The present invention relates to an improved method of producing extensible films made of polyethylene especially for the palletization of products which must transpire, such as fruit and vegetable products.

More particularly the invention relates to an improvement to the method disclosed in the Patent Application No. RM98A000531, filed on August 7,1998 in the name of the same Applicant.

The above Patent Application in the name of the same Applicant discloses a method and the relative apparatus capable of perforating a film of polyethylene on a continuous basis by cutting the same so as to provide a kind of"tongues"or edges that are bound to the film by a certain length.

According to such known method, this is carried out by passing the polyethylene film between a roller provided with punches and a counterroller with a surface of elastic material so that the film is punched, as desired, without removing material.

Even if high production rates of the order of 250-300 m/min are reached, such method has not negligible drawbacks due to the fact that the counterroller of elastic material undergoes a remarkable wear so that it should be replaced at most every two running hours.

This is due to the fact that, if the elastic material is too hard, its useful life is kept and the notchings in the film are not regular enough, while if the counterroller is coated with soft elastic material, the processing is perfect but the wear of the same cannot be accepted.

The invention seeks to provide a method and an apparatus that allow a polyethylene film provided with C-or U-shaped notchings to be accomplished without removing material, thus overcoming the problems and the drawbacks discussed above.

This is achieved by a method capable of providing C- or U-shaped notchings, i. e. not closed like a loop, in a polyethylene film by passing the latter, after being tensioned, about a roller provided with cutting members and rotating at a tangential speed which is different from the speed of the film so as to provide a relative movement between roller and film causing the latter to be partially torn by the cutting members of the roller.

A better understanding of the invention will result from the following detailed description with reference to the accompanying drawings that show schematically some preferred embodiments thereof only by way of a not limiting example.

In the drawings: Fig. 1 shows the winding direction of the film after

the end of the processing; Fig. 2 is a perspective view of the curved notchings in the film; Fig. 3 is a detail of one of the saw teeth; Fig. 4 shows the processing for stiffening the film; and Fig. 5 shows schematically the apparatus for carrying out the method of the invention.

With reference to the figures and as discussed above, the invention provides several curved notchings 2 in film 1 of extensible material such as polyethylene.

Such notchings 2 are then characterized by a"tongue" or edge bound to film 1 by a certain length so that the tongue can keep at a distance from the surface of the film.

In order to provide such notchings, the method described causes the film to pass over the teeth 4 arranged on the surface of a roller 3 and provided with a serrated portion 5 arranged along an arc seen in radial direction.

As clearly shown by the figures, the arched indentation 5 has its convexity oriented to the rotation direction of roller 3.

According to a peculiar feature of the invention, the tangential speed of teeth 4 of roller 3 is lower than

the traverse speed of film 1 which is in contact with the teeth. Thus, there is provided a relative movement causing film 1 to be partially torn by teeth 4 so that the tearings forming the notchings 2 are obtained.

Such roller 3 rotates in a direction which is in accordance with the feed of film 1.

Fig. 5 is a diagram of the apparatus carrying out the method according to the invention. Film 1 is unrolled from reel B, passes over a rubber roller 6 by which it is stretched and over a tension roller 7 by which it is kept taut in the correct position with respect to the next, already described serrated roller 3. After having undergone controlled tearings causing the desired notchings 2, film 1 is pressed by pressure rollers 8 and 9 and wound on a storage reel 5.

According to a peculiar feature of the disclosed method, before being pressed by pressure roller 8 and 9 the film is passed across a narrowing which causes a plastic deformation in the plane of the film serving as continuous longitudinal stiffening. Such narrowing is preferably formed by two pulleys 11 that rotate in the plane of film 1.

In order to further stiffen film 1 without adding further material, according to the invention there are provided two further assemblies of three wheels 10 causing a further longitudinal curliness 12 in the central area of the film. Each wheel assembly is formed by a pair of wheels opposed to a larger wheel, all of the wheels being inclined with respect to the plane of the film. The pair of wheels is arranged on

one side of the film and the opposite wheel is arranged on the other side of the film so that the film is pressed tightly between the large wheel and the two smaller wheels, thus causing a small plastic deformation.

A further peculiar feature of the invention consists in that the arrangement of the teeth or punches 4 on roller 3 is such that there is only one tooth in contact with film 1 at any moment. Thus, any tearing of the film which could be caused in case several teeth engage the film in the same area is avoided.

In order to achieve such result, each saw tooth 4 is arranged on a different generating line and is machined so that indentation 5 is tangential to the side surface of one ideal cylinder which is coaxial with roller 3.

At last, it should be noted that in order to change the width of the holes 2 during the processing, it is sufficient to change the difference between the traverse speed of film 1 and the tangential speed of roller 3 and teeth 4.

In conclusion, it should be noted that the arrangement of the teeth and the pattern of the whole apparatus are such as to provide notchings 2 with an edge hanging down and directed with its free portion to the feed direction of film 1.

This is particularly useful during the actual use of the film processed according to the invention. In fact, the film is unrolled from the reel to a direction such that it is the fixed portion of each

tongue that is earlier removed from the reel, thus avoiding any break of the film. The break could be caused by the accidental adherence of the free ends of the edges to the reel in case the film has been wound in the opposite direction with respect to the direction indicated by the figures.

The present invention has been described and illustrated according to a preferred embodiment thereof, however, it should be understood that those skilled in the art can make equivalent modifications and/or replacements without departing from the scope of the present industrial invention.