GB740458A - Improvements relating to the manufacture of tubular articles of thermoplastic material - Google Patents

Abstract

<PICT:0740458/IV(a)/1> Self-supporting tubular films are made from thermoplastic material by extruding the heat-fluidized material in tubular form, progressively cooling it to a temperature at which it is no longer plastic, while drawing it away from the extrusion point by traction applied in a region in which it has ceased to be plastic, controlling the cross-sectional shape of the tube by drawing it, on its way to said region, through an aperture between a pair of contacting, freely rotatable profiled rolls having their axes perpendicular to the direction of travel of the tube and controlling the temperature of the tube so that, on reaching said aperture, it is soft enough to be deformed, but is not limp and, on leaving the aperture, is hard enough to retain the cross-sectional shape imparted by the aperture. The material emerges hot from the extrusion die 10 in the form of a circular tube 14 and is drawn through shaping rolls 20, 21 by a pair of draw rolls 15, 16 revolving at such a speed that the tube is stretched between the die 10 and the rolls 20, 21. The temperature of the material is regulated by air jet cooling rings 17, 18, so that when it reaches the rolls 20, 21 it is soft enough to be deformed but is not limp, and by further cooling rings 22, 23 so that it is no longer plastic when it reaches the draw rolls 15, 16. The shaping rolls 20, 21 are so shaped and relatively disposed that the aperture between them has the same shape as that required for the cross-section of the tube, e.g. elliptical or polygonal, but is very slightly larger, its perimeter, however, being less than the circumference of the die 10. The shaping rolls are made of steel and have roller bearings, the upper roll 21 being carried on a frame 33 which is pivotally mounted and urged by a spring 36 to press the upper roll down towards the lower. To assist the rapid cooling of the tube during, and especially after, its passage between the shaping rolls, the latter are preferably hollow and internally cooled by circulating water. Two or more pairs of shaping rolls in series may be used. The draw rolls 15, 16 are of steel covered with rubber and are slightly profiled to grip the tube, being urged together by springs of which the compression is adjustable. To adjust the speed of the rolls 15, 16 so as to stretch the tube as mentioned above they are driven through infinitely variable gearing. The air cooling rings 17, 18, 22, 23 have numerous apertures round their inner surfaces through which jets of air blow on the tube, supplied from a compressed air reservoir under the control of throttle valves by which the temperature of the material may be regulated as mentioned above. The die has a central passage 12 through which air, warmed by an electric heater, enters the inside of the tube to remove vapour resulting from the volatilization of the plasticizer used which would otherwise condense on the cooler parts of the tube. The air is supplied from a reservoir under the control of a valve to keep it at a pressure slightly above atmospheric, so as to support the tube without expanding it, the volume being sufficient for that purpose without the far end of the tube being plugged. An inert gas such as nitrogen or carbon dioxide may be used instead of air. Internal discs may be fitted to counteract the tendency of the tube to collapse on leaving the die. The invention is described with particular reference to the use of cellulose acetate as the material, but also suitable are other cellulose derivatives, thermoplastic polyvinyl compounds, thermoplastic polymers of acrylic acid derivatives, and thermoplastic linear condensation polymers. Specifications 691,125 and 702,176 are referred to.