Patent GB-788133-A (GB)
Title: Method and apparatus for shaping plastic foams
Inventor: MCCURDY JOHN LLOYD
IPC: B29C47/00
Filed: 1956-01-03
Abstract: 788,133. Making plastic foam sheets. DOW CHEMICAL CO. Jan. 3, 1956, No. 217/56. Class 87(2). Plastic foam sheets are made by a method in which a flowable gel of a thermoplastic resin having a normally gaseous agent dissolved therein is fed to a device comprising a pressureresistant chamber having an extrusion orifice and situated immediately in front of the extrusion orifice a shaper which has members spaced apart to form a slot-like passageway therebetween, the members extending laterally for a sufficient distance beyond the sides of the orifice for the stream of extruded material to be laterally unconfined and to expand to a selfsustaining plastic foam. The shaper, Fig. 4, comprises rollers 3 to 5 and 6 to 8 with their ends supported in diverging plates 10, 12 at one end and in similar plates, not shown, at the other end. The material is extruded from the pressure resistant chamber 15 through an orifice which can be opened and closed by rotating a plug 16 and expands against the rollers, the pressure of which determines the thickness of the cellular sheet. The roller bearing supports, e.g., the plates 10, 12, are bolted to end plates, e.g. 14, the bolts passing through slots in the plates which permit adjustment of the spacing between the two sets of rollers. Only a pair of rollers may be used or there may be any desired number. As shown in Fig. 6, the shaper through which the material is extruded from the orifice 1 comprises plates 19, 20 with clamps 21 for adjusting the distance between their outer lips. In Fig. 8 the shaping plates 19, 20 have layers 25, 26 of polytetrafluoroethylene over parts of their surfaces to reduce resistance to the flow of extruded material. The plate 19 has a hinged end-section 19a which is pressed downwards by a spring 27 of which the pressure may be adjusted by a screw 30 mounted on a bracket 28. The pressure of this plate (to the region of which the layers 25, 26 do not extend) provides frictional resistance to forward movement of the material which causes sideways spreading of the extruded gel, the spring 27 regulating the resistance inversely with the rate of extrusion and counteracting the effects of surges. The thickness of the sheet can be varied by changes in the spacing between the plates 19, 20 and the width by changes in the load applied by the spring 27. The freshly-extruded gel flows over a portion of a half-round bar 23 attached to the head plate 18 on one side of the orifice 1, a similar bar being situated on the other side and the stream of gel is thereby grooved at its sides. During its passage between the shaping members 19, 20, the material expands and eliminates the grooves with the result that the sides of the resulting sheet are planar and do not bulge as they otherwise would due to the expansion. A spring loaded plate similar to 19a in Fig. 8 may be applied against a plastic foam sheet at the delivery end of a device as in Fig. 4 or Fig. 6. Polystyrene is particularly mentioned as the thermoplastic resin of which the foamed sheets are made and examples are given of the extrusion and shaping of gels of polystyrene and methyl chloride under specified pressures and temperatures. Other resins are mentioned, e.g., solid polymers and copolymers of monoalkenyl aromatic compounds containing a vinyl or isopropenyl radical, methylmethacrylate, vinylacetate, and copolymers of compounds with one another.