Abstract:
A stretch film for the packing of goods, particularly for the packing of goods stacked on a pallet, comprises a prestretched main film ( 10 ). The main film ( 10 ) is reinforced by reinforcement strips ( 12 ) extending in the longitudinal direction. The reinforcement strips ( 12 ) are made from film strips of prestretched film which have been folded a plurality of times in the longitudinal direction. Between two adjacent reinforcement strips ( 12 ), the main film ( 10 ) can comprise two mutually staggered rows of holes ( 16,18 ). This stretch film with perforations ( 14 ) is particularly useful for the packing of food items.

Description:
BACKGROUND 
     The invention relates to a stretch film and method for producing a stretch film. 
     Stretch films are produced by prestretching a film. Prestretching the film will increase the strength of the film. Further, the prestretching generates a memory effect so that the film upon further stretching will remember its previous condition, i.e. the film will contract. Stretch films are used for packing goods for shipment. Particularly, stretch films are utilized for tying together goods on a pallet or the like. For this purpose, the goods are stacked on a pallet, and a stretch film is wrapped completely around them to thus hold the goods safely on the pallet. 
     Due to different tensions in the film while being wrapped around the goods and due to sharp edges of the goods, such stretch films can easily tear at the edges. Because of the inherent tension in the film, this will cause the film sheet to tear off, so that the goods have to be wrapped again. For reducing the danger that the outer edges of the stretch film might tear, it is known from EP 0 728 102 to fold the two outer edges of the longitudinal film inwards. Also in such a stretch film, sharp edges or projecting edges of a carton may tear holes in the film. Because of the tension in the longitudinal direction of the film, these holes will enlarge along the whole width of the film so that a safe hold of the goods on a pallet will not be guaranteed anymore. 
     SUMMARY 
     It is an object of the invention to provide a stretch film and a method for producing a stretch film wherein the danger of tearing is reduced. 
     The stretch film according to the invention comprises a prestretched main film and reinforcement strips extending in the longitudinal direction of the main film. As provided by the invention, the reinforcement strips comprise film strips of prestretched film which are folded in the longitudinal direction. The main film has a plurality of reinforcement strips connected thereto in the longitudinal direction of the main film. The distances between the reinforcement strips can be selected in dependence from the number of the reinforcement strips and the width of the film. Even a tearing of the film between two reinforcement strips will thus cause merely a small hole which cannot grow until extending along the whole width of the film. 
     Since the reinforcement strips provided as folded film strips, the reinforcement of the main film can be determined by the number of the folds. The film strips are folded preferably twice, and particularly four times, in the longitudinal direction. 
     With usual film widths of 400 to 500 mm, preferably at least three mutually parallel reinforcement strips are connected to the main film. By providing at least three reinforcement strips made from film strips that have been folded at least four times in the longitudinal direction, the holding force of the stretch film can be increased at least five times. This advantageous particularly when packing heavy goods. 
     In a particularly preferred embodiment of the stretch film, the main film comprises perforations. The main film can be perforated before or after being connected to the reinforcement strip. Preferably, the perforations are formed after the application of the reinforcement strips because this will reduce the danger of a tearing of the stretch film during perforation. Such perforated stretch films with reinforcement strips will reliably provide for an interchange of air and thus are suited particularly for the packing of food items. 
     To increase the strength of the film and to guarantee a safe connection of the reinforcement strips with the main film, the main film is perforated preferably only in the regions between the reinforcement strips. 
     To decrease the danger that even the perforated main film might tear, it can be provided, in addition to the reinforcement strips, that the main film comprises a plurality of longitudinal rows of holes which are staggered relative to each other. Thus, a large strength of the stretch film is provided along with a good venting of the packed goods. 
     In the inventive method for producing the stretch film provided with the reinforcement strips, film strips are supplied which are made from a first prestretched film. Thereafter, reinforcement strips are produced by folding these film strips at least once in the longitudinal direction. Then, a second prestretched main film is supplied, and the reinforcement strips are connected to the main film in such a manner that the reinforcement strips will extend in parallel to each other in the longitudinal direction of the main film. 
     Producing the reinforcement strips through the folding of film strips offers the particular advantage that the main film and the film strips can consist of the same film and that a considerable increase of the strength is achieved by the folding. Using the same film is further advantageous in that the connection between the reinforcement strips and the main film can be performed in a simple manner. 
     Preferably, the film strips are produced by cutting a prestretched film. Thus, two identical film rolls can be used for producing the stiffened stretch film, one of the films serving as a main film while the reinforcement strips are produced from the other film. 
     In a preferably preferred variant of the method, the main film will be perforated. The perforation of the main film can be performed before or after connecting the main film to the reinforcement strip. Preferably, the perforation is carried out after the connection with the reinforcement strips. Because of the perforations in the main film, the stretch film of the invention is particularly useful for the packing of food items. 
     Preferably, only the regions of the main film between the reinforcement strips are perforated. Thus, the main film will not be weakened in the regions where the reinforcement strips are connected to the main film. On the other hand, especially in case of very small holes, it is possible and advantageous under the aspect of production technology to perforate the main film prior to the connection with the reinforcement strips. 
     Preferably, the perforation of the main film is performed to the effect that two respective reinforcement strips have a plurality of rows of holes arranged therebetween which extend in the longitudinal direction of the main film. These rows of holes are positioned in a mutually staggered arrangement to thus increase the strength of the stretch film. Such a perforation will improve the venting of the packed goods, particularly in case of food items. If particularly strong and tear-resistant stretch films are required, only one row of holes is provided between the reinforcement strips. A further increase of the strength of the stretch film with continued reliable venting can be achieved by providing holes only between each second pair of reinforcement strips so that the regions between the reinforcement strips are alternately perforated and non-perforated. 
     The perforating of the film can be performed e.g. by punching the holes. Particularly in thin films, it is possible to carry out the perforation by exposure to heat. For this purpose, use can be made e.g. of heated protrusions which have a temperature above the melting point of the film, e.g. preferably above 200° C. and particularly above 250° C. Such heated protrusions, which preferably comprise aluminum, can be arranged on a roller so that the perforating of the film is carried out during the transport of the film over that roller. Preferably, immediately after perforating the film by application of heat, a cooling by air is performed. The formation of the holes by melting the film is further of advantage in that the melting of the film will generate reinforced edges on the holes. Thus, tearing of the film on the holes is prevented. 
     To connect the reinforcement strips to the main film, at least one of the films can include additives. By use of such additives, the connection between the two films, i.e. the main film and the reinforcement strips, is reinforced. 
     Preferably, the main film and the reinforcement strips have different temperatures so that the films will adhere to each other already due to the differences in temperature and will enter a sufficiently strong connection. 
     Of particular advantage is the connecting of the reinforcement strips and the main film by application of heat. In this case, heat is supplied to at least one roller among a plurality of pressure rollers provided to press the reinforcement strips and the main film together. To avoid an impairment of the characteristics of the stretch film, the supplied heat must not be so high that the two films melt to each other or become welded to each other. 
     The films used are preferably films produced by the sheet blowing method or flat-molded films. The film material can be selected from a group comprising polyethylene, polyvinylchloride, ethylene-vinyl-acetate, ethylene-methylacetate and polyethylene. 
     To allow for a processing of film lengths as large as possible, the device for producing a reinforced, preferably perforated stretch film can be combined with a device for stretching the film. The device for reinforcing and perforating the film is then arranged directly downstream of the device for stretching the film. Winding up the stretched film prior to reinforcing and perforating the same is not required in this case. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in greater detail hereunder in connection with a preferred embodiment with reference to the accompanying drawings. 
         FIG. 1  is a schematic plan view of a stretch film according to the invention, 
         FIGS. 2   a  and  2   b  show a schematic view of a device for producing the stretch film of the invention, 
         FIG. 3   a  is a schematic sectional view of a folded film strip, taken along the line I in  FIG. 2   a,    
         FIG. 3   b  is a schematic sectional view of a folded film strip, taken along the line II in  FIG. 2   a,    
         FIG. 3   c  is a schematic sectional view of a folded film strip, taken along the line III in  FIG. 2   a , and 
         FIG. 3   d  is a schematic sectional view of a folded film strip, taken along the line IV in  FIG. 2   a.    
     
    
    
     DETAILED DESCRIPTION 
     The stretch film comprises a main film  10  which is provided as a prestretched film. A plurality of reinforcement strips  12  are connected to the main film  10  in the longitudinal direction thereof. The reinforcement strips  12  comprise film strips  34  ( FIG. 2   a ) folded in the longitudinal direction and likewise made from prestretched film. Provided between respectively two adjacent reinforcement strips  12  are two rows of holes  16 , 18  comprising a plurality of holes  14 . The holes  14  of the row of holes  16  are staggered relative to the holes  14  of the row of holes  18  by respectively half the distance between adjacent holes  14  of a row of holes  16 , 18 . The holes  14  of the row of holes  16  are thus positioned to fill the gaps relative to the holes  14  of the row of holes  18 . 
     The reinforcement strips  12  comprise the same material as the main film  10 . To achieve a stronger reinforcement, the film for generating the reinforcement strips  12  is folded a plurality of times so that the reinforcement strips  12  comprise a plurality of layers. Preferably, prior to being cut into individual strips to be folded, the film from which the reinforcement strips  12  are produced has the same width as the main film  10 . Thus, use can be made of identical films both for the main film  10  and for the production of the reinforcement strips  12 . 
     For producing the reinforcement strips  12  ( FIG. 2   a ), a first prestretched film sheet  22  is wound off a supply roll  20 . The film sheet is supplied via a guide roller  24  to a cutting unit  26 . 
     The cutting unit comprises cutting blades  30  arranged on a holding support  28 . By the transport of the film sheet  22  in the direction of the arrow  32 , the film sheet  22  is cut into individual film strips  34  by the cutting blades  30 . 
     The film strips  34  are supplied, via a guide roller  36 , to a curve roller  38 . Curve roller  38  comprises curve elements  42  arranged for rotation on a shaft  40 . By the curve elements  42  and the tension in the film strips  34 , the outer longitudinal edges of the film strips  34  are folded upwards in the Figure so that each film sheet  34  is substantially U-shaped in cross section ( FIG. 3   a ). 
     Thereafter, the film sheets  34  being U-shaped in cross section are supplied to a folding unit  44 . The folding unit  44  comprises shaping rods  46 . The shaping rods  46  are oriented transverse to the moving direction of the film strips and arranged at an inclination. For setting the width of the folded regions, the shaping rods can be adjusted transversely the moving direction  32  of the film strips. During the transport of the film strips  34  in the moving direction  32 , the outer legs  48  ( FIG. 3   a ) of the U-shaped film strips  34  are folded inwards. Thus, the film sheet  34  will assume the cross section shown in  FIG. 3   b.    
     In the next step, the film strips  34  are guided around a guide roller in such a manner that the outer legs  48  of the film strips  34  point in the direction of the guide roller  50 . Thereby, the outer legs  48  are pressed onto the region of film strip  34  arranged between the legs  48  ( FIG. 3   c ). 
     The subsequent folding unit  52 , also provided with shaping rods  54 , will again fold the film sheets  34  in the longitudinal direction and thus lend them the shape illustrated in  FIG. 3   d . Thereafter, film strip  34  is pressed together in a pressing unit, thus generating reinforcement strips  12  folded four times. The pressing unit  56  comprises two pressure rollers  58  arranged to press against each other with the required force. In this manner, trapped air will be pressed out from the reinforcement strips  12  and the individual film layers will be connected to each other. 
     From pressing unit  56 , the film strips  12  are guided via guide rollers  60  into that region of the device where the reinforcement strips  12  are to be connected to the main film  10  ( FIG. 2   b ). 
     As a main film  10 , a second prestretched film is wound off a supply roll  62  and guided via a guide roller  64  to a pressing unit  66 . The reinforcement strips  12  are also supplied to the pressing unit  66  while moving in parallel to the longitudinal direction of main film  10  and respectively in parallel to each other. Similar to pressing unit  56 , the pressing unit  66  comprises two rollers  68  arranged to press against each other with a predetermined force. The connection of the reinforcement strips  12  to the main film  10  is performed in that either the reinforcement strips  12  and/or the main film  10  includes an additive. Further, the pressing rollers  68  of the pressing units  66  can be heated so that the reinforcement strips  12  and the main film  10  will be connected to each other under the influence of heat. In this regard, care must be taken that the temperature of the pressing rollers  68  does not cause a melting of the reinforcement strips  12  or the main film  10 . 
     In the next step, perforating rollers  70 , 72  will be operated to form holes into the main film  10  between the reinforcement strips  12 . For this purpose, perforating roller  72  is provided with cylindrical protrusions  74  distributed about its periphery. Since the illustrated embodiment is shown as a stretch film comprising three reinforcement strips  12  and consequently two rows of holes, perforating roller  72  is provided with two rows of cylindrical protrusions  74 . The corresponding perforating roller  70  is formed with cylindrical recesses  76  corresponding to the cylindrical protrusions  74 . The cylindrical recesses  76  are likewise arranged in two rows. One or both of the two perforating rollers  70 , 72  are heated. By the heating, the holes  14  are molded into main film  10 . The holes are thus not generated by punching so that no film waste is generated. 
     The perforating roller  70  can also be omitted. In this case, the perforating is performed solely by means of the perforating roller  72  with the protrusions  74 . Because of the tension in the film, the film only slightly deviates from the perforating roller  72 , thus adhering sufficiently close to the perforating roller  72  or the protrusions  74  for having the perforations formed therein. 
     Apart from circular holes  14 , any desired other shape can be selected for the holes. Particularly advantageous is an oval or elliptic hole shape having its longer semiaxis extending transversely to the longitudinal direction of the film sheet so that the shorter semiaxis is oriented in parallel to the tension direction prevailing in the stretch films. 
     After forming the holes  14  in the main film  10 , the film sheet is deflected a plurality of times via guide rollers  78 . In this manner, there is provided a cooling path in which the film sheet will be cooled again sufficiently to keep it from becoming bonded to itself when being wound up afterwards. After the cooling path, the film sheet wound up to form a roll  80 . Roll  80  moves vertically to the transport direction of the film sheet in the direction of the arrow  82 , i.e. in a horizontal direction. This will safeguard that the reinforcement strips  12  when being wound onto roll  80  will not be arranged completely above each other. By the reciprocating movement of the wind-up roll  80 , the roll has a substantially identical thickness throughout its width. Further, to guarantee a uniform winding onto roll  80 , a press-on roller  84  is provided.