Shrink-wrap packaging system

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

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] FIG. 1 is a pictorial view of a shrink-wrap packaging system in accordance with the present invention;

[0013] FIG. 2 is a pictorial view of an enlarged break-away section of the shrink-wrap packaging system of FIG. 1;

[0014] FIGS. 3A-3D 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] FIG. 4 is a front elevation of a hot knife constructed in accordance with the present invention;

[0016] FIG. 5 is an enlarged front elevation of the hot knife of FIG. 4 with parts broken away;

[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. 7A-7C are enlarged side views of alternate embodiments of the hot knife shown in FIG. 5;

[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] 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] FIG. 10 is an elevation of an enlarged portion of the tunnel system of FIG. 9 with parts broken away;

[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] FIG. 12 is a top view of an alternate embodiment of a portion of the tunnel system of FIGS. 8 and 9;

[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] 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] 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. 3A-3D 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. 7A-7C, which depicts alternative hot knives 29A, 29B and 29C, show that other shapes are contemplated. FIG. 7A shows blade 80A having rounded blunt edges 82A. FIG. 7B shows blade 80B having extra sharp edges 82B formed from concave tapered cuts. FIG. 7C shows blade 80C having extra blunt edges 82C formed from concave tapers that terminate in flat surfaces.

[0039] FIGS. 6 and 7A-7C show hot knives 29 and 29A-29C, 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] 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] 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.

Claims

1. A hot knife comprising: a blade having an extended surface and an operative edge; an electrical heater element fixed to said extended surface adjacent said edge; and an outer layer of electrical insulation covering said heater element.

2. The hot knife of claim 1, wherein said heater element includes an inner layer of electrical insulation fixed directly to said extended surface and an electrical resistive film fixed to said inner layer and extending adjacent said edge.

3. The hot knife of claim 2, wherein said electrical resistive film has a pair of electrical contacts with a serpentine strip of resistive material stretching between said contacts.

4. The hot knife of claim 3, wherein said blade is ceramic, said inner and outer layers of insulation are glass, and said serpentine strip of resistive material is deposited as a thick-film resistor on said inner layer.

5. The hot knife of claim 3, wherein said blade is metal and said inner and outer layers of insulation are rubber.

6. The hot knife of claim 3, wherein said edge tapers to a sharp edge coated with a non-stick material.

7. The hot knife of claim 3, wherein said edge tapers to a blunt surface coated with a non-stick material.

8. The hot knife of claim 3, wherein said hot knife further includes a second operative edge spaced from said first-mentioned operative edge, and with said electrical heater element located between said first-mentioned edge and said second edge to form a double-edged hot knife.

9. An electrical oven for heating items comprising: a tunnel having an exit port, an entrance port and opposed side walls; conveyor means mounted in said tunnel between said side walls for transporting items from said entrance port to said exit port; a heat exchanger mounted adjacent said side walls and extending between said entrance and exit ports on either side of said conveyor means, said heat exchanger including an extended permeable surface with a plurality of electrical surface heater elements fixed thereto; and an air circulating system mounted adjacent said tunnel, said air circulating system including a blower having a blower input and a blower output, an intake ductwork communicating with said blower input and said tunnel adjacent said exit port, and an output ductwork communicating with said blower output and said tunnel via said heat exchanger.

10. The electrical oven of claim 9, wherein said heater elements each include an inner layer of electrical insulation fixed directly to said extended permeable surface and an electrical resistive film fixed to said inner layer.

11. The electrical oven of claim 10, wherein said electrical resistive film has a pair of electrical contacts with a serpentine strip of resistive material stretching between said contacts.

12. The electrical oven of claim 11, wherein said permeable surface is metal, each of said heater elements is covered with an outer layer of electrical insulation, and said inner and outer layers of insulation are rubber.

13. The electrical oven of claim 11, wherein said heat exchanger includes a plurality of parallel flanges projecting into said output ductwork and said electrical surface heater elements mount on said flanges.

14. The electrical oven of claim 13, wherein said heat exchanger includes a plurality of radial flanges projecting into said output ductwork and said electrical surface heater elements mount on said flanges.

15. A shrink-wrap system for packaging items comprising: a supply of shrink-wrap film; a sealer system having a hot-knife means for encapsulating items in said film, said hot-knife means includes at least one elongated blade with an operative edge, a first electrical heater element fixed to said blade and an outer layer of insulation covering said heater element; and an electrical oven system having a tunnel, and a heat exchanger having at least one extended permeable surface mounted adjacent said tunnel and a plurality of electrical surface heater elements mounted on said extended permeable surface.

16. The shrink-wrap system of claim 15, wherein said first electrical heater element includes a first layer of electrical insulation fixed directly to said blade and a first electrical resistive film fixed to said first layer and extending adjacent said edge.

17. The shrink-wrap system of claim 16, wherein said plurality of electrical surface heater elements each include a second layer of electrical insulation fixed directly to said extended permeable surface and a second electrical resistive film fixed to said second layer.

18. The shrink-wrap system of claim 16, wherein said first electrical resistive film and said second electrical resistive films each has a pair of electrical contacts with a serpentine strip of resistive material stretching between said contacts.

19. The electrical oven of claim 18, wherein said heat exchanger includes a plurality of parallel flanges with a plurality of said second electrical surface heater elements mounted thereon.

20. The electrical oven of claim 18, wherein said heat exchanger includes a plurality of radial flanges with a plurality of said electrical surface heater elements mounted thereon.