GB1442041A - Method of and apparatus for making plastic precision mouldings for medical and dental purposes - Google Patents

Abstract

1442041 Injection moulding ETAB DENTAIRE IVOCLAR 15 March 1974 [15 March 1973] 11658/74 Heading B5A In a method of making a precision moulding for medical or dental purposes, a polymerizable plastics material 19 is injected from a store into a mould cavity 23 and polymerized at a rate which decreases towards the injection aperture 8, and additional polymerizable material is injected from the store into the cavity 23 to compensate for shrinkage. Preferably the gradient of the rate of polymerization is achieved by producing a temperature gradient from the remote end of the cavity via the injection aperture 8 to the store, by thermally insulating the source of heat from the surface of the mould near the injection aperture and from the store. Fig. 1 shows apparatus for making a denture, comprising a mould 12, 12<SP>1</SP> made of plaster of Paris, mounted in two half casings 1, 1<SP>1</SP> closed by covers 5, 5<SP>1</SP> of material of low thermal conductivity. Teeth 24 of plastics material may be inserted into the mould cavity prior to injection moulding. The half casings 1, 1<SP>1</SP> have neck parts 2, 2<SP>1</SP>, and recesses to receive two halves 6, 6<SP>1</SP> of an insulating member. An insulative sleeve 9 is inserted in the neck parts 2, 2<SP>1</SP>, being spaced therefrom by an annular air gap 7. The mould parts are clamped together by means of a ram 14 which can be actuated by a screw press or an hydraulic press acting on a spring housing 15 and a set of cup springs 16. The force exerted on the housing 15 can be maintained by rotating a wedge ring 18 with respect to a fixed wedge ring 17 and securing the rings in the adjusted position. Polymerization can be carried out in a heating bath. The moulding material may be housed in a storage container 20 inserted in the sleeve 9. A cup-shaped insert 21 may protect the injection piston from the polymerizable material. This may be a mixture of at least one monomeric and at least one polymeric ester of acrylic acid or methaorylic acid. Modifications.-Instead of using a temperature gradient, the gradient of the rate of polymerization may be produced by increasing the concentration of a catalyst in the material initially injected, or by irradiating the material for polymerization, the radiation being more intense at the part of the mould remote from the injection aperture.