GB1244887A - Improvements in or relating to a method and device for biaxially stretching preformed thermoplastic films - Google Patents

Abstract

1,244,887. Biaxially stretching preformed thermoplastic films KADEE N.V. 24 Dec., 1968 [30 Dec., 1967], No. 61289/68. Heading B5B. An apparatus for biaxially stretching a web of pre-formed film (consisting of a synthetic organic polymeric thermoplastic material such as soft polyvinyl chloride) in a continuous manner as shown in plan view in Fig. 1 and in side elevation in Fig. 2, comprises a pair of feed rolls 2 and 2<SP>1</SP> of which 2 is a driven roll and 2<SP>1</SP> a press roll and a pair of draw rolls 3 and 3<SP>1</SP>, of which 3 is a roll driven at a higher peripheral speed than roll 2 and 3<SP>1</SP> is a press roll, whereby the film web is stretched longitudinally. The difference between the peripheral speeds of the driven rolls 2 and 3 is controlled by an arrangement comprising a binary control device R and two revolution counters T 2 and T 3 which measure the speed of rotation of the rolls 2 and 3 respectively. The signals of the counters T 2 and T 3 are subtracted in the control device R and compared with a preset value. The output signal E, which is a function of the extent to which the difference between the signals of the counters differs from the preset value, controls the drive of the roll 3. The means for stretching the film web transversely comprises a pair of endless travelling belts (one of which is indicated by the reference numeral 6) diverging in the direction of travel of the film web. The endless belts travel as guide and drive rolls 4 and 5 and are provided with pins 7 (Fig. 3) adapted to penetrate the edge portions of the film web. The driving member of the roll 4 is coupled with that of roll 2 so that the peripheral speeds of these rolls are the same. Similarly, the peripheral speeds of rolls 5 and 3 are the same. As shown in Fig. 3, the belts each comprise a plurality of metal plates or links 8 interconnected by helical springs 9 and guided in the slot defined between two longitudinal guide members 10, 10<SP>1</SP>. The pins 7 are fastened to the plates 8. The guide for each endless belt is pivotable about a fixed vertical shaft, one of which is indicated in Fig. 1 by the reference numeral 14. By displacing the point of attachment 13 of one end of a spring 12, the other end of which is fastened to the guide 10, 10<SP>1</SP> near the end of the guide remote from the pivot 14, the divergence of the two endless belts and hence the degree of transverse stretching of the film web 1 can be adjusted. The distance of the point 13 from the members 10, 10<SP>1</SP> is adjustable, whereby the tension of the spring 12 and hence the transverse tension applied to the film web 1 may be preset. Micro-switches 11, 11<SP>1</SP> are both mounted on one of the two pairs of guide members 10 and 10<SP>1</SP> and therefore move with the free end of this pair of longitudinal guide members adjacent to the draw roll 3. The micro-switches 11 and 11<SP>1</SP> are closed by being brought into contact with one or the other of a pair of fixed stops (not shown), of which one is on one side and the other on the other side of the free end of the guide members 10, 10<SP>1</SP>. If the plasticity of the film is too high or too low, the spring 12 will contract or stretch respectively so that the switches 11 or 11<SP>1</SP> respectively are closed. When the switch 11 is closed, a signal is applied to the control of the drive of the feed roll 2 so that the speed of this roll is increased. The speed of the roll 3 is correspondingly increased by the device R and, as the result of the increase in speed of the rolls, the period of residence of the film web in the heating zone will decrease so that the plasticity of the film will decrease. The closure of the switch 11<SP>1</SP> has the reverse effect.