Abstract:
A method of producing a plastic sheet-strap includes the steps of providing first and second work rollers such that the work rollers rotate in opposite directions and at different lineal surface velocities. The work rollers are spaced apart with respect to each other so as to define a work roller nip therebetween. A pinch roller is provided that is operably associated with the first work roller. The pinch roller and the first roller defining a deaerating nip therebetween. A solid sheet of material having a thickness is fed into the deaerating nip. The material is pinched against the first work roller and moves with rotation of the first work roller to the work roller nip between the first and second work rollers. The material is simultaneously milled and stretched substantially at the nip during a single pass of the solid sheet material through the nip as the rollers rotate in opposite directions and at different lineal surface velocities to form a milled and stretched oriented sheet. The second roller has a concave profile to work the edges of the sheet more than a central region of the sheet. A chill roller is disposed downstream of the second work roller to cool the milled and stretched oriented sheet.

Description:
BACKGROUND OF THE INVENTION 
   The present invention is directed to a method and apparatus for producing oriented plastic strap. More particularly, the present invention is directed to a method and apparatus for milling and stretching a plastic sheet into strap stock material having a predetermined and consistent desired thickness. 
   Plastic strap is often used to package items in a bundle, on a pallet or in a crate for shipping, storage and merchandising. The strap is applied in a tensioned loop by an automatic or manually operated strapping machine. In a typical manufacturing process, a cast sheet of thermoplastic material, for example, polypropylene, is first reduced in size by rolling it through a pair of closely spaced milling rollers or cylinders that rotate in opposite directions. After the thickness of the sheet is reduced, the sheet is drawn and stretched out of the milling rollers by a series of orienting rollers or a bridle assembly to its final desired size. 
   One specific method that is commonly used is a process called the short gap method. An apparatus for carrying out this method includes an entry bridle, a stretching assembly and an exit bridle. A slow speed, heated bridle assembly advances a cast sheet of material, usually a film, to a stretching assembly. The stretching assembly includes a pair of rollers or cylinders set a distance apart. The first roller rotates at the same speed as the entry bridle. The second roller is rotating faster than the first roller and at the same speed as the exit bridle. Thus, as the film passes through the assembly, it is stretched to its final desired size. 
   Still another known method is called the zero gap method. In this method, the gap between the steps of milling and stretching are eliminated. Thus, the steps are carried out substantially simultaneously. 
   The zero gap method overcame many of the disadvantages of prior known methods, vis-à-vis limited increases in strength without significant decreases in other desired properties. In addition, the zero gap method reduced the necking that otherwise occurred as the sheet was stretched over the distance between the rollers. 
   It was however found that other concerns arose with use of the zero gap method. For example, it was noted that the lateral ends or sides of the sheet-strap tended to be thicker than the central sheet-strap regions. That is, there was less reduction in thickness or less flattening at the edges than in the middle or central region of the sheet-strap. Although the zero gap method in some respects addressed this problem by “pinning” the edges of the sheet-strap between the rollers, the lessened edge reduction nevertheless occurred. 
   In addition, it was noted that air tended to accumulate at the juncture of the strap or sheet and the roller. The accumulated or entrained air, which is generally “caught” by surface roughness or imperfections in the sheet-strap, moved within the apparatus, to between the sheet-strap and the milling rollers. This resulted in uneven thermal, e.g., heat transfer, properties across the lateral width of the roller/sheet interface. 
   It was also noted that in certain instances, the sheet-strap continued to be worked, e.g., elongated, even after it exited the last work roller. As a result, the amount of elongation of the sheet-strap (as during the working of the sheet-strap) was not as controlled as desired. 
   Accordingly, there is a need for an improved method for producing oriented strap. Such a method results in consistent strap thickness across the width of the strap. Desirably, in such a method, air accumulation at the first work roller is reduced or eliminated, resulting in consistent thermal and heat transfer properties at the strap/roller interface. 
   BRIEF SUMMARY OF THE INVENTION 
   An improved method of producing a plastic sheet-strap includes the steps of providing first and second work rollers such that the work rollers rotate in opposite directions and at different lineal surface velocities. The work rollers are spaced apart with respect to each other so as to define a work roller nip therebetween; 
   A pinch roller is operably associated with the first work roller. The pinch roller and the first roller define a deaerating nip therebetween. A solid sheet of material, having a thickness, is fed into the deaerating nip such that the solid sheet of material is pinched against the first work roller and moves with rotation of the first work roller to the work roller nip. Movement of the sheet into the nip between the first and second work rollers simultaneously mills and stretches the solid sheet of material substantially at the nip during a single pass of the solid sheet material through the nip as the rollers rotate in opposite directions and at different lineal surface velocities to form a milled and stretched oriented sheet. 
   The pinch roller can be formed having a convex surface to facilitate reducing the entrainment of air in the center of the sheet-strap. Alternately, the pinch roller can be formed having a concave surface to facilitate reforming the edges of the sheet-strap. 
   The deaerating nip formed by the pinch roller and the first work roller serves to flatten surface irregularities to prevent air from being entrained between the sheet and the work rollers. This has the effect of reducing thermal and heat transfer inconsistencies between the rollers and the sheet. 
   In a preferred method, the pinch roller is rotated at a speed to match the lineal surface velocity of the pinch roller with the lineal surface velocity of the first work roller. To effect preventing air entrainment, the sheet is compressed between the pinch roller and the first work roller. The pressure exerted by the pinch roller on the sheet-strap can be controlled by an air regulator (controlling a pneumatic cylinder), or like means. 
   The method can include milling and stretching the sheet between a flat first work roller and a concave-profile second work roller. This configuration “works” the edges of the sheet to a greater extent than the central region of the sheet. In addition, the method can include cooling the sheet immediately as its exits the milling and stretching operation, that is immediately downstream of exiting the second work roller. This prevents over-working or over-stretching the sheet. Cooling can be carried out by contact with a chill roller. An apparatus for carrying out the method is also disclosed. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The benefits and advantages of the present invention will become more readily apparent to those of ordinary skill in the relevant art after reviewing the following detailed description and accompanying drawings, wherein: 
       FIG. 1  is a simplified partial side view of an apparatus for producing an improved oriented strap in accordance with the principles of the present invention; 
       FIG. 2  is an enlarged side view of the milling and stretching rollers of the zero gap assembly and showing an inlet or upstream pinch roller and an exit or downstream chill roller; 
       FIG. 3  is a partial cross-sectional view of the milling and stretching rollers of  FIG. 1 , taken along line  3 - 3  of  FIG. 1 ; and 
       FIGS. 4   a  and  4   b  illustrated alternate profiles for the pinch roller. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   While the present invention is susceptible of embodiment in various forms, there is shown in the figures and will hereinafter be described a presently preferred embodiment with the understanding that the present disclosure is to be considered an exemplification of the invention and is not intended to limit the invention to the specific embodiment illustrated. 
   It should be further understood that the title of this section of this specification, namely, “Detailed Description Of The Invention”, relates to a requirement of the United States patent Office, and does not imply, nor should be inferred to limit the subject matter disclosed herein. 
   Illustrated schematically in  FIG. 1 , is an apparatus  10  for milling and stretching or elongating a sheet or workpiece W into a thin strap stock material. The general arrangement and operation of such an apparatus  10  and method are described in Van Erden et al., U.S. Pat. No. 5,387,388, which is commonly assigned with the present application, and which is incorporated herein by reference. 
   Although the present invention is discussed with only a single sheet or workpiece W, it will be understood that more than one sheet or workpiece can be passed through the assembly at a time. 
   The illustrated apparatus  10  includes a zero gap assembly  12 . The phrase “zero gap” refers to the concept of substantially eliminating any gap between a step of milling and a step of stretching a sheet or workpiece W. That is, the steps of milling and stretching are accomplished substantially simultaneously. The zero gap assembly  12  is located between a feeding assembly  14  and an exit bridle assembly  16  on a frame or support  18 . 
   The feeding assembly  14  may take any of several forms, and as shown in  FIG. 1 , can include an extruder  20  for extruding a sheet or workpiece of stock material and an entry bridle assembly  22 . The extruder  20  produces a sheet or workpiece of suitable material, such as polypropylene, to the entry bridle assembly  22  for feeding into the zero gap assembly  12 . The sheet W can be pre-heated in the entry bridle assembly  22  to enhance the working properties of the sheet material. 
   The entry bridle  22  includes a plurality of rollers or cylinders  24 ,  26 ,  28 ,  30  that properly deliver the sheet W for feeding into the zero gap assembly  12 . Typically, the rollers  24 ,  26 ,  28 ,  30  do not substantially contribute to the stretching or the milling of the sheet W. As illustrated, the rollers  24 ,  26 ,  28 ,  30  are arranged along two rows with bottom row rollers  26 ,  30  spaced between and at a distance beneath the top row rollers  24 ,  28 . Rollers  24 ,  28  rotate in a clockwise direction while rollers  26 ,  30  rotate in a counterclockwise direction so that when the sheet W is wound around the entry bridle assembly  22 , it travels through the rollers  24 ,  26 ,  28 ,  30 . Each of the rollers  24 ,  26 ,  28 ,  30  are rotated at a uniform speed by, for example, a motor and shaft assembly (not shown). All of the rollers  24 ,  26 ,  28 ,  30  rotate at essentially the same speed or lineal surface velocity as a top roller  32  in the zero gap assembly  12 . 
   After the sheet W passes through the feeding assembly  14 , it advances to the zero gap assembly  12  for milling and stretching into a finished sheet S having a predetermined desired thickness. The zero gap assembly  12  includes first and second work rollers or cylinders  32 ,  34 , respectively (illustrated as an upper or top roller  32  and a lower or bottom roller  34 ) that are rotatably mounted in opposing relationship. A nip  36 , that is, the minimum distance between the work rollers  32 ,  34 , can be varied greatly depending on the desired finished thickness of the sheet S. As discussed in detail in the aforementioned U.S. Pat. No. 5,387,388 to Van Erden et al., the zero gap rollers  32 ,  34  may be solid or hollow and may be heated to enhance the stretching properties of the sheet material. 
   It has been observed that air can be entrained or trapped by the moving sheet W as it approaches the top roller  32 . Air tends to collect or accumulate along the surface of the sheet W and can accumulate within minor surface imperfections in the sheet W. Because of the “stretching” of the sheet W about the top roller  32 , the air accumulates between the sheet W and the roller  32  as the sheet W moves over the roller  32 . The trapped air tends to form a pocket that moves around between the sheet W and the roller  32  at the sheet/roller interface, as indicated at  38 . 
   In that the air has considerably different heat transfer properties than the roller  32  and the sheet W, heat transfer from the roller  32  to the sheet W is adversely effected, i.e., inconsistent. This in turn results in varying temperatures in the sheet W (small temperature variations at various locations on the sheet W). These temperature variations can result in changes in thickness of the sheet S after orientation, in the direction of flow. 
   In order to prevent air from being entrained between the sheet W and the top roller  32 , a high pressure pinch roller  40  is positioned at the inlet  42  to the zero gap assembly  12 , just downstream of the point at which the sheet W begins to wrap around the top roller  32 . In a present apparatus, the pinch roller  40  is configured to compress the sheet W, as much as 2/1000 inches (2 mils) to flatten surface roughness of the sheet W to prevent air entrainment. An alternate pinch roller  240  (see  FIG. 4   b ) can be formed with a convex surface to reduce the entrainment of air at the center of the sheet W. 
   In a present embodiment, the pinch roller  40  exerts a pressure on the sheet (against the top roller  32 ) by actuation of a cylinder  41 , such as a pneumatic cylinder. The pressure can be controlled or varied to achieve a desired compression by control/variation of the pneumatic pressure. Other methods to achieve pressure pinch roller  40  control will be recognized by those skilled in the art. 
   It is also contemplated that the application of pressure by the pinch roller  40  can reform the shape of the sheet W before it is oriented. This can, in turn, flatten edge beads or tapers and provide a more uniform thickness (improved flatness) across the milled and stretched sheet S. The pinch roller  40 , as seen in  FIG. 1 , rotates in a direction opposite of the first work roller  32 . 
   It will also be appreciated by those skilled in the art that the work rollers  32 ,  34  principally “work” the edges of the sheet W, rather than the central region of the sheet W. To this end, the present apparatus  10  uses a concave bottom roller  34  in the zero gap assembly  12 . As seen in  FIG. 3 , a present bottom roller  34  has a diameter d 44  at the middle  44  of the roller  34  that is about 10/1000 inches (10 mils) to about 20 mils, and preferably about 14 mils to about 16 mils less than the diameter d 46  at the edges  46  of the roller  34 . In this manner, the edges of the sheet S are flatter and the thickness of the sheet S across the sheet width is more consistent than that of sheet formed in flat roller assemblies. The concave surface of the roller  34  also allows air that may get past the pinch roller  40  to pass through the nip  36  rather than accumulate at the nip  36 . Alternately still, as seen in  FIG. 4   a , the pinch roller  140  can have a concave surface to further “work” the edges of the sheet W. 
   After the finished sheet S exits the zero gap assembly  12 , it winds around the exit bridle assembly  16 . The illustrated exit bridle assembly  16  includes a plurality of rollers or cylinders  48 ,  50 ,  52 ,  54 ,  56 ,  58  that pull the sheet S out of the zero gap assembly  12  properly. The rollers  48 ,  50 ,  52 ,  54 ,  56 ,  58  do not substantially contribute to any stretching of the sheet S. It will however be appreciated by those skilled in the art that the rollers  48 ,  50 ,  52 ,  54 ,  56 ,  58  can add some stretch to the sheet W. As illustrated, the rollers  48 ,  50 ,  52 ,  54 ,  56 ,  58  are arranged along two rows with bottom row rollers  50 ,  54 ,  58  being spaced between and at a distance beneath the top row rollers  48 ,  52 ,  56 . Rollers  48 ,  52 ,  56  rotate in a clockwise direction while rollers  50 ,  54 ,  58  rotate in a counterclockwise direction so that when the sheet S is wound around the exit bridle assembly  16 , it travels through the rollers  48 ,  50 ,  52 ,  54 ,  56 ,  58 . The rollers  48 ,  50 ,  52 ,  54 ,  56 ,  58  are rotated at a uniform speed by, for example, a motor and shaft assembly (not shown). All of the rollers  48 ,  50 ,  52 ,  54 ,  56 ,  58  rotate at essentially the same lineal surface velocity as the bottom roller  34  in the zero gap assembly  12 . 
   Although the rollers  48 ,  50 ,  52 ,  54 ,  56 ,  58  do not substantially contribute to any stretching of the sheet S, the elevated temperature of the sheet S, in conjunction with the “pulling” tends to continue elongation of the sheet S. As a result, control of the final thickness of the sheet S (and thus the subsequently formed strap) may not be as great as desired. To this end, a chiller roller  60  is positioned proximal to the outlet  62  of the zero gap assembly  12 , prior to entry into the exit bridle  16 . The chiller roller  60  reduces the temperature of the sheet S which in turn reduces the extent of undesirable, post-milling elongation. This provides greater control over the final sheet S (and thus strap) thickness. 
   Although the embodiment shown and described herein, is configured having the mill (work) rollers  32 ,  34  situated in a top-bottom arrangement, it is to be understood that the rollers  32 ,  34  may be placed in a side-by-side arrangement in which arrangement, the top roller  32  becomes the first roller the sheet contacts while the bottom roller  34  becomes the second roller the sheet contacts. 
   A method for producing oriented strap S includes the steps of forming, as by extrusion, a sheet W and conveying the sheet W to the entry bridle assembly  22  of a forming apparatus  10 . The sheet W winds around the entry bridle rollers  24 ,  26 ,  28 ,  30  for proper alignment for feeding into the zero gap assembly  12 . The sheet W then feeds between a high pressure pinch roller  40  and the top roller  32  of the zero gap assembly  12 , feeding around the top of the top roller  32 . In this manner, the sheet W is compressed, flattening the surface roughness of the sheet W, to preclude air from entering the zero gap assembly  12  with the sheet W, between the sheet W and the top roller  32 . The sheet W is driven by the top roller  32  and the entry bridle rollers  24 ,  26 ,  28 ,  30  which are driven at the same lineal surface velocity. 
   As it traverses into and through the zero gap assembly  12 , the sheet W travels around the circumference of the top roller  32  until it reaches the work roller nip  36  defined between the top and bottom work rollers  32 ,  34 . The faster rotating bottom roller  34  pulls the sheet W through the nip  36  while the slower rotating top roller  32  and the mill reduction brakes the speed of the lower surface of the sheet W. Thus, the sheet W accelerates through the nip  36  and is simultaneously milled and stretched to its final predetermined thickness as it passes through the nip  36 . 
   The sheet W is worked, e.g., milled and stretched between a flat top roller  32  and a concave bottom roller  34 . This tends to work the edges of the sheet W to a greater extent than the central region of the sheet W, which results in forming a sheet W having edges with a more consistent thickness (relative to the central region of the sheet W). 
   As it exits the zero gap assembly  12 , i.e., following milling and stretching, the sheet S passes over a chill roller  60 . The chill roller  60  reduces the temperature of the sheet S as it exits the assembly  12 , and thus stops or at the least retards continued elongation of the sheet S. The sheet S is pulled from the assembly  12  (over the chill roller  60 ), by the exit bridle assembly  16 . Pulling the now cooled sheet S produces a thin, flat, oriented sheet S that is ready to be surface treated and/or heat treated as desired and sliced into thin straps as required for use in strapping packages and the like in accordance with known procedures. 
   The above described apparatus and method produce a high quality, consistent thickness strap compared to known apparatuses and methods. 
   All patents referred to herein, are hereby incorporated herein by reference, whether or not specifically done so within the text of this disclosure. 
   In the present disclosure, the words “a” or “an” are to be taken to include both the singular and the plural. Conversely, any reference to plural items shall, where appropriate, include the singular. 
   From the foregoing it will be observed that numerous modifications and variations can be effectuated without departing from the true spirit and scope of the novel concepts of the present invention. It is to be understood that no limitation with respect to the specific embodiments illustrated is intended or should be inferred. The disclosure is intended to cover all such modifications as fall within the scope of the claims.