A method for the production of tubular films of thermoplastic plastics with a defined volume shrinkage with the aid of the blowing process

摘要

1351973 Heat-shrinkable tubular films CHEMISCHE WERKE HULS AG 19 Nov 1971 [21 Nov 1970] 53794/71 Heading B5B Heat-shrinkable tubular films which are suitable for use in packaging foodstuffs and at the temperature of subsequent processing shrink in volume whilst retaining their predetermined internal diameter (i.e. they shrink only in the longitudinal direction) are prepared by a process which involves longitudinally stretching a tubular film at a temperature substantially below the melting range of the synthetic organic thermoplastic film material whilst at the same time maintaining the internal diameter of the film constant in the stretching zone. The films are prepared by longitudinally stretching a conventionally melt-extruded tubular film between feed godet rollers 9 arranged in stretcher stand 8 and draw godet rollers 11 arranged in a second stretcher stand 12, the film being inflated to its predetermined internal diameter by the injection of a gaseous or liquid medium at A and heated to and stretched at a temperature below its melting range in a heating channel 10. Variations in the width of the lay-flat film wound up on roller 13 are monitored by a mechanical or optical sensing device (not shown) which is operatively connected to motor 14 and spindle 15 which move the second stretching stand 12 from C to B or from C to D to accommodate any variations in the pressure of the inflating medium and thus maintain the internal diameter of the tubular film constant in the stretching zone. The thermoplastic material may be a polyolefin, a polyester or a polyamide which may be stretched at a ratio of from 1À1 to 6. Examples relate to the preparation of heat shrinkable films of polylauryl lactam and polyethylene which are heated in the heating channel 10 to between 80‹ C. and 100‹ C. and 115‹ C. to 120‹ C. respectively. The film inflating medium may be replaced in the stretching zone by a mandrel.