3306 . 
   As a result, when the apparatus is reconfigured to accommodate a smaller frame, arms  3307 – 3309  move as shown in  FIG. 33B . Arm  3307  moves leftward to clamp the rightmost edge of frame segment  30   b . Arm  3308  moves downward to clamp the topmost edge of frame segment  30   c , and moves leftward by the same distance as arm  3307 , so that the right end of arm  3308  meets arm  3307 . Arm  3309  moves downward by the same distance as arm  3308 , so that the top end of arm  3309  meets arm  3308 . Arm  3306  continues to clamp the bottommost edge of frame member  30   a , and arm  3309  continues to clamp the leftmost edge of frame member  30   d.    
   The basic orientation of the apparatus  3103  is vertical, with a slight incline to allow for the frame members  30   a – 30   d  to be supported by the back plate,  3302 , so as the frame  30  comes in from the right hand side of the  FIG. 33A , the frame comes in under arm assembly  3307 . Clamp Arm assembly  3307  lifts (in a direction away from the backplate  3302  shown in  FIG. 34 ) and allows the frame to travel along the back plate  3302 . As gravity pulls the frame down to Arm  3306  the frame  30  rests along the fixed clamp plate  3373  as shown on  FIG. 37 . There is a slot  3303  that is shown on the back plate  3302 . 
   Depending on if it&#39;s a new size or the same size as the one that was just finished, the arms  3307 – 3309  may either open fully in the case of a new screen size to a home position as shown in  FIG. 33A , or it will just open slightly, just enough to be ready for the next frame  30  without opening up more than it needs to. 
   Optionally, a bar code reader or keypad may be used to scan or type in an indication of the frame size. Linear encoders attached to the rodless cylinders that move the arms  3307 – 3309  (or other measuring means) may be used provide a means for determining the location of the arms, and the current opening size at any time. By identifying the destination size to the system, and measuring the current size, it is possible to avoid opening the arms fully to their home positions (shown in  FIG. 33A ), even when changing frame sizes. 
   The next frame is then pushed into the machine  3102  and the clamp gate opens allowing the frame  30  to pass under Arm  3307 . Arm  3309  of the gate will have already shut and the little conveyor through that slot  3382  slides the screen frame assembly over to the gate of clamp Arm  3309 . It will be a hard stop. An air cylinder pushes the gate up to its final position. Arm  3307  moves over to the left just until it touches the screen bar  30   b , thus clamping and/or straightening out the pre-bow from screen bar  30   b . The air cylinder  3329  at the bottom of the apparatus actuate Arm  3307  and clamps the vertical members  30   b  and  30   d  of the screen frame. Arm  3308  also comes down and clamps or touches the top frame member  30   c  and clamps the two (optionally pre-bowed) horizontal frame members  30   a  and  30   c  straight, thus clamping and straightening the frame  30  on all four sides. Arm  3308  is actuated by an air cylinder  3330  along the left side. 
   Although the exemplary system moves the frame  30  into place in the insertion apparatus  3102  by inserting the frame through the gap  3382  ( FIG. 34 ) on the side of the apparatus, alternative embodiments are contemplated in which the frame is loaded from the front. To front-load the frame into the apparatus, the arms  3307 – 3309  are either open to their home positions ( FIG. 33A ), or if the frame size is identified to the system (e.g., via bar code or keypad entry), the arms can be opened by a small amount beyond the size of the frame, the frame moved into position, and the arms repositioned to clamp the frame. 
   Loading the frame  30  into the insertion apparatus  3102  from the right side is only an example. One of ordinary skill can readily configure the apparatus so that it is possible to load the frames from above or below, or from left or right. 
   Note that the movement of the arms  3307 – 3309 , as described above, may either be sequential or simultaneous. 
   The frame remains clamped between arms  3306 – 3309  while the ventilation cloth  34  is being inserted in the screen bar grooves. The cloth  34  is inserted into the groove, and then pushed out to the left under arm  3309 . After that, the clamp gate assembly lifts up (away from the backplate  3302 , in a direction normal to the loaded frame) and allows the finished screen frame assembly to pass through. 
   Understanding of the details of  FIGS. 33A–39  will be facilitated by the brief parts list immediately following this paragraph. This is only a partial list; conventional fasteners, finishing coatings and the like are omitted, for brevity. 
   Parts list for  FIGS. 33–35 : General Arrangement of arm assemblies
       3301  SUPPORT FRAME     3302  BACK PLATE     3303  LOAD/UNLOAD SLOT     3304  SMALL SCREEN (IN POSITION)     3305  LARGEST SCREEN (IN POSITION)     3306  LOWER HORIZONTAL ARM ASSEMBLY (ARM  1 )     3307  RIGHT AND VERTICAL ARM ASSEMBLY (ARM  2 )     3308  UPPER HORIZONTAL ARM ASSEMBLY (ARM  3 )     3309  LEFT HAND VERTICAL ARM ASSEMBLY (ARM  4 )     3310  SUPPORT  1       3311  SUPPORT  2       3312  SUPPORT  3       3313  SUPPORT  4       3314  RIGHT HAND VERTICAL GUIDE RAIL     3315  LEFT HAND VERTICAL GUIDE RAIL     3316  UPPER HORIZONTAL (STATIONARY) GUIDE RAIL     3317  ARM  3  HORIZONTAL GUIDE RAIL     3318  ARM  1  RAIL     3319  ARM  2  RAIL     3320  ARM  3  RAIL     3321  ARM  4  RAIL     3322  ARM  1  CORNER YOKE     3323  ARM  2 A CORNER YOKE     3324  ARM  2 B CORNER YOKE     3325  ARM  3 A CORNER YOKE     3326  ARM  3 B CORNER YOKE     3327  ARM  4  CORNER YOKE     3328  YOKE LINEAR BEARING BLOCK (TYPICAL)     3329  HORIZONTAL RODLESS CYLINDER     3330  VERTICAL RODLESS CYLINDER     3331  ARM  2  STABILATION CABLE ASSEMBLY     3332  ARM  3  STABILIZATION CABLE ASSEMBLY     3333  ARM  2  PULLEY # 1       3334  ARM  2  PULLEY # 2       3335  ARM  2  PULLEY # 3       3336  ARM  2  PULLEY # 4       3337  ARM  2  PULLEY # 5       3338  ARM  2  PULLEY # 6       3339  ARM  3  PULLEY # 1       3340  ARM  3  PULLEY # 2       3341  ARM  3  PULLEY # 3       3342  ARM  3  PULLEY # 4       3343  ARM  3  PULLEY # 5       3344  ARM  4  PULLEY # 6       3345  ARM  2  LOWER CABLE CLAMP/RODLESS CYLINDER ATTACHMENT     3346  ARM  2  UPPER CABLE CLAMP     3347  ARM  3  LEFT HAND CABLE CLAMP/RODLESS CYLINDER ATTACHMENT     3348  ARM  3  RIGHT HAND CABLE CLAMP     3349  HORIZONTAL LINEAR ENCODER     3350  VERTICAL LINEAR ENCODER     3351  PRESS CYLINDER SUBASSEMBLY     3354  ARM MEMBER     3375  CLAMP BAR SUPPORT COLUMN   

   Parts list,  FIG. 36  TYPICAL PRESS CYLINDER ASSEMBLY (ONE AT EACH END OF EACH ARM ASS&#39;Y)
       3351  PRESS CYLINDER SUBASSEMBLY     3352  PRESS CYLINDER     3353  ADAPTER BLOCK     3354  PRESS ARM MEMBER     3355  CARRIAGE     3356  LINEAR BEARING BLOCKS     3357  LINEAR RAIL     3358  CYLINDER SUPPORT ARMS (2 PER CYLINDER)     3359  MAIN SUPPORT PLATE     3360  SUPPORT ARM MEMBER     3361  PRESS TRAVEL STOP (ADJUSTABLE)     3362  PIN ASSEMBLY     3363  PIN (TYPICAL EVERY ⅝ INCH)     3364  PIN SPRING     3365  PIN BLOCK     3366  PIN BLOCK MOUNT     3367  COOLING AIR MANIFOLD     3368  AIR DELIVERY PORTING     3369  AIR NOZZLE     3370  SUPPLIMENTAL AIR MANIFOLD     3371  SUPPLIMENTAL AIR NOZZLE   

   Parts List,  FIG. 37  STATIONARY CLAMP PLATE ASSEMBLY
       3372  SCREEN BAR     3373  REPLACEABLE CLAMP BAR     3374  CLAMP BAR MOUNT     3375  CLAMP BAR SUPPORT COLUMN (2 PER ARM)     3376  CLAMP BAR HEIGHT FINE ADJUST MECHANISM   

   Parts List,  FIG. 38  MOVING (GATE) CLAMP PLATE ASSEMBLY
       3377  MOVING CLAMP BAR SUPPORT COLUMN (2 PER ARM)     3378  MOVING CLAMP BAR SUPPORT COLUMN RAIL     3379  MOVING CLAMP BAR SUPPORT COLUMN BEARING BLOCK     3380  MOVING CLAMP CYLINDER     3381  CORNER SHIELDS     3382  SPACE   

   Arm  3307  of the screen clamp assembly  3102  is a movable assembly that moves up and down (i.e., normal to the plane of the loaded frame  30 ). It&#39;s actuated by 2 different cylinders as shown on  FIG. 38 , and this allows the screen to be loaded and slid along the main plate  3302 . All of the Arms  3306 – 3309  and supporting superstructure, is supported on four posts that are approximately in the  4  corners of the machine so these are Item numbers  3310 ,  3311 ,  3312 , and  3313 . One of these support columns (Item  3312 ) can be seen in  FIG. 34 . 
   So Arm assembly  3306  includes a rail  3318  and a support arm member  3360 . This is a stationary rail member, and at the top of the apparatus is another stationary rail member  3316 . Arm assembly  3307  slides and includes rail  3319 . Assembly  3307  slides to the left and right on rail  3316  at the top and rail  3318  at the bottom. Arm  3307  is connected to each of these with the yolks at the bottom. At the bottom is yolk  3323  which has slide bearing blocks in the bearing rails. At the top arm  3307  is connected with another yolk which  3324 . Yolks  3323  and  3324  give Arm  3307  the horizontal action. This horizontal motion is accomplished with an air cylinder  3329  at the bottom of the machine. Cylinder  3329  is a rodless air cylinder that is connected to yolk  3323  via an Arm- 3307 -lower-cable-clamp/rodless-cylinder attachment  3345 . There&#39;s also another cable clamp  3346  at the other end of Arm  3307 . 
   This cable system performs the function of keeping Arm  3307  vertical; i.e., to make it perfectly perpendicular to Arm  3306  and Arm  3308  and parallel to Arm  3309 . The cable system ties the top yolk  3324  and the bottom yolks  3323  together, so arm  3307  doesn&#39;t have a tendency to tilt or twist. The exemplary mechanism includes cables and pulleys; alternatively, timing belts, or drive shafts with a rack and pinion, a spline shaft assembly, or air cylinders, or one of many different mechanisms could be used. 
   In the exemplary system, as the Arm  3307  moves to the left, the cable is fed down to the left and goes, or proceeds around pulley  3333 . The cable then travels back inside of Arm support member  3360  which is a hollow tube, and goes back and travels around pulley  3338 . From pulley  3338 , the cable travels up the machine to pulley  3337 . The cable travels around pulley  3337  and ends at the cable clamp  3346  on the right side of the cable clamp. Each end of the cable, is attached to this cable clamp, with a threaded adjustment for fine tuning of the alignment of the Arm  3307 . 
   When the air cylinder moves to the right then the tension on that cable pulls the top of Arm  3307  to the right. If Arm  3307  moves to the left, then another cable which goes from attachment  3345  in the other direction (which is to the right) and proceeds around pulley  3334 . Then the cable goes up to pulley  3335  which takes it inside of the supporting member  3316  and goes around pulley  3336 . And from pulley  36  it returns back to the cable clamp  3346  for Arm  3307 . When arm  3307  moves to the left, the air cylinder which is down at the bottom pulls the bottom of Arm  3307  to the left and pulls the cable which then pulls the top to the left. These two cables in conjunction with each other keep Arm  3307  perpendicular to Arm  3306  and keep the squareness of the machine. 
   In a similar manner, Arm  3308  moves in two directions. It moves horizontally back and forth and also vertically up and down (within a plane parallel to the loaded frame  30 ). To keep Arm  3308  square in a vertical motion, another Arm  3317  serves as horizontal guide rail for Arm  3308 ; Arm  3317  is a guide rail which moves only vertically up and down (in a plane parallel to the loaded frame). It does not move horizontally back and forth but it provides the horizontal alignment for Arm  3308 . It is supported by a corner yolk  3326  on the right hand side and corner yolk  3326  travels on a vertical rail  3314 . Rails  3314  and  3317  are positioned further from the base plate  3302  than the arm assemblies  3306 – 3309 . 
   Arm  3308  has a horizontal guide rail  3317  that supports it on the right hand side. On the left hand side it is supported through a corner yolk  3327  which is also the corner yolk for Arm  3309 . The corner yolk  3327  travels in a vertical direction up and down a left hand vertical guide rail  3315 . Comer yolk  3327  also supports Arm  3309  and allows Arm  3308  to pass underneath the corner yolk so that smaller screen frames can be accommodated. 
   Also, the vertical travel is controlled through guide rail  3317 . Arm  3308  is supported on each end by the corner yolks  3326  and  3327 . On the left side of the machine (in  FIG. 33A ) is a vertical air cylinder  3330  which is attached to the vertical cable clamp  3347 , and this is shown in  FIG. 35 . The air cylinder  3330  moves this whole mechanism up and down (relative to the backplate  3302 , normal to a plane of the loaded frame). A counter weight mechanism is also provided. This air cylinder  3330  may be only strong enough to move the mechanism but is not required to support the weight of the mechanism in a neutral position. Thus, there is a counter weight (not shown) to the system, which just equals the weight of the vertical travelling members. 
   The counter weight assembly (not shown) may include a cable attached to corner yolk assembly  3327 . The cable passes up the machine, over the top of the machine, and then back down and either attaches to a spring or a counter weight, a physical weight, or an air cylinder that just will counter the weight of the clamping arm assemblies. 
   Arm  3308  is clamping the screens so it must remain parallel to Arm  3306  (for a screen frame that is rectangular). A cable arrangement similar to that described above is provided. This cable arrangement starts with cable clamp  3347  on the left side of the machine ( FIG. 33A ). A cable comes out the bottom of clamp attachment  3347  and proceeds down to pulley  3342 . The cable returns up through the inside of the supporting member for guide rail  3315 . The cable travels up to pulley  3343  then it travels to the right hand side and joins up with and goes around pulley  3344 . The cable then travels down to the cable clamp  3348  on the other end of Arm  3308 . This cable provides the tension to hold up the right hand side of Arm  3308  when there&#39;s nothing in the system. When the screen is clamped, to keep this right hand of Arm  3308  down, another cable comes out the bottom of cable clamp  3348 . The cable comes down around pulley  3339 , the cable passes back up around pulley  3340  and travels over to the left to pulley  3341 . From pulley  3341  it travels to cable clamp  3347 . These two cables in conjunction with each other keep the horizontal guide rail  3317  for Arm  3308  horizontal. 
   On the left side of the screen ( FIG. 33A ) there is a vertical Arm assembly  3309 . Arm  3309  is guided vertically on the left hand vertical guide rail  3315 . Arm  3309  is connected with yolk  3327  at one end. At the bottom end Arm  3309  is attached with the corner yolk  3322  for Arm  3306 . Yolk  3322  holds some bearing blocks on it and allows the Arm  3309  to travel vertically up and down (the up and down directions in  FIG. 35 , towards or away from the backplate  3302 ) and maintain a vertical orientation. The vertical member  3315  is attached at the top of the machine to support Arm  3316  via a little corner plate  3313 . 
   There are stationary rails on the outer periphery of the machine on the right side, supporting stationary supporting Arm  3308  and the top rail which is right at the very top of the machine is supporting Arm  3307 , the bearing block for Arm  3307  at the one end and at the other end its at the post end at  3313  supports member  3315 . In turn, rail  3315  is supported at the other end. Rail  3315  supports Arms  3308  and  3309  through yolk  3327 . Arm  3306  at the bottom of the machine is supported by the support post  3310  at the bottom left corner of the machine  3102 . 
   Also shown in  FIG. 35  is a corner shield  3381 . The corner shield  3381  can be pneumatically actuated into, or out of place, to prevent the pins from trapping the screen cloth at the corners, which can result in the cloth fully entering or being inserted into the groove  32  of the screen bar. 
     FIGS. 36–39  show details of the clamping/press mechanism.  FIG. 36  shows the air manifolds and the pins and a cylinder  3352  that activates the pressing action.  FIG. 37  shows the way in which the arrangement of the pins  63  in relationship to the screen bar  30  in relation to the clamping assembly.  FIGS. 38 and 39  shows a special case where the clamp plates  3373 ,  3374  moves up and down under control of cylinder  3380 . 
   Because the machine  3102  allows for side loading of the screen frames as well as front loading, the screen frames pass through the gap  3382  shown  FIG. 34 . For the screens to go through the gap, the clamp plates have to move out of the way. The two side clamp plates act as gates as well as the clamp bars, to keep the screen frame  30  in position. The clamping mechanism  3373  and  3374  on Arm  3307  and Arm  3309  can be raised and lowered by an actuating cylinder  3380  (as shown in  FIG. 38 ). By raising the clamping mechanism  3373 ,  3374  out of the way, the frame can be loaded in from the side. Once the frame  30  is past the arm  3307 , the clamping mechanism is lowered again. Arm  3306  and Arm  3308  can be stationary as shown in  FIG. 37 , and the moving clamp mechanisms on Arm  3307  and Arm  3309  are shown in  FIGS. 38 and 39 , respectively. 
   Optionally, if the frame is to be loaded from the top or bottom, then arms  3306  and  3308  can be configured to be movable up and down (in the direction parallel to the insertion devices  3363 ), and arms  3307  and  3309  would not require up and down movement (normal to the plane of the loaded frame). To insert the frame from the top or bottom, the clamping mechanisms  3373  and  3374  of arms  3308  and  3309  would be raised away from the backplate  3302 , the frame is inserted through the top or bottom, and the arms  3306  and  3308  would return to their normal plane. 
   The slot  3303  is in the back plate  3302 . Slot  3303  provides a mechanism to push the frames into the machine  3102  and out of the machine. Two rodless air cylinders with little fingers slide the screen frames into the machine and out of the machine. Once the frame  30  is in position, as shown in  FIG. 38 , the screen cloth is suspended or draped down. Preferably, the screen cloth  34  is automatically fed down to cover the area of the screen frame  30 . After the screen cloth  34  is stopped in position, it is draped in that area. The press  3102  closes; the pins  3363  come down and push the screen cloth into the groove  32  in the screen bar  30 . 
     FIG. 36  shows the press assembly  3102  and the cylinder that activates the press. The exemplary gap between the pins  3363  and the groove  32  of the screen bar  30  is about 10 centimeters (4 inches). Other gap sizes may be used. The exemplary gap of 10 centimeters allows enough room to get the screen frame  30  in and out, but is not so far so that excess time is required to close the press. So, just before the screen cloth is in position for the next assembly to be completed, the (previous) finished screen frame from the previous cycle is ejected out of the machine  3102 . 
   In the example, the outside of the frame  30  is clamped and the pins  3363  are part of the clamping assemblies, as shown in  FIGS. 36–39 . Because the pins  3363  are physically attached to the clamp Arms  3306 – 3309  by a fixed distance, the pins are now in location when the frame  30  is clamped as shown in  FIGS. 37–39 . A modular, replaceable clamp bar  3373  is designed to suit the particular screen bar profile being used (The profile is the distance from the outside edge of the screen bar to the center of the spline groove  32 ). 
   Note that in apparatus  3102  the registration of the frame is performed from the outside of the screen bar as the frame is clamped, to position the pins for insertion. (In contrast, the apparatus described above with respect to  FIGS. 20–30  uses registration with reference to the inside edges of the screen bar). 
     FIG. 40  is an elevation view of an apparatus similar to that of  FIG. 31A . The apparatus of  FIG. 40  includes a smaller oven  4004  that is capable of holding two frames (one behind the other) simultaneously.  FIG. 40  shows the transport mechanism in greater detail. 
   At the right side of  FIG. 40  a pair of transfer (rodless) cylinders  4020  actuate a pair of carts  4010  and  4011 . Each cart  4010  and  4011  holds a respective frame  30 . By actuating cylinders  4020 , the carts  4010 ,  4011  are moved from right to left to move the frame into the right end of the oven  4004 , and outside the left end of the oven. 
   An exemplary cart  4010  is shown in  FIG. 41 . Cart  4010  has a generally L-shaped bracket  4102 , for fitting around a corner of a frame  30 . The cart  4010  has a plurality of insulated pads  4104 . A spring-loaded finger  4108  rotates into the position shown in  FIG. 41  to lock the frame  30  to the cart  4010 . 
   The insulated pads  4104  avoid hot spots in the frame that might melt the glue. The insulated pads may be sized to provide a light interference fit, but should be sufficiently sized to avoid denting the frames. The three insulated pads  4104  clip the frame  30  while allowing any size screen without touching the corner keys (thus avoiding blemishing the corner keys). The frame can be bigger than the cart  4010 . 
   In a small frame, the center of gravity of the frame is close to the left clip  4104  in  FIG. 41 . The frame sits and is supported by the clips  4104  without touching the spring loaded finger  4108 . As the screen size increases and the center of gravity of the frame moves away from the cart  4010 , the frame  30  tends to lift off of the right clip, and thus the finger  4108  stabilizes the frame by preventing the frame from lifting up out of the cart. The large frame rests against the finger and the bottom of the left clip. The clip  4104  on the vertical member of cart  4010  stabilizes the frame in the vertical orientation, and provides a means to push the frame forward. 
   An actuator  4110  may be provided to rotate the finger  4108  out of the way to load the frame onto or off of the cart  4010 . Alternatively, the finger can be spring loaded to bias finger  4108  into the position shown, and the finger can be opened manually to insert or remove a frame. Alternatively, the finger can be configured so as to be mechanically triggered to close when a frame is placed in the position shown in  FIG. 41 . Alternatively, the finger  4108  may be a retractable finger, and may be actuated mechanically or pneumatically, instead of a passive spring-loaded device. 
     FIG. 42  is a side elevation view of the oven  4004  shown in  FIG. 40 , taken viewed along section line  42 — 42 . The oven  4004  has an hot air feed duct  4202  into which the hot air is fed at a temperature between about 150 and about 300 degrees C., depending on the desired cycle time and the type of adhesive used. The hot air is forced up through the oven  4004 , and exits through an outlet duct  4203 . Air from outlet  4203  is fed through a heater and returned to the inlet duct  4202 . 
     FIG. 43  is an enlarged detail of  FIG. 42 . As shown in  FIG. 43 , cart  4010  may be higher than cart  4011 . This difference in the height of the carts  4010 ,  4011  is optional, and may be used if there is insufficient room for the two rodless cylinders  4020  to be positioned side-by-side at the same height. For example, if a wider oven  4004  is used, then the rodless cylinders  4020  may be placed further apart, and it would not be necessary to locate the carts  4010  at different heights.  FIG. 43  also shows a pair of screen frame support rails  4210  and  4211 . In the example, rail  4210  is in the same plane as the insertion apparatus  3102 . 
   Referring again to  FIG. 40 , the heated frames are pushed out of the oven  4004 , and are transferred to the back support frame  4002 . A folding finger gate  4040  lets the frame pass through and prevents the frame from traveling backwards. Preferably, the folding finger is a passive device, as described below. 
     FIG. 44  shows the hand-off between the oven conveyor and the transfer conveyor. The screen frame  30  is pushed to the left by its respective cart  4010  (or  4011 ). A pair of guide rollers  4404  bring the frame  30  from an out-of-plane position to an in-plane position (These positions are shown in  FIGS. 43 and 44 ). Alternatively, instead of rollers, a guide bar or plate may be used. When the frame  30  pushes against the folding finger  4040 , the finger pivots clockwise about its axis  4048 , as shown in  FIG. 46 . A spring  4042  biases the finger to the position shown in  FIGS. 44 and 45 . When the frame  30  passes to the left of the folding finger  4040 , the spring  4042  pulls the finger back to its rest position, as shown in  FIG. 45 . When finger  4040  is in this position, the frame  30  cannot move backwards. A block  4044  prevents the spring from pulling the finger  4040  back past its rest position. Finger  4042  is mounted to the carriage  4046  of a rodless cylinder  4402  (shown in  FIG. 44 ). Once the frame  30  is in the position shown in  FIG. 45 , the rodless cylinder  4402  moves the carriage  4046  (and thus the frame  30 ) into the clamping apparatus  3102 . 
   A second folding finger assembly  4050  pushes the finished screen frame assembly out of the insertion apparatus  3102 . The operation of the assembly  4050  is the same as described above with reference to assembly  4040 . 
     FIGS. 47 and 48  show an exemplary automatic screen cloth feed for the insertion apparatus  3102 .  FIG. 47  is a rear elevation view of the apparatus shown in  FIG. 33A .  FIG. 48  is a right side elevation view of the apparatus shown in  FIG. 47 . Details of the insertion apparatus are omitted from both  FIGS. 47 and 48  for easier viewing of the cloth feeding apparatus. 
   The exemplary apparatus includes five different feeds for feeding any one of five different widths of screen cloth  34  at any given time. This allows rapid switching between screen assembly widths. Although is also possible to use a single cloth feed with the cloth that corresponds to the widest assembly made on the apparatus, using different cloth widths substantially reduces wasted cloth. 
   Each width of cloth is fed from a roller  4730 . At the top of the assembly, a plurality of nip rollers  4710  and  4711  feed the cloth. Rollers  4710  are beneath each piece of cloth, and rollers  4711  are above each piece of cloth. A plurality of drive units  4720  (which may be reversible quarter horsepower motors) drive the rollers to feed the cloth. There is one lower sensor (such as a photoeye  4750 ) at the bottom of the clamping assembly  3102 . When the cloth  34  is being fed out, the motor  4720  stops when the cloth reaches photoeye  4750 . 
   Only one of the rolls  4730  of cloth  34  pays out at any given time. When the insertion apparatus is being reconfigured to change to a new size of screen frame, the previously used cloth is retracted, and the next cloth to be used is paid out. To retract the cloth from the last screen frame assembly, the motor  4720  corresponding to that cloth is operated in reverse. A plurality of sensors (such as photoeyes  4740 ) are positioned, one below each nip roller  4710 . When the cloth  34  retracts so that the photoeye for that roll of cloth no longer detects the presence of cloth below roller  4710 , then the motor  4720  for that roll of cloth is turned off. 
   One of ordinary skill in the art understands that additional aspects of the apparatus of  FIGS. 31–48  may be configured using the teachings of the apparatus of  FIGS. 20 to 30  and the description thereof provided above. In addition, various features of the apparatus in  FIGS. 20 to 30  may be replaced by features of the apparatus of  FIGS. 31–48 . 
   Adhesive 
   Adhesive is applied in the groove  32  of the screen bar  30   a  or against or close to the base of the step  32 ′ of the screen bar  30   a ′. In either case, the adhesive  36  is applied along the base  32   a  of the respective tensioning step  32 . As is described below, the adhesive  36  may be applied as a film or bead. 
   In either the embodiment shown in  FIG. 6  or that shown in  FIG. 8 , the adhesive  36  is secured to the screen bar  30   a  along the base  32   a  of the respective tensioning step  32 . The term “secured” or the term “bonded” as used herein is intended to include the generally accepted terms for adhesion of one material to another, i.e., mechanical interlocking, the formation of direct chemical bonds across the interface of the materials and electrostatic attraction, as discussed in Engineered Materials Handbook, Vol. 3, “Fundamentals of Adhesives and Sealants Technology”. ASM International Handbook Committee, page 40. By far, the dominating adhesion mechanism, especially in the absence of reactive groups, is the electrostatic attraction of the adhesive to the screen bar as the adherent and vice versa. These are primarily dispersion forces (London forces) and forces arising from the interaction of permanent dipoles. These forces provide much of the attraction between the adhesive and adherent and contribute significantly to the cohesive strength of the adhesive polymer. Mechanical interlocking is assisted by the roughness and porosity of the adherent, in this case, the screen bar. The formation of covalent chemical bonds requires that there be mutually reactive chemical groups tightly bound on the adherent surface and in the adhesive. 
   Preferably, the adhesive  36  is applied while the screen bar  30   a  or  30   a ′ is being made. The screen bar substrate itself may be made from metal, plastic, composites, wood and the like. By way of example, the screen bar  30   a  or  30   a ′ may be made by either roll-forming or extruding metal (or by extruding plastic) into a segment of screen bar  30   a  or;  30   a ′ and forming groove  32  (or step  32 ′) along one side of the screen bar segment  30   a  (or  30   a′ ). 
   Equivalent methods may be used for other materials. At this time, adhesive  36  or  36 ′ is applied in the groove  32  of the segment of screen bar  30   a  (or along the base of the step  32 ′ of the segment of screen bar  30   a ′.) However, if desired, the adhesive  36  may be applied in a separate (“off-line”) operation subsequent to the manufacture of the segment of screen bar  30   a  or  30   a′.    
   During roll-forming, for example, the adhesive may be applied to the flat strip, before it passes through the rollers of the roll former, or, preferably, at or near the exit end after the screen bar has been shaped. If the adhesive is applied to the flat strip, however, the adhesive must be allowed to cool before roll-forming, which takes time and space, and it is more difficult to position the film or bead of adhesive correctly. In the case of extruded screen bar, the adhesive can only be applied after the screen bar has been formed, or off-line. 
   In each of the above cases, adhesive may be applied to the screen bar using a standard hot melt adhesive applicator using a bulk melter and a constant displacement pump or the like. Alternatively, a screw-type extruder may be used for this application. Either a film or a bead of adhesive having a desired thickness can be applied. For both types of applications (bulk melter or extruder), the hot melt adhesive (in bulk, pellet or granular form) is heated above the melting point and pushed through a small orifice (nozzle) to stream into the groove  32  of the screen bar  30   a – 30   d  or along the base of the step  32 ′ of the screen bar (or to its final location, if applied onto the flat strip before the strip is roll-formed), which is driven under the nozzle at a constant speed. The molten adhesive is allowed to cool to room temperature, and the finished screen bar with applied adhesive can then be stored. Typically, roll-forming lines run at a speed between 100 and 400 feet per minute and slightly less for aluminum extrusion. Off-line application typically runs at 100 to 300 feet per minute. By way of example, the reapplication of a 0.05″ diameter bead of adhesive having a specific gravity of 1.02 (typical for polyamide) will need to be supplied at 8 pounds per hour to meet a 100 feet per minute line speed and 48 pounds per hour for a 300 feet per minute line speed. 
   Alternatively, the adhesive may be pre-extruded as a solid ribbon. The cooled solid ribbon of adhesive may be roll-formed into the screen bar during the roll-forming process. Near the end of the roll-forming process, when the screen bar material is close to its final shape, the ribbon of adhesive is introduced, and the material forming the screen bar may be bent around the ribbon of adhesive to retain the adhesive. The solid adhesive may also be pressed into the roll formed bar after the roll-forming is complete. Preferably, any roll-forming lubricants that may be present in the groove or tensioning step are removed before applying the ribbon of adhesive. Although applying the adhesive in a sold, pre-extruded form may add an extra step to the screen bar roll-forming process, it eliminates the need to heat the screen bar above 60° Celsius to obtain good adhesion between the screen bar material and the adhesive. 
   Preheating the screen bar just prior to application of the adhesive, to between about 40 and about 150° C., greatly improves the adhesion between the adhesive and the screen bar. Flame treatment of the surface of the screen bar also improves this adhesion. Therefore, when applying the adhesive, it is preferable to heat the screen bar at the location of adhesive application. Heating the side of the screen bar that the adhesive contacts significantly lowers the viscosity of the adhesive and allows it to flow easily at the heated interface. This provides a mechanical bond (interlocking) on a microscopic scale, in that the adhesive flows into any minute imperfections in the screen bar, as well as an electrostatic bond. It is preferable to heat the screen bar to a temperature in the range of about 40° C. to about 150° C., with about 60° C. to about 120° C. being preferred and about 60° C. to about 100° C. being most preferred. A propane flame or like heating element can be used to heat the screen bar in this manner. Corona treating, as is routinely used in the plastic and adhesive industry may also improve bond strength, depending upon the substrate. 
   Mechanical bonding also can be effected by perforating the bottom  32   a  of the groove  32  or the bottom  32   a ′ of the screen bar  30   a ′ adjacent to the step or lip  32   b.  When applied, the low viscosity adhesive flows through these openings to some extent and forms rivet-shaped beads or heads on the underside of the screen bar. When solidified, these beads mechanically lock the screen to the screen bar. These openings may be on the order of 1/32″ (0.08 cm) round or square. This dimension may be varied as desired. 
   Further, adhesive bond can be lost if, for example, residual processing lubricants are not removed prior to applying the adhesive to the screen bar, if extreme and sudden temperature changes occur, if improper surface treatment or improper preheating of the screen bar is done, or if the adhesive is applied while too cold. For these reasons, both mechanical and electrostatic bonding are preferred. If, for example, the electrostatic bond is lost because of excess processing lubricants, the mechanical interlocking assures bonding. As discussed above, perforations in the screen bar adjacent to the step are the preferred mechanical interlock. 
   The adhesive is allowed to cool and set in the groove  32  of the screen bar  30   a  or along the step  32 ′ of screen bar  30   a′.  Then, the segment of screen bar  30   a  or  30   a ′, which includes the adhesive  36  or  36 ′, can be stored for any desired time period, and used at a later date. Typically, the screen bar and adhesive assembly is sold in a standard lineal format typically 12 feet (3.6 meters) long. As discussed above, the lineals are cut to size and made into screen frames using corner keys or otherwise, in accordance with conventional practice. 
   Another aspect of the invention is the re-melting characteristic of the adhesive used. Generally speaking, a preferred adhesive (1) is applied easily, in liquid (e.g., melted (preferred) or solvated) form, (2) solidifies after application to the screen bar (for storage, shipment, assembly of the screen frame, etc.) and then (3) can be re-melted or reactivated (liquefied) during application of the screen to secure the screen to the screen frame. 
   The adhesive family known generally as “hot melt adhesives” have been found to have these attributes, since they can be applied in liquid form, solidify and then can be remelted or “re-activated” at the time of securing the screen (i.e., screen assembly). 
   Hot melt adhesives in a solvated, liquid form, can also be used. They are liquefied by the use of solvents such as toluene, MEK (methyl-ethyl-ketone), acetone, and the like. Once solvated, they are applied in liquid form and solidify upon solvent evaporation. They can then be re-melted in the same way the non-solvated forms are. The solvated forms, however, are less desirable, since the solvents add costs, and the evaporated solvents are typically toxic when inhaled. 
   The curable type of hot melt adhesives, known as “hot melt polyurethane adhesives” (i.e., PUR&#39;s or HMPUR&#39;S) can also be used for this invention, if the adhesive is re-activated (at the time of securing the screen) before it cures. The window of time available, between application to the screen bar and cure, depends upon the adhesive formulation. For instance, Henkel macromelt adhesive A4676 is a hot melt polyurethane adhesive which has approximately four days before it is cured to the point where reactivating cannot occur, effectively. Also available, with similar s characteristics, is HL9527 available from European Fullers, Rangeview Road, Mississauga, Toronto, Ontario. Essentially, these adhesives react with the moisture in the air, causing permanent molecular cross-linking and thus become un-meltable (thermoset). The act of curing or cross-linking of the polyol and the isocyanate in these adhesives precludes the resultant polyurethane from remelting. 
   The A4676 adhesive, for example, has an acceptable application melt temperature of 110° C. and a green strength (tensile strength, before cure) of 4 to 5 pounds per linear inch of screen) which is more than adequate to secure the screen, once applied. The adhesive, upon curing, has a tensile strength of 2300 lb., a heat resistance temperature of 300° C. and a viscosity of 100 poise at 230° C. The advantage to this type of adhesive is the low application temperature and the relatively high heat resistance temperature, once cured. The disadvantage is the fact that the assembly must be completed shortly after the application of the adhesive to the screen bar. Thus, this type of adhesive has limited use. For the majority of applications, when the screen bar is stored for prolonged periods before screen assembly, the regular hot melt (non-curing type) adhesive must be used. For this reason, the regular hot melt type of adhesive is most preferred for practicing this invention. 
   The temperature during remelting of the adhesive is typically limited to below 400° F., preferably at 350° F., to prevent smoke (from PVC coated screen cloth). Hotter temperatures may be used, if any fumes exuded by the screen and/or adhesive are evacuated, trapped, and filtered or recycled. 
   The use of B-stage epoxy adhesive appears to be not nearly as practical for this invention. They could be made to work if formulated to be applied in a high enough viscosity state to allow handling, once applied to the screen bar; to have a high enough tack or green strength to secure the screen before cure; and to have a long enough shelf life, once applied to the screen bar, to allow screen assembly in time before natural crosslinking occurs. All of these conditions, however, make these adhesives difficult to work with in this environment. Another major drawback with these adhesives is the need for a long cure time at elevated temperatures. Typically, this requires the use of an oven. High intensity lasers have been used to greatly speed up the cure time, but may be impractical, from a cost perspective, for this invention. 
   As noted above, it is particularly desirable to reduce cycle time by extracting the insertion device (e.g., insertion pins) as soon as the adhesive in the vicinity of the pins solidifies. For a clean appearance, it is necessary to wait until the pins can be extracted without formation of strings of adhesive during extraction. The choice of adhesive can influence the cycle time. In particular, adhesives that tend to shear without forming strings are preferred based on this criterion. A preferred material is Henkel Macromelt 6071 adhesive, which has a heat resistance temperature of 70 C, and a melting temperature below 100 C. 
   An acceptable degree of bonding can occur without encapsulation of the strands of the screen-into the adhesive. Therefore, encapsulation is not essential to this invention. It is, however, preferred to encapsulate the strands of the screen using the adhesive, since this results in mechanical bonding as well as adhesive bonding. Further, encapsulation allows visual assurance that full melting and bonding have occurred. 
   For straight adhesion, without encapsulation, the adhesive can be applied as a film in a layer having a thickness between about 0.0005 to about 0.020 inches, and preferably, between about 0.003 to about 0.020 inches. The film option, if deemed acceptable by users, has the advantage of faster application speed and less cost. Whether a film or a bead of adhesive is used is really a matter of the degree of bond certainty that is desired by the particular user. When using a bead of adhesive, a layer having a thickness between about 0.020 to about 0.250 inches is preferred. When a bead is used, it is preferred to apply the adhesive in an amount to provide a layer having a thickness between about 0.030 to about 0.150 inches. This amount is sufficient to provide encapsulation. 
   An advantage of using a bead of adhesive in a groove (over a film of adhesive in a groove or along a bottom of a step or lip) is that the bead can be mechanically trapped by the walls of the groove, if the walls of the groove are tapered slightly to form a smaller spacing at the top (opening) than at the bottom. 
   In the exemplary embodiments of the invention, the primary mode of cooling at the time of screen assembly (as opposed to the time of application of the adhesive) to the screen bar occurs by conduction of heat into the aluminum substrate (screen bar) and secondarily, by convection/conduction into the surrounding air. Although it is also possible to allow cooling to occur naturally to minimize process complexity, forced cooling (by methods such as forced ambient or chilled air) is quicker. If forced air cooling is used, it may be either attached to the insertion tool (as in  FIGS. 2–5 ) or in the form of a general fan or blower blowing air over the entire assembly or focused on the screen bar. 
   Forced cooling may be desired when hot ambient conditions exist or if the screen bar is preheated. Also, the screen bar must be cool enough to avoid remelting of the adhesive after the adhesive has cooled. 
   Because the preferred mechanism of cooling includes heat sinking into the screen bar, it is important to use a minimum amount of adhesive to avoid a thick barrier of low conducting adhesive that would interfere with heat flux from the hot adhesive to the screen bar. 
   For the adhesive to bond to the strands of the screen, it is necessary for the adhesive to cool below its melt point. For this reason, in this embodiment, it is preferred to utilize an adhesive (such as a crystalline adhesive) having a sharp melt point, so that the adhesive solidifies soon after cooling begins. 
   The adhesive also must provide adequate holding strength over the full range of service temperatures. Hot melt adhesives, particularly, polyester and polyamide adhesives have been shown to offer good flow and adhesive characteristics over the full temperature range experienced in service. Additionally, and when desired, these adhesives also provide good encapsulation (mechanical anchoring of the screen strands) characteristics. 
   Generally speaking, conventional thermoplastic pure polymer resins such as polyamide, polyester, polycarbonate and the like tend to have higher melt flow viscosities than is acceptable, resulting in lower screen holding strength than desired, because it is difficult to embed the strands of the screen in these adhesives. Straight polyamide (e.g., nylon) and polyester (PET) polymer resins (plastics) work only to a limited degree, since the viscosity and melt temperatures are higher with these pure resins. Also, these resins include none of the desirable additives, which lower viscosity and melt temperature and improve surface wetting (via surfactants). Although pure tensile holding strength may be achieved with high viscosity resins and adhesives, the lack of adequate holding strength puts a greater demand on the electrostatic or adhesive bonding component. 
   The polyester and polyamide families of adhesives have shown good performance at elevated service temperatures. Therefore, these adhesives are preferred. Nevertheless, this invention is not limited to these adhesives. Rather, any suitable hot melt or equivalent adhesive or thermoplastic resin having the required heat resistance temperature, bond strength and viscosity characteristics can be used. 
   Most manufacturers follow ANSI and CGSB standards for load requirements. Experiments show that in order to pass the CAN/CGSB 79.1 type II standard, a retention strength of approximately 9 pounds per inch width of screen is required when the load is applied in the plane of the screen (i.e., tensile loading). This value was obtained from tests conducted at room temperature. This value was measured using a 1 inch (2.5 cm) long screen bar sample with a piece of screen 1 inch by 2 inches (2.5 cm×5.1 cm) attached. A tab attached to the screen bar and coplanar with the screen was inserted into one jaw of an Instron tensile testing machine while the screen was inserted into the other jaw. Samples were then loaded to the break point, which was recorded. 
   Existing spline retention technology which meets this load requirement of 9 pounds at room temperature was measured to drop to approximately 4 pounds per inch at 60° C. At −40° C., there was not a significant change in retention strength compared to room temperature measurements. The strength of hot melt adhesives also decreases at elevated temperatures, but may increase at slightly lower temperatures. In experiments, a strength of 30 to 35 pounds per inch was obtained at room temperature conditions using the Henkel 6206 adhesive. At 60° C., the strength was measured to be 20 pounds per inch. The present invention thus gives over three times higher retention strength over current spline technology over the range of service temperatures. This was unexpected! 
   In choosing a hot melt adhesive or thermoplastic resin to meet the requirements of hot weather conditions, one should consider various temperature values specified by the manufacturers of these adhesives or resins. Specific values include melt and glass transition temperatures as measured using differential scanning calorimetry (DSC ASTM test #E 698), heat resistance temperature using ASTM test method #D 2293 and softening point, usually determined using the ball and ring test, ASTM #E 28. Generally, the ball and ring temperature is approximately 8 to 10° C. greater than the melt temperature for polyester and polyamide adhesives. 
   The most important temperature value relating to selection of materials for this invention is the heat resistance temperature, since this value indicates the temperature at which movement under load occurs. This is referred to as “creep”. Typically, a 500 gram load is used on a 1 inch by 1 inch (2.5 cm×2.5 cm) lap seam (as opposed to a butted seam). The heat resistance temperature is an indication of when an adhesive begins to rupture under loaded conditions. 
   In short, the theoretical minimum heat resistance temperature allowable is the design ambient temperature. Nevertheless, practically speaking, it is generally necessary to have a heat distortion temperature to perform in the ambient conditions expected. In most areas (excluding tropical climates), this temperature is considered to be about 35 to about 45° C. Although it is most preferred to have adequate strength to hold screen tension up to 85° C. for shipping in closed containers (as per MIL-STD A10), a reasonable upper ambient limit (desert) temperature is considered to be about 50° C., where full performance strength is required. With the sun directly hitting dark colored screen bars, an additional 20° C. can be reached. Thus, a preferred minimum heat resistance temperature is about 70° C. for service, and about 85° C. for shipping. In temperate climates, it is generally acceptable to have a heat resistance temperature of about 55° C. This compensates for a 35° C. upper limit on ambient temperatures and a 20° C. differential for sunshine on dark colors. In tropical climates, these values are 45° C. plus a 20° C. differential, which yields a minimum of about 65° C. 
   Because the upper limit for ethylene vinyl acetate (EVA) type adhesives is generally considered to be about 75° C., this type of adhesive is acceptable from a temperature standpoint. However, EVA hot melt adhesives are not preferred because plasticizer migration from the screen may occur at elevated ambient temperatures resulting in loss in structural integrity, i.e., tensile strength. 
   In the adhesive industry, a 15 to 20° C. margin of safety is generally recommended between the heat resistance temperature of the adhesive used and the expected service temperature. Thus, an 85° C. service temperature expectation would suggest that the adhesive have a heat resistance temperature of about 100 to about 105° C. Adhesives in the polyamide or polyester family of hot melts meet this criterion. It is, however, more preferred to have an adhesive with a heat resistance temperature of about 120° C. This gives a 35° C. margin of safety over the 85° C. shipping temperature and 50° C. above the 70° C. dark color desert conditions under direct sunlight. Again, polyamide and polyester hot melt adhesives meet these values. 
   Thus, the adhesive should have a heat resistance temperature of not less than about 35° C. A heat resistance temperature between about 55° C. and about 180° C. is preferred, with between about 85° C. and about 150° C. being more preferred and between about 100° C. and about 130° C. being most preferred. Thermoplastic (hot melt) adhesives or resins are acceptable. These adhesives allow replacement of the screen by using a hot tool to first liquefy and allow removal of the old screen, and then replacement in a manner discussed herein. If desired, replacement screen also could be attached using conventional spline techniques, when using screen bar that has a spline groove. For this reason, a groove is preferred over a simple step. 
   The melting point value specified by the adhesive manufacturers is also important. This value is the temperature at which the adhesive begins to liquefy and flow under shear stress. 
   Although heating the adhesive by convection is preferred, a heated tensioning tool may be used. Because the preferred tensioning tool includes a plurality of pins that remain in the adhesive till the adhesive re-solidifies, the use of heated pins is expected to increase the cooling time. Nevertheless, if a heated insertion tool is used, it is important to use an adhesive having a low enough melt temperature (e.g., about 100° to about 225° C. (maximum)) to allow a heated tool temperature within an operating range, which limits smoke production. Smoke can be generated from either the adhesive or the coating on the screen. This range is about 200° C. to about 500° C. (with about 200° C. to about 400° C. being preferred, about 200° C. to about 300° C. being more preferred and about 250° C. to about 300° C. being most preferred) with minimum smoke production. The corresponding maximum ball and ring temperatures of the adhesive are about 210° C. (acceptable), about 150° C. (preferred) and about 120° C. (most preferred). Hot melt adhesives selected from the group consisting of polyester, polyamide, polyolefin, polypropylene, polyurethane, butyl and ethylene vinyl acetate (EVA) give satisfactory bond strength at s room temperature (about 20° C. and below). However, only the polyester and polyamide adhesive families seem to perform particularly well at elevated temperatures. Although the EVA&#39;s may generally work well, they are not preferred due to excessive plasticizer migration, which may occur at elevated ambient temperatures. This causes loss of bond strength. 
   Table I shows polyamide and Table 2 shows polyester hot melt adhesives that meet the high temperature requirements and melt flow characteristics. In these tables, the Macromelt adhesives are available from Henkel, Elgin, Ill., whereas the Bostik adhesives are available from Bostik, Middleton, Mass. and the letter “a” indicates “acceptable” while the letter “p” indicates “preferred”. 
   
     
       
         
             
             
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
                 
               Ball and 
               Heat 
               Viscosity/ 
               Tensile 
             
             
               Polyamide 
               Ring 
               Resistance 
               (temp.) 
               Strength 
             
             
               Adhesive 
               Temp. ° C. 
               Temp. ° C. 
               Poise/(° C.) 
               psi 
             
             
                 
             
           
          
             
               Macromelt 6000-a 
               200 
               115 
                4/(200) 
               1900 
             
             
               Macromelt 6202-p 
               150 
               110 
               50/(210) 
                450 
             
             
               Macromelt 6206-a 
               180 
               145 
               40/(210) 
               1100 
             
             
               Macromelt 6211-a 
               145 
               125 
               25/(210) 
                370 
             
             
               Macromelt 6212-a 
               110 
                80 
               35/(200) 
                500 
             
             
               Macromelt 6071-a 
                95 
                70 
               10/(160) 
                210 
             
             
               Bostik 7239-p 
               150 
               115 
               35/(200) 
                385 
             
             
               Bostik 4252-p 
               150 
               110 
               22/(205) 
                580 
             
             
               Bostik 6240-a 
               185 
               145 
               16/(230) 
               N/A 
             
             
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
             
             
           
             
               TABLE 2 
             
             
                 
             
             
                 
               Ball and 
               Heat 
               Viscosity/ 
               Tensile 
             
             
               Polyester 
               Ring 
               Resistance 
               (temp.) 
               Strength 
             
             
               Adhesive 
               Temp. ° C. 
               Temp. ° C. 
               Poise/(° C.) 
               psi 
             
             
                 
             
           
          
             
               Bostik 4101-p 
               120 
                95 
                145(200) 
               3400 
             
             
               Bostik 4103-p 
               135 
               110 
                425(225) 
               2290 
             
             
               Bostik 4156-a 
               160 
               137 
                23(215) 
               2700 
             
             
               Bostik 4175-a 
               200 
               N/A 
                900(225) 
               N/A 
             
             
               Bostik 4178-a 
               145 
               120 
               1000(215) 
               3000 
             
             
               Bostik 5182-a 
               150 
               N/A 
                900(215) 
               N/A 
             
             
               Bostik 7116-p 
               150 
               N/A 
                340(200) 
               N/A 
             
             
               Bostik 7199-a 
               190 
               170 
                200(215) 
                700 
             
             
                 
             
          
         
       
     
   
   Another property that may be important, and one that separates thermoplastic (hot melt) adhesive from thermoplastic resins (plastics) is surface wetting. In this respect, melt viscosity is one of the most important properties of a hot-melt adhesive. In general, for a given adhesive, as the temperature increases, its viscosity decreases. Therefore, for a given hot-melt adhesive formulation, the temperature of the adhesive during application controls the viscosity, which greatly influences the extent of surface wetting. The bond formation temperature is a minimum below which surface wetting is inadequate. A hot-melt adhesive is applied at a running temperature, at which the viscosity is sufficient to wet surfaces. See the  Engineered Materials Handbook,  Vol. 3, “Adhesives and Sealants”, ASM International Handbook Committee, page 80. 
   Preferably, the adhesive not only melts and flows, but also has a wetting action to spread easily over the surface of the strands of the screen to secure and/or encapsulate them. Adhesive manufacturers add waxes and plasticizers as surfactants to promote surface wetting. The amounts of these additives remain proprietary to the adhesive manufacturers. Loads applied to the screen must be carried by the adhesive. The adhesives listed in Tables 1 and 2 give acceptable bond and tensile strength to meet the load requirements of the installation. Preferably, the tensile strength of the adhesive is over 200 psi, but many adhesives having a lower tensile strength can still effectively carry the loads. Strand encapsulation enhances bond strength between the screen and the adhesive and mechanical interlocking between the adhesive and the screen is preferred to ensure full bond potential. Perforations in the screen bar, discussed above, is the preferred method of mechanical interlocking. 
   There was an initial concern that polyamide adhesives and EVA would soften over time while in contact with plasticized PVC. screen, due to the potential plasticizer migration. (Polyester adhesives do not have the same susceptibility to plasticizer migration and thus, softening characteristics.) This concern with polyamide adhesives and EVA, however, has not been demonstrated in practice. It is believed that the amount of plasticizer available for migration is very low. For this reason, polyamides are, along with polyester adhesives, preferred. 
   Good weathering characteristics are advantageous, because many screen assemblies are exposed to full sunlight and extreme whether conditions. Industry standards generally demand mechanical properties to be, maintained over a ten year period. However, twenty years is preferred. 
   To enhance weatherability, it is generally known to add to the adhesive carbon black for blocking ultraviolet (UV) light, as well as light absorbers and light stabilizers. Also, adding enough carbon black to make the adhesive opaque is sufficient to block UV light. Generally, 0.5 to 2% by weight of the adhesive is adequate to block UV light, and 1 to 1.5% by weight is sufficient to make the adhesive opaque. Diminishing returns are experienced above 2%, and mechanical properties also can be adversely affected. Carbon black is preferred from a cost and performance standpoint. Alternatively, instead of adding carbon black to the adhesive to block UV from the sun, TiO 2  may be used. This would achieve a white color. 
   Benzotriazole is a suggested additive to act as a UV absorber for both polyamide and polyester adhesives. An example is Tinuvin 234, available from Ciba-Geigy, which is a 100% active chemical. This chemical may be added to the adhesive in an amount of 0.05% to 0.3%, with 0.1% be a typically specified amount, by weight. 
   Products which act as “hindered amine light stabilizers” (HALS) may also be added to the adhesive, in an amount between 0.05 to 0.3% by weight. 0.1% is a typically specified amount. Tinuvin 622, available from Ciba-Geigy, is a 100% HALS and is recommended for polyamide and polyester adhesives. 
   It is believed that using the accepted adhesives in a foamed form (with 20%–70% lower density) has an advantage of giving a larger bead size, for example, for a given mass per unit length—thus, lowering cost. A larger diameter bead increases the bonding area, which improves the bond strength. Also, the insertion speed is theoretically increased, as less mass is heated and melted from a given bead size. A Nordson model FM190 hot-melt dispensing unit is designed to apply foamed adhesives in bead form. Nitrogen is generally used as the foaming agent in such foamed adhesives. 
   The screen bars of this invention are designed to meet both the Canadian and U.S. type II standards for load resistance and pull out strength. (ANSI-SMA SMT 31- and CAN-CGSB-79.1-M91). In Canada, the load resistance test for a type II screen requires that a 75 lb. weight, or 37 lb. for a type I screen distributed over a one foot square diameter, be placed in the center of a three foot by three foot pre-clamped screen. The Canadian pull out test resembles a tensile test in which a one inch section of screening and screen bar are subjected to tensile loading in, for example, an Instron tensile testing machine. To satisfy this pull out test, screen samples must demonstrate at least 9 lb./inch resistance to tensile loads. If the spline or glue joint separates under a 9 lb. load, the screen fails the pull out test for type II screens. 
   The screen bars of this invention were designed to meet the customary screen dimensions as follows: 
   
     
       
         
             
             
             
           
             
                 
                 
             
             
                 
               BayForm B516 
               BayForm B38 
             
             
                 
                 
             
           
          
             
                 
               D-.17 inches 
               D-.235 inches 
             
             
                 
               T-.020 inches 
               T-.023 inches 
             
             
                 
                 
             
          
         
       
     
   
   The above dimensions, shown in  FIG. 6 , are typical in the screen industry, whereby “D” represents the height of the tensioning step, “T” represents the thickness of the bar material, which is typically aluminum, and E represents Young&#39;s modulus of the screen bar material (10.3×10 6  psi for aluminum, 30×10 6  for steel). It is known through experience that a B516 aluminum screen bar generally fails the 75 lb. load test if its thickness (T) falls below 0.018 inches. Similarly, an aluminum screen bar manufactured to the B38 standard generally is known to fail the 75 lb. load test if its thickness (T) falls below 0.020 inches. When the gluing methods of the present invention are employed, however, instead of the prior art&#39;s spline technique, a thickness “T” of less than 0.018 inches for the B516 bar, and a thickness “T” of less than 0.020 inches for a B38 bar was sufficient to meet the 75 lb. load test. Moreover, the present gluing technique was tested in accordance with the Canadian 79.1 type II standard pull out test parameters. Under this test, a B38 type screen bar must meet at least 9 lbs. per inch in tensile load before the spline pulls out, or the screen separates. Using spline technology, a B38 bar thickness “T” was reduced from 0.023 inches to 0.018 inches for a standard spline product, and this product resulted in a tensile load of 6 lbs./inch tensile force test result, thus failing the test. When a B38 style bar having a thickness of only 0.016 inches and a glued joint pursuant to the teachings of this invention was similarly tested, it had a tensile force of 25 lbs., passing the test by a factor of safety of almost 3.0 (or of almost 6.0 for a type I screen). 
   Accordingly, the screen bars of this invention can be made thinner and stronger than prior art screen bars using splines. According to solid mechanics analysis, the conventional spline screen bar cross-sectional ratio “D(in.)/T 2 (in. 2 )E(psi)” should be no greater than 
   41.3×10 −6  to meet the 75 lb. test. Using the present invention, the inventor contemplates achieving a ratio greater than 41.3×10 −6  to meet the CGSB-CAN 79.1 type II specification, and even 48.5×10 −6  or greater, with ratios as high as 65×10 −6  without failing the pull out test. Below in Table 3, examples of pull out test results for various thicknesses and tensions step heights employing a spline (Sets 1, 2 and 3) and the adhesive method of this invention (Sets 4, 5 and 6) are provided, easily demonstrating that the improved method of this invention increases the performance of screens subjected to a tensile load. 
   A screen and frame when so joined by a method according to the invention can pass a 37 lb. load test in accordance with break load at a thickness “T” at least about 10% less than the thickness “T” of a passing spline-retained screen and frame of like material undergoing said load test. For example, in Table 3, Set 2 specifies a spline type screen that failed the test, using 0.019 in. thick material. Set 5 specifies a screen according to the invention that passes the test with only 0.016 in. thick material. Because 0.016 is less than 0.019 (a failing spline thickness) by at least 10%, and a passing spline frame would require thickness greater than 0.019, an assembly according to the invention can easily be at least 10% thinner than a passing spline-retained screen frame of like material. 
   A screen and frame when joined according to the invention has a break load test value of at least 50% greater than a spline retained screen of like thickness “T” and like tensioning step height “D”. For example, in Table 3, Set 3 specifies a failing 0.016 spline with a 0.23 in. step height. The largest pull out load in sample set 3 is 5.769 lb. Set 5 specifies a passing frame screen assembly according to the invention, having the same thickness and the same tensioning step height. The minimum break load in sample set 5 is 18.22 lb., which is more than three times the pull out load of the spline type assembly of set 3. 
   
     
       
         
             
           
             
               TABLE 3 
             
             
                 
             
             
               PULL OUT/BREAK LOAD TEST ANALYSIS 
             
             
                 
             
           
          
             
               Set 1: T = 0.018 in., D = 0.200 in. with spline, D/T 2 E = 59.9 × 10 −6   
             
          
         
         
             
             
             
          
             
                 
               Sample code 
               Pull Out load 
             
             
                 
                 
             
             
                 
               FM1 
               5.922 
             
             
                 
               FM2 
               6.276 
             
             
                 
               FM3 
               7.713 
             
             
                 
               FM4 
               8.056 
             
             
                 
               FM5 
               7.683 
             
             
                 
               FM6 
               6.824 
             
          
         
         
             
          
             
               Set 2: T = 0.019 in., D = 0.200 in., with spline, D/T 2 E = 54 × 10 −6   
             
          
         
         
             
             
             
          
             
                 
               Sample code 
               Pull Out load 
             
             
                 
                 
             
             
                 
               FP1 
               8.236 
             
             
                 
               FP2 
               7.731 
             
             
                 
               FP3 
               6.156 
             
             
                 
               FP4 
               8.851 
             
             
                 
               FP5 
               7.570 
             
             
                 
               FP6 
               5.503 
             
          
         
         
             
          
             
               Set 3: T = 0.016 in., D = 0.230 in., spline, D/T 2 E = 87.2 × 10 −6   
             
          
         
         
             
             
             
          
             
                 
               Sample code 
               Pull Out load 
             
             
                 
                 
             
             
                 
               016P 
               −15.769 
             
             
                 
               016P 
               −25.603 
             
             
                 
               016P 
               −35.557 
             
             
                 
               016P 
               −44.416 
             
             
                 
               016P 
               −55.103 
             
             
                 
               016P 
               −63.850 
             
          
         
         
             
          
             
               Set 4: T = 0.0235 in., D = .230 in., Bostik 4156 polyester adhesive, 
             
             
               D/T 2 E = 40.4 × 10 −6   
             
          
         
         
             
             
             
          
             
                 
               Sample code 
               Break load 
             
             
                 
                 
             
             
                 
               IB4145-1 
               30.94 
             
             
                 
               IB4145-2 
               24.21 
             
             
                 
               IB4145-3 
               29.66 
             
             
                 
               IB4145-4 
               26.01 
             
             
                 
               IB4145-5 
               26.78 
             
             
                 
               IB4145-6 
               24.91 
             
          
         
         
             
             
          
             
                 
               B516 = D = 0.17, T = 0.020 
             
             
                 
               B38 = D = 0.230, T = 0.0235 
             
          
         
         
             
          
             
               Set 5: T = 0.016 in., D = 0.230 in., 6206 Henkel adhesive, 
             
             
               D/T 2 E = 87.2 × 10 −6   
             
          
         
         
             
             
             
          
             
                 
               Sample code 
               Break load 
             
             
                 
                 
             
             
                 
               31.64 
             
             
                 
               19.83 
             
             
                 
               18.22 
             
             
                 
               20.52 
             
             
                 
               22.62 
             
             
                 
               24.93 
             
          
         
         
             
          
             
               Set 6: T = .0235 in., D = 0.230 in., with Henkel 6206 with adhesive, 
             
             
               D/T 2 E = 40.4 × 10 −6   
             
          
         
         
             
             
             
          
             
                 
               Sample code 
               Break load 
             
             
                 
                 
             
             
                 
               28.15 
             
             
                 
               30.56 
             
             
                 
               28.08 
             
             
                 
               27.14 
             
             
                 
               25.38 
             
             
                 
               30.19 
             
             
                 
                 
             
          
         
       
     
   
   Although hot melt adhesives and thermoplastic resins are discussed above, the inventor contemplates that pressure sensitive adhesives and like bonding agents that provide acceptable results also could be used, if desired. 
   Tape 
   Although the exemplary assembly described above uses an adhesive that is applied as a film or as a strip, an adhesive tape may be used. 
   According to an embodiment shown in  FIG. 13A , a tape  1331  is laid on the mounting surface  1330   a  of the frame  1330 , with an adhesive surface of the tape facing away from the frame. Tape  1331  has adhesive on both sides. The tape may have: (1) a fixed portion  1331   a  that is fixedly attached to the mounting surface  1331   a;  and (2) an extended flap  1331   b  that is not adhered to the mounting surface of the frame. In  FIG. 13A , a piece of non-adhesive tape  1332  is inserted between the flap  1331   b  and the mounting surface  1330   a . The bottom surface of flap  1331   b  adheres to the non-adhesive tape  1332 . This prevents the bottom surface of flap  1331   b  from adhering to the mounting surface  1330   a . The flap  1331   b  is free to be folded over the edge of the screen fabric  1334 , as shown in phantom in  FIG. 13A . Thus, the screen fabric  1334  is adhered between two layers of tape  1331   a  and  1331   b.    
     FIG. 13B  shows a variation of the embodiment of  FIG. 13A . A tape  1331 ′ having only a single adhesive surface may be used. The tape  1331  is applied to the mounting surface  1330   a ′ of the screen bar segment  1330 ′, with the adhesive surface of the tape facing up, away from the mounting surface of the screen bar segment. A separate adhesive layer  1333  is used on the bottom of one half  1331   a ′ of the tape, to fix that half of the tape to the mounting surface  1330   a ′. The resulting screen bar segment and tape combination is similar to the example of  FIG. 13A , in that one half  1331   a ′ of the tape  1331 ′ is fixedly mounted to the mounting surface  1330   a ′ of the screen bar segment  1330 ′, and the other half  1331   b′  of the tape  1331  is a movable flap; the flap  1331   b ′ can be folded over to capture the screen material  1334 ′ between two halves of the tape strip  1331 ′ (as shown in phantom in  FIG. 13B ). 
   Alternatively, as shown in  FIG. 15A , a non-adhesive plastic film or tape  35  may be interposed between the adhesive  36  and the pins  54  or other inserting tool (e.g., roller) during the insertion process. The tape  35  should be capable of withstanding high temperatures. The tape  35  may be, for example, cloth or polymeric tape. The tape  35  may be dispensed after the adhesive is melted, but before driving the pins  54  into the adhesive  36 . In this case, the apparatus may be substantially as described above with reference to  FIGS. 2–5 . When the pins  54  insert the screen fabric  34  into the groove  32 , the film or tape  35  shields the pins from contact with the adhesive. The film or tape  35  may be left in the groove after assembly, as shown in  FIG. 15B . In a further variation of this method, other techniques may be used for melting the adhesive with the tape or film  35  in place, such as by microwaves, or by heating the frame to indirectly heat the adhesive. 
   Other Inserting Apparatus 
   Although the exemplary inserting apparatus is described above as a plurality of pins, other inserting apparatus may be used. It may be desirable to use one or more rollers instead of a plurality of pins. Insertion methods using a roller are described in greater detail in the parent application Ser. No. 08/997,737, which is incorporated by reference herein. 
   A roller can be manually or automatically actuated to travel along the length of a side of the frame. An example is shown in  FIG. 14A . One, two or more sides of the screen may be inserted into the adhesive simultaneously. To simultaneously insert more than one side of the screen, a plurality of rollers are actuated by a plurality of actuators (not shown). 
   The roller may be heated to melt the adhesive. To avoid continuous increase in the roller temperature while the roller passes through the heated adhesive in successive assemblies, it may be desirable to cool the roller(s) in between sides. 
   As in the case of pins, a release coating, such as TFE may be used on the roller to prevent the adhesive from sticking to the roller. Alternatively, the roller wheel may have a permanent TFE coating. If the roller doesn&#39;t contact the adhesive, no release coating is required. 
   A cleaning device may be used at the end of each machine cycle to remove glue build-up from the roller.  FIGS. 14B and 14C . show an example of a device  1403  having a groove  1404  shaped like the inserting edge  1402  of the roller. The device  1403  is placed adjacent to the roller  1402 . The roller  1402  is then passed through the device  1403 , so the adhesive is squeezed and scraped off of the roller  1402 . 
   Another device for removing the adhesive from the roller is shown in  FIG. 14D . Tool  1405  is in the form of a sharpened hollow cylinder. This cleaning tool  1405  may be used for an inserting wheel that has an open side. The cleaning tool  1405  has a circular cutting or scraping edge  1406  very slightly larger in diameter than the roller  1402 ′. Tool  1405  can fit over the roller  1402 ′ in the axial direction, scraping the adhesive off in the process. 
   One skilled in the art can readily provide other tools for cleaning the adhesive off of the inserting roller  1402 . 
   The roller may optionally be mounted to an apparatus (not shown) for dispensing adhesive, so that the roller trails behind the ribbon of adhesive by a predetermined distance; if the apparatus moves along the groove or tensioning step at a constant speed, then the roller inserts the screen material in at a predetermined time after the adhesive is dispensed in the groove or tensioning step. Alternatively, the apparatus may be stationary, and the frame may be mounted on an X-Y table, so that the same relative motion is provided between the frame and the roller. 
   In a further variation of this apparatus, a nozzle may be mounted behind the roller. The nozzle may provide heated gas if a thermosetting adhesive is used, or the nozzle may apply cooled gas if a pre-heated thermoplastic adhesive is used. As the apparatus moves relative to the groove or tensioning step, a ribbon of adhesive is applied, then the roller follows, and finally a jet of heated or cooled gas is applied to the adhesive. 
   In still another variation, the inserting apparatus may be a pin-roller (not shown) including a plurality of pins attached to a roller, and extending outwardly from the surface of the roller, in a radial direction. The roller may include a bearing assembly to provide smooth rolling action. Preferably, the pins are evenly spaced. The pins are spaced apart from each other so that the outer tips of any two successive pins are about 1.25 cm (0.5 inch) apart. The pins may be any of the types described above. Preferably, the pins are coated so that the adhesive does not stick to the pins. A release coating, such as TFE, may be applied to the wetted surfaces. The pin-roller may be about the same width as the diameter of the plurality of pins. 
   The pin-roller combination allows use of an application technique very similar to that described above with respect to a smooth roller, yet yields results similar to those achieved using a plurality of pins. For example, the screen frame may be preheated (to melt the adhesive therein) and blocked with pre-loading blocks. The screen cloth is placed on the frame, and the pin-roller is rolled through the groove of the frame to insert the screen. This may be done manually, or by machine. Alternatively, local heating may be used. A nozzle may trail behind the pin-roller. The nozzle may dispense a cool gas or fluid, which may be air, carbon dioxide, water, mist, etc. The cool gas or fluid cools the adhesive until the adhesive re-solidifies, completing the bonding operation. Alternatively, the frame may be permitted to cool by natural convection, or by forced convection from a large fan. Other cooling methods known to those skilled in the art may also be used. 
   Another exemplary roller type insertion device may have a corrugated or fluted edge (not shown). When the corrugated or fluted roller passes through the groove  32  of a screen bar segment, the insertion device makes an impression in the general form of a sine wave. Alternatively, a plain roller (of a type shown in  FIG. 14A ), may be used. Similarly, a corrugated blade may be used. 
   Still another exemplary method according to the invention includes a continuous feed process for inserting the screen fabric into one or more grooves of the frame. According to this embodiment, a frame is formed from four (or more) screen bar segments, each of which has a respective groove. Each groove in each screen bar segment extends across the entire length of the screen bar frame, from edge to edge, including both the length of the screen bar segment and the corner key. The grooves on each pair of adjacent screen bar segments continue onto, and intersect in, the corner key (not shown). For example, a four-sided frame should have a set of grooves in the general shape of a tic-tac-toe board, or a pound sign (#) with orthogonal sides. 
   A frame having grooves that extend from edge to edge can be continuously fed by a conveyor into an apparatus having a pair of insertion devices (preferably rollers, pin-rollers or corrugated rollers as described above) spaced apart from each other. By this method, the two rollers (or pin-rollers or corrugated rollers) simultaneously fit into the two parallel grooves on two opposite sides of the frame. One of the insertion devices may be fixed, and the other movable (in the direction perpendicular to the groove), to accommodate differently sized frames. The two insertion devices can each have a heat source just ahead of the insertion device, to melt the adhesive just before insertion. Optionally, a nozzle may blow ambient air on the adhesive just behind the insertion device to speed up the cooling. Once the screen cloth  34  on the first two sides is inserted, the frame is rotated by 90 degrees, and the remaining two sides of the screen cloth  34  are inserted in the same manner. 
   Alternatively, instead of feeding the frame through a stationary insertion apparatus, the frame may be held still, and two (longitudinally) movable insertion devices (preferably, rollers, pin-rollers or corrugated rollers) may be passed through the grooves simultaneously. Further, although the exemplary frames described above include the grooves or tensioning steps on the face of the frame, the grooves may be located on the side edges and ends of the frame. 
   In a variation of this exemplary process, the frame may be loaded onto a conveyor, which transports the frame through an oven. The frame is pre-heated in the oven. The heated frame exits the oven on the conveyor and moves to a press having insertion devices similar to those described above. The conveyor stops when the frame is positioned at the insertion devices. Two movable arms and two stationary arms straighten the frame for tensioning. The screen cloth is placed in position over the frame (with the edges over the grooves), preferably using a gantry or pick-and-place type robot. Other types of positioning apparatus may be used. The screen material may optionally be pre-cut to a final installed size before placing the screen on the frame. At least one, but preferably four, insertion devices (one on each side of the frame) are simultaneously inserted in the grooves, inserting the screen cloth into the grooves, simultaneously pushing the screen into fixative contact with the adhesive on each side member of the first frame. Ambient air may then be blown over the frame to reduce the cycle time. Using this variation of the exemplary method, the entire assembly process can be automated. 
   Having the groove extend all the way to both edges of the frame may be advantageous for the above described batch type insertion process, as well as the continuous process described immediately above. With the groove extending all of the way to the edge, there is no need to retract the bayonet pins (shown in  FIG. 17 ) at the corners of the frame during the batch insertion process; the insertion device can be applied over the corners in the same way as in the middle of the frame. 
   With at least two movable arms and two fixed arms (each fixed arm being located opposite a respective movable arm), it is easy to form a second screen assembly having a second frame, wherein the second frame has a different size from the first frame. More generally, any number of differently sized frames may be made with the same apparatus, using the same conveyor, with size changes between any two consecutive frames. The adjustment may either be controlled by the operator entering arm positions. However, it is preferred to use a more automated process, in which a process controller identifies all of the screen sizes to be produced and the positions of the arms needed to form the appropriately sized screen for each assembly. 
   When using an oven to pre-heat the frame, particular attention must be given to the frame corners. Conventional frames are typically assembled using corner keys. The corner key material and adhesive must be selected so that the adhesive melts at a temperature below the temperature at which the corner key melts or creeps significantly. Comer keys made of a high temperature plastic (e.g., nylon) may be used, but may be substantially more expensive than polypropylene corner keys. Another alternative is to use an oven having an average temperature below 212 degrees may be used, with additional heat sources directed at the adhesive (but not on the corner keys). For example, an oven having, with infrared radiation focused on the adhesive (but not on the corner keys) may be used. Another alternative is to have a narrow slot in the ceiling of the oven, directing heated air on the frame or adhesive, but not the corner keys. 
   Still another alternative is to form the frame from a single piece of screen bar stock with folded mitered corners, in which case at most one corner key (which may be a high temperature plastic) is used. In particular, the inventor has discovered that a more stabile frame is formed if the mitered corners are cut to between 44.0 and 44.5 degrees, instead of 45 degrees. An exemplary mitered frame using a metal internal corner key achieved good results. It is believed that the smaller miter angles place the mitered corners in compression, for greater rigidity and stability. 
   Other Screen Bar Configurations 
   Although the exemplary embodiments described above include a groove or tensioning step, other screen bar configurations may be used. For example, the screen bar may be flat. Alternatively, the screen bar may have a ridge. 
   Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claim should be construed broadly, to include other variants and embodiments of the invention which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.