Method and apparatus for making a corrugation-free foam

摘要

The invention provides a method and apparatus for extruding a corrugation-free polystyrene foam with a blowing agent by extruding the extrudate though an annular die opening 38 so that it contacts a choke ring inner surface 31 within a contact time of 1 to 20 milliseconds. A foam made with the apparatus according to the invention will have no visible corrugations and an average cell size of less than 0.35 mm. The opening 38 is provided by an extrusion die 32 and surface 31 is provided by a choke ring 30 in which at least a portion of the die 32 is received. The blowing agent may be 100% carbon dioxide. The opening 38 is spaced from an axis 44 of die 32 by a first distance t d whilst the surface 31 is spaced from the same axis by a second distance t c . The difference between the two distances provide the gap through which the extrudate travels.