Abstract:
A heat sealing wheel assembly of a shrink wrap machine reduces thermoplastic film waste during the side cutting and sealing process. The relative diameters of the heat wheel and heating hub of the heat sealing wheel assembly are configured to create an annular region towards the outer circumference of the heat wheel. This creates a space to permit closer engagement of the gripping mechanism for grasping and guiding the edge of thermoplastic film next to the point of contact of the opposing heat wheel edges, and avoids the obstruction normally caused by the heating hub. The closer gripping engagement reduces the amount of film that must extend out from the point of sealing contact of the heat wheels, resulting in less waste during the side sealing process.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from provisional patent application Ser. No. 60/540,372 filed Jan. 30, 2004, entitled APPARATUS FOR INCREASED EFFICIENCY IN CUTTING AND SEALING FILM EDGES. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to apparatuses for wrapping and sealing articles in a folded thermoplastic film. More specifically, the invention is directed to a mechanism for cutting and sealing the side edges of the plies of plastic enclosing the wrapped article. 
     There exist a wide variety of automated apparatuses, known as shrink-wrap machines, which provide a continuous supply of thermoplastic film for wrapping a series of articles fed through the machine. Machines of this type are well known in the art and are described generally in Stork U.S. Pat. No. 5,956,931. Typically, a shrink-wrap machine will include a conveyor system for transporting the objects to be packaged through the machine in serial fashion. Thermoplastic film is continuously dispensed from a supply roll disposed in close proximity to the conveyor belt. The film is usually dispensed as a single sheet, and the machine has mechanisms for folding the sheet to form two plies, placing the sheet onto the conveyor system, and depositing the article for packaging between the plies of the folded sheet. The conveyor system sends the film-draped articles on to further mechanisms in the machine where cutters and sealers cut and seal the plastic around the article, thereby enclosing it within the film to form the finished, packaged product. Typically, the cuts and seals are effected by subjecting the thermoplastic sheet to a heated knife bar or a heated sealing wheel. A knife bar is used to make transverse cuts, such as those for creating the end seals between individual packaged articles carried along the conveyor system. The shrink-wrap machine is usually automated through software programs that control the speed of the conveyor system, the timing of the stroke of the knife bar, and the height of the cutters, all of which function to control the dimension of the finished package. 
     The open side edges of the folded thermoplastic sheet are sealed together as the sheet is transported by the conveyor system. A heated sealing wheel is optimally used to make the side seal because it can effect a continuous seal and is not dependent upon the dimensions of the final sealed package. To ensure that the plastic sheet is driven through the sealing wheel at a constant uniform rate and that it is fed straightly and evenly, certain mechanisms such as rollers and grip chains are employed to grasp and urge the plastic sheet along. These mechanisms must be placed in close proximity to the heating wheel in order to be effective. However, the available space adjacent to the heating wheel is limited and these grabbing mechanisms are usually required to be substantially offset from the sealing point. Thus, in order for the plastic sheet to be engaged by these grabbing mechanisms, a substantial extra amount of the plastic sheet edge is required to be extended beyond the sealing edge of the heating wheel. This has always resulted in excess waste of thermoplastic material. It would be desirable to eliminate this waste of material, and provide a means for decreasing the amount of thermoplastic sheet edge material that must extend laterally beyond the heating wheel when effecting a side seal in the shrink-wrap process. 
     SUMMARY OF THE INVENTION 
     By means of the instant invention there is provided an improved shrink-wrap machine apparatus for reducing waste of thermoplastic sheet material during the sealing process. The invention provides for closer access of gripping mechanisms at the area of the heat seal wheel assembly that facilitate the conveyance of the thermoplastic sheet at a steady rate and in an even alignment as the sheet passes through the heat seal wheel assembly. The gripping mechanisms are vital to prevent misalignment or slippage of the sheet that can occur during the packaging and sealing operations carried out by the shrink wrap machine. Accordingly, it is necessary to grip the outer edge of the sheet on the outside of the heat seal wheel during the sealing process. 
     The heat seal wheel assembly comprises a pair of heat conducting discs whose peripheral edges oppose and meet each other in a common plane, forming a continual point of contact where the sealing of the side edges of the thermoplastic sheet occurs. The discs are driven by separate, outboard-facing axles. A heating element hub is disposed centrally on each of the discs and provide heat that is conducted radially to the peripheral edges of the discs. The heating element hubs necessarily have a certain thickness and their outer surfaces are offset from the outer edges of the discs. There is no available space in the shrink wrap machine for disposing the heating element hubs on the inboard side of the discs. The gripping mechanism, usually a dual, interlocking chain, grips the exposed edge of the thermoplastic sheet emanating from between the opposing peripheral edges of the discs. To minimize waste, it is essential that the gripping chains be positioned as close to the planar side of the discs as possible. 
     By means of the instant invention, a modified heat seal wheel assembly is provided which brings the gripping mechanism closer to the heating discs than has heretofore been capable. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective schematic view of a shrink wrap machine. 
         FIG. 2  is a schematic side-elevation view of a conveyor system and film side-sealing apparatus of a shrink wrap machine, with other components of the shrink wrap machine removed for clarity. 
         FIG. 3  is a perspective view focusing on the interior aspect of the side-sealing apparatus of the shrink wrap machine, with the thermoplastic film sleeve approaching the side sealing apparatus, with a portion of the conveyor bed cut away to show the lower portion of the side sealing apparatus. 
         FIG. 3   a  is another perspective view focusing on the interior aspect of the side-sealing apparatus of the shrink wrap machine, with an end of the side sealed thermoplastic film sleeve exiting the side sealing apparatus. 
         FIG. 4  depicts a prior art side-sealing apparatus for a shrink wrap machine, and is a view in side elevation thereof taken from an end view of the shrink wrap machine as products would exit the apparatus. 
         FIG. 5  is a view in side elevation of the inventive side-sealing apparatus of the shrink wrap machine as products exit the apparatus, with the gripping mechanism removed to permit better viewing. 
         FIG. 6  is a perspective view of the exterior aspect of the side-sealing apparatus of the shrink wrap machine. 
         FIG. 7  is a view in side elevation of the exterior aspect of the side-sealing apparatus of the shrink wrap machine. 
         FIG. 8  is a top plan view of a prior art side sealing apparatus of a shrink wrap machine. 
         FIG. 9  is a top plan view of the inventive side sealing apparatus for a shrink wrap machine. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     The invention comprises apparatus used in connection with a machine for wrapping and packaging articles in a thermoplastic film. The machine, commonly known as a shrink wrap machine, is shown in  FIG. 1  and is generally indicated by the reference numeral  10 . Its general structure and operation are well known to those having skill in the art, and therefore only structural and functional details relevant to the instant invention shall be discussed. In brief explanation, the shrink wrap machine  10  is generally comprised of a conveyor system  12 , a supply  14  of thermoplastic film, a side sealing assembly  16 , and a cross-cutting assembly  18 . Articles  20  are fed through the shrink wrap machine on conveyor system  12 , where they are first placed within a folded section of thermoplastic film  22 . Mechanisms are provided for inverting an edge of the film  22  to create a two-ply layer of film between which the article  20  is placed. A continuous sheet  23  of folded-over, two-ply film, with articles  20  placed between, is conveyed through shrink wrap machine  10  where it is subjected to sealing and cutting for creating individually wrapped articles  24 . 
     The folded thermoplastic film  23  is first sealed along its side edges  26  and  28  by side sealing assembly  16  to create a continuous sleeve  32  around the articles  20  as shown in  FIG. 2 . Transverse cuts and seals are subsequently made along sleeve  32  within the shrink wrap machine to enclose articles  20  within their own discrete packages  24 . This further processing is beyond the scope of this invention and need not be discussed in any further detail. 
     The instant invention focuses on the side sealing assembly  16  for creating the continuous seal along edges  26  and  28  of thermoplastic film  23 . As can be seen in  FIGS. 3 and 3   a,  the side sealing assembly  16  comprises a pair of wheel assemblies  34  and  36  having circumferential opposing edges aligned in a common vertical plane. The edges  26  and  28  of thermoplastic film  23  are advanced along the conveyor system by rollers  30  and fed between the wheel assemblies  34  and  36  where a seal  38  is effected, thereby creating continuous thermoplastic film sleeve  32 . Each wheel assembly is comprised of a nip roller  40 , heat wheel  42  and heating hub  44 . A gripping mechanism  46 , such as interlocking chains, is also disposed adjacent to the exterior aspect of the wheel assembly, which is outboard with respect to the conveyor system  12 . Nip rollers  40  serve to draw the side edges  26  and  28  of film  23  evenly together and perpendicular to the plane of the wheel assemblies to promote an even seal. This is necessary when accommodating various sizes of packaged articles. As the dimension of the article to be packaged increases, upper film edge  26  will fall out of perpendicular alignment to the plane of the wheel assemblies and must be brought back into even registry thereto by nip roller  40  to prevent uneven sealing. The opposing heat wheels  42  engage the film edges tightly together for effecting the side seal. Heat wheel  42  is heated by heating hub  44 , and the heat acts to melt and seal the film edges  26  and  28  together and also to trim off the excess film edge  48 . In a preferred embodiment, gripping mechanism  46  is comprised of interlocking chains  50  and  52  which grip film edge  48  therebetween. Upper chain  50  runs a continuous loop around wheel assembly  34  and is driven by a pair of sprockets  60  and  62 , and is maintained in its circuitous path by guide plates  64  and  66  positioned adjacently to heating hub  44  of the wheel assembly. Lower chain  52  is symmetrically opposite to upper chain  50  and is similarly positioned with respect to the lower wheel assembly, except that it is positioned slightly offset from upper chain  50 . The offsetting of the chains permit the individual links of the opposing chains to interlock, effectively gripping the edge of the thermoplastic film therebetween. #35 chain is commonly used for this purpose. Gripping mechanism  46 , however, is not limited to interlocking chains, and other mechanisms known to those skilled in the art may be used. The excess film  48  that results from the sealing and trimming is thus carried off to a take-up spool  54 . Gripping mechanism  46  is also necessary for assisting in drawing thermoplastic film  23  through the side sealing assembly evenly and at a steady rate. 
     A limitation in prior art side sealing assemblies is the excess waste  48  of thermoplastic film generated as a consequence of the use of the gripping mechanism. While it is necessary to place the gripping mechanism as close to the heat wheel  42  as possible, heating hub  44  has always presented a spatial obstacle as can be seen in heating wheel assemblies of the prior art shown in  FIGS. 4 and 8 . The gripping mechanism is prevented from getting any closer to the heat wheel  42  than permitted by the outer side of the heating hub  44 . Accordingly, the amount of film waste  48  generated during the side sealing process has heretofore never been decreased less than the width represented by the thickness of the heating hub itself. When factoring in the width of the heating hub, and the width of the gripping mechanism, the excess trim  48  of thermoplastic film generated by prior art side sealing mechanisms can exceed 1.60 inches. This equates, over time, to a substantial amount of film. 
     PREFERRED EMBODIMENT 
     The heating wheel assembly  100  of the instant invention, shown in  FIG. 5 , has a modified structure which permits the gripping mechanism to grab the thermoplastic film edge in close proximity to the heat wheel, unimpeded by the heating hub. Specifically, the relative diameter of the heat wheel  142  is increased with respect to the diameter of heating hub  144 . This effectively provides an annular region  70  on the outer region of heat wheel  142  that extends beyond the outer circumferential edge of the heating hub  144 . Heat wheel  152  of the opposing heating wheel assembly also has its relative diameter increased with respect to the diameter of heating hub  154 , effectively providing an annular region  72  on the outer region of heat wheel  152  that extends beyond the outer circumferential edge of the heating hub  154 . In the opposing positions of the heat wheel assemblies, annular regions  70  and  72  combine to create gap  76 . In creating the annular regions  70  and  72 , either the diameter of the heat wheel may be increased or the diameter of the heating hub may be decreased (or both conditions may occur), so long as the relative diameter of the heat wheel is increased with respect to the heating hub. The heat wheels should have sufficient heat conducting qualities for conducting the heat from the heating hubs over the annular distances  70  and  72  to their circumferential edges to generate sufficient heat for effecting the heat sealing of the film edges. 
     Gap  76  provides clearance for gripping mechanism  146  to more nearly approach the point of contact  80  between heat wheels  142  and  152  and avoid heating hubs  144  and  154 , as can be clearly seen in  FIG. 5 . Interlocking chains  150  and  151  meet in gap  76 , as seen in  FIGS. 6 and 9 , where they grip the trimmed excess film  148 . Similarly, annular regions  70  and  72  provide clearance for guide plates  160  and  162 , respectively, for chains  150  and  151  to have an unimpeded, circuitous path through gap  76  and around the heating hubs  144  and  154 . Guide plates  160  and  162  are provided with cut-out regions  164  and  166 , respectively, to permit their placement over and around heating hubs  144  and  154 . This enables the guide plates, and therefore, the gripping mechanism chains to gain closer access to heat wheels  142  and  152 . Each annular region should have a radius dimension sufficient to accommodate the combined width of the chain and portion of the guide plate at the Twelve O&#39;clock position of heat wheel  142 , and the Six O&#39;clock position of heat wheel  152 . This will then also permit sufficient clearance at the area of interlocking engagement of the chains at the Six O&#39;clock position of heat wheel  142  and the Twelve O&#39;clock position of heat wheel  152 . The annular region of the outer heat wheel may be manufactured to have a dimension of any particular size, but in any event, must be sufficient to accommodate the combined width of the interlocking chain and the portion of the guide plate carrying the chain. As an example, heat wheel  142  may have a diameter of 4.50 inches. If the interlocking chain  150  of gripping mechanism  146  has a width (top to bottom dimension of each link) of 0.5 inches, and the portion of guide plate  160  at the Twelve O&#39;clock position has a width (top to bottom) of 0.2 inches, it would be desirable to create an annular region of 0.8 inches to permit sufficient clearance of the chain and guide plate at the Twelve O&#39;clock position. To create the annular region of 0.8 inches, heating hub  144  should have a diameter of 3.70 inches or less. The combined annular regions of heat wheels  142  and  152  would then create a gap  76  of 1.60 inches at the Six O&#39;clock position of heat wheel  142 . It should be understood that the respective dimensions of the heat wheel, heat hub, and gripping mechanism guide plate are variables, any of which can be varied with respect to the other to create an appropriate sized annular region. Furthermore, the configuration of the gripping mechanism may also influence the desired dimension of the annular region. 
     Through this closer placement to the heat wheels, the gripping mechanism may engage the excess film edge along points closer to the heat wheels, preferably less than 1.50 inches from the contact point  80  between the wheels. Accordingly, the placement of the thermoplastic film on the conveyor system may be adjusted so that less of the film edge protrudes from the exterior aspect of the heating wheel assembly as film sleeve  32  is conveyed and processed through the shrink wrap machine. 
     Accordingly, the amount of thermoplastic film required to be extended from the heat wheels for engagement with the gripping mechanism can be significantly decreased, resulting in less waste during the side sealing process. Even one-half an inch less film wasted translates into a substantial cost saving in film used over time. The following example highlights this point. Assuming that for a 20-inch roll of thermoplastic film costing $200, the proportionate cost per width inch is $10. If one-half of an inch less is wasted during the edge sealing process, $5 per roll is saved. Typically, four rolls of thermoplastic film may be used during an operational shift of the shrink-wrap machine, and two shifts are run per day. Accordingly, each day $40 of film waste is avoided. If this use trend is extended over an entire year of two shift-days, and 52 five-day weeks, a savings of $10,400 per year may be realized. Over the life of the shrink wrap machine, which can be ten years, a savings of over $100,000 may be made possible. 
     Various changes and modifications may be made within this invention as will be apparent to those skilled in the art. Such changes and modifications are within the scope and teaching of this invention.