Patent Publication Number: US-2009218732-A1

Title: System and method for edge heating of stretch film

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a system and method for edge heating of stretch film to a predetermined temperature for shrinking the sides of the film, smoothing out a cut edge of the film and/or thickening the edge. 
     2. Description of Related Art 
     Various processes for making stretch film with reinforced edges are known. U.S. Pat. No. 5,531,393 describes a stretch film having folded edges applied. The roll of film is prestretched along the length thereof and the edges are folded so as to strengthen the edges. The film can be rolled to minimize the piling up of the folded edges. 
     U.S. Pat. No. 6,592,699 describes a process for making stretch film having heat-sealed edges. A folding apparatus folds the edge portions of a film to form folded edges. The folded film is then passed through a bridle mechanism having two heated rolls. The heated rolls are heated to a temperature in the range of 150° F. to about 250° F. The heated rolls heat the entire face of the film. The hot stretch process performs a dual process of stretching the film and heat sealing the folded edges. 
     The above-described patents have the disadvantage that the edge thickness is determined by doubling of thickness of the film by folding over the edge thereby requiring extra material. It is desirable to provide a method for more precisely controlling the edge thickness and reducing the amount of material needed for forming a thicker edge on the film. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method and system for edge treatment of a film in which one or both edges of the film is heated to a temperature above a softening point of a DSC onset temperature. In one embodiment, the edges of film are heated to a temperature above the peak DSC onset temperature. A central portion of the film between the edges remains at a lower temperature. In one embodiment, an edge heat treatment apparatus having a heated channel is used to receive an edge portion of the film and heat the edge portion. 
     In one embodiment, the edge heat treatment apparatus is used before a pre-stretching of the film. A second heat treatment apparatus can be used after pre-stretching of the film. 
     The invention will be more fully described by reference to the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a system for edge heating of stretch film in accordance with the teaching of the present invention. 
         FIG. 2  is a schematic diagram of an edge heat treatment apparatus receiving an edge of film. 
         FIG. 3  is a graph of an example DSC curve. 
         FIG. 4  is a schematic diagram of a top plan view of the system for edge heating of a stretched film. 
         FIG. 5  is a schematic side view of the film after heat treatment with the edge heat treatment apparatus. 
         FIG. 6  is a schematic diagram of a rough edge of an edge portion of the film. 
         FIG. 7  is a schematic diagram after heating the rough edge with the edge heat treatment apparatus to provide a smooth edge. 
         FIG. 8  is a schematic diagram of an alternate embodiment of an edge heat treatment apparatus. 
         FIG. 9  is a schematic diagram of a system for edge heating of a stretch film including pre-stretching. 
         FIG. 10  is a schematic diagram of a system for edge heating of a stretch film including a blade. 
         FIG. 11  is a schematic diagram of a roll of film including a thickened edge. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in greater detail to a preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts. 
       FIG. 1  is a schematic diagram of a system for edge heating of stretch film  10  in accordance with the teaching of the present invention. Film  12  is received over roller  14 . For example, film  12  can be a stretchable plastic film manufactured by the blown film or as cast film process. Film  12  can be formed of one or more layers and/or blends having any known compositions such as, for example, polyethylene, linear low density polyethylene, very low density polyethylene, polyvinyl chloride, ethylene vinyl acetate, ethylene methyl acetate, ethylene methyl acrylate, metallocene linear low density polyethylene, high density polyethylene, polypropylene, enable metallocene polyethylene, and ethylene copolymers with high alpha olefins. 
     Film  12  is received in edge heat treatment apparatus  15 . In one embodiment, edge heat treatment apparatus  15  comprises heated channel  20  which receives edge portion  13  of film  12 , as shown in  FIG. 2 . Edge portion  13  is heated to a temperature above the onset point of the differential scanning calorimetry (DSC) and more preferably above the peak DSC temperature. An example DSC curve for a stretch film is shown in  FIG. 3 . For example, film  12  can be heated to a temperature in the range of about 120° C. to about 140° C. Film  12  can shrink upon reaching the peak DSC temperature for releasing internal stress in film  12 . 
     Heating film  12  with edge heat treatment apparatus  15  can increase the thickness of edge portion  13 . Referring to  FIG. 4 , upon heating edge portion  13 , with edge heat treatment apparatus  15 , width W 2  is reduced from width W 1  before edge heat treatment apparatus  15  for shrinking film  12 . For example, width of film  12  can be reduced in an amount in the range of about 1/32 to about ¼ inches. The reduction in width of film  12  results in an increase in edge thickness. For example after heating edge portion  13 , has a thickness T 1  which is greater than a thickness T 2  of central portion  16  of film  12 , as shown in  FIG. 5 . For example, the edge thickness T 1  can be increased by an amount in the range of about 10% to about 40%. After edge heat treatment apparatus  15 , film  12  is received under roller  17 , as shown in  FIG. 1 . 
     In one embodiment, edge portion  13  can have a rough edge  30  produced by slitting or cutting with a blade, as shown in  FIG. 6 . Upon heating rough edge  30  with edge heat treatment apparatus  15 , rough edge  30  is smoothed out to form smooth edge  32 , as shown in  FIG. 7 . 
     In an alternate embodiment, edge heat treatment apparatus  45  is mounted over roller  46 , as shown in  FIG. 8 . Edge heat treatment apparatus  45  has a curved shape matching the shape of roller  46 . Edge heat treatment apparatus heats edge portion  13  of film  12 . Film  12  is received under roller  47 . Film  12  proceeds to edge heat treatment apparatus  45 . After edge heat treatment apparatus  45 , film  12  is received under roller  48 . 
       FIG. 9  is a schematic diagram of a system for edge heating of stretched film  50 . System for edge heating of stretched film  50  includes pre-stretch apparatus  52 . Pre-stretch apparatus  52  includes a plurality of rollers  53 . Film  12  is received in edge heat treatment apparatus  55   a  after being rolled with roller  56 . Edge portion  13  of film  12  is heated by edge heat treatment apparatus  55   a  before being received in pre-stretch apparatus  52  for providing a stronger edge of film  12  while film  12  is proceeding through pre-stretch apparatus  52 , thereby reducing tearing and allowing a greater amount of pre-stretch. 
     Blade  57  is used to cut film  12  for making two film webs, as shown in  FIG. 10 . Referring to  FIG. 9 , film  12  is received in edge heat treatment apparatus  55   b  after pre-stretch apparatus  52 . Edge portion  13  of film  12  is heated by edge heat treatment apparatus  55   b  after pre-stretch apparatus  52  for increasing the strength of edge portion  13 . Film  12  is rolled by roller  59  before being wound on roll  60 . 
     In one embodiment, film  12  comprises multi-webs of film. Multi-web film  14  is received in edge heat treatment apparatus  15 , edge heat treatment apparatus  45  or edge heat treatment apparatus  55 . The multi-web of film can be slit with blade  57 . The cut film is received in edge heat treatment apparatus  15 , edge heat treatment apparatus  45  or edge heat treatment apparatus  55 . 
     In one embodiment, film  12  comprises double wound sheeting. In an alternate embodiment, tubing  60  is used instead of film  12 . Tubing  60  is received in edge heat treatment apparatus  15 , edge heat treatment apparatus  45  or edge heat treatment apparatus  55 . 
       FIG. 11  is a schematic diagram of roll  100  of film  12  including edge portion  13  having increased thickness. 
     It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments, which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.