Patent GB-858616-A (GB)
Title: Production of seamless thermoplastic tubing
Inventor: DYER GERALD BENJAMIN
IPC: B29C48/10
Filed: 1959-01-03
Abstract: <PICT:0858616/IV(a)/1> <PICT:0858616/IV(a)/2> <PICT:0858616/IV(a)/3> <PICT:0858616/IV(a)/4> <PICT:0858616/IV(a)/5> Seamless tubing is made by continuously dry-extruding molten thermoplastic material in tubular form, flattening the tubing at a point spaced from the point of extrusion, maintaining a substantially constant continuous bubble of a gaseous medium in the section of tubing extending from the point of extrusion to the point of flattening, the gaseous medium in the bubble being under a differential pressure such that the pressure in the zone nearest the point of extrusion is sufficient to expand the tubing while in the plastic formative state to the desired diameter at a point beyond the point of extrusion, while the pressure in the zone beyond the point of expansion to the desired diameter is sufficient to keep the tubing expanded but insufficient to cause further expansion, and cooling the tubing between the point of extrusion and the point of expansion to the final desired diameter, the rate of withdrawing the tubing, the degree of expansion of the tubing and the degree of cooling the tubing all being correlated in accordance with predetermined desired physical characteristics of the tubing.  The thermoplastic material may be polyethylene, polypropylene, a polyester, a polyamide, a polyvinyl halide, a polyvinylidene halide, or a copolymer of a vinyl or vinylidene halide with another polymerizable organic compound. The bubble of gaseous medium may be isolated or may be open to the atmosphere at a point before the point of extrusion.  A controlled volume of cooling medium may be directed on to and around the exterior of the tubing, and the interior of the tubing may be cooled by means of a continuously circulating liquid cooling medium not in direct contact with the tubing.  Apparatus as shown comprises in each case an annular extrusion die 1, means for extruding a molten thermoplastic material through the lips of the die 2 in the form of seamless tubing 9, means 3, 6 for admitting a gaseous medium into said tubing and means 4, 5 for causing a back pressure in said tubing in the vicinity of the point of extrusion to cause expansion of the tubing to a predetermined diameter, said means for causing a back pressure may comprise (Fig. 2) a circular plate 5 attached to the die at a point beyond the point of extrusion, the plate having an opening therein within which is positioned a fan 12 designed to blow the gaseous medium in the direction of the die face and countercurrent to the flow of gaseous medium at the point of emission thereof into the tubing. Alternatively, the means for admitting the gaseous medium into the tubing may comprise a tube (3) inserted through the centre of the die in which tube is positioned a second tube 4 of smaller diameter, the second tube being attached at a point beyond the point of extrusion to a concentric circular plate 5 (Fig. 3). Alternatively (Figs. 5 and 6) the back pressure causing means may comprise a shaped cooler 5 attached to the die and provided with inlet and outlet means 14, 15 for circulating a liquid cooling medium through the interior of the shaped cooler.  The shaped cooler may have an opening through its central portion (Fig. 6) within which is positioned a fan (not shown, but as in Fig. 2) designed to blow the gaseous medium in the direction of the die face and countercurrent to the flow of gaseous medium at the point of its emission into the tubing Alternatively (Fig. 5) the means for admitting gaseous medium into the tubing may comprise a tube 3 inserted through the centre of the die and adjacent to the lower portion of the shaped cooler in which tube is positioned a second tube 4 of smaller diameter, the second tube being attached at a point beyond the point of extrusion to that part of the shaped cooler furthest from the point of extrusion of the thermoplastic material.  In operation, inflating air entering zone P1 emerges in the tubing 9 before the plate or shaped cooler 5 and builds up a back pressure in this zone until tubing 9 expands enough to create a gap between the tubing and the plate or cooler. Air then escapes to zone P2, keeping the tubing from touching the plate or cooler, and this air, in the apparatus of Figs. 1, 3 and 5, is bled off to atmosphere via tube 4, a valve or hydrostatic leg of liquid being positioned at point 8 to control the pressure in zone P2 so that it is just sufficient to keep the tubing 9 inflated to the proper diameter but insufficient to cause further expansion.  In contrast to this "open-bubble" system, Figs. 2 and 6 illustrate a closed or "isolated-bubble" system in which inflating air is admitted through tube 3 to expand tubing 9 to the desired diameter and the valve 11 (not shown in Fig. 6) is then closed.