GB1143689A - - Google Patents

Abstract

1,143,689. Sheathing filaments by extrusion. E. I. DU PONT DE NEMOURS & CO. 7 Dec., 1967 [12 Dec., 1966], No. 55791/67. Heading B5A. [Also in Division G2] One or more light transmitting filaments, such as are described in Specification 1,037,498, are covered with synthetic resin or rubber to form a light-guide, by passing the filament or filaments along a conduit 18 in extrusion crosshead 1 to enter a resin or rubber tube formed between annular die 9 and mandrel 12, see also Figs. 3A-3C (not reproduced). The distance between orifice 2 and the outlet 14 of mandrel 12 is adjusted so that firstly, the tube is an interference fit with the filaments and secondly, insufficient back-pressure is created to cause flow of material rearwardly within conduit 18. The second requirement is particularly necessary when an irregularly crosssectioned bundle of filaments is used, Fig. 4 (not shown), and the first requirement ensures that such a bundle is embedded in the tube, Fig. 5 (not shown). Extrusion through orifice 2 gives a circular cross-section to the light-guide irrespective of the cross-section of the filament bundle. Mandrel 12 is adjustable axially by rotation of screw thread 19 within fixed nut 22 and is held in position by lock nut 20. Vacuum is applied between the filaments and the tube via conduit 24 and perforations 13 to both draw the tube on to the filaments and to produce a cooling effect. Typically the lightguide has a diameter between 2 and 4 mm. Resinous covering materials mentioned are synthetic rubber, low or medium density polyethylene, polypropylene and vinyl chloride polymers or copolymers. As described in Specification 1,037,498 the filaments may be of polymethylmethacrylate sheathed in fluoroalkyl methacrylate or polystyrene sheathed in polymethylmethacrylate.