Patent GB-1470349-A (GB)
Title: Process for preparing a foamed thermoplastic article
IPC: B29C48/30
Filed: 1974-06-19
Abstract: 1470349 Extruding foamed thermoplastics SEKISUI KASEIHIN KOGYO KK 19 June 1974 [20 June 1973] 27205/74 Heading B5A [Also in Division C3] A board of foamed thermoplastics is produced with a surface appearance simulating the longitudinal grain of wood, by extruding the thermoplastics with foaming agent from an elongate slit die outlet 4, Fig. 4, which has opposed boundaries 5 comprising recesses spaced transversely of the extrusion direction, to produce a foamed extrudate comprising opposed surfaces comprising ribs of complementary shape to the recesses, and which are then flattened, whilst softened, to produce surface layers of alternate bands extending longitudinally of the board and which are of greater and lesser density at locations respectively corresponding to and intermediate the ribs. The bands are enhanced by colouring of the thermoplastics. The board is of constant density interiorly of the surface layers, producing a greater bending strength across the board width than is obtained from a board comprising coalesced separately extruded strands. The flattening may be effected by continuous passage of the extrudate directly from the die between opposed rolls, or by pressing between opposed rolls or platens after the extrudate has been cooled and its surfaces reheated. The recesses in the die taper in the upstream direction from the die outlet, and may be semicircular, Fig. 1 (not shown); may be pointed triangular with one side n being longer than the other; may be rectangular, Fig. 5 (not shown); or may comprise a central rectangular configuration on one surface, Fig. 6, of which the remainder and the other surface comprises rounded triangular recesses of the same height, but of decreasing width towards the edges of the die. In the Fig. 7 embodiment, the ribs of the extrudate may optionally be cooled, immediately downstream of the die outlet 4, by contact with pipes 7 through which cooling air is circulated, the extrudate then passing through a guide 9, a frame 10 and through a water cooling trough 11 between opposed lateral rollers 12 which limit its width dimension and opposed transverse rollers 12 which flatten the surface ribs 81 to form the board. To maintain a uniform extrusion pressure across the width of the die 2, the thermoplastics is delivered to the die outlet 4 through the perforations of a plate 14, which offer a resistance to flow which progressively decreases from the centre of the die towards the edges. Such may be effected by upstream or downstream portions of the perforations being enlarged over a length which decreases towards the edges, ar alternatively by unevenly distributing the perforations. The temperature of the thermoplastics is controlled by heating or cooling medium, circulated through a die jacket 150 and a torpedo 17. The die 2 is also surrounded by heaters, Fig. 2 (not shown). The thermoplastics may comprise a homopolymer or copolymer of ethylene, styrene, propylene, vinyl chloride, or methyl methacrylate, or a mixture thereof, and the foaming may be effected by specified decomposable compounds, or hydrocarbons, or chlorinated hydrocarbon, added prior to or during extrusion. The examples relate to polystyrene containing butane, talc, and brown pigment using the Fig. 1, Fig. 5, and Fig. 6 die, and polypropylene containing pentane, talc, and blue pigment using the Fig. 4 die.