Patent GB-1378319-A (GB)
Title: Moulding of plastics articles
IPC: B29C51/02
Filed: 1972-02-29
Abstract: 1378319 Forging sintered plastics DOW CHEMICAL CO 29 Feb 1972 [1 March 1971] 9394/72 Heading B5A [Also in Division B8] A thermoplastic article, e.g. a container, is formed by compressing thermoplastics powder into a coherent briquette, Fig. 1, which, at a temperature between the alpha transition and melting temperature of the powder, is sintered and then forged directly into the article, Fig. 1A (not shown), or forged into a preform which is subsequently formed into the article. The alpha transition temperature is defined, for amorphous polymers, as the glass transition temperature, and for crystalline polymers as the temperature between the glass transition and melting temperature at which the first energy loss peak is observed. The powder is preferably one that compresses at 30,000 p.s.i. to at least 85% of its molten density, and the forging temperature is preferably 2‹ or 5‹ C. below the melting point, with forging effected in a mould at the same temperature as the briquette. A preform may be cooled for subsequent forming, or directly transferred to a thermoforming mould in a clamp ring with the centre of the preform maintained at its forming temperature and the periphery within the ring being below the alpha transition. The heating of the powder may be effected prior to, or subsequent to its formation into the briquette, e.g. in an oven in which the sintering is effected. The preform may be formed to the article by the differential air pressure means 20 shown, or by forming by vacuum, matched dies, plug assist vacuum, drawing, impact, rubber pad, or fluid. Orientation in the article may be increased by fusion occurring earlier in the forging process. The examples relate to the moulding of containers from: (1)-(3), high density polyethylene; (4) and (5), foamed high density polyethylene; (6) and (7), the same high density polyethylene with different proportions of vinylidene chloride/ methyl acrylate co-polymer to form a barrier container as in Fig. 4; (8) and (9), a surface layer of the material of Ex. (6) with either an inner layer of the co-polymer of Ex. (6), or of the material of Ex. (4) to form a barrier container as in Fig. 5; (10) and (14), acrylonitrile butadiene styrene terpolymer; (11), vinyl chloride/ vinyl acetate co-polymer; (12) the solution recrystallized material of Ex. (1); (13) the material of Ex. (1), solution polymerized and precipitated; (15), a surface layer of the material of Ex. (1), an inner layer of the material of Ex. (4). Other polymers which may be employed are polystyrene, polyvinyl chloride, polyvinylidene chloride, vinyl chloride/vinyl acetate co-polymers, and co-polymers of vinylidene chloride with vinyl chloride, and acrylonitrile. During forging, a lubricant may be employed such as potassium oleate, silicone grease and oils wiped on the mould or briquette, or particulate P.T.F.E. or zinc stearate sprayed on to the mould, or a P.T.F.E. coat baked on to the mould, or a film of low density polyethylene laid loosely around a polyvinylidene briquette. With a foamable material, a dry blended mixture of the powder and a chemical blowing agent is employed, with foaming accuring after opening of the forging press. The powder may incorporate pigments, flame-retardants, metals, glasses, ceramics, glass-fibres or platelets, or magnetic particles. Powder mixtures may also comprise polystyrene with acrylonitrile homopolymers or co-polymers. Articles comprising multiple layers of different materials, are formed by sequential compression of each layer in the briquette. The briquette may be inked or distortion printed. The articles may also comprise gears, bushings, bearing bushings, and fan blades which change pitch as a function of temperature.