GB1253315A - Process and apparatus for forming thermoplastic articles - Google Patents

Abstract

1,253,315. Sinter moulding. H. B. CARTER. 26 Sept., 1969 [2 Oct., 1968], No. 47445/69. Heading B5A. A particulate thermoplastic material, e.g. polyethylene or ethylenevinylacetate co-polymer is heated and sintered in a rotating mould inclined to the vertical and through which it is continuously fed, with heated unsintered particles passing through the mould being recirculated thereto. The moulded articles preferably comprise pipes or ducting, which may have bell-shaped or enlarged ends to facilitate joining, but may comprise elongated tubular hoppers with at least one partially closed end, and their cross-section may be cylindrical, elliptical, rectangular or hexagonal, or the articles may taper. The wall thickness of the article may be uniform throughout, but may be varied either by employing a mould having uniform heating and varying wall thickness, or by varying the heating along its length. The longitudinal axis about which the mould 1 is rotated may be horizontal, with the particles displaced therethrough by air from a blower, but in the preferred embodiment the particles pass gravitationally through a cylindrical mould 1 which is rotated, by a motor 19 via gears 15, 13, in a position slightly inclined to the horizontal, e.g. at 5 degrees, upon pairs of support rollers 11 which engage between guide flanges on the mould exterior. The mould 1 is heated by flame burners 25, and particulate material is supplied to the upper end 5 thereof, via a shoot 31, from a hopper 27 provided with a variable size outlet opening and variable speed feed screw 35. Heated unsintered material passing out through the lower end 7 of the mould 1, is recirculated to the hopper 27 either via a conveyer belt 37 and vertical screw 39, or by a feed screw extending either between the outlet 7 and the mould upper end 5, or the hopper 27. Liquid or powdered additions may be dispensed directly into the hopper 27 wherein they may be mixed with the material by an agitator (not shown), or may be dispensed directly into the mould 1 by spray nozzles 49 supplied from metering pumps 47, or may be added to the material during the recirculation thereof by a screw conveyer, or may be dispensed by a vibrating feed. Removal of the article from the mould is facilitated by a mould constructed in two sections, by coating the mould with silicone release agent, and by continuing rotation during cooling and shrinkage of the article, after the heating has been terminated, e.g. by automatic means operable from means sensing when the sintering is completed. The moulded articles may comprise two or more layers of materials, each having different properties provided by additives, e.g. colour, or an outer layer comprising carbon black ultra-violet inhibitor. The additives may also comprise flame retardants which do not form gas bubbles in the article during sintering, e.g. tetrabromoethane, tetrabromocyclododecane, hexabromocyclododecane, pentabromodiphenylether, or antimony trioxide, employed at 5 to 20% of the total weight of the additive and particulate material. The additives may also comprise liquid or powdered cross-linking agents for polyethylene, e.g. ditertiarybutylperoxide, dicumylperoxide, and tertiarybutylcumylperoxide employed as 0À5 to 5 parts by weight to 100 parts polyethylene.