| 摘要 |
1441586 Controlling extrudate surface roughness IMPERIAL CHEMICAL INDUSTRIES Ltd 3 July 1973 [12 July 1972] 32559/72 Heading B5A The occurrence of a visible surface roughness effect, known as "sharkskin", in an extruded thermoplastics is reduced by cooling an external layer of the thermoplastics in passage through an extrusion die so that it emerges from the die at a temperature above that of its solidification, and below the temperature at which the effect is a maximum. The extrudate may comprise a solid section, or a parison for blow moulding. The thermoplastics may comprise polyvinyl chloride, low and high density polyethylene, acetal copolymers, acrylic polymers, acrylonitrile/styrene copolymer, and polymethyl methacrylate, and the examples relate to polyethylenes of density 939 and 970 kg./m.<SP>3</SP> and of specified molecular weights. The outer layer is preferably extruded at a temperature of 15‹ C. or less above the solidification temperature of the thermoplastics. In the Fig. 1 apparatus, the outer layer of a circular rod extruding from the die 11 of a heated and insulated barrel 13, is cooled by cooling of the die 11 by compressed air from the orifices 18, or the continuous slot of a ring 17. A thermocouple 16 is provided. The air is preferably refrigerated, or water, or air/ water coolant may alternatively be employed. A sheet die may be employed. In the Fig. 4 embodiment, for extruding blow moulding parisons, the outer layer is cooled by cooling of the die by a heat exchanger 44, through passages 45 of which refrigerated water, e.g. at 0‹ C., is circulated, and which at 48 is insulated from the die by an asbestos sheet, and at 46 is provided with a silicone grease layer to promote heat transfer. The heat exchanger may alternatively be built into the die, and the mandrel 42 may also be cooled by a pipe, or by an exchanger similar to that of Fig. 4, to produce an extruded pipe having smooth internal and external surfaces. The cooling may be varied to produce a patterned effect, e.g. a sheet having many alternating lines of glossy and matt areas obtained by oscillating spaced apart air nozzles along the die. |
