Patent GB-1406273-A (GB)
Title: Production of sheets of foamed plastics materials
IPC: B29C48/08
Filed: 1972-11-24
Abstract: 1406273 Foamed fibre web: web forming machines DYNAMIT NOBEL AG 24 Nov 1972 [26 Nov 1971] 54543/72 Headings D1R and D1N [Also in Divisions B5 and C3] A sheet of foamed thermoplastic plastics material is continuously formed by feeding thermoplastic plastics material and a liquid blowing agent through inlets 5, 9 respectively into a heated screw extruder 1, extruding the mixture of material and blowing agent through a plurality of extrusion orifices 15 at the same rate and pressure to form a plurality of filaments, allowing the blowing agent to vapourise and foam the filaments by passing the filaments through a hot air shaft (20, Fig 3), passing the filaments through a pair of oscillating deflector plates (23) to produce layers of filaments on a conveyor (25, Fig 3) and passing the layers of filaments through heating, pressing and cooling zones (Fig 4) on endless belts to remove residue of blowing agent and to compress and set the layers of filament into a sheet. The blowing agent is a solvent for the plastics material. The mixing by the extruder screw 2 is effected under a pressure at which the blowing agent is liquid at a temperature which is above the boiling point of the liquid at normal pressure. Each orifice 15 is provided with a gearwheel pump 18 driven by a motor having an adjustable speed. Each orifice 15 may be provided with an annular air nozzle 17 for cooling the filaments to 60-90C. A barrier 16 may be provided upstream of the orifices 15 to distribute the material towards the pumps 18. The screw 12 is fitted with a brake ring 10 to prevent the blowing agent moving upstream thereof. The plastics material may be fed to the extruder 1 by a screw 4 and hopper 8. The blowing agent may be introduced into the extruder by a metering pump 12. The heating, pressing and cooling zones may comprise an initial drying and pressing zone (29, Fig 4), a further drying zone 34, final drying and pressing zone (43) and a cooling and pressing zone 44. Drying is achieved by blowing air at a temperature of 95-120<SP></SP>C, under a pressure of 150-200 mm of water and at a rate of 10000 m<SP>3</SP> per hour through the filaments from above and/or below. Final pressing of the filaments may be carried out at 0.2 kg/cm<SP>2</SP> and cooling at 40-50C. The filaments may be transported through the zones upon endless chain conveyors (31, 35, 41) which may be provided with suction devices (28, Fig 3) for retaining the filaments and for reclaiming the blowing agent. Overhead and side endless belts (41, 42, Figs 4, 5, 6) carry out the pressing of the filaments. The conveyors may be of steel and provided with pressing rollers 46 (Fig 5). The blowing agent which may be present in 30-45 % by weight of the plastics material may comprise acetone, methylene chloride, 'benzene, ethyl acetate, monofluorotrichloromethane or mixtures thereof. The plastics material may be polystyrene, polyethylene, cellulose acetate or P.V.C. containing lead stearate and sulphate. The filaments may be extruded at a rate of 3-4 m/sec through six or twelve nozzles having a 1 mm diameter and may be expanded to a diameter of 6-10 mm.