Abstract:
Rolls of pre-stretched cling film are used to manually wrap and unitize bulky loads such as boxes on a pallet. Using film that has previously been plastically stretched simplifies the wrapping operation. Unfortunately, however, the pre-stretched rolls of film have edges which are prone to fail during use. The disclosed invention concerns rolls of pre-stretched cling wrap having improved edges that are less prone to fail, and the method and apparatus for producing such rolls.

Description:
FIELD OF THE INVENTION  
         [0001]    The current invention concerns a method, apparatus, and product comprising a pre-stretched thermoplastic film having improved edges.  
         BACKGROUND OF THE INVENTION  
         [0002]    The use of thermoplastic stretch or cling films has become a commonplace method of securing bulky load such as pallets filled with boxes. Generally, there are two types of film and wrapping methods, one referred to as machine wrap, the other as hand wrap. Both types of these films are typically made largely from the various polyethylene resins, and maybe made as single or multilayer products. Frequently an additive known as a tackifier or cling agent, e.g. polyisobutylene, will also be added to ensure that adjacent layers of film will adhere to each other.  
           [0003]    Machine wrap films are typically used in powered pallet wrap machines, e.g. see U.S. Pat. No. 5,040,356 for an example. On the other hand, hand wrap utilizes an operator to stretch the film manually around the load. As the efficient use of the wrap requires that it be plastically stretched around the load, various devices, e.g. U.S. Pat. Nos. 4,600,163 and 4,752,045, have been devised to help the operator in this task. More recently, however, various methods of pre-stretching the film have been devised, e.g. U.S. Pat. No. 5,458,841 and WO89/06594, which will simplify application of the wrap to the load. With these pre-stretched hand wrap films, the operator merely wraps the film around the load without plastically stretching the film. As the elastic memory of the film recovers, the film contracts about the load.  
           [0004]    One problem, however, with these pre-stretched hand wraps is that the edges of the film are subject to damage resulting in tearing of the film during use. Typically, the edges of the pre-stretched hand wrap film have been prepared by transversely slitting individual roll widths of film from a wider width by means of a conventional sharp edge slitter assembly. The film edges so prepared are anything but uniform, and will contain numerous small defects such as rips and tears. These rips and tears, plus the relatively limited ductility remaining in the heavily pre-stretched material leads to failure during application. One method of reinforcing the edges of the pre-stretched film has been devised, folding the edges of the material to form a hem, e.g. see U.S. Pat. Nos. 5,565,222 and 5,531,393. These hems, however, cause difficulties winding a uniform roll of film due to the essentially doubled thickness of the hemmed edge as compared to the remainder of the film. U.S. Pat. No. 5,531,393 discloses axially oscillating the roll during winding to minimize piling up of the hemmed edges on the roll.  
         OBJECTS OF THE INVENTION  
         [0005]    It is an object of the present invention to provide a novel roll of pre-stretched hand wrap film having edges resistant to mechanical damage that results in tearing of the film during application to the load. Additionally, the film will have edges of uniform thickness that will simplify winding of uniform rolls.  
           [0006]    A further object of the present invention is to provide a new method and apparatus for manufacturing pre-stretched hand wrap film having improved edges. A still further objective of the present invention is to maximize the useful width of pre-stretched hand wrap film manufactured by longitudinally slitting a greater width of stretch film into individual narrower rolls by avoiding the necessity of folding over or hemming the edges of the individual rolls to prevent tearing when applied to a load.  
         SUMMARY OF THE INVENTION  
         [0007]    According to the present invention, a roll of pre-stretched hand wrap film having improved edges is produced by longitudinally slitting and annealing a previously stretched width of thermoplastic stretch wrap either by thermally slitting or by slitting and applying heat and then winding. By thermally slitting or slitting with the application of heat, the edges of the resulting narrower rolls are thermally annealed, resulting in edges of greater ductility and tear resistance than the remainder of the width of the film.  
           [0008]    According to another aspect of the invention, there is a method for producing rolls of pre-stretched stretch wrap comprising the steps of directing a moving web of previously stretched film to a slitting means, slitting the web longitudinally to produce one or more individual webs having a narrower width dimension than the original web, and thermally annealing the slit edges of the web. In the preferred method, there will be an additional step of winding the slit and edge annealed web to produce the roll of improved wrap.  
           [0009]    The apparatus comprises a guide means arranged largely transversely to the direction of motion of the previously stretched film to support the film and direct it into contact with a slitting blade and annealing means. Preferably, the guide means constitutes a guiding and support surface over which the film passes, said guiding and support surface having apertures into which a heated blade assembly may pass to contact, sever and anneal the film edges.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 shows a roll of pre-stretched film, including a microscopic view of a slit edge of film that has been slit by a conventional sharp edged blade and wound, all in accordance with the prior art.  
         [0011]    [0011]FIG. 2 shows a roll of film, including a microscopic view of a slit edge of film, that has been slit by a thermal slitting and annealing blade and then wound, all according to the present invention.  
         [0012]    [0012]FIG. 3 is a plan view of a thermal slitting arrangement in accordance with the present invention.  
         [0013]    [0013]FIG. 4 is an elevation view of a thermal slitting arrangement in accordance with the present invention.  
         [0014]    [0014]FIG. 5 is a drawing of the thermal slitting and annealing blade used in the apparatus shown in FIGS. 3 and 4.  
         [0015]    [0015]FIG. 6 is an elevation view of a thermal slitting and annealing blade according to another embodiment of the present invention.  
         [0016]    [0016]FIG. 7 show a further embodiment of the present invention in which the slitting and annealing means comprise separate means.  
         [0017]    [0017]FIG. 8 is a detail on the construction of the annealing means of FIG. 6. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    [0018]FIG. 1 shows a roll of film  10 , including a microscopic view of a slit edge  13  and hemline  12  of a roll  10  of pre-stretched film produced according to prior art methods. As shown, the roll  10 , which was wound on core  15 , has tapered or conical ends  11  resulting from axial oscillation of the roll during winding. Additionally, the microscopic view shows the fold  12  for the hem, and the actual slit edge of the film  13 . There are numerous defects  14  that can serve as initiation points for failure during use.  
         [0019]    [0019]FIG. 2 shows a roll of pre-stretched film  55 , including a microscopic view of an edge  20 ′, of pre-stretched film  20  produced by slitting and annealing with a heater blade in accordance with the present invention. As shown, the roll has remarkably square edges  56 , as there was no need to axially oscillate the roll during winding to distribute hemmed material. Additionally, thermally slit and annealed edge  20 ′ is essentially uniform and defect fee.  
         [0020]    [0020]FIG. 3. and FIG. 4 show the apparatus for producing an improved film roll in accordance with the present invention. A previously stretched film web  20  is directed into slitting and annealing station  30  over guiding means  25  such as conventional idler rolls, stretcher rolls or bowed rolls. Slit and annealed individual webs,  20   a  through  20   e  in FIG. 3 are discharged from the slitting and annealing station  30 , over bowed roll  40  to winder  50  or winders  50   a  through  50   e  where the webs are wound into individual rolls. Additionally, narrow strips of material  20   z , which formed the original edges of web  20 , are either discarded or recycled.  
         [0021]    As shown, slitting and annealing station  30  comprises two main assemblies, a transversely oriented guide and support surface  31  and slitting and annealing knife assembly  33 . Guide and support surface  31  preferably has a convex surface over which pre-stretched web  20  passes and one or more openings  32  in support surface  31 . Additionally, there will be at least one heated slitting and annealing blade  33 . One blade  33  is required for each longitudinal slit. As shown in FIG. 3 and FIG. 4, there are six heated slitting and annealing blades as pre-stretched web  20  is to be subdivided into five final webs,  20   a  through  20   e  and two edge trims  20   z . Active surface  35  of heated slitting and annealing blade  33  is brought into contact with web  20  and further recessed into openings  32  of support surface  31 . As web  20  contacts and passes heated slitting and annealing blade  34 , blade  34  will heat and cut through web  20  producing two edges,  20 ′ and  20 ″, as well as heat affected zones, also referred to as annealed zones  21 ′ and  21 ″ adjacent to the slit edge  20 ′ and  20 ″ respectively. As is well understood to those familiar with the web handling art, heated slitting and annealing blades  33  will be mounted to a support structure, preferably one that will permit blades  33  to be moved from an inactive position to an active position in contact with web  20 . In the interest of clarity, however, the support structure is not shown in the Figures.  
         [0022]    Also as shown in FIG. 3 and FIG. 4, a bowed roll or other web spreading device  40  is adjacent to the discharge side  39  of slitting and annealing station  30 . Bowed roll  40  is used to separate the slit edges  20 ′ and  20 ″ of web  20  to ensure that slit edges  20 ′ and  20 ″ do not stick together in case of contact prior to complete cooling. Following slitting, the now individual webs  20   a  through  20   e  are directed to one or more winding machines  50 . As will be understood to those familiar with winding technology, many different types of winding devices may be used. As shown in FIG. 3 and FIG. 4, however, individual staggered winders  50   a  through  50   e  are shown. With this arrangement, each individual slit and annealed web  20   a  through  20   e  will be wound on its own respective winder  50   a  through  50   e.    
         [0023]    [0023]FIG. 5 shows a closer view of slitting and annealing blade  33 . As previously indicated, slitting and annealing blade has an active surface  35  that contacts web  20 . Contact of web  20  with active surface  35  produces slit edges  20 ′ and  20 ″ as well as annealed zones  21 ′ and  21 ″ adjacent slit edges  20 ′ and  20 ″ respectively. Body portion  34  of blade  33  contains one or more cavities  36  containing electrical cartridge heaters  37  that are used to heat blade  33  to its operating temperature. Operating temperature will depend upon material thickness and the speed at which web  20  passes blade  33 . Preferably blade  33  will also contain a temperature measuring device such as thermocouple  38 , the output of which will be directed to a temperature control device, as is well known to those versed in the art.  
         [0024]    An alternate type of heated slitting and annealing station  30 ′ is shown in FIG. 6. In this instance the transverse guide means  31  takes the form of an upstream idler roll  31   a  and downstream idler roll  31   b . Web  20  passes over idler rolls  31   a  and  31   b , and is contacted by heated slitting and annealing blade  33  in the space between rolls  31   a  and  31   b . This blade is manufactured from an electrically resistive material such as Chromel-A, and is directly heated by passing electrical current through the blade. The blade has an active edge  35 ′ which will contact web  20 , to produce slit edges  20 ′ and  20 ″ along with the associated annealed zones  21 ′ and  21 ″. Temperature of blade  33 ′ is controlled by varying the current flowing through the blade  33 ′. Current in turn is controlled by supplying power to blade  33 ′ from a variable supply such as a Variac.  
         [0025]    [0025]FIG. 7 shows another configuration of a slitting and annealing device useful in the practice of the current invention. As shown, the device consists of a transverse web guiding and support device shown generally as  31 ″, a separate annealing device  70 , and a slitting device  33 ″. In operation, web  20  passes over the first roll  31   a ′ of the web guiding and support device  31 ″, where a narrow annealed zone  21  is produced in the web  20  by annealing device  70 . Annealing device  70  preferably takes the form of an arc shaped shoe which closely conforms to the outside diameter of roll  31   a ′ and which discharges heated air onto web  20 . Alternately, annealing device  70  could be an arc shaped shoe closely conforming to the roll that is heated and which would heat web  20  by radiation or convection. Web  20  then encounters slitter blade  33 ′″ which forms a longitudinal separation  21  of web  20 . Slitter blade  33 ′″ is aligned with annealing device  70  so that the longitudinal separation is centered within annealed band of web  20 . Slitter blade  33 ′″ may be any device typically used for longitudinally slitting a web, including a sharpened blade  33 ′″ as shown in FIG. 7. Heated blades of the types shown in FIG. 3 and in FIG. 4 as  30  or if FIG. 5 as  30 ″ may also be used. After slitter  33 ′″, web  20  passes over a second roll  31   b ″, and then to winders  50 .  
         [0026]    [0026]FIG. 8 shows the preferred form of annealing device  70 . As shown in FIG. 8, annealing device  70  will have a hollow body  71  having an air inlet  72 , a heating chamber  73 , and a heated air distribution chamber  74 , which will apply heated air to web  20 . Heating chamber  73  will contain one or more cartridge heaters  75  to heat the flowing from inlet  72  to distribution chamber  74 . As previously indicated, heated air distribution chamber  74  is arc shaped to conform closely to roll  31   a ′. Additionally, distribution chamber  74  has a porous wall  76  that faces web  20 . The actual discharge of heated air from the distribution chamber  74  to web  20  is through porous wall  76 . Web  20  is thoroughly heated by annealing device  70 , which anneals a narrow band  21  of web  20 .  
         [0027]    As can be seen in FIG. 3, the improved rolls of pre-stretched cling film of the present invention are produced as follows. The pre-stretched film web  20  is brought from a source of supply into the slitting and annealing unit over a series of rolls or other appropriate web guide devices. The source for web  20  may be a master roll of pre-stretched and wound film, or it may be the discharge from a pre-stretching unit such as a machine direction orientation device or a cold pre-stretcher of the types identified in U.S. Pat. No. 5,458,841 or WO89/06594. Within the slitting and annealing unit  30  the film is brought across the surface of the transverse film guide surface  31  and contacted by heated slitting and annealing knife  33 .  
         [0028]    Under the influence of the heated blade  33 , the film will be separated longitudinally. Additionally, immediately adjacent to the longitudinal separation the web will have seen sufficient heat to anneal, i.e. reverse the effects of the prior stretching, the film. The optimum temperature at which to operate the heated slitting and annealing knife  33  will depend upon the material and thickness of web  20  and the speed at which it passes through the slitting and annealing unit  30 . At the very least, however, the temperature of the slitting and annealing knife  33  must exceed the melting temperature of web  20 . For the types of materials from which these pre-stretched cling films are prepared, melting temperature are frequently on the order of 120° C.  
         [0029]    The now longitudinally slit web  20  leaves the transverse guide surface  31  over discharge bowed roll  40 . Bowed roll  40  is used to separate the slit edges  20 ′and  20 ″ to prevent the possibility of these edges sticking together in case of contact with each other while sill tacky. After the bowed roll the individual webs  20   a  through  20   e  are led over a series of nip rolls and idler rolls to one or more winders  50 . As is well known to those involved with the web handling and winding arts, there are many different ways in which the separate webs may be wound. As shown in FIG. 3 and FIG. 4, however, each web goes to a separate winder. Other possibilities include all webs going to individual cores on a single winder shaft, or alternate webs being wound on cores on alternate shafts, a process referred to as offset winding.  
         [0030]    It will be understood by those generally familiar with handling plastic webs that the particular embodiments of the invention disclosed in this document are illustrative and are in no way meant to limit the invention. Numerous changes and modifications may be made and the full use of equivalents made without departing from the spirit or scope of the invention as defined in the following claims.