GB2106434A - Precision casting of metals - Google Patents

Abstract

Die-casting or injection moulding dies are usually expensive due to the need for skilled machining and hand finishing. The invention enables castings of high precision to be produced at comparatively low cost. Resin-coated granular refractory material is packed against a model of the desired casting, for example a plaster cast, and heated, preferably under pressure, to cure the resin to form a mould against which metal can be cast e.g. to produce a cast iron die for die-casting or plastics injection moulding. The granular material is preferably packed against the model in layers, each of a different grain size, and the packing may be effected around tubes which will serve for conducting cooling fluid during the metal casting. A plurality of moulds may be made, each corresponding with a different part of the desired surface, and the different parts cast by the moulds being assembled to form the casting die e.g. assembled under clamping pressure and heated to remove distortion. <IMAGE>

Description

SPECIFICATION Precision casting of metals Precision casting of metals, by the methods hitherto employed, is difficult and/or expensive.

Die-casting, of course, is well-known, and can be used to produce castings of high precision, particularly in light metal alloys, but the steel or other dies required are expensive to manufacture since they usually require skilled machining and hand finishing. Cast metal (e.g. iron) dies are known, but do not usually reproduce the patterns from which they are cast with a high degree of precision or economy. Known processes which enable more precise reproduction to be achieved require a substantial energy input in heating and involve the use of expensive equipment and the consumption of expensive moulding materials.

For example, in the so-called "Shaw" and "Unicast" processes, the refractory material, in the form of a liquid slurry suspended in an ethyl silicate binder, together with a catalyst, is poured over the pattern and hydrolyses initially into a rubbery solid form. This is stripped from the pattern and then must be stoved at about 9000C for a prolonged period to burn off the organic components and convert the silicate into a solid ceramic form. Furnaces to achieve such a temperature are costly and consume substantial energy. Partial recovery of the silicate refractory material is possible, but not economical unless very large quantities are used.

In the "Corning" or "Shell Moulding" process, moulding sand coated with a thermosetting resin is poured over a heated pattern. The sand adjacent to the pattern is heated by the pattern and its resin coating is first softened, uniting together the grains of sand near to the pattern, and is then cured to form a solid shell over the surface of the pattern. Sand more remote from the pattern is unaffected by the heat and can be poured back into a storage receptacle. As the sand is not significantly compacted around the pattern, the resultant shell mould does not usually produce a good surface finish on castings produced in it and cannot reproduce fine detail. Dimensional changes may also occur during the curing of the resin so that the mould does not accurately reproduce the shape of the pattern.Because of these difficulties, shell moulding is unsuitable for producing cast dies.

The invention enables finely detailed precision castings, such as cast dies, to be produced at a significantly lower cost than has hitherto been thought possible and without a high degree of skill.

According to the invention a method of precision casting of metals, employing a model including a representation of at least a part of the surface of the desired casting, comprises the steps of: packing at least a part of the surface of the model with a thermosetting resin-coated granular refractory moulding material; heating the packed moulding material to cure and set the resin to form a mould; separating the mould from the model; and casting metal against the mould.

The invention preferably includes the step of applying pressure to the packed granular moulding material whilst it is heated.

Various thermosetting resin-coated granular moulding materials are known and are suitable for use in performing the invention. For example, the above-mentioned resin-coated moulding sands used in shell moulding may be used. Preferred moulding materials are resin-coated zircon or chromite sands or mullite.

In packing the model with the resin-coated granular refractory moulding material, the surface of the model is preferably first coated with a release agent such as a silicone composition. The granular moulding material is poured into cavities in the model or into a box in which the model is mounted. Preferably a box is used in any case. The model or the box is vibrated during packing to cause the grains of the moulding material to settle and compact against the model and the surface of the moulding material is levelled off. Preferably, pressure is then applied to the levelled surface, either by placing an array of loading plates over the surface or by means of a pneumatic bag, for example of silicone rubber, trapped against the surface and inflated. A pressure of a few hundred grams per square centimetre (a few pounds per square inch) is adequate.The pressure is maintained whilst the packed model and moulding material are heated in an oven to cure the resin. A temperature of about 1 So0C for two hours is usuallly sufficient. During heating, the resin coating of the grains of refractory material is first softened, forming a lubricating film enabling the grains to move more closely to the model and to each other under the influence of the applied pressure. The resin flows on the surface of the model, forming a continuous compacted layer of grains and resin over the surface and reproducing substantially every detail. The resin then sets on - curing, solidifying the refractory material. The model is preferably removed from the refractory material before curing is complete and the refractory mould is heated further to complete the curing.In this way, more even heating of all parts of the mould can be achieved.

Because the granular moulding material is simply poured against the model during packing, it is possible to reproduce very fine or delicate details of shape in a model without causing them to break. Such details are wholly enveloped in the packed moulding material before any pressure is applied, so they are not unduly stressed.

The refractory mould so produced is used in conventional manner to produce a metal casting.

For example, one or more such moulds may be inserted into cavities in a moulding flask and iron may be poured against them.

If a desired casting is of a complex shape, separate models may be made of parts of its surface and these may be assembled to form a surface against which the refractory material is packed. Alternatively, the separate models may be packed separately, to produce separate refractory moulds which are then assembled in a moulding flask to define the shape of the desired casting.

After the cast metal has solidified, the refractory mould(s) is/are removed or may be broken away to release the casting. The used moulds can be fired to burn off the resin, enabling a substantial proportion -- about 95 per cent or more -- of the refreactory material to be recovered for re-coating with resin for further use.

The cost of materials for performing the invention is thus substantially reduced in comparison with .known methods in which the refractory material cannot be used more than once, except as lowgrade refractory fillers.

Cast metal dies for die-casting of metals or plastics injection moulding may be produced by means of the invention. A single die or a pair or set of dies may be required, depending upon the shape of the die castings or mouldings to be produced. Two or more models may be made, each having a surface reproducing a part of the surface of the casting or moulding which will be defined by one die of a pair or set of dies and with adjacent surfaces corresponding to the desired mating surfaces of the die pair or set when assembled, and preferably including countersinks or other locating means.A model for a single die, our a pair or set of models for a die pair or set, may be produced by a conventional plaster casting or similar method from a pattern of the die-castings or injection mouldings to be produced or an actual specimen of a die-casting or injection moulding.

The model or models may be cast in plaster, such as Plaster of Paris, or cementitious material reactive with water to form a set mass, or clay or the like. Cast dies are produced from these models though the intermediary of refractory mouldings as described, and can then be assembled together to define the required die cavity for casting or injection moulding.

These dies may suffer from slight distortion of the mating faces, but this can be rectified in known manner by clamping them in their assembled position under substantial pressure and heating, for example to a temperature of about 900"C for cast iron dies, and allowing them to cool.

Cast iron dies produced by the invention have been found to reproduce with precision the shape defined by the original model or models. Likewise, if the models are cast in plaster or the like from a pattern or specimen as described, the castings or mouldings produced in the dies faithfully reproduce the surface features of the pattern or specimen at a substantially lower cost than has hitherto been thought possible. Worn or broken die castings, such as parts of obsolete machinery, can be restored to their original shape by means of resin orotherfilling material and used as patterns for producing dies by this method. The original parts can then be reproduced, enabling such machinery to be brought back into useful service, which could not previously have been contemplated on economic grounds.

To control the thermal conductivity of the moulds, the granular refractory moulding material may be packed in layers of different grain size, and the surface finish of the moulds, and thus of the castings produced from them, can be improved by packing an initial layer of fine grain material, for example such as will pass through a 150 micron mesh seive, which may be backed up by layers of coarser grain size. The thermal conductivity of the moulds may be regulated further by inserting tubes into the granular moulding material as it is packed against the model. Cooling fluid may then be passed through the tubes during the casting of metal against the moulds.

Claims (8)

1. A method of precision casting of metals, employing a model including a representation of at least a part of a surface of the desired casting, comprising the steps of: packing at least a part of the surface of the model with a thermosetting resin-coated granular moulding material; heating the packed moulding material to cure and set the resin to form a mould; separating the mould from the model; and casting metal against the mould.

2. A method as claimed in Claim 1 including the step of applying pressure to the packed granular moulding material whilst it is heated.

3. A method as claimed in Claim 1 or Claim 2 wherein the granular moulding material is packed against the model in layers of different grain size.

4. A method as claimed in any of Claims 1 to 3 including the step of inserting cooling tubes into the granular moulding material as it is packed against the model.

5. A method of precision casting of metals substantially as hereinbefore described.

6. A method of producing cast metal dies for die-casting metals or injection moulding plastics materials comprising the method claims in any preceding claim in which the model is a cast made from a representation or specimen of the die casting or injection moulding to be produced.

7. A method of producing a cast metal die for metal die-casting or plastics injection moulding, comprising the steps of: making a plaster cast of at least a part of the surface of a pattern of the required die-casting or injection moulding; packing the plaster cast with a thermosetting resin-coated granular moulding material; heating the packed moulding material to cure and set the resin to form a mould reproducing the surface or said part of the surface of the pattern; separating the mould from the plaster cast; and casting metal against the mould to form the die.

8. A method as claimed in Claim 7 including the step of applying pressure to the packed granular moulding material whilst it is heated.