GB2126148A - Mould making - Google Patents

Abstract

A method of removing polystyrene foam from a composite polystyrene/wax pattern in a mould is described using superheated steam. The polystyrene foam shrinks on heating to allow expansion of the wax. The pattern shown in Figure 1 comprises large volume runners and risers 2, 4 which are made from expanded polystyrene foam while the pattern 6 requiring higher accuracy is made from wax. In an alternative embodiment a pattern comprises expanded polystyrene around which wax is injected in a die to form a coating. <IMAGE>

Description

SPECIFICATION Mould making This invention relates to the making of moulds for the casting of components.

In the conventional "lost wax" process of making a mould there is first fabricated a wax structure consisting of a representation of the component to be cast, together with the necessary feeder and runner passages required for casting. A body of refractory ceramic material such as Silica with a chemical binding agent is then built up around the wax structure and the wax is removed or "lost" (e.g. by heating so as to melt the wax) to leave a mould with internal cavities defining the component and feeder and runner passages.

Difficulties arise in the handling of such wax structures because of their high overall weight, since the volumes required for the feeder and runner passages may, especially in the case of components having larger cross-sectional areas, or in multi-component moulds, be relatively large in order to minimise the introduction of undesirable properties in the material of the finished component.

It has been proposed that, in order to facilitate handling of structures for defining mould cavities, the portions of the structure which may have a large volume and do not require such high dimensional stability and such good surface finish as the component portion (e.g. the feeder and runner passage portions) be made from an expanded synthetic resin material such as expanded polystyrene which is considerably lighter than wax. However, the use of polystyrene as part of a structure in this way has been hindered by the difficulty of removing it from the mould.

It has been proposed to use a chemical solvent to remove the polystyrene but the required solvents are chlorinated hydrocarbons which are hazardous to use since they are toxic and often inflammable. It has also been proposed to vaporise the polystyrene from the mould when pouring the molten material to cast the component, but this is not possible where wax has to be removed from the mould since the wax must be removed before the mould is used for casting. It has also been proposed that the polystyrene be burnt out of the mould when the mould is fired, but again this is not desirable where the mould contains wax since the wax expands and may crack the mould.

It is one object of the present invention to provide a method of removing an expanded synthetic resin material from a mould whereby the above disadvantages may be overcome.

Another problem which arises with the use of an expanded synthetic resin material, such as expanded polystyrene, is the difficulty of securely attaching the wax patterns which form the casting cavities, to the polystyrene runner system.

It is a further object of the invention to provide a lightweight composite mould pattern including an expanded synthetic resin material to which additional wax pattern can be securely attached by conventional methods.

In accordance with the invention as claimed in the appended claims, synthetic resin material, which either wholly or partially forms a pattern for a casting mould, is removed from a casting mould by subjecting the mould to a superheated steam environment.

We have found that the expanded synthetic resin material, and in particular expanded polystyrene, shrinks to a fraction of its volume when contacted by superheated steam. This reduction in volume allows room for any wax parts of a composite pattern material to expand without cracking the surrounding mould material and thus makes possible the provision of lightweight composite moulds using both wax and expanded synthetic resin materials.

A preferred lightweight material is expanded polystyrene and this can be used in various forms, for example a foam, or a plurality of small beads held together in a binder.

The pressure and temperature of the superheated steam are not critical since expanded polystyrene shrinks significantly at temperatures above 1 00 C. For rapid removal of both wax and polystyrene, however, it is preferable to use superheated steam at elevated pressure and this is conveniently carried out in a steam autoclave.

When the expanded polystyrene has shrunk in volume to an acceptable extent, usually down to a small compact lump, it may be removed bodily from the mould, or the mould may be further subjected to the high pressure steam environment to evaporate it completely from the mould.

Alternatively, the remaining compact lump of polystyrene can be left in the mould to be vaporised when the mould is fired to sinter it before casting.

In one preferred form of composite wax and synthetic resin mould, expanded polystyrene is used as a lightweight chill around which a conventional wax pattern material is injected in a die to form a runner system. Additional wax patterns of the desired cast article can then be simply joined to the wax outer surface of the runner by a conventional technique of using a hot knife to melt the surfaces to be joined.

Examples of the present invention, including composite patterns and a method of removing them from casting moulds, will now be more particularly described with reference to the accompanying drawings in which: Figures 1 and 2 illustrate one form of composite pattern for casting turbine blades for gas turbine engines, and, Figures 3 and 4 illustrate an alternative form of composite pattern for casting other components for a gas turbine engine.

Referring now to Figures 1 and 2, there is shown a lightweight composite pattern for making a mould for casting turbine blades. The pattern consists of four vertical runners 2 interconnected at their bases by horizontal runners 4 and 5 at the base and top respectively and all of which are made from expanded polystyrene foam formed into a die of appropriate shape. Attached to the runner system are eight wax patterns 6 of turbine blades. The wax patterns are attached by melting the surfaces of the patterns and pressing them into position, or by means of a wax or other resin adhesive.

In order to provide a smooth surface on the runner and to aid in the attachment of the wax patterns 6, it may be dip-coated in a suitable resin or wax, or may be placed in a further die into which wax is injected to provide a thin, smooth coating over its surfaces. Preferably one surface of the polystyrene forming the horizontal runner 4 is left uncoated for ease of removal of the polystyrene.

In order to form a casting mould from the composite pattern, it is coated in conventional manner (e.g. by dipping) with a slurry of refractory ceramic material (e.g. Silica in a suitable binder) which is built up in layers and dried.

In order to remove the pattern, the mould is placed in a steam autoclave so that the uncoated surface of the expanded polystyrene is exposed to superheated steam at a temperature of approximately 1 700C and a temperature of 7 bars.

At this temperature and pressure the expanded polystyrene rapidly shrinks down to a small fraction of its expanded size allowing room for expansion of the wax portions of the pattern as the wax heats up to its melting temperature, and allowing a passage for the melted wax to run out of the mould. Any polystyrene remaining in the mould may either be physically removed, or the mould may be left in the autoclave until the polystyrene completely evaporates. Alternatively, the polystyrene may be left in the mould to be burned out when the mould is fired.

The temperature and pressure of the steam in the autoclave may vary widely, the main criteria being that the steam should cause shrinking of the polystyrene before significant heating and expansion of the wax takes place through the mould walls. The optimum for any pattern arrangement can be simply established by experiment.

Figures 3 and 4 show a different composite pattern in which expanded polystyrene is used as a lightweight chill to reduce the weight of a runner system of relatively large dimensions. In this arrangement the runner system includes a pouring cup 10 feeding horizontal runners 12 down each side and transverse runners 14. The runners are arranged to feed three casting cavities.

This pattern is made first by forming a chill in a die from expanded polystyrene beads held together in a low temperature binder and subsequently placing the chill in a second die and injecting wax into the die to surround the chill on all but one surface to a thickness of about 0.0625 in. The surface left uncoated is usually the surface 1 5 which forms the opening of the pouring cup 10.

The casting cavities are defined by wax patterns (not shown) which are joined to the runner system at the three square surfaces 1 6.

Recesses 18, 20 are formed in the surfaces 1 6 into which projections on the wax patterns fit for location of the wax pattern on the runner system.

Because the surface of the runner is wax, it is a simple matter to join the wax pattern to the square surfaces 1 6 by a conventional hot knifing technique.

To form a casting mould from this lightweight composite pattern, it is simply invested in ceramic slurry as is common practice. The pattern is removed, as described above, by placing the dried mould in an autoclave and exposing the polystyrene to superheated steam at appropriate temperature and pressures (e.g. 1 700C and 7 bars), whereupon it shrinks down to a fraction of its volume and allows the wax first of all to expand on heating and then to run out.

Claims (12)

1. A method of making a casting mould comprising the steps of: making a pattern for defining a cavity in the mould, said pattern being formed wholly or in part from an expanded synthetic resin material.

coating the pattern with refractory material leaving part of the pattern surface exposed, and, removing the pattern, characterised in that the step of removing the pattern includes subjecting the mould containing the pattern to a superheated steam environment to shrink the expanded synthetic resin material.

removing the pattern, characterised in that the step of removing the pattern includes subjecting the mould containing the pattern to a superheated steam environment to shrink the expanded synthetic resin material.

2. A method as claimed in Claim 1 and in which the expanded synthetic resin material comprises polystyrene foam.

3. A method as claimed in Claim 1 and in which the expanded synthetic resin material comprises beads of expanded polystyrene in a binder.

4. A method as claimed in Claim 1 and in which the mould is subjected to superheated steam until the expanded resin material has reduced in volume to the extent that it can be physically removed from the mould.

5. A method as claimed in Claim 4 and in which the mould is further subjected to superheated steam until the resin material has completely vaporised.

6. A method as claimed in Claim 1 and in which the pattern is a composite pattern comprising a portion made from an expanded synthetic resin material and a portion made from wax joined together.

7. A method as claimed in Claim 6 and in which the portion made from an expanded synthetic resin material is coated with a wax or resin material to which the portion made from wax is joined.

8. A composite pattern for making a casting mould comprising a portion made from wax and a portion made from an expanded synthetic resin material joined together.

9. A composite pattern for making a casting mould comprising a portion made from an expanded synthetic resin material and coated with wax.

10. A composite pattern as claimed in Claim 8 or Claim 9 and in which the expanded synthetic resin material is polystyrene.

11. A method of making a casting mould substantially as hereinbefore particularly described with reference to the accompanying drawings.

12. A composite pattern for making a casting mould substantially as hereinbefore particularly described with reference to the accompanying drawings.