Patent Publication Number: US-6713010-B1

Title: System for stretch-wrapping

Description:
This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/IE99/00056 which has an International filing date of Jun. 21, 1999, which designated the United States of America. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a wrapping system for stretch-wrap plastic film and in particular linear low density polyethylene films. 
     2. Discussion of the Background 
     Stretch-wrap plastic film has been widely used for some time for wrapping and securing a load on a pallet for example. More recently the provision of a pre-stretched wrapping film has been proposed. The use of a pre-stretched wrapping film advantageously uses dramatically less material. The processes and apparatus for producing such a pre-stretched plastic film for wrapping have not however been entirely successful and such pre-stretched films have not gained widespread acceptance in use. 
     SUMMARY OF THE INVENTION 
     The present invention is directed towards providing an improved wrapping system for stretch-wrap plastic film which is simple and efficient to use. 
     According to the invention there is provided a wrapping system for stretch-wrap plastic film, comprising: 
     partially pre-stretching a plastics film by a desired amount, 
     relaxing the film, and 
     further stretching the partially pre-stretched film immediately prior to using the pre-stretched film for wrapping. 
     Advantageously the second stretching of the film immediately prior to application reactivates the memory of the plastic material causing shrinkage after application to firmly wrap and secure a load on a pallet for example. 
     In a preferred embodiment after relaxing the film, the film is wound on a core, the further stretching being carried out as the film is drawn from the core for use. 
     The ratio of the elongations of the film during the pre-stretching step and during the further stretching step is about 3:1. 
     In one embodiment of the invention the total stretching of the film is between 150% and 250%. Most preferably the total stretching of the film is about 200%. 
     Preferably the pre-stretching and relaxing elongates the plastics film by about 140-160% and the final stretching of the film immediately prior to application gives about a 15-20% further elongation of the pre-stretched film. It has been found that this controlled double stretching of the plastics film gives better results than the prior art in which the plastics film is fully stretched in one step. 
     In a preferred embodiment the pre-stretching of the plastics film is carried out in a number of steps. Ideally the pre-stretching is carried out in three steps giving in step  1  about 100% elongation, in step  2  about 30% elongation and in step  3  about 20% elongation. 
     In a further embodiment the film is stretched by feeding the film from a supply roll around two or more spaced-apart film stretching rollers, each roller rotating at a greater speed than the roll or roller immediately upstream to stretch the film between the rollers. 
     In a particularly preferred embodiment the pre-stretched film is loosely wound onto the core. This advantageously prevents collapse of the core due to the shrinkage in the film after winding. Preferably the film is loosely wound onto the core by feeding film off the final stretching roller at a speed greater than the speed at which the film is wound up on the core. 
     A further disadvantage with existing pre-stretched wrapping film systems is that when the film is pulled from the core for wrapping edge hang-ups tend to cut the film if it is drawn off the core at an angle which is often the case when wrapping a pallet or the like. Edge hang-ups are generated because the pre-stretched film is extremely thin and is not a problem generally with conventional stretch film. This problem can be overcome according to the invention by leading the film off the core whilst maintaining the line of take-off where the film parts from the core substantially parallel to the central rotational axis of the core. Thus the film is evenly drawn oft the core initially prior to application about the goods to be wrapped. 
     Preferably the film is drawn from the core around a take-off roller which rotatably engages against an outer surface of the core. This ensures that the film is evenly drawn from the core and it can be then applied at any desired angle onto the materials to be packaged downstream of the take-off roller. 
     In a preferred embodiment the final stretching of the film is carried out by leading the film from the core around a final stretch roller, the speed of the final stretch roller being greater than the speed of discharge of film from the core. 
     Conveniently the final stretch roller is drivably connected to the take-off roller through a gear drive which ensures rotation of the final roller at a desired speed greater than the take-off roller to achieve the desired final stretch of the film. Alternatively the final stretch roller may be geared to a rotational support for the core to achieve the same result. 
     In another aspect the invention provides a method for producing a partially pre-stretched plastics film for subsequent stretching and wrapping about a load. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly understood by the following description of some embodiments thereof, given by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic illustration of apparatus for partially pre-stretching plastics film according to the invention; 
     FIG. 2 is perspective view of a hand-held film applicator for applying a secondary stretch to the film immediately prior to wrapping an object with the film; 
     FIG. 3 is an elevational view of apparatus for producing pre-stretched film according to the invention; and 
     FIG. 4 is a plan view of the apparatus. 
    
    
     DISCUSSION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, a system for producing pre-stretched plastic wrapping film will be described. A plastic film  25  is produced in conventional fashion and wound up on a roll  10 . The roll  10  is rotatably mounted on a support  11  and film  25  is delivered through film stretching apparatus  12  to pre-stretch the film  25  by a desired amount prior to winding up the pre-stretched film  25  on a core  14 . 
     The film stretching apparatus  12  comprises a set of stretching rollers, in this case comprising three rollers, namely a first roller  15 , second roller  16  and third roller  17 . Each roller  15 , 16 , 17  has an associated idler roller  18  to guide the film about the roller  15 ,  16 ,  17  in frictional engagement with the roller  15 , 16 , 17 . The rollers  15 , 16 , 17  are driven at different speeds, the third roller  17  having a greater speed than the second roller  16  which in turn has a greater speed than the first roller  15 . Thus the first roller  15  applies a stretch of about 100% to the film, the second roller  16  further stretches the film by about 30% and the third roller  17  further stretches the film by about 20%. In this way the plastics film  25  is pre-stretched in three steps prior to winding up on the core  14 . 
     The film  25  is loosely wound up on the core  14  to allow for subsequent shrinkage of the film  25 . To achieve this a take-off roller  28  downstream of the stretching apparatus  12  rotates at a speed slightly less than the speed of the third stretching roller  17 . 
     For wrapping objects with the film  25  a pre-stretched film roll  30  is mounted on a hand-held dispenser  32  the roll  30  being rotatably mounted on a support  33  carried on a handle  34 . Also mounted on the handle  34  are a pair of rollers, namely a take-off roller  36  and a final stretch roller  38 . The rollers  36 , 38  are mounted between support brackets  39 . At one end of the rollers  36 , 38  the rollers  36 , 38  are interconnected by a gear drive  40  which ensures rotation of the final stretch roller  38  at a desired speed greater than the take-off roller  36  to achieve the desired final stretch of the film  25  as it is drawn from the roll  30 . 
     In use, an operative grips the handle  34 . Having attached a free end of the film  25  to the object to be wrapped the operative walks around the object drawing film  25  off the roll  30  and applying it around the object. As the film  25  is drawn from the roll  30  it is given the final stretch between the take off roller  36  and the final stretch roller  38  prior to application to the object to reactivate the memory of the plastic material so that the film will subsequently shrink onto the object securely wrapping and gripping the object. It will be noted that the film  25  is led from the roll  30  around the take-off roller  36  which engages against the roll  30  to maintain the line of take-off where the film  25  parts from the roll  30  substantially parallel to the central rotational axis of the roll  30 . Thus the film  35  is evenly drawn from the roll  30  without damage due to hang-ups at the edge of the roll  30 . 
     It will be appreciated that the system of the invention applies a desired optimum stretch to the film and it can be easily used by even an unskilled operative to wrap an object. 
     It will be appreciated also that while a hand-held dispenser is particularly convenient to use the system may be used with an automatic wrapping device if desired. 
     Referring now to FIGS. 3 and 4 there is shown apparatus for producing rolls of partially stretched film according to the invention, the apparatus being indicated generally by the reference numeral  50 . The apparatus  50  comprises a conventional extruder  51  and associated die assembly  52  for forming plastics film in a bubble configuration  53  as it is delivered up from the die  52 . A conventional collapsing device  54  collapses the bubble  53  which is then delivered through a pair of nip rollers  55  to form the flat film  25  which is then delivered to the stretching apparatus  12 . It will be noted that the bubble  53  is collapsed into essentially two overlapping sheets joined at their edges. Also, the temperature of the plastics material at the nip rollers is sufficiently hot to allow blocking or fusing together of said sheets to form a composite fused flat film  25 . The film  25  is then delivered through the stretching apparatus  12  in which the film is partially stretched by about 200% overall in three stages as previous described. Downstream of the stretching apparatus  12 , the partially stretched film is fed to a core winder  58  in which the partially stretched film is relaxed by about 25% before being wound up on a core as previously described. Two core winders  58  are shown in the drawing. The film  25  may be split longitudinally as shown in FIG. 4 at an outlet of the stretching apparatus  12  to form two narrower films  25  which are then wound up on the separate core winders  58  if desired. 
     It will be appreciated that any other method and apparatus may be employed for forming the flat plastics film which is subsequently delivered to the stepped stretching apparatus  12 . 
     The invention is not limited to the embodiments hereinbefore described which may be varied in both construction and detail within the scope of the appended claims.