| 摘要 |
<PICT:1009842/C3/1> A method of producing a sheet of foamed plastics material composed of a multitude of closed cells, in which the cells towards one surface of the sheet are smaller than those towards the other surface, comprises extruding fluid-foamed thermoplastics material through a circular die orifice to form a tube, rapidly cooling the inner surface of the tube adjacent the orifice by impinging a jet of gas at a temperature substantially lower than that of the plastics material against an annular zone of the inner wall of the tube as it issues from the orifice and before full expansion of the cells and thereafter slitting the tube longitudinally to form a sheet. The treated surface has a lower coefficient of friction than the opposite surface. In one embodiment, foamable polystyrene beads are transferred by screw 12 through the barrel 11 of the screw extruder, heated to a temperature at which the beads melt and which is above the atmospheric boiling-point of the expanding agent. The melt is forced through extrusion channel 19 and, in leaving orifice 20, with simultaneous formation of closed cells due to the volatilization of the blowing agent, forms an annulus which passes over cylindrical mandrel 21 to form closed tube 23. Gas, especially air, is fed through pipe 24, channels 26 and 27 and through a radial passage 32 between plate 30 and flange 28, emerging through the peripheral slot 33. The gas is directed on to the inner surface of the film 23 while the plastic is still in a heated deformable state and before full expansion of the cells has occurred. The tube is slit by cutter wheel 35 and the slit tube is opened out to form a sheet. The gas from slot 33 produces a pressure which expands and supports the hot deformable tube and provides a cushioning film between the tube and the mandrel. The material is useful in the production of nested, tapered cups. |
