| 摘要 |
1,167,436. Moulding configured sheets; laminates. W. R. GRACE & CO. 21 Aug., 1967 [19 Aug. 1966], No. 38363/67. Headings B5A and B5N. [Also in Division B6] A thermoplastic sheet for use in a dielectric moulding or embossing process has a dielectric loss factor which decreases from one face to the other. Consequently the face having the higher factor is heated more rapidly than the rest of the sheet and the time taken to raise this face to the softening temperature is less than when the temperature of the whole sheet is raised uniformly. The sheet may consist of a single-layer containing a non-uniformly dispersed conductive or dielectric additive (see Fig. 1, not shown) or may comprise a plurality of layers each having a uniform dispersion of additive, the amount of additive decreasing from one layer to the next (see Figures 2, 3 and 4, not shown). The first of the above types of sheet may be made by coating or extruding P.V.C. containing a uniformly dispersed additive such as carbon black and subjecting the sheet, whilst still fluent, to a vibrating or electric field through one surface, and the second of the above types may be made by laminating preformed layers or by successive casting and gelling in a mould. This latter process is used to give a sheet having an upper layer of carbon containing P.V.C., a middle layer of carbonless P.V.C., and a lower layer of P.V.C. containing finely-divided sand. Such sheets are moulded or embossed by locating them together with a configured die between the plates of a conventional dielectric press, vacuum being applied to the sheets to enhance the flow of material into the configured die. In the preferred embodiment, the plates are moved towards each other to form a vacuum seal after which movement is continued until the plates just contact the sheet. Dielectric heating is then applied for a short interval after which the plates are further forwarded to effect the moulding or embossing. |
