GB650916A - Improvements in or relating to the production of cast or otherwise moulded enlarged or reduced facsimiles of objects - Google Patents

Abstract

Enlarged or reduced facsimiles of objects are produced by (a) forming from a highly-plasticized polyvinyl resin a mould of the object to be copied, (b) removing the object from the mould, (c) steeping the mould in an organic solvent which swells or shrinks the mould, and (d) casting or otherwise moulding a facsimile in the swollen or shrunk mould. The resin may be that containing 15-25 per cent of polyvinyl chloride and 85-75 per cent of plasticizer, sold under the Registered Trade Mark "Vinamold". Organic solvents that swell the resin are ketones, such as acetone and methyl cyclohexanone, certain chlorinated hydrocarbons, e.g. ethylene dichloride and trichloroethylene, and esters, such as amyl acetate. Organic solvents that shrink the resin are toluene, benzene, ethyl alcohol, kerosene, petrol, paraffin, white spirit and certain chlorinated hydrocarbons, e.g. carbon tetrachloride. By mixing the solvents, various degrees of swelling or shrinking can be obtained. The swelling action of methyl cyclohexanone is rather rapid, and it is preferred to dilute this solvent with an equal volume of white spirit. The period for steeping the mould in the solvent is in general at least 3 hours. The maximum enlargement of the mould is to about 1 1/3 its original size and the maximum shrinkage is to about 3/4 its original size. If the steeped moulds, whether swollen or shrunk, are allowed to stand in the open air they shrink, perhaps unevenly, to about 1/2 of the original size, and for this reason they are stored in the solvent. To produce very large or very small moulds for making very large or very small facsimiles, a first mould is prepared as described above, then an intermediate facsimile is cast in the first mould and removed therefrom, then a second mould is prepared from the intermediate facsimile, and this second mould is swollen or shrunk, the process being repeated until a sufficiently large or small mould has been obtained. A mould is made by pouring the molten resin over the object, e.g. a coin, medal, cameo, button or entomological or botanical specimen, and allowing it to cool; if the object is porous it is first coated with a solution of vinyl acetate in toluene or with a thermosetting phenol-formaldehyde lacquer. The material used for making the facsimiles may be plaster, wax, gelatine or those synthetic plastics which can be melted at temperatures below the softening point of the resin. When plaster is used it may be mixed with a 5 per cent solution of a wetting agent, to prevent blow-holes; this retards setting but produces a harder plaster. A plaster facsimile thus made may be coated with paraffin wax and used to make a larger mould and a larger facsimile. Two examples are given.