Abstract:
A thermoforming apparatus for forming sheets of sheet plastic onto a substrate in which a retractable substrate heater is able to be positioned over a substrate loaded into the press and energized while a plastic sheet is being heated in an oven included in the thermoformer apparatus. The plastic sheets are cut from a strip pulled from a roll of sheet material, the roll supported for side to side movement and driven by a servomotor responsive to error signals generated by a strip edge sensor to compensate for wandering of a strip edge to thereby insure accurate location of plastic sheets cut from the unwound strip.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional of U.S. Ser. No. 09/721,298 filed on Nov. 22, 2000, now U.S. Pat. No. 6,648,043, issued on Nov. 18, 2003. This application claims the benefit of provisional application Ser. No. 60/167,589 filed on Nov. 26, 1999. 

   BACKGROUND OF THE INVENTION 
   This invention concerns thermoforming, a widely practiced manufacturing process involving the forming of preheated thin plastic sheets into molded parts. In this process, the plastic sheets are preheated in an oven, and then positioned aligned with tooling in a thermoforming press, the sheet drawn against tooling mold surfaces, using a pressure differential or vacuum to force the heated plastic sheet into conformity with the mold surfaces. A plug assist opposite the mold tooling is often used to preform the sheet into closer conformity to the mold surfaces when press platens are moved together to assist in the process of thermoforming the sheet against the tooling mold surfaces. 
   Often, in automated machines, sheet material is fed off a large roll, cut to a proper length, then clamped about its perimeter in a clamping frame. The clamping frame and sheet is advanced into an oven where the sheet is preheated to a forming temperature. The clamping frame is then advanced into a press having platens mounting mold and plug assist tooling which are brought together to cause the heated sheet to be formed into a part. 
   Vacuum or a differential air pressure is applied at the same time to form the sheet into conformity with the molding surfaces. 
   After forming, the press is opened and the completed part is removed or transferred to another station for further processing. 
   In twin sheet thermoforming, two sheets are preheated and formed as described, and then fused together to form a unitary part. 
   In another version of the thermoforming process, a preheated sheet is drawn onto a preformed porous substrate by a vacuum applied to the undersurface of the substrate. Such process has been used for applying a cover skin layer to an automotive interior trim piece. 
   The cover layer can be adhesively bonded to the substrate. While heat activated adhesives can provide a superior bond, to have a separate oven and coordinated handling of the substrate as well as the sheets would add considerably to the complexity and cost of the machine. 
   In automating this process, it is usual to feed sheet material off a roll fed into a loading area where a proper length is cut off, the cut sheet clamped into a clamping frame for transport to an oven and thereafter to the forming press. Sometimes material rolls are wound such that the edge of the web wanders laterally to some extent as the sheet is pulled off the roll. This will cause some mispositioning of the cut sheets which can cause faulty parts to be produced. 
   It is the object of the present invention to provide the capability of preheating a substrate to bond a thermoformed sheet thereto without requiring separate additional handling of the substrate. 
   It is a further object of the present invention to provide an accurately controlled feeding of sheet material into a thermoforming apparatus when being unwound from a roll of sheet material. 
   SUMMARY OF THE INVENTION 
   The above objects of the present invention and others which will become apparent upon a reading of the present specification and claims are achieved by providing a movably mounted substrate heater for each substrate tool which heater can alternatively be positioned in an advanced position extending within the forming press and cantilevered over the substrate, or in a retracted position withdrawn from the press. A powered drive arrangement is operated to shift the heater between the respective positions during cycling of the apparatus. 
   While the sheet to be bonded to the substrate is within the oven and being preheated, the substrate heater is in its advanced position within the forming press, and energized to heat the substrate adhesive. 
   When the sheet being heated in the oven reaches a proper degree of softening, the substrate heater is retracted out of the way to allow the heated sheet to be advanced into the press and positioned over the now heated substrate. 
   The thermoforming operation then proceeds by cycling of the press to mold and bond the sheet to the substrate surface to complete the part. 
   In order to prevent mispositioning of the sheets when cut from a roll of material, a roll stand is provided which is movable back and forth laterally. An edge sensor is provided which generates an error signal used to control a positioning drive system coupled to the stand to drive the same in either direction so as to compensate for any sensed mispositioning of the web edge. An electric servo motor with position feed back sensors is preferably used as the drive system. 
   The apparatus may be used to produce left and right parts by providing simultaneous parallel processing of two parts in the same machine. 
   Two side-by-side clamping frames are alternatively loaded with sheets successively cut from the roll and deposited on side-by-side loading tables positioned in front of a common sheet preheating oven. The loading tables are preferably turn tables which can rotate the cut sheets and clamping frames to reorient the grain of the sheet material to be in a desired proper relation to the substrate. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic plan view of the thermoformer apparatus according to the present invention. 
       FIG. 2  is a side elevational view of a portion of the apparatus depicted diagrammatically in  FIG. 1 . 
       FIG. 3  is a top view of the top view of the apparatus of  FIGS. 1 and 2 . 
       FIG. 4  is a fragmentary front elevational view of the forming press portion of the apparatus with the retracting substrate heaters. 
       FIG. 5  is an end view of a sheet material roll stand included in the apparatus of  FIG. 1 . 
       FIG. 6  is a front view of the roll stand shown in  FIG. 3 . 
       FIG. 7  is an enlarged front view of one of the retracting substrate heaters, mounting, and positioning drive system. 
       FIG. 8  is a plan view of the radiant heater and related components shown in  FIG. 6 . 
       FIG. 9  is an end view of the radiant heater and related components shown in  FIGS. 6 and 7 . 
   

   DETAILED DESCRIPTION 
   In the following detailed description, certain specific terminology will be employed for the sake of clarity and a particular embodiment described in accordance with the requirements of 35 USC 112, but it is to be understood that the same is not intended to be limiting and should not be so construed inasmuch as the invention is capable of taking many forms and variations within the scope of the appended claims. 
   Referring to the drawings and particularly  FIG. 1 , the apparatus  10  according to the present invention includes a feed roll stand  12  and a feed system for feeding a strip of plastic sheet material  14  unwound from the roll over each of a pair of aligned rotary tables  16 . A movable cut off sheet support  18  has a traveling cutter  19 , which is operated to grip and pull a segment of plastic sheet from the roll, cut the same off, and position a cut off piece  20 A,  20 B on a respective table  16 A or  16 B. 
   Clamping frames  22 A,  22 B are shuttled to and from ovens  24  to be positioned over the tables  16 A,  16 B. A common holder may be used to shuttle the separate clamping frames  22 A,  22 B to and form the various stations. 
   The sheets  20 A,  20 B are rotated on the tables  16 A,  16 B to be properly oriented with respect to preformed substrate panels  26 A,  26 B which have been loaded onto tooling  28 A,  28 B in a lower platen  29  of a thermoforming press  30 . 
   The tables  16 A,  16 B are thereafter elevated to position the sheets  20 A,  20 B within a respective clamping frame  22 A,  22 B, which has clamps operated to secure the sheets  20 A,  20 B within the respect frame in the well known manner. After lowering of the tables, the clamping frames  22 A,  22 B are shuttled back into the oven  24 . A common holder can be used to move both frames  22 A,  22 B at once. Such equipment is well known in the art and is not described here in any detail. 
   According to the one feature of the present invention, the roll stand  12  is mounted to be movable laterally in either direction by operation of a drive motor  32  so as to maintain the position of the web edge, which is monitored by a suitable sensor  34  positioned adjacent to one edge. Thus, if the web shifts laterally as it is unwound from the roll, there is a compensating shift laterally of the roll stand  12  in the opposite direction to bring the edge back into proper position prior to cut off of a sheet  20 A,  20 B. 
   According to another feature of the present invention, substrate heaters  36 A,  36 B are mounted adjacent the press  30  to be movable to an advanced position within the press  30  cantilevered out over substrates  26 A,  26 B loaded onto the tooling  28 A,  28 B on the lower platen  29 , the heaters  36 A,  36 B thereafter retracted out of the press  30  by operation of respective positioning drive systems  38 A,  38 B. 
   The heaters  36 A,  36 B, preferably of the radiant quartz type, are advanced over the substrates  26 A,  26 B and energized after the substrates  26 A,  26 B are loaded onto the tooling and during the time when the corresponding sheet pieces  20 A,  20 B are being preheated in the oven  24 . When the sheet pieces  20 A,  20 B begin to be advanced into the press  30 , the heaters  36 A,  36 B are deenergized and retracted out of the way. 
   Certain further details of such an apparatus can be seen in  FIGS. 2–4 . The rotary cut off tables  16 A (and table  16 B not shown) include ball support bearings  40  to support the weight of an under plate  42  and top plate  44 . Strip heaters  46  may be provided on the under surface of the top plate  44  to prewarm the plastic sheets. 
   Elevator drives  48  enable raising the table  16 A after a cut off plastic sheet  20 A is deposited thereon and rotated by rotary drive  50 . 
   A traveling cut off frame  52  with a gripper bar  54  and traversing cut off wheel  56  ( FIG. 3 ) is operated by a drive system  58  to grip and pull off a section of plastic material from a material roll on the roll stand  12 , and cut off a correct length to form the sheets  20 A,  20 B. 
   The clamping frames  20 A,  20 B are positioned over the table  16 A,  16 B when shuttled out of the oven  24  by a clamping frame drive motor system  60  mounted on a holder for the clamping frames  20 A,  20 B. 
   The tables  16 A,  16 B are elevated to position the sheets  20 A,  20 B within a respective clamping frame  20 A,  20 B with a series of clamping cylinders actuated to hold the sheets  20 A,  20 B about their perimeters, in the manner well known to those skilled in the art. 
   The oven  24  is of a well known type having a top heater  62  and bottom heater  64  supported on an elevator mechanism  66  which lowers the bottom heater  64  as the heating cycle continues to compensate for sag of the sheets  20 A,  20 B as they are warmed. 
   The clamping frames  22 A,  22 B are driven into the thermoformer press  30  after being heated to the proper degree, between the lower platen  29  and an upper platen  68 . 
   The lower platen  29  is mounted to be pivoted out as shown to position the substrate tooling  28 A,  28 B in a position to allow loading of the substrates  26 A,  26 B and unloading of a completed part, one or more power cylinder  70  used to carry out the tilting. 
   The upper platen  68  is driven up and down on threaded posts  72  by an electric motor drive system  74 , in the well known manner. 
   Tooling (not shown) is provided on the upper platen  68  to carry out the thermoforming, and may include a vacuum box for forming a concavity in the sheet to assist in conforming the sheet to the substrate shape, also in the well known manner. 
   The substrates  26 A,  26 B rest on the bottom platen tooling  28 A,  28 B, which is provided with vacuum passages for creating a vacuum forming of the sheets against the substrates  26 A,  26 B, again in the well known manner. 
   The bottom platen  29  (shown raised in  FIG. 4 ) is similarly supported on threaded posts  76  and driven up and down thereon by an electric motor powered drive system  78 . 
   The substrate heaters  36 A,  36 B are each mounted on a framework  80 A,  80 B cantilevered from a respective side of the press frame work  82 , reinforced by a preheater frame  81 A,  81 B. 
   As best seen in  FIGS. 7–9 , the heater units  84 A,  84 B are suspended from a respective holders  85 A,  85 B supporting opposing sets of rollers  90 A,  90 B which can roll along bars  91  welded to the bottom of the framework  82 , driven by an electric motor drive system  86 A,  86 B. The heater drive system include a sprocket driven toothed belt  88 A,  88 B passing around on sprockets  94 A,  94 B to be able to be advanced into the press to be cantilevered in a position projecting over a respective substrate while the sheet pieces are being heated in the oven  34 . The belts  88 A,  88 B are attached at either end to tensioners  93 A,  93 B. 
   The feed roll stand  12  (best seen in  FIGS. 5 and 6 ), is of a design able to accommodate multiple rolls of sheet material on a wheel  96  having four detachable support rods  98  each able to hold a roll  102  of sheet stock. End plates  100  are notched to receive a respective rod end, held by latches  104 . An axle shaft  106  is revolvably supported on end frames  108  and a bottom frame  120  supported on roller wheels  110  resting on floor plates  112 . An air motor drive  122  controlled by a manually actuated valve enables rotation of the wheel  98  to bring a selected roll  102  into proper feed position. 
   A VFD motor  114  drives threaded shafts  116  received in drive nuts  118  attached to the underside of the bottom frame  120  to enable side-to-side or lateral compensating adjustment of the roll stand position in response to error signals received from the sensor  34 . Suitable position feed back sensors (not shown) are typically used with the positioning motor in the well known manner to produce a stable, responsive feed back control system. A servo motor could also be used instead of a VFD motor.