This invention relates to a method and an apparatus for transversely stretching a web or film of plastic material. Such transversal stretching accorplishes either or both of the following purposes:

• 1) obtaining a pre-stretched film suitable for shrinking onto an enveloped product;
• 2) permanently extending the width of a plastic film. Plastic films or foils are usually stretched either longitudinally or transversely, in order to improve their ^quali- ties, to reduce the thickness, to impart desired properties along a chosen direction, etc. Two known stretching techniques are commonly employed, i.e. a longitudinal stretching and a transversal stretching. Longitudinal stretching is particularly suitable to be carried out by means of a continuous process wherein the plastic web is driven between a group of rolls rotating at different speeds.

Transversal stretching is not easy to be obtained in a continuous process since it requires that the moving edges of the film are gripped, laterally extended and then released. To this purpose several apparatuses have been proposed that only differ for the modalities and the members employed for progressively gripping, stretching and releasing-the edges of the moving web. A known apparatus of this kind is provided, on both sides, of a series of pliers movable along an endless conveyor and forming an angle with the foil advancement direction, these pliers being able to engage the foil edges for only a length of the path and then releasing them after the film has been spread out to the desired width. It is evident that such devices are rather complex, raise the costs of the apparatus and furthermore they are noisy, difficult to be properly adjusted and have mechanical limits for the operating speed.

There has also been proposed a system wherein relatively large pulleys are provided, engaging the film between the pulley groove and a belt or rope, but with unsatisfactory results.

It is therefore an object of the present invention to provide for a method and an apparatus for transversely stretching a film of plastic material whereby the web edges are gripped and temporarily held by simple but effective means without any discontinuity and in a smooth way.

It is a further object of the invention to realize an apparatus whereby the width of a given film web can be increased through a transversal stretching of the continuously running film.

These and further features of the invention, as well as the associated advantages, will become evident from the following description of a preferred non-limiting embodiment illustrated in the attached drawings in which:

• FIGURE 1 is a schematic front view of the apparatus employing two diverging assemblies according to the invention;
• FIGURE 2 is a side view of one of the assemblies, also illustrating the input and output rolls;
• FIGURE 3 and 4 show two details of the rolls mounted on the assemblies;
• FIGURE 5 Illustrates the film path between the rolls; and
• FIGURE 6 is a cross-section of the assembly along the line I-I of Fig. 1.

Fig. 1 schematically shows a front view of the apparatus for transversely stretching a web of plastic film 1 advancing along the direction shown by arrow A. The film unwinds from a supply roll and is successively wound onto another roll (not shown) passing through a group of incoming rolls 15 and a group of outcoming rolls 18 (only one for each group shown in Fig. 1). The incoming web has a width indicated by Li that is lesser than the width Lu of the outcoming film after the stretching. The gripping means for the stretching includes two assemblies 5 positioned at both sides of the web and forming an angle with the advancement direction of the film. More particularly the longitudinal axes of the assemblies form an angle a (adjustable according to the desired degree of stretching as will be illustrated later) with respect to the film axis.

The assemblies 5 are pivotally mounted by pins 31 on base members 37 movable along guide shafts 43 and the thraded rod 34 with respect to the main frame 30. Threaded red 34 can be rotated by motor means (not shown) for the lateral positioning of the assemblies 5 along guide rods 43 in order to fit the apparatus to the width of the incoming film. At their lower portion each assembly is provided with a rack sector 40 engaging with a pinion 44 rotated by a motor 48 to vary the divergence angle a of the assembly, i.e. the degree of stretching. Preferably the apparatus is erclosed in a tunnel 39 maintaining a desired temperature.

With reference to Fig. 2 there is illustrated a preferred structure of one of the diverging assemblies 5. Each assembly is formed by three elements 105 disposed one after the other and rigidly connected between each other. The assembly includes a common support member 2 carrying cylindrical rolls 6 having their axes parallel. Each element 105 includes a body or frame 3 carrying cylindrical rolls 8, of a smaller diameter, having their axes parallel to each other and to-the ones of rolls 6. Two rolls 8 are provided for each roll 6, the axes of rolls 8 being symmetrically positioned with respect to the axis of each roll 6. The assembly is pivotally mounted on base member 37 by means of a pin 31 so that the divergency angle a can be varied according to the desired degree of stretching. The frame members 3 are movable with respect to support member 2 in order to position the film edge between the rolls before starting the apparatus. Rolls 6 are driven at a constant speed thanks to known driving means, not shown in the drawings for sake of simplicity, whereas rolls 8 are idle rolls rotating about their axes thanks to the friction engagement with belts that will be described later.

In the figures th_f rolls are shown in the working configuration wherein rolls 8 are substantially tangential to rolls 6 and with their axes offset. Two endless belts 9 are fitted between the rolls and suitable roller return and tensioning-means comprise pulleys 115, 116, 117, the latter being mcvable to adjust the belt tension. The upper portions 130 of each element 105 receive the necessary supplies of fluid under pressure for the operations and of a heating fluid schematically shown with the reference numeral 140.

In operation the edges of foil 1 are driven between rolls 6 and 8,thanks to the friction engagement with belts 9, mainly at the tangency points, that are, as a matter of fact, tangency arcs. The belts are of an elastically deformable material able to retain its properties even at a rather high temperature and are compressed at the tangency points holding the foil edges against rolls 6 so that the film is compelled to advance. In correspondence of the upper arc Tl the foil is kept into contact with the surface of rolls 6 whereas in correspondence of the arc T2 a certain release is allowed between the belt and the foil (see Fig. 5).

From the above description it is evident that the edges of the foil are effectively held between the groups of rolls 6 and 8`so as to advance the plastic film and at the same time spreading the same toward the sides realizing a desired degree of transversal stretching.

Figs. 3 and 4 show one of the rolls 8 and a detail of the contact area between rolls 6 and 8, respectively. According to the preferred embodiment shown in the drawings, rolls 8 are provided with two circumferential grooves 11 partially housing belts 9 that have a circular or elliptical cross-section. As can be seen from Fig. 4 the distance between the axes of rolls 6 and 8 is not such as to cause a real contact between the rolls, but the gap is small enough that foil 1 is engaged between the rolls surfaces when the belts are compressed and squashed against tte rolls. More particularly the belt, than can be of neoprene, is subjected to a transversal deformation (as shown in Fig. 4) wedging itself in the gap between the rolls surfaces to an extent depending upon the force required to maintain the engagement. This way one obtains a progressive and smooth gripping preventing any breakage of the foil 1 that might otherwise happen due to the resistance opposed by the foil when the edges are spread apart. The system further allows for the regular passage of edge portions that are torn or damaged ensuring a much more effective gripping compared with that of the pliers employed in the art. As in the prior art the outer edges of the film are subsequently trimmed of about 10-15 cm on each side. Anyhow the prior art devices are generally unable to correctly stretch the film in presence of peripheral defects, often causing even the breakage of the film. In such cases all these inconveniences are overcome by the apparatus of the invention, thus improving the quality of the product and the veld.

There are provided two belts 9, both to evenly distribute the stresses and as a safety since in case a belt breaks the operation is not stopped. As an alternative, a single flat belt extending for a substantial length of the rolls can replace the illustrated rounded belts.

It is preferable, but not strictly required, that the rolls surfaces are smooth and still preferably the diverging assemblies are enclosed in a tunnel kept at a predetermined constant temperature in a known manner, but other heating means can be used.

According to the invention the plastic foil 1 is heated before reaching the assemblies 5 and to this purpose some of the advancing rolls 15 are heated. In some of the rolls a heated fluid is circulated, e.g. oil under pressure so that after passing the rolls 15 the temperature of foil 1 is substantially equal to the one required for the stretching operation (between 70°C and 150°C according to the materials and to the desired stretching features).

The foil is then applied to the diverging assemblies 5, the rolls 6 of which can also be heated by a heating fluid circulating inside them to maintain the film web at the desired terperature. On coming out from assemblies 5 the stretched film is carried by another group of driven rolls 18, substantially similar to those 15 having the purpose of cooling the stretched film. Preferably at least some of these rolls are supplied with a cooling fluid circulating inside them. The cooling is preferably a fast one in case the film is stretched to obtain a shrinkable foil, whereas such cooling is slower in case the foil has to definitively retain the transversal size. The diametersof rolls 15 and 18 are relatively small so that they can be positioned as near as possible to theassemblies 5 for a better control of the stretching conditions. Thus it is possible to avoid any undue cooling of the film when passing from rolls 15 to the rolls 6 of the assemblies. In certain applications rolls 6 need not to be heated since the film does not significatively cools travelling along the assemblies and the structure of the apparatus is much simplier. The diameters of rolls 6 and 8 depend upon several conditions and upon the desired stretching degree. The best results were obtained, as for the smoothness of the engagement, using rolls 6 having a diameter of 2-5 times that of rolls 8.

As stated above, the process is preferably carried out at a temperature ranging from 70 to 150°C and this is the temperature of rolls 15 and in case of rolls 6. Belts 9 are subjected to some wearing that is generally compensated by their elasticity and anyhow these parts are to be replaced after a certain number of working hours in view of their low cost and easy replacement.

The apparatus is quite advantageous not only for stretching foils of plastic materials that are later to be shrinked onto wrapped articles. By suitably choosing the working temperature it is possible to obtain a permanent enlargement of the film, i.e. a non shinkable web having a width larger than the original film. For exemple when a PVC film is used, if the stretching process is carried out at a temperature of 70-80°C one obtains a stretched film that substantially shrinks back to the original size when subsequentely heated. In case the stretching is carried out at a temperature of about 130-140°C the deformation is substantially permanent and the film does not appreciably shrinks even if later heated. This is a very important feature since the standard widths of plastic foils have commonly been in the order of 100-110 cm and the production machines were dimensioned to obtain (after trimming the outer edges) a foil having such a width. More recently there has been a request for wider foils, in the order of 130-140 cm and the newer machines have been built for producing web of this measure,-or. in case a measure that is double of the standard and will be longitudinally cut to form two films. The old machines would therefore become obsolete for a numler of applications since it is not possible to replace the rolls with longer ones. By using the apparatus of the present invention, it is possible to permanently stretch, i.e. enlarge, a 110 cm foil produced by an oli machine up to a width of 140 cm without resorting to a new machine, of course giving up to the shrinking properties. This use of the apparatus is commercially very interesting since it allows for a reconditioning of machines already installed enlarging their field of use.

The apparatus is noiseless due to the absence of jerks movement, reliable even on ruined film edges and exhibits a high operating speed without any difficult adjustement or tuning.

Although the invention has been disclosed with reference to a preferred embodiment, it is not to be limited to it. The scope of the invention extends to all the obvious changes and/or modifications that will be evident to the one skilled of the art and that anyhow provide for the transversal stretching of a moving plastic web by means of two groups of co-operating rolls and at least an elastically deformable belt for the engagement with each edge of web to be transversely stretched.