Patent GB-1138324-A (GB)
Title: Production of tubing from thermoplastics
IPC: B29C48/03
Filed: 1966-04-22
Abstract: 1,138,324. Tubular films; stretching extruded tubing. BADISCHE ANILIN- & SODAFABRIK A.G. 22 April, 1966 [24 April, 1965], No. 17655/66. Headings B5A and B5B. In a process for the extrusion of thermoplastic tubing said tubing is extruded from the die into a chamber wholly or partly filled with a liquid in which the external pressure exerted on the tubing may be regulated by varying the height of the column of liquid. A cushion of gas having a predetermined pressure may be maintained between the die and the liquid. Thus the tubing may be extruded into a chamber partly filled with liquid whose level can be raised or lowered by applying a regulatable excess or reduced pressure between the plane of the die and the level of the liquid. The tubing may be stretched in the chamber containing liquid, or after it has left the chamber containing liquid. The apparatus of Fig. 1, comprises an extruder having an annular die 3, the die plate 3a being made of insulating material to lessen heat transfer. A J-shaped chamber 1 is secured at its shorter arm to the die-plate 3a and is filled with a liquid, the column 2 of liquid in the right hand open arm of the chamber having a level which corresponds to the maximum pressure to be produced at the die-plate 3a, having regard to the specific gravity of the liquid. The extruder forces the plastic tubing 4 continually into the liquid 2 and immediate collapse of the tubing is prevented by a gaseous pressure medium introduced through pipe 5. The pressure of the liquid column 2 in the immediate vicinity of dieplate 3a and the gas pressure inside the tubing are about in equilibrium. The cooled tubing is drawn through the J-shaped chamber 1 by draw-off means 6 over guide rolls 6<SP>1</SP>, passed if necessary through stripping means to remove adherent liquid and is then wound up on storage rolls. In a modification, not shown, the cooled tube is removed from chamber 1 by a shorter route from a horizontal branch of its movement through a packing-lip device provided in the wall of chamber 1. In Fig. 2, a cushion of gas 7 is maintained in the short arm of chamber 1 between the plane of the die-plate and the level of the liquid column, a supply pipe 8 for the gaseous pressure medium being introduced through the die-plate 3a. Pipe 8 is provided with a stop valve 8a preventing escape of the gas cushion trapped in the short arm of the chamber 1. In carrying out the process for the production of tubing using plastics having relatively high melting point ranges, it is advantageous to reduce gas cushion 7 to a height of a few millimetres. It then serves substantially only as a particularly good heat insulating layer between the die-plate and the liquid. In Fig. 3, the free end of the chamber 1 rigidly connected to the die-plate 3 dips into a trough 1a filled with liquid. The same hydrostatic conditions obtain as in the apparatus of Figs. 1 and 2. The space surrounding the tubing 4 is evacuated to such an extent through the pipe 8 in the die-plate 3 that the liquid level rises to the desired height. The pressure conditions prevailing in the gas-filled part of vessel 1 and the liquid-filled part of vessel 1 and the regulatable pressure in the interior of the plastic tubing exerted through pipe 5, cause hot stretching of the tubing immediately beyond the dieplate. In Fig. 4, the tubing 4 is withdrawn upwardly. After having been regulatably supported within liquid column 2, the tubing passes through an annular heating means 9 arranged above the level of the liquid and is brought to the hot stretching or cold stretching temperature adapted to the particular material. Nip rolls 6 close the tubular sheeting 4 gas tight. By raising and regulating the gas pressure inside the tubing it is hot stretched or cold stretched in the usual way depending on the temperature resulting from the heat radiated by heating means 9. In general, hot stretching may be effected by continually heating the column of liquid or continually circulating liquid in the liquid column. The cross-section of the extruded tubing may be changed, and especially enlarged, after the tubing has passed through the liquid column corresponding to a predetermined regulatable pressure, by a method other than those hitherto used. By means of the regulatable column of liquid outside the tubing and the regulatable gas pressure inside the tubing, the higher pressure necessary for cold stretching is set up, the tubing being regulatably supported in its plastic range against the higher internal pressure used by the external column of liquid from the die onwards. A further modification is for the column of liquid bearing against the die-plate to be composed of layers of liquids having different densities and different properties, in order either to achieve additional treatment effects, or by the use of liquids having a density greater than 1, to extend still further the range of pressure variation which can be achieved with a J-shaped vessel of a given height. An example describes the treatment of tubular polyethylene film.