GB892490A - Method and apparatus for stretching films of thermoplastic materials - Google Patents

Abstract

A continuously advancing film of thermoplastic organic material is simultaneously stretched in the longitudinal and the transverse directions while it is being heated, in the absence of <PICT:0892490/IV (a)/1> <PICT:0892490/IV (a)/2> <PICT:0892490/IV (a)/3> <PICT:0892490/IV (a)/4> <PICT:0892490/IV (a)/5> any prior longitudinal stretching without transverse stretching, the edges of the film being each provided with a bead or thickened edge to facilitate gripping thereof during stretching. The invention is particularly applicable to films of high molecular weight highly crystalline polymers, especially polypropylene. The operation may be carried out in an apparatus comprising two groups of rollers each comprising two or more rollers for advancing the film in a longitudinal direction, the peripheral speed of the rollers in the group through which the film first passes being lower than that of the rollers in the group through which the film subsequently passes and the ratio of the speeds corresponding to the desired longitudinal stretching ratio, guide means located along each side of the film, adapted to engage the edges of the film and being disposed at an angle to one another so that the film during its passage through the apparatus undergoes simultaneous longitudinal and transverse stretching, and means for heating the film during this stretching. The guide means may be straight with the transverse stretching increasing at a constant rate during the whole of the travel of the film between the two groups of rollers, or may be curved so that transverse stretching takes place during only part of the travel of the film between the two groups of rollers, the heating means being located solely over that part of the apparatus where such transverse stretching takes place. As shown in Figs. 5 and 6, the apparatus comprises a pair of feed rolls A, A1, a pair of draw rolls B, B1 and two fixed rails P (Fig. 7) which include an angle a (Fig. 6) and support driving means for the pairs of rollers C, C1 which engage the edges of the film. The upper roller C1 is an idler roller and the lower roller C is driven at a speed which increases with successive pairs so that the speed of the film gradually increases during its travel from the feed to the draw rolls. Each driving means or power transmission gear (Fig. 7) comprises a box S, a sprocket wheel I, a bevel gear H, a device V for adjusting the pressure exerted on the film edge by the rollers C, C1 and a nut D for fixing the box S to the support P. The sprockets I are driven by two endless chains K lying in a horizontal plane and driven by sprockets T. The gear boxes S are rotatable about their vertical axes a-a and be fixed in any given position by the nut D in order to dispose the axes of the roller couples at right-angles to the direction of the resultant of the longitudinal and transverse rates of stretch of the film. As shown in Fig. 11, the guides for transversely stretching the film each comprise two superposed longitudinal channel members L, L1 in which the film clamping rollers C, C1 are not rotated by a positive drive means but are free to rotate in links M of a driven endless chain by means of which the rollers C, C1 are caused to travel along the guides. The links M are connected to one another by pivots P which are provided with rollers Pr to reduce friction between the chain and the guide channels. The speed of the chain is intermediate between the peripheral speeds of the feed and draw rolls. Films of highly crystalline polymers have their mechanical properties improved in both the longitudinal and transverse directions when stretched according to the invention.