| 摘要 |
644,429. Extruding polythene. DU PONT DE NEMOURS & CO., E. I. Aug. 28, 1947, No. 23841. Convention date, Aug. 28, 1946. [Class 87 (ii)] [Also in Group II] A process for improving the extrusion properties of polythene, especially with a view to its use in coating wires, comprises forcing polythene at a temperature above 100‹ C. through a confined space between concentric surfaces, one of which is rotating with respect to the other at a peripheral speed of at least 90 feet per minute, so that the polythene is subjected to a shearing force at right angles to the direction of flow, the confined space having a clearance factor of at least 40. The concentric surfaces are formed by a cylinder 4, known as a smearhead, rotating in a cylindrical tube, and the clearance factor is defined as the length 20 of the smearhead multiplied by the reciprocal of the clearance between the smearhead and the tube. The polythene is introduced .at 1, preferably in powdered form, and is forced towards the smearhead by an Archimedean screw 2. To produce the maximum working effect on the polythene the temperature should be as low as practicable and a cooling chamber 7 is provided round the smearhead. A jacket 5 is used to heat or cool the polythene, as required, in its advance along the Archimedean screw. The treated polythene is discharged through an opening 21 or it may be passed directly through a die to shape it according to the ordinary method of extrusion. The Archimedean screw may have a diminishing pitch, as shown. To give greater flexibility of operation the smearhead 4 and screw 2 may be separately driven at different speeds and in different directions. The Specification describes a method of testing the suitability of a polythene for coating wires according to which it is extruded through a standard type of wire coating machine under stated conditions of die dimension, temperature, extrusion pressure, and wire diameter and the wire is drawn through at increasing speeds. As the speed of the wire rises the surface of the polythene coating it becomes wavy and the point at which the waviness is unsatisfactory is the measure of the suitability of the polythene for wire coating. The Specification also mentions making sheets from polythene treated according to the invention.
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