METHOD AND APPARATUS FOR THE PRODUCTION OF THERMOPLASTIC MOULDINGS WITH A SOLID SKIN AND A CELLULAR CORE

1442766 Injection moulding cellular articles INSTITUTE PO METALOZNANIE I TECHNOLOGIA NA METALITE 20 Sept 1973 [25 Sept 1972] 44300/73 Heading B5A In a method of making moulded articles having a cellular core and an integral skin by injecting a thermoplastic material containing a blowing agent into a mould 3 against a gas counter-pressure sufficiently high to prevent foaming, wherein the counter-pressure is then relieved to allow framing and excess foamed material is released into a storage chamber 6, each article, after the first one, is made by filling 50 to 90% of the volume of the mould cavity with unfoamed material, the remaining volume being filled up with recompressed excess foamed material from the storage chamber. Apparatus for carrying out the method comprises a valve 2 for connecting the mould 3 to an injection cylinder 1 or to the chamber 6 containing a piston 7. The mould is pressurized by a reservoir 4. The first article is made by filling the mould 3 direct from the injection cylinder. Subsequent mouldings may be made (i) by filling the chamber 6 from the injection cylinder 1 and then transferring this charge plus the foamed material just under the piston 7 to the mould, or (ii) by introducing the non- foamed material directly from the cylinder 1 into the mould and then topping up with material from the chamber 6.