Abstract:
The invention relates generally to the field of plastics manufacturing. In particular, but not by way of limitation, the invention relates to a system and method for manufacturing co-extruded plastic film and products using same. In one embodiment, a first plastic layer having a relatively high melting temperature is co-extruded with a second plastic layer having a relatively low melting temperature. Embodiments of the invention also disclose manufacturing processes for end products that exploit the improved co-extruded film. One embodiment is a process for manufacturing a bag stack with releasable bonds between adjacent bags in the bag stack. Another embodiment is a process for manufacturing a stack of plastic sheets with releasable bonds between adjacent sheets in the sheet stack.

Description:
FIELD OF INVENTION 
       [0001]    The invention relates generally to the field of plastics manufacturing. In particular, but not by way of limitation, the invention relates to a system and method for manufacturing co-extruded plastic film and products using same. 
       BACKGROUND 
       [0002]    Consumable thermoplastic (plastic) products, such as polyethylene bags and sheets, are often sold in stacks. The stack format facilitates distribution, and also allows a consumer to individually dispense a bag or sheet as needed. For instance, a bag stack may be suspended from a rack near the point of sale in a retail store, and bags can be individually separated from the stack. 
         [0003]    Discrete dispensing requires that each item can be easily separated from the stack. While perforated attachment, for example to a stack header, is often acceptable, some applications require a releasable bond between adjacent bags or sheets. Many plastic welding techniques are known. But conventional processes that form releasable bonds are often difficult to control during manufacturing. 
         [0004]    Improved materials and/or manufacturing processes are needed for forming releasable plastic bonds. 
       SUMMARY OF THE INVENTION 
       [0005]    Embodiments of the invention seek to overcome one or more of the limitations described above. In one embodiment, a first plastic layer having a relatively high melting temperature is co-extruded with a second plastic layer having a relatively low melting temperature. During manufacturing of a stack of bags, sheets, or other plastic products, a releasable bond can be formed between at least portions of adjacent second plastic layers without bonding adjacent first plastic layers. 
         [0006]    In a first embodiment, the first plastic layer of a co-extruded material is high-density polyethylene (HDPE) and the second plastic layer is ethylene vinyl acetate (EVA) or ethylene methyl acrylate (EMA). In a second embodiment of the invention, the first plastic layer is a blend of HDPE and linear low density polyethylene (LLDPE), and the second plastic layer is EVA or EMA. In a third embodiment, the first plastic layer of a co-extruded material is HDPE or a HDPE/LLDPE blend and the second plastic layer is a blend of LLDPE and polyolefin plastomer (POP). In a fourth embodiment, the first plastic layer of a co-extruded material is HDPE or a HDPE/LLDPE blend and the second plastic layer is a blend of HDPE and POP. In a variation of the third or fourth embodiment, a polyolefin elastomer (POE) could be used in place of the POP. Alternatively, in a variation of the third or fourth embodiment, the second layer is POP only. 
         [0007]    Embodiments of the invention also disclose manufacturing processes for end products that exploit the improved co-extruded film. One embodiment is a process for manufacturing a bag stack with releasable bonds between adjacent bags in the bag stack. Another embodiment is a process for manufacturing a stack of plastic sheets with releasable bonds between adjacent sheets in the sheet stack. 
         [0008]    These and other features are more fully described in the detailed description section. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Embodiments of the invention are described with reference to the following drawings, wherein: 
           [0010]      FIG. 1A  is a flow diagram of a co-extrusion process with reference to functional manufacturing components, according to an embodiment of the invention; 
           [0011]      FIG. 1B  is a flow diagram of a co-extrusion process with reference to functional manufacturing components, according to an embodiment of the invention; 
           [0012]      FIG. 2A  is a flow diagram of a co-extrusion process with reference to functional manufacturing components, according to an embodiment of the invention; 
           [0013]      FIG. 2B  is a flow diagram of a co-extrusion process with reference to functional manufacturing components, according to an embodiment of the invention; 
           [0014]      FIG. 3  is a flow diagram of a bag stack manufacturing process, according to an embodiment of the invention; 
           [0015]      FIG. 4  is a perspective view of a bag stack, according to an embodiment of the invention; 
           [0016]      FIG. 5  is a flow diagram of a sheet stack manufacturing process, according to an embodiment of the invention; 
           [0017]      FIGS. 6A-6C  are side sectional views of a sheet stack before and during a dispensing operation, according to an embodiment of the invention; and 
           [0018]      FIGS. 7A-7D  are perspective views of a dispensing container for a sheet stack before and during a dispensing operation, according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The drawings are not to scale. Some features illustrated in the drawings have been exaggerated for descriptive clarity. Sub-headings are used in this section for organizational convenience; the disclosure of any particular feature(s) is/are not necessarily limited to any particular section or sub-section of this specification. The detailed description begins with the co-extrusion process. 
       Plastic Film Co-Extrusion 
       [0020]      FIGS. 1A ,  1 B,  2 A, and  2 B illustrate four alternative processes for producing co-extruded film with a first plastic layer having a relatively high melting temperature and a second plastic layer having a relatively low melting temperature. 
         [0021]      FIG. 1A  is a flow diagram of a co-extrusion process with reference to functional manufacturing components, according to an embodiment of the invention. As shown therein, high-density polyethylene (HDPE) pellets are fed from the hopper  105  to the extruder  110 . The extruder  110  meters the input of HDPE pellets, melts, mixes, and pumps liquefied HDPE through the filter  115  and flow heater(s)  120  to the co-extrusion tooling  150 . Likewise, ethylene vinyl acetate (EVA) or ethylene methyl acrylate (EMA) pellets are fed from the hopper  125  to the extruder  130 . The extruder  130  meters the input of EVA or EMA pellets, melts, mixes, and pumps liquefied EVA or EMA through the filter  135  and flow heater(s)  140  to the co-extrusion tooling  150 . The co-extrusion tooling  150  also receives cold air from the cooling blower  145 , and outputs co-extruded film  155 . The co-extrusion tooling  150  is preferably in the form of concentric rings, and the co-extruded film  155  is preferably in the form of blown film (tube) to achieve the desired material thicknesses. The co-extruded film  155  includes HDPE on an inner layer, and EVA or EMA on an outer layer. The co-extruded film  155  may then be treated at the corona treatment station  160  to improve adhesion at subsequent welding and/or printing steps. 
         [0022]    The melting points of HDPE, EVA, and EMA are approx. 266, 176, and 216 deg. F., respectively. Because the melting temperatures of EVA and EMA are lower than the melting temperature of HDPE, it may be easier to produce releasable bonds between adjacent EVA or EMA layers during subsequent manufacturing without bonding adjacent HDPE layers. In embodiments of the invention, the HDPE layer of the co-extruded film  155  is much thicker than the EVA or EMA layer. For instance, the HDPE layer may be 5 to 300 microns (micrometers) thick, whereas the EVA or EMA layer may only be 1 to 60 microns thick. In other words, the thickness of the inner HDPE layer may be five times the thickness of the outer EVA or EMA layer. Other thickness ratios are also possible. The relative thickness of the EVA or EMA layer enables bonds between adjacent EVA or EMA layers to be predictably released according to application requirements. 
         [0023]    Variations to the process illustrated in  FIG. 1A  are possible. For example, in embodiments of the invention, hoppers  105  and/or  125  may also include recycled polymeric material, filler, color concentrate, and/or other additives without changing the principle process flow and benefits that are described above with reference to  FIG. 1A . 
         [0024]      FIG. 1B  is a flow diagram of a co-extrusion process with reference to functional manufacturing components, according to an embodiment of the invention. The process flow is the same as the embodiment in  FIG. 1A  except as described below. HDPE pellets are fed from hopper  165  to the mixer  175 . Likewise, linear low density polyethylene (LLDPE) pellets are fed from hopper  170  to the mixer  175 . The mixer  175  mixes the HDPE and the LLDPE; and then the blender  180  blends the HDPE and the LLDPE into a predetermined HDPE/LLDPE blend. The HDPE/LLDPE blend may be stored in the blended batch hopper  185  before it is fed to the extruder  110 . The HDPE/LDPE blend may be preferable to HDPE alone due to improved material flow characteristics and/or other properties. The co-extruded film  155  includes an HDPE/LLDPE blend on an inner layer, and EVA or EMA on an outer layer. 
         [0025]    The melting points of HDPE, LLDPE, EVA, and EMA are approx. 266, 248, 176, and 216 deg. F., respectively. Because the melting temperatures of EVA and EMA are lower than the melting temperature of the HDPE/LLDPE blend, it may be easier to produce releasable bonds between adjacent EVA or EMA layers during subsequent manufacturing without bonding adjacent HDPE/LLDPE blend layers. In embodiments of the invention, the HDPE/LLDPE blend layer of the co-extruded film  155  is much thicker than the EVA or EMA layer. For instance, the HDPE/LLDPE blend layer may be 5 to 300 microns (micrometers) thick, whereas the EVA or EMA layer may only be 1 to 60 microns thick. In other words, the thickness of the inner HDPE/LLDPE blend layer may be five times the thickness of the outer EVA or EMA layer. Other thickness ratios are also possible. The relative thickness of the EVA or EMA layer enables bonds between adjacent EVA or EMA layers to be predictably released according to application requirements. 
         [0026]    Variations to the process illustrated in  FIG. 1B  are possible. For instance, in embodiments of the invention, hoppers  165 ,  170  and/or  125  may also include recycled polymeric material, filler, color concentrate, and/or other additives without changing the principle process flow and benefits that are described above with reference to  FIG. 1B . 
         [0027]      FIG. 2A  is a flow diagram of a co-extrusion process with reference to functional manufacturing components, according to an embodiment of the invention. The process flow is the same as the embodiment in  FIG. 1A  except as described below. HDPE or a HDPE/LLDPE blend is disposed in hopper  230  and fed to the extruder  110 . Linear low density polyethylene (LLDPE) pellets are fed from hopper  205  to the mixer  215 . Likewise, polyolefin plastomer (POP) pellets are fed from hopper  210  to the mixer  215 . The POP pellets may be, for example, Dow Affinity™ POP. The mixer  215  mixes the LLDPE and the POP; and then the blender  220  blends the LLDPE and the POP into a predetermined LLDPE/POP blend. The LLDPE/POP blend may be stored in the blended batch hopper  225  before it is fed to the extruder  130 . The co-extruded film  155  includes HDPE or an HDPE/LLDPE blend on an inner layer, and a LLDPE/POP blend on an outer layer. 
         [0028]    The melting points of HDPE, LLDPE, and pure POP are approx. 266, 248, and 133 deg. F., respectively. Because the melting temperature of the LLDPE/POP blend is lower than the melting temperature of HDPE or an HDPE/LLDPE blend, it may be easier to produce releasable bonds between adjacent LLDPE/POP blend layers during subsequent manufacturing without bonding adjacent HDPE or HDPE/LLDPE blend layers. The LLDPE/POP blend may be preferable to EVA or EMA (discussed with reference to  FIGS. 1A and 1B ) due to lower material costs or other material properties. The LLDPE may be, for example, 0-90% of the LLDPE/POP blend, and the POP may be 10-100% of the LLDPE/POP blend. In embodiments of the invention, the HDPE or HDPE/LLDPE blend layer of the co-extruded film  155  is much thicker than the LLDPE/POP blend layer. For instance, the HDPE or HDPE/LLDPE blend layer may be 5 to 300 microns (micrometers) thick, whereas the LLDPE/POP blend layer may only be 1 to 60 microns thick. In other words, the thickness of the inner HDPE or HDPE/LLDPE blend layer may be five times the thickness of the outer LLDPE/POP blend layer. Other thickness ratios are also possible. The relative thickness of the LLDPE/POP blend layer enables bonds between adjacent LLDPE/POP blend layers to be predictably released according to application requirements. 
         [0029]    Variations to the process illustrated in  FIG. 2A  are possible. For instance, an alternative POP material or a polyolefin elastomer (POE) could be used in place of the Dow Affinity™ POP, according to design choice. In an alternative embodiment, the outer layer could be POP alone. In addition, hoppers  230 ,  205  and/or  210  may also include recycled polymeric material, filler, color concentrate, and/or other additives without changing the principle process flow and benefits that are described above with reference to  FIG. 2A . 
         [0030]      FIG. 2B  is a flow diagram of a co-extrusion process with reference to functional manufacturing components, according to an embodiment of the invention. The process flow is the same as the embodiment in  FIG. 1A  except as described below. HDPE or a HDPE/LLDPE blend is disposed in hopper  230  and fed to the extruder  110 . HDPE pellets are fed from hopper  235  to the mixer  215 . Likewise, POP pellets are fed from hopper  210  to the mixer  215 . The POP pellets may be, for example, Dow Affinity™ POP. The mixer  215  mixes the HDPE and the POP; and then the blender  220  blends the HDPE and the POP into a predetermined HDPE/POP blend. The LLDPE/POP blend may be stored in the blended batch hopper  225  before it is fed to the extruder  130 . The co-extruded film  155  includes HDPE or an HDPE/LLDPE blend on an inner layer, and a HDPE/POP blend on an outer layer. 
         [0031]    The melting points of HDPE and pure POP are approx. 266 and 133 deg. F., respectively. Because the melting temperature of the HDPE/POP blend is lower than the melting temperature of HDPE or a HDPE/LLDPE blend, it may be easier to produce releasable bonds between adjacent HDPE/POP blend layers during subsequent manufacturing without bonding adjacent layers of HDPE. The HDPE/POP blend may be preferable to EVA or EMA (discussed with reference to  FIGS. 1A and 1B ) due to lower material costs or other material properties. The HDPE may be, for example, 0-90% of the HDPE/POP blend, and the POP may be 10-100% of the HDPE/POP blend. In embodiments of the invention, the HDPE or HDPE/LLDPE blend layer of the co-extruded film  155  is much thicker than the HDPE/POP blend layer. For instance, the HDPE or HDPE/LLDPE blend layer may be 5 to 300 microns (micrometers) thick, whereas the HDPE/POP blend layer may only be 1 to 60 microns thick. In other words, the thickness of the inner HDPE or HDPE/LLDPE blend layer may be five times the thickness of the outer LLDPE/POP blend layer. Other thickness ratios are also possible. The relative thickness of the HDPE/POP blend layer enables bonds between adjacent HDPE/POP blend layers to be predictably released according to application requirements. 
         [0032]    Variations to the process illustrated in  FIG. 2B  are possible. For instance, an alternative POP material or a polyolefin elastomer (POE) could be used in place of the Dow Affinity™ POP, according to design choice. In an alternative embodiment, the outer layer could be POP alone. In addition, hoppers  230 ,  235  and/or  210  may also include recycled polymeric material, filler, color concentrate, and/or other additives without changing the principle process flow and benefits that are described above with reference to  FIG. 2B . 
       Bag Stack Manufacturing 
       [0033]    The co-extruded film described above with reference to  FIGS. 1A ,  1 B,  2 A, and  2 B improves the ability to form releasable plastic bonds between the relatively low melting temperature layers. In bag stack manufacturing applications, it is sometimes desirable to form releasable bonds between adjacent bags in the bag stack. For instance, where a welded releasable bond exists between the back of a first bag and the front of a second bag in a suspended bag stack, removing the first bag from the bag stack will cause the second bag to open in preparation for loading. This may be desirable, for example, to speed checkout at a retail point of sale. 
         [0034]      FIG. 3  is a flow diagram of a bag stack manufacturing process, according to an embodiment of the invention.  FIG. 4  is a perspective view of a bag stack, according to an embodiment of the invention. With reference to  FIGS. 3 and 4 , the process begins in step  305  and then forms a co-extruded tube with relatively low melting point material on an outer layer of the tube in step  310 . Next, the process forms gusseted side walls in a portion of the tube to produce a gusseted tube in step  315 . The process then welds a bottom edge of the gusseted tube in step  320  to form a bottom seal  415 . In step  325 , the process cuts and welds the tube at a predetermined distance from the bottom edge to form a bag. In step  330 , the process punches the bag to form handles  425 , a center tab  430 , and a frangible header  440 . Step  330  may also punch tooling holes in the frangible header  440  to facilitate stacking. In step  335 , the process stacks multiple bags to form a bag stack  405 . Step  335  may be accomplished, for example using a wicketer. In step  340 , the process forms a permanent bond between adjacent bags in the bag stack in the frangible header  440 , for instance at locations  445 . The process forms releasable bonds between outer layers of adjacent bags in the bag stack, for instance at a location  435  near the center tab  430 , in step  345 . Step  345  may be or include, for example, thermoplastic welding. The heat applied in step  345  is sufficient to form the releasable bonds between adjacent layers of relatively low melting temperature material in the co-extruded film, but is not sufficient to form a bond between adjacent layers of relatively high melting temperature material. The process terminates in step  350 . 
         [0035]      FIG. 4  shows a portion of the top weld  420  from step  325  that was not removed during punch step  330 .  FIG. 4  also illustrates that the side gussets  410  preferably span the width of the handles  425  so that each handle  425  is essentially a loop of 2-ply thermoplastic for increased strength. 
         [0036]    Variations to the manufacturing process illustrated in  FIG. 3  and the resulting bag stack  405  shown in  FIG. 4  are possible. For instance, releasable bonds may also be desirable in bags that do not include gussets. In one embodiment, the cut and weld of cut/weld step  325  could be performed simultaneously. In an alternative embodiment, the cut/weld step  325  could include separate cut and weld steps, and the order of cutting and welding could be varied according to design choice. In addition, alternative embodiments may perform stacking step  335  prior to punching step  330 , according to known bag stack manufacturing methods. 
       Sheet Stack Manufacturing 
       [0037]    The ability to more easily form releasable plastic bonds can also be beneficial for manufacturing a stack of dispensable plastic sheets (such as a deli sheet product).  FIG. 5  provides a manufacturing process for such a product.  FIGS. 6A-6C  and  7 A- 7 D illustrate additional product features, as well as an end-user dispensing operation. 
         [0038]      FIG. 5  is a flow diagram of a sheet stack manufacturing process, according to an embodiment of the invention. With reference to  FIG. 5 , the manufacturing process begins in step  505  and then forms a co-extruded sheet with relatively low melting point material on one layer in step  510 . Step  510  may be executed, for instance, using any of the alternative co-extrusion processes described above with reference to  FIG. 1A ,  1 B,  2 A or  2 B. Next, in step  515 , the process cuts multiple sheets of predetermined length from the co-extruded tube. In step  520 , the process folds each of the multiple sheets, for instance about a short dimension, except for a portion of each sheet at a header end. The process then stacks the multiple folded sheets in step  525 . The orientation of each sheet in the stack is such that the relatively low melting temperature layers are adjacent to each other. The process then welds the stack of multiple folded sheets at a non-folded (header) portion to form permanent bonds in step  530 . Next, the process cuts a perforation line to define a header dimension in step  535 . The process then welds the stack of multiple folded sheets at a folded portion to form releasable bonds between adjacent sheets in the stack in step  540 . The heat applied in step  540  is sufficient to form the releasable bonds between adjacent layers of relatively low melting temperature material in the co-extruded film, but is not sufficient to form a bond between adjacent layers of relatively high melting temperature material. The process terminates in step  545 . 
         [0039]    Variations to the manufacturing process illustrated in  FIG. 5  are possible. For example, instead of partially folding multiple sheets in step  520  and then stacking the multiple folded sheets in step  525 , each sheet could be folded and then added to the stack individually. The sequence of welding step  530 , cutting step  535 , and welding step  540  could be changed, according to design choice. The manufacturing process could also include disposing the sheet stack in a dispensing container, as illustrated in  FIG. 7A . 
         [0040]      FIGS. 6A-6C  are side sectional views of a sheet stack, according to an embodiment of the invention.  FIGS. 6A-6C  illustrate three sheets  605 ,  610 , and  615 . Each sheet is co-extruded to produce two layers. Sheet  605  includes a first layer  620  with a relatively high melting point and a second layer  625  with a relatively low melting point. Likewise, sheet  610  includes a first layer  630  with a relatively high melting point and a second layer  635  with a relatively low melting point. Sheet  615  includes a first layer  640  with a relatively high melting point and a second layer  645  with a relatively low melting point. 
         [0041]    In  FIG. 6A , sheets  605 ,  610  and  615  are shown partially folded and stacked, the result of the manufacturing process described above with reference to  FIG. 5 . Permanent bonds  665  and  670  affix adjacent sheets in the stack at the header area  660 , which is defined by the perforation line  675 . Releasable bonds  650  and  655  bond adjacent sheets between the second (relatively low melting point) layers of each co-extruded sheet. Note that adjacent layers of the relatively high melting point layers are not bonded in the folded portion. For example, in sheet  610 , a first portion of layer  630  is not bonded to a second portion of layer  630  in the proximity of releasable bonds  650  and  655 . The reason for this is that the temperature used in step  345  is not sufficient to melt the relatively high melting point material of layer  630 .  FIGS. 6B and 6C  illustrate a portion of an end-user dispensing operation. In  FIG. 6B , a user has extended sheet  605  in a direction  680  away from the header  660 . In  FIG. 6C , the user has further extended sheet  605  in direction  680 . This action has caused the separation of sheet  605  from the header  660  at the perforation line  675 , and has also caused partial extension of sheet  610 . 
         [0042]      FIGS. 7A-7D  are perspective views of a dispensing container for a sheet stack, according to an embodiment of the invention. In one respect,  FIGS. 7A-7D  illustrate that a manufactured sheet stack may be disposed in a dispenser  710  that includes an opening  715 . The dispenser  710  could be rigid or soft, and the size and shape of the opening  715  could vary, according to design choice. In another respect,  FIGS. 7A-7D  illustrate a portion of an end-user dispensing operation.  FIG. 7A  is a view prior to dispensing, where a sheet stack (not visible) is disposed inside the dispenser  710 . In  FIG. 7B , a user has extended sheet  605  in a direction  680  from the header  660  and partially through the opening  715 . In  FIG. 7C , the user has removed the sheet  605  from the dispenser  710 . Because of releasable bond  650 , this action has also extended sheet  610  in the direction  680 . In  FIG. 7D , a user has fully separated sheet  605  from the sheet stack. 
       SUMMARY 
       [0043]    Embodiments of the invention thus provide an improvement in the composition of co-extruded plastic materials. The improved materials can utilize known manufacturing equipment, reduce material costs, and improve the repeatability of manufacturing steps that produce releasable plastic bonds. Embodiments of the invention also provide manufacturing processes for a bag stack and a sheet stack that exploit the releasable bond feature. 
         [0044]    Those skilled in the art can readily recognize that numerous variations and substitutions may be made in the invention, its use and its configuration to achieve substantially the same results as achieved by the embodiments described herein. Accordingly, there is no intention to limit the invention to the disclosed exemplary forms. Many variations, modifications and alternative constructions fall within the scope and spirit of the disclosed invention as expressed in the claims.