GB1231535A - - Google Patents

Abstract

1,231,535. Extruding foamed thermoplastics. LAVORAZIONE MATERIE PLASTICHE S.p.A. 7 March, 1969 [23 March, 1968], No. 12206/69. Heading B5A. [Also in Division C3] In a process of extrusion moulding foamed thermoplastics, a gaseous nucleating agent is injected into the barrel of the extruder to be dispersed in the molten resin upstream of a point where a foaming agent is injected into the molten resin, the injection points being between zones where the resin is compressed sufficiently to form hydraulic sealing plugs. Resins specifically mentioned are polystyrene, polyethylene and polypropylene, foaming agents mentioned are butane, pentane, hexane, trichlorofluoromethane and dichlorodifluoromethane and the gaseous nucleating agents specified are carbon dioxide, nitrogen, air, methane, ethylene and acetylene. In the embodiment of extruder described, at least two co-rotating intermeshing screws 14 are provided which define within barrel 10 a melt zone A, an injection zone C with one or more injectors 32 for the nucleating agent, an injection zone E with one or more injectors 34 for the foaming agent, and a metering zone G having externally a jacket 22 for cooling water and a coil 24 which is the evaporator of a refrigerating system, and which is cooled internally by fluid flow within the screws. Further zones B, D and F are sealing zones, zone B being formed by smooth cylindrical portions 40 on the screw shafts, and zones D and F by reverse flight portions 42 and 44 which are preferably interrupted. Zones E and F may be duplicated or triplicated. Generally, in zones A, C, E and G, the screws may have constant or variable diameter cores and flights of constant of gradually decreasing pitch. The screw axes may be parallel or otherwise. Heaters 16, 18 and 20 are provided.