Patent GB-725100-A (GB)
Title: Improvements in hollow moulded plastic articles and method of forming same
IPC: B29C41/04
Filed: 1953-08-24
Abstract: A method of making a flexible seamless hollow article having a rubber-like resinous outer wall and an integral resinous thermally-insulating lining comprises: (i) heating a hollow metal mould having a mould cavity of the shape to be imparted to the article (ii) pouring a plastisol into the hot mould and pouring out the plastisol that does not adhere as a skin to the mould walls (iii) gelling the skin by heat (iv) pouring into the hot mould, the walls of which are covered with the gelled skin, a plastisol containing a blowing agent and pouring out the plastisol that does not adhere to the skin (v) further heating of the mould to blow the second plastisol layer and bond them integrally together.  The first plastisol is prepared by dispersing finely divided polyvinyl resin powder in a liquid plasticizer to form a creamy liquid, to which a stabilizer and colouring pigment are added.  The ingredients of the liquid plasticizer, comprising dioctyl phthalate, dioctyl adipate and a polyester resin, the preparation of which are defined in the Specification, are mixed together in a suitable container, then the resin is added slowly with high speed stirring.  The second plastisol is the same as the first but a blowing agent and a wetting agent, as specified, are added before the resin.  It is stated that the preparations result in a suitable plastisol viscosity: a viscosity of about 8000 centipoises at 80 DEG F. is preferred for the first plastisol and about 2500 centipoises for the second.  The mould is first heated to a temperature of about 175 DEG F., though alternatively it may be heated after the plastisol has been poured into it.  The thickness of the skin which forms is proportional to the time; suitably the plastisol is poured out after it has been in the mould about 4 seconds.  A suitable time for gelling the skin by heat is 90 secs., at a temperature of 420 DEG F.  After the second plastisol has been poured in, the time for blowing it and fusing it to the deposited skin is 5-10 minutes at about 420 DEG F., though the temperature varies with the resin used and the amount of plasticizer. Another, inner, skin may be deposited on the second layer, from the same plastisol as the first, and fused with that layer when it is blown.  The layers may be of different colours, if desired.  The metal mould may be made in the following stages: (i) a model with the shape of the finished article is made of wood or plastic (ii) a flexible shell is formed on the model, of a low-temperature-setting elastomeric moulding compound (iii) plaster is cast in this shell to make a former or any number of formers (iv) the former is sprayed with nylon to give it a smooth surface (v) the former is then sprayed with hot molten aluminium, resulting in the required metal mould. The method is described especially with reference to making footwear and hot water bottles, and is stated to be an improvement on previous slush moulding processes for footwear because the cellular lining gives warmth.  Other articles mentioned are gloves or mittens, gaiters, and insulated food containers.