Patent GB-656735-A (GB)
Title: Improvements in or relating to a method of making plastic tubes and films
IPC: B29C48/08
Filed: 1948-05-03
Abstract: <PICT:0656735/IV (a)/1> Plastic tubing and film is made by continuously extruding a tube of fused normally-crystalline vinylidene chloride polymer at 120-190 DEG  C., withdrawing the tube downwardly from the die orifice through a cooling bath, the surface of which is less than about 6 inches below the orifice and the temperature of which is maintained at a constant value between 0 DEG C. and about 40 DEG  C. to supercool the tube, at a speed greater than the speed of the polymer through the die orifice, while maintaining a predetermined head of a lubricating liquid within the tube to regulate the diameter and wall thickness of the supercooled tube and to lubricate its inner surface, flattening the tube and advancing it through the nips of two pairs of pinch rolls, which pairs are stationed at a distance from one another at least 5 times the diameter of the super-cooled tube, introducing and maintaining a gas in the portion of the tube between the pairs of rolls in amount more than sufficient to fill the tube to its supercooled diameter and sufficient to establish a positive internal pressure and to distend the tube a portion of the distance from the second pair of rolls back towards the first pair of rolls, to from 3 to 5 times the diameter of the supercooled tube, but insufficient to distend the tube to such enlarged diameter the entire distance between the pairs of rolls, thereby, cold-stretching the tube in a zone intermediate between and removed from the two sets of rolls, the amount of gas employed being so limited that introduction of an additional quantity thereof results in lengthening of the distended portion of the tube and displacement of the cold-stretching zone towards the first pair of rolls, without material increase in the diameter of the distended portion of the tube, and driving the second of the two pairs of pinch rolls at a peripheral speed of from 2 to 4 times that of the first of the two pairs of pinch rolls.  The finished tubing may then be slit lengthwise to form a film if desired.  The cooling bath may be maintained at a temperature between 0 DEG  and 20 DEG  C. and the tube after cooling may be warmed to 20-50 DEG  C. before being inflated and stretched.  The surface of the cooling bath is preferably 0.5-1.75 inches below the extrusion orifice.  In an example a copolymer prepared from a mixture of 85 per cent vinylidene chloride and 15 per cent vinyl chloride and containing a plasticizer, is extruded at 170-173 DEG  C. through a ring die orifice 12 to form a soft plastic tube 13 which while still hot is conducted into a bath of water 14 maintained at 2-7 DEG  C.  A quantity of high-boiling petroleum oil 19 is maintained inside the tube so that its surface is 0.5-1 inch above that of the cooling bath.  The tube 13 is passed between guides 25, and then between a pair of driven pinch rolls 15 rotating at a peripheral speed of 9-10 feet per minute, passes round the guide roll 29 and is withdrawn vertically from the bath and drawn between the second pair of pinch rolls 20 rotating at a peripheral speed of 23-26 feet per minute.  Compressed air is introduced, by means of a hollow needle, into the section of the tube situated between the surface of the cooling bath and the second pair of pinch rolls 20 at a pressure of 1-1.5 lbs. per square inch until a portion of the section of the tube is distended with the formation of a bubble having a diameter of about 12 inches, that of the supercooled portion of the tube, after passing through the first set of pinch rolls being 2.37 inches.  The longitudinally and transversely stretched and flattened tube issuing from between the second pair of pinch rolls 20 is passed under considerable tension over a series of smoothing rolls (not shown) and eventually wound on a drum (not shown) for storage.  A section of the cold-stretched tube is slit longitudinally, unfolded and smoothed to form a film having a width of over 37 inches.  Two other embodiments of the invention are described and illustrated. In these the supercooled tube is withdrawn from the cooling bath in a flattened form and fed to the first of the two pairs of pinch rolls between which the tube is inflated.  The tubes are said to be strong both along the longitudinal axis and also transversely thereto and consequently to have a high bursting strength. Similarly films prepared from the tubes by slitting are said to have high tensile strength in all directions.  Specifications 431,619, 524,777 and U.S.A.  Specification 2,183,602 are referred to.