| 摘要 |
In the production of sheets or films of foamed plastics material by extrusion, plastics material is extruded through a slot die, is allowed to expand without confinement and thereafter the opposite surfaces of the extrudate are successively subjected to longitudinal "hotstretching" of not more than 20%, with consequent flattening, by passage over a smooth surface. The stretching may be effected by two or more cylindrical or barrel-shaped rollers with, in the simplest case, the extrudate being led in an S-shaped path over two rollers. Preferably the temperatures of the rolls are controllable to ensure that at the last contact between a roll and the extrudate the non-contacting surface thereof is above the softening point of the plastics material. External heaters may be provided, e.g. infra-red lamps, to assist this (see also Specification 1,091,624). However, in some cases the extrudate may be force-cooled using cold air blowers. After stretching, the extrudate may be heated to reduce the elongation of cells caused by the stretching. Plastics materials exemplified are polystyrene and polypropylene, with pentane being used as a blowing agent, and mixtures of sodium bicarbonate and citric acid or azodicarbonamide respectively as modifiers. |
