The production of tubular articles

摘要

In the production of tubing by extruding hot thermo-plastic material in tubular form through an annular die, the cross-sectional shape and size of the extruded tube are determined by drawing it, while still plastic, across and past shaping edges carried by a stem which projects from the central core of the die, without touching the inside of the tube, the shaping edges being spaced along the interior of the tube within a region in which the tube is progressively cooled to a temperature at which it is no longer plastic, and the outside of the tube being unconfined throughout the region (see Group V). As shown, cellulose, acetate or other thermoplastic material is extruded through a die 10 over a core 11 and the tube is drawn by rollers 27, 28 over a series of discs 15-20. The tube passes through three regions: the first immediately after leaving the die, when it is very plastic and not self-supporting; the second where it passes over the discs, being then cooler and less plastic but still capable of being shaped; and the third where the traction is applied, the tension being transmitted without substantial change of shape of the tube, which has passed from the plastic to the rigid state. To support the tube between the die 10 and the series of discs 15-20 an additional disc 14 may be placed in front of the die, a short distance beyond an air cooling ring 22. Further cooling rings 23-26 surround the tube, the last ring 26 cooling the material to the temperature of the third region. The size of the discs and the number and arrangement of the air cooling rings depend on the size and material of the extruded tube, but the Specification gives some general directions. Instead of a series of discs on a stem, a one-piece shaping member may be used, formed with lands and grooves, the lands making line-contact with the tube. A helical shaping surface may also be used. A suitable material for the discs is brass, and lubrication p may be achieved by charging the cooling air with lubricant. The shaping member or members may be of a material to which the hot plastic material does not stick, e.g. polytetrafluorethylene, and the surface may then offer a larger area of contact. According to a modification, the tube is not cooled so much while being shaped and therefore stretches and acquires a smaller cross-section as it is drawn off the shaping means. The shaping members may be of other cross-sections than circular, e.g. elliptical or polygonal. Other suitable thermoplastic materials which may be used are cellulose propionate, cellulose acetatepropionate, cellulose acetate-butyrate, ethyl and benzyl cellulose; solid polymers and co-polymers of ethylene; polyvinyl compounds such as polystyrene, and co-polymers of vinyl chloride with a minor proportion of vinyl acetate, of vinylidene chloride with a minor proportion of vinyl chloride, and of vinyl chloride or vinylidene chloride with acrylonitrile or methacrylonitrile; thermoplastic polymers of acrylic acid derivatives, e.g. polyethyl acrylate and polymethyl methacrylate; and thermoplastic linear condensation polymers such as the nylons, polyethylene terephthalate and poly-4-amino-1, 2, 4-triazoles. Cellulose esters derived from carboxylic acids containing 2 to 4 carbon atoms, and especially cellulose acetate, are particularly suitable. Specification 666,727 is referred to.