Abstract:
A shrink-wrap packaging system comprising a supply of shrink-wrap film, a sealer system and an oven. The sealer system includes a hot-knife assembly, which encapsulates items being packaged by cutting, joining and sealing layers of shrink-wrap film that an operator fits around the items. The hot knife includes a double-edged blade having an electrical surface heater element fixed thereto. The heater element comprises a first layer of electrical insulation fixed directly to the blade, a serpentine resistive film deposited on the first layer and a second layer of insulation covering the resistive film. The oven includes a tunnel in which the encapsulated items are heated to form a shrink-wrap package. The oven includes heat exchangers that mount adjacent the tunnel. The heat exchangers generate heat via an array of electrical surface heater elements similar to those mounted on the hot knife.

Description:
CLAIM TO PRIORITY  
       [0001]    This application claims priority of U.S. provisional patent application serial No. 60/245,494; filed Nov. 3, 2000 and entitled “HOT KNIFE, SURFACE HEATER TECHNOLOGY (SHT)/SHRINK PACKAGING TUNNELS (SPT)”, which is incorporated by reference herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention generally relates to thermal techniques for cutting, sealing, joining or otherwise processing materials with heat. More particularly, it relates to improved shrink-wrap packaging systems using surface heater technology.  
           [0004]    2. Description of the Prior Art  
           [0005]    Conventional shrink-wrap systems generally comprise a shrink-wrap sealer system mounted in-line with a shrink-wrap tunnel system and a supply of shrink-wrap film. Shrink-wrap film typically comes as a roll of tough, transparent plastic material that shrinks when heated to form a tightfitting covering for items being packaged. The heart of a shrink-wrap sealer system is a device designed to cut, join and seal layers of shrink-wrap film that an operator has snugly fitted around items being packaged. A tool having one or more heated edges often performs the cutting, joining and sealing functions.  
           [0006]    A typical shrink-wrap tunnel system includes a heated tunnel with entrance and exit ports, and a conveyor that transports packages through the tunnel. After using the sealer system to encapsulate the items into a loose-fitting sealed package, an operator (or robot in a fully automated system) directs the sealed package onto the conveyor at the entrance port of the tunnel. As the conveyor transports the sealed package through the tunnel, the shrink-wrap film heats and shrinks, forming a tightfitting sealed package, i.e., a “shrink-wrap package.” 
           [0007]    Those concerned with the development of shrink-wrap packaging systems have long recognized the need for improved techniques of shrinking, cutting, joining and/or sealing plastic films. Although conventional packaging systems have served the purpose, they have not proved entirely satisfactory because of their high operating costs, which are primarily associated with the production of high operating temperatures. In addition, the active surfaces of conventional thermal cutting and/or sealing tools found in many known packaging systems are not always capable of accurately maintaining uniform operating temperatures, a critical requirement for a high-performance shrink-wrap system. Still further, a need exists for improved techniques of heating shrink-wrap ovens, which must operate uniformly at suitable baking temperatures to achieve high quality packages at high production rates.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention satisfies these needs in the art by providing apparatus having critical elements heated with electrical surface heaters that are capable of being accurately controlled to provide uniform operating temperatures with minimal use of electrical energy. One aspect of the invention comprises a hot knife having a blade with an extended surface and an operative edge. An electrical heater element mounts on the extended surface adjacent the edge. An outer layer of electrical insulation covers the heater element. In addition, the heater element includes an inner layer of electrical insulation fixed directly to the extended surface and an electrical resistive film fixed to the inner layer adjacent the edge. The resistive film follows a serpentine path between a pair of electrical contacts.  
           [0009]    Another aspect of the invention involves an electrical oven for heating items. The oven comprises a tunnel having an exit port, an entrance port and opposed side walls. A conveyor mounts in the tunnel between the side walls for transporting items from the entrance port to the exit port. A heat exchanger mounts adjacent the side walls and extends between the entrance and exit ports on either side of the conveyor. The heat exchanger includes an extended permeable surface with a plurality of electrical surface heater elements fixed thereto. An air circulating system mounts adjacent the tunnel. The air circulating system includes a blower, an intake ductwork and an output ductwork. The intake ductwork communicates with the input of the blower and the tunnel adjacent the exit port. The output ductwork communicates with the output of the blower and the tunnel via the heat exchanger. In addition, each of the electrical surface heater elements includes a serpentine strip of resistive film embedded in insulating layers that mount directly on the permeable surface of the heat exchanger.  
           [0010]    A further aspect of the invention comprises a shrink-wrap system for packaging items. The system comprises a supply of shrink-wrap film, a sealer system and an electrical oven system. The sealer system has a hot-knife assembly for encapsulating items in the film. The hot-knife assembly includes at least one elongated blade with an operative edge, an electrical heater element fixed to the blade and an outer layer of insulation covering the heater element. The electrical oven system has a tunnel and a heat exchanger. The heat exchanger has at least one extended permeable surface mounted adjacent the tunnel and a plurality of electrical surface heater elements mounted on the extended permeable surface. In addition, the heater elements each include an inner layer of electrical insulation fixed directly to the blade or the extended surface and an electrical resistive film fixed to the inner layer, which extends adjacent the edge. Each of the resistive films follows a serpentine path between a corresponding pair of electrical contacts. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0012]    [0012]FIG. 1 is a pictorial view of a shrink-wrap packaging system in accordance with the present invention;  
         [0013]    [0013]FIG. 2 is a pictorial view of an enlarged break-away section of the shrink-wrap packaging system of FIG. 1;  
         [0014]    FIGS.  3 A- 3 D are schematic top views of a portion of the shrink-wrap packaging system of FIG. 1, showing successive stages of a sealing process in accordance with the present invention;  
         [0015]    [0015]FIG. 4 is a front elevation of a hot knife constructed in accordance with the present invention;  
         [0016]    [0016]FIG. 5 is an enlarged front elevation of the hot knife of FIG. 4 with parts broken away;  
         [0017]    [0017]FIG. 6 is an enlarged sectional view taken on the line  6 - 6  of FIG. 5 and looking in the direction of the arrows;  
         [0018]    FIGS.  7 A- 7 C are enlarged side views of alternate embodiments of the hot knife shown in FIG. 5;  
         [0019]    [0019]FIG. 8 is a top sectional view of a tunnel system in accordance with the invention taken on the line  8 - 8  of FIG. 9 looking in the direction of the arrows;  
         [0020]    [0020]FIG. 9 is a sectional view of the tunnel system of FIG. 8 taken on the line  9 - 9  of FIG. 8 and looking in the direction of the arrows;  
         [0021]    [0021]FIG. 10 is an elevation of an enlarged portion of the tunnel system of FIG. 9 with parts broken away;  
         [0022]    [0022]FIG. 11 is a sectional view taken on the line  11 - 11  of FIG. 10 and looking in the direction of the arrows;  
         [0023]    [0023]FIG. 12 is a top view of an alternate embodiment of a portion of the tunnel system of FIGS. 8 and 9;  
         [0024]    [0024]FIG. 13 is a sectional view taken on the line  13 - 13  of FIG. 12 and looking in the direction of the arrows;  
         [0025]    [0025]FIG. 14 depicts an alternate embodiment of a portion of the tunnel system of FIGS. 8 and 9 in a top sectional view taken on the lines  14 - 14  of FIG. 15 and looking in the direction of the arrows; and  
         [0026]    [0026]FIG. 15 is a sectional view taken on the line  15 - 15  of FIG. 14 and looking in the direction of the arrows. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]    Referring now to FIGS. 1 and 2, shrink-wrap system  10  includes sealer system  11  mounted in-line with tunnel system  12 . Sealer system  11  comprises bench  15  on which a roll of two-ply folded shrink-wrap film  17  mounts. Shrink-wrap film  17  has two adjacent edges  14  and a folded edge  16 . Edges  14  and  16  extend longitudinally on opposite sides of shrink-wrap film  17 .  
         [0028]    Hinges  25  pivotally mount sealer bracket  27  on bench  15 . Sealer bracket  27  carries a pair of hot knives  29  via blade holders  28  and bolts  26 . Sealer bracket  27  positions hot knives  29  in an L-shaped configuration. Also located on bench  15  directly below hot knives  29  are L-shaped, non-stick pads  18  and  19 . The operating edges of hot knives  29  align with the longitudinal centerlines of pads  18  and  19  when bracket  27  pivots down. Pads  18  and  19  act as supports for film  17  during cutting and sealing operations with hot knives  29 . In many situations, resilient materials, e.g., Teflon-coated felt, may be suitable for fabricating pads  18  and  19 . In other situations, e.g., in automated, high-production applications, more rigid materials, e.g., Teflon-coated metals, would be more suitable. Mounting tolerances, however, would be more rigid in the latter instance.  
         [0029]    Conveyor  21 , which nests with pads  18  and  19 , has an upper surface that moves toward tunnel system  12  as indicated with arrow  23 . Conveyor  30 , which communicates with conveyor  21 , extends between sealer system  11  and tunnel system  12 . The upper surface of conveyor  30  moves on a downward slope from conveyor  21  in the direction of arrow  31 . The front face of bench  15  includes a conventional control  13 , which an operator uses to control various heating elements, conveyors, blower motors, etc.  
         [0030]    With reference to FIG. 1, tunnel system  12  includes bench  41  on which shrink oven  43  mounts. Shrink oven  43  contains tunnel  42  with respective entrance and exit ports  45  and  47 . Flexible, heat-insulating door flaps  51  hang down from the top of entrance and exit ports  45  and  47 . Conveyor  53  extends along the top of bench  41  through the floor area of tunnel  42 . The upper surface of conveyor  53  moves in the direction of arrow  55 , i.e., away from sealer system  11 . The interior of shrink oven  43  is described below in detail with respect to FIGS.  8 - 15 .  
         [0031]    FIGS.  3 A- 3 D illustrate successive stages in a sealing process as performed by sealer system  11  during packaging of item  60 . FIG. 3A shows an unrolled portion of shrink-wrap film  17  with item  60  located between the bottom and upper plies thereof. After placing item  60  between the plies of shrink-wrap film  17 , an operator lays the open free end of film  17  on pad  18 , as shown in FIG. 3B. Next, the operator pivots sealer bracket  27  (see FIGS. 1 and 2) down toward L-shaped pads  18  and  19  until hot knife  29  presses shrink-wrap film  17  against pad  18 . Hot knife  29  melts shrink-wrap film  17  along a fine line, causing the end of shrink-wrap film  17  to separate from the remaining roll. An operator usually discards the separated end as scrap. The applied heat causes the layers of film to fuse at their edges, forming thin sealed edge  61  (see FIG. 3C). The operator next returns sealer bracket  27  to its up position (see FIG. 1).  
         [0032]    At this point, the operator further unrolls shrink-wrap film  17  until item  60  nests between pads  18  and  19  (see FIG. 3C). The operator again lowers sealer bracket  27  until knives  29  compress shrink-wrap film  17  against pads  18  and  19 . Again, knives  29  melt, cut, fuse and seal shrink-wrap film  17 , this time along L-shaped edges  62  and  63  (see FIG. 3D). At this point, shrink-wrap film  17  has essentially encapsulated item  60  within a sealed package, designated here with reference numeral  70 . This last cutting operation also leaves the free end of shrink-wrap film  17  with new sealed edge  61 .  
         [0033]    The operator next rests package  70  on conveyor  21 . The operator activates conveyor  21 , via control  13 , causing package  70  to move onto conveyor  30 , which in turn transports package  70  to conveyor  53 . Package  70  pushes flexible flap  51  up as conveyor  53  moves package  70  into tunnel  42 . In a manner described below in detail, heating elements maintain the temperature of tunnel  42  at an appropriate operating level. The heat in tunnel  42  causes the plastic film to shrink and form a tough, tightfitting, sealed package, i.e., a “shrink-wrap package,” which exits tunnel  42  at exit port  47 .  
         [0034]    Referring now to FIGS.  4 - 6 , hot knife  29  includes blade  80  having opposed operating edges  82  coated with Teflon or other non-stick material. Surface heater  84  mounts on blade  80 . Surface heater  84  includes a first electrical insulating layer  85  fixed directly to the surface of blade  80 . Heater element  86  mounts on insulating layer  85  and extends over a substantial length of hot knife  29  following a serpentine or winding path. Although a serpentine shape is preferred, heater element  86  may have different shapes, including a linear shape. Insulating layer  85  electrically insulates heater element  86  from blade  80  while providing a suitable surface to which heater element  86  fixes.  
         [0035]    Second electrical insulating layer  81  joins to the outer surfaces of electrical insulating layer  85  and heater element  86 . Heater element  86  terminates at opposite ends in exposed electrical contacts  87 , which lie adjacent access holes  89  cut in blade  80 . As shown schematically in FIG. 4, control  13  provides electrical power to heater element  86  via contacts  87 . Blade  80  also includes mounting holes  88 . Bolts  26  attach hot knife  29  to blade holder  28  via mounting holes  88  (see FIG. 2).  
         [0036]    Fabricators may construct hot knife  29  using a variety of conventional processes, including thick-film deposition and thin-film deposition techniques. As one example, hot knife  29  may be fabricated as follows: a machinable ceramic is cut to form blade  80  having edges coated with Teflon; glass is fused to one of the flat sides of the ceramic blade to form first insulating layer  85 ; a conventional thick-film carbon based material is deposited on first insulating layer  85  to form heater element  86 ; and glass is fused to layer  85  and heater element  86  to form second insulating layer  81 . As a second example, hot knife  29  may be fabricated as follows: a metal, such as steel, tungsten or the like is machined to form blade  80  with Teflon coated edges; a layer of silicon rubber that fixes to the metal of blade  80  forms first insulating layer  85 ; heater element  86  is fabricated from a carbon based material, which fixes to first insulating layer  85 ; the silicon rubber material is again used to fabricate second insulating layer  81 .  
         [0037]    U.S. Pat. No. 6,037,574 issued Mar. 14, 2000 to Lanham et al describes a quartz substrate heater fabricated with thick-film and thin-film deposition processes that are suitable for fabricating surface heaters in accordance with the present teachings. Since surface heater  84  mounts directly on blade  80 , the thermal path from the heat source, i.e., from heater element  86 , to the point of application, i.e., to edge  82  of hot knife  29 , is greatly shortened. As a result of this shortened thermal path, melting, cutting, fusing, sealing, etc. of plastic films, such as polyolfins, polyvinyl chloride, other vinyls, and the like can be performed very efficiently and effectively. As such, the wattage required, in comparison to conventional heater designs, is considerably reduced. In addition, required cut and/or seal temperatures can be more accurately controlled, which can result in a marked increase in seal quality and per cycle seal rates for a given film material. Further, the close physical relationship between heater element  86  and edge  82  permits an operator to more accurately control performance through the application of a wide range of voltages, frequencies and polarities via control  13 . Thus, results that are more consistent are attainable in a wide range of materials not necessarily limited to packaging films of polyolfins, polyvinyl chloride and other vinyls. For instance, hot knives fabricated with the present techniques can find a variety of applications, including the manufacture of cauterizing surgical scalpels.  
         [0038]    While operating edges  82  of blade  80  in FIG. 6 taper to sharp pointed edges  82 , FIGS.  7 A- 7 C, which depicts alternative hot knives  29 A,  29 B and  29 C, show that other shapes are contemplated. FIG. 7A shows blade  80 A having rounded blunt edges  82 A. FIG. 7B shows blade  80 B having extra sharp edges  82 B formed from concave tapered cuts. FIG. 7C shows blade  80 C having extra blunt edges  82 C formed from concave tapers that terminate in flat surfaces.  
         [0039]    [0039]FIGS. 6 and 7A- 7 C show hot knives  29  and  29 A- 29 C, each having double edges with the same shape. Since only one edge of a hot knife operates at a time, the operator may reverse the edges when the one in use wears out. It is also contemplated that the double edges  82  of a given hot knife  29  may have different shapes for use during different applications. To change a worn-out edge  82  or reposition an edge with a different shape, a user inverts hot knife  29  by simply unbolting it from blade holder  28  and reattaching it with the edges interchanged.  
         [0040]    Referring now to FIGS. 8 and 9, the major structures of tunnel system  12  are essentially symmetric about a vertical plane that includes line  9 - 9  of FIG. 8. Tunnel  42  includes upper partition  91 , and side walls  92  and  93 . FIG. 8 depicts a number of packages  70  being transported through tunnel  42  via conveyor  53 . Blower  94  mounts on upper partition  91  in chamber  97 , which communicates with vertical air ducts  95 . Air ducts  95  communicate with tunnel  42  via air intake grills  96  located in side walls  92  and  93  near the exit of tunnel  42 . While the drawings, in FIG. 9, show only one intake grill  96 , i.e., in side wall  92 , tunnel  42  also includes a similar intake grill (not shown) in its side wall  93  in a symmetrical position.  
         [0041]    Blower  94  blows air into chamber  99  via its air output port  98 . Vertical partitions  101  and  102 , upper tunnel partition  91 , and the top, upper side and front walls of shrink oven  43  define chamber  99 . Opposed slanted baffles  103  and  104  direct blowing air from chamber  99  into vertical air ducts  105 , which in turn direct air into tunnel  42  via grilled heaters  106 . Grilled heaters  106  sit on either side of tunnel  42  in side walls  92  and  93 . Although the drawings show heater grills  106  located only in side wall  92 , similar heater grills  106  (not shown) mount in a corresponding symmetrical location in side wall  93 . Arrows  90  indicate the direction of air flow in chambers  97  and  99 .  
         [0042]    [0042]FIGS. 10 and 11 depict a portion of one of the heater grills  106 . Heater grills  106  each include a rectangular rigid wall  107  that may be a part of a tunnel side wall, or may be individually fabricated and placed in appropriate openings cut in tunnel side walls  92  and  93 . Heater grills  106  each comprise an array of spaced fluted openings  108  in wall  107 . A parallel array of surface heaters  84  (see FIGS.  4 - 6 ) mount directly on the surface of walls  107 , with each heater mounted between a different row of openings  108 .  
         [0043]    The close proximity between heater grills  106  and tunnel  42  greatly shortens the thermal path between the heat source and the point of application of heat to packages  70 . Because of this shortened thermal path, heat shrinking plastic films, such as polyolfins, polyvinyl chloride, other vinyls, and the like can be performed efficiently and effectively. Again, the wattage required, in comparison to conventional oven designs, is considerably reduced.  
         [0044]    [0044]FIGS. 12 and 13 illustrate heat exchanger  110 , which is an alternate heat source suitable for use in place of or in combination with heater grill  106 . Heat exchanger  110  includes a plurality of elongated metal flanges  111  that join to the inside walls of air duct  105 ′, an alternate embodiment of air duct  105  (see FIG. 8). Surface heaters  84  mount on both sides of flanges  111 . Tunnel wall  92  and an outer vertical wall of shrink oven  43  that is parallel to tunnel wall  92  define the inner and outer walls of air duct  105 ′. At its top, air duct  105 ′ opens to chamber  99  (see FIG. 8). Tunnel wall  92  includes openings  112 , which form a grill that permits the inside of air duct  105 ′ to communicate with tunnel  42 . Arrows  115  illustrate the direction of flow from chamber  99  through heat exchanger  110  and into tunnel  42 .  
         [0045]    Another alternate embodiment comprises radial heat exchanger  120 , which forms modified air duct  105 ″. Radial flanges  121 , which extend from the inside surface of cylinder  122 , carry surface heaters  84  on both sides thereof. Cylinder  122  has a plurality of openings  127  that permit the interior of air duct  105 ″ to communicate with tunnel  42 . Arrows  125  indicate the direction of flow through heat exchanger  120 .  
         [0046]    Various modifications of the invention are contemplated. It is to be understood, therefore, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.