Abstract:
A shrink tunnel assembly for wrapping an article with polymeric shrink film includes a shrink tunnel having a heating zone, and conveyor means for carrying an article to the heating zone and away from it. While in the heating zone, polymeric shrink film covering an article avoids contacting any solid surface, thereby protecting the article from marring and destruction.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the field of packaging products in heat shrinkable film, and more particularly to shrink tunnels for applying such film to product packages.  
         BACKGROUND OF THE INVENTION  
         [0002]    A shrink tunnel is a device for supplying heated air to an enclosed chamber for the purpose of shrinking a polymeric film around a package for physical protection and enhanced appearance. Shrink tunnels in commercial use today generally include a mechanical conveyer for carrying packages through the chamber.  
           [0003]    One weakness of prior art shrink wrap methods is that conveyor belts are generally made of a metal mesh or silicone rollers that become so hot that some heat sensitive packages are marred or destroyed. Also, the weight of the product package pressing against the shrink film will prevent the shrink film from shrinking evenly around the package. The portion of the product lying on the conveyor will hold the shrink film and its weight will prevent the film from shrinking as quickly and uniformly as in areas not affected by the conveyor.  
           [0004]    Another disadvantage of existing shrink wrap tunnels is that they are built to handle a large variety of product sizes. Therefore, when running small packages the tunnel is operating with a large volume of unneeded hot air. This represents wasted energy.  
           [0005]    A principal objective of the present invention is to provide a shrink tunnel assembly that avoids damaging or marring the appearance of film covered packages.  
           [0006]    A related objective of the invention is to provide a shrink tunnel assembly that operates more efficiently by reducing the volume of wasted hot air. This reduces energy costs for the tunnel operation, associated costs for conditioning the operating environment and, in an embodiment having no moving parts, reduces noise pollution and maintenance.  
           [0007]    Additional objectives and advantages of my invention will become apparent to persons skilled in the art from the following detailed description of a preferred embodiment.  
         SUMMARY OF THE INVENTION  
         [0008]    In accordance with the present invention there is provided a shrink tunnel assembly for shrink wrapping articles with a polymeric shrink film. Some examples of the types of articles commercially wrapped in shrink film include pharmaceutical packages, gift-wrap, window shades, door molding, and food products. Polymeric films used for shrink-wrapping generally contain at least one polymer selected from polyvinyl chloride, polyethylene, and styrene-butadiene copolymers.  
           [0009]    The shrink tunnel assembly of the invention includes a shrink tunnel including a heating zone, means for conveying heated air into the heating zone, a first conveyor means for conveying film covered articles to the heating zone, and a second conveyor means for carrying film covered articles away from the heating zone. The shrink tunnel preferably has an exterior wall, an interior wall spaced inwardly of the exterior wall, and insulating material adjacent the exterior wall.  
           [0010]    The interior wall of the shrink tunnel defines a chamber having an inlet opening, an outlet opening spaced downstream from the inlet opening, and a heating zone between the inlet opening and the outlet opening. The heating zone includes an entrance side and an exit side spaced downstream from the entrance side. The interior wall preferably also includes a side opening for conveying heated air into the heating zone, thereby to heat the polymeric shrink film to an elevated temperature. The film is preferably heated to a temperature of about 121-177° C. (250-350° .F) in the heating zone.  
           [0011]    The means for conveying heated air into the heating zone includes an electric or gas powered heater, and an electric blower or compressed air capable of blowing heated air into the heating zone at a velocity greater than 2000 feet per minute, preferably greater than 3000 feet per minute.  
           [0012]    The first and second conveyor means preferably each include a conveyor belt mounted on pulleys, both powered by an electric motor. The conveyor belt may have either steel belting, rollers, or a polymeric upper surface for conveying articles. The nature of the article to be wrapped determines the type of conveyor to be used.  
           [0013]    An important feature of the present invention is that in the heating zone, polymeric film covering the articles does not contact any solid surface. In the particularly preferred embodiment shown and described herein, this objective of the invention is accomplished by locating the first and second conveyor means outside the heating zone. In one embodiment, first conveyor is spaced from the second conveyor by a distance d 1 , and articles heated in the heating zone each have a length d 2  that is greater than d 1 . The articles thereby avoid dropping down into a gap between the conveyor belts.  
           [0014]    In the shrink tunnel assembly of the invention, convection heating of the shrink film results in the film being brought up to an appropriate temperature before any substantial heating of the article takes place. Any heating of the merchandise that does occur results from heat conduction through the plastic film. Plastics like polyvinyl chloride, polyethylene, and styrene-butadiene copolymers used in shrink-wrapping are generally poor conductors of heat. Accordingly heating of the merchandise is minimized. 
       
    
    
     BRIEF OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a perspective view of a preferred shrink tunnel assembly of the invention.  
         [0016]    [0016]FIG. 2 is a cross-sectional view taken along the lines  2 - 2  of FIG. 1.  
         [0017]    [0017]FIG. 3 is a cross-sectional view taken along the lines  3 - 3  of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]    Referring to FIG. 1 there is shown a shrink tunnel assembly  10  of the present invention. The assembly  10  includes a shrink tunnel  11  having a metal interior wall  12 , a metal exterior wall  13 , and insulating material  14  outside the exterior wall  13 . The interior wall  12  defines an elongated, generally cylindrical chamber  15  having an inlet opening  17  and an outlet opening  18  spaced downstream from the inlet opening  17 . A first support stand  19  supports the tunnel  11  adjacent the inlet opening  17  and a second support stand  20  supports the tunnel  11  adjacent the outlet opening  18 .  
         [0019]    The assembly  10  also includes an electric heater  25  and compressed air or an electric fan  26  for blowing hot air into a side duct  27  extending through the tunnel exterior wall  13  and communicating with the chamber  15 . A series of holes around the perimeter of the interior wall  12  allows heat to enter the chamber  15  and completely the article  30  to be heated. One suitable electric heater  25  for practice of the present invention is a 36 kw heater sold under the trademark “Sureheat 36,000” by Osram Sylvania. The electric fan  26  or compressed air is capable of blowing heated air into the chamber  15  at a velocity of at least 3000 fpm.  
         [0020]    Referring now to FIGS. 2 and 3, articles  30  are delivered through an inlet opening  31  of the shrink tunnel  11  by a first conveyor or conveyor means  32  having a steel mesh belt  33  rotating around spindles  34 ,  35 . An outside spindle  34  is rotated by an electric motor (not shown) and the belt  33  rotates passively around the inside spindle  35 . The articles  30  are covered by a shrink film  38  having a thickness of about 0.5 to 3 mils. A polyvinyl chloride (PVC) shrink film is particularly preferred but other plastic films will work.  
         [0021]    Articles  30  are carried on the first conveyor  32  to a heating zone  40  where the shrink film  38  is heated to a temperature of approximately 300° F. (149° C.) by hot air flowing through a side duct  27  extending through a side opening  39  in the tunnel walls  12 ,  13 . Heating the film  38  softens it so that the film  38  shrinks into close proximity with the article  30 . The heating zone  40  is bounded by an entrance side  41  near the first conveyor  32  and an exit side  42  spaced downstream of the entrance side  41 , near a second conveyor  47 . The first conveyor  32  and the second conveyor  47  are both located outside the heating zone  40 . Accordingly, articles  30  do not contact a solid surface located inside the heating zone  40 . The second conveyor  47  carries articles  30  away from an outlet opening  48  spaced downstream from the inlet opening  31 .  
         [0022]    As shown in FIG. 2, the first conveyor  32  is spaced from the second conveyor  47  by a distance d i , and packaged articles  30  carried by the conveyors  32 ,  47  have a length d 2  that is greater than d 1 . If desired, a fixed support (not shown) may be placed in the space  40  below the articles  30  to prevent them from falling downwardly between the conveyors  32 ,  47 .  
         [0023]    In an alternative embodiment of the invention not shown in the drawings, the first conveyor  32  is placed at a higher elevation than the second conveyor  47  so that articles  30  fall downwardly through the heating zone  40 . In this alternative embodiment, sizes of the articles  30  are less critical than when the conveyors  32 ,  47  are at approximately the same elevation. Another variation includes a vertically oriented shrink tunnel together with a horizontally mounted first conveyor  32  and second conveyor  47 . In this variation articles  30  fall from the first conveyor  32  through the shrink tunnel  11  to the second conveyor  47 . This arrangement allows for very short articles  30  to pass through the shrink tunnel  11 .  
         [0024]    The foregoing description of my invention has been made with reference to some preferred embodiments. Persons skilled in the art understand that numerous changes and modifications can be made without departing from the spirit and scope of the aims.