GB2148760A - Casting metal in a sand backed shell mould - Google Patents

Abstract

A method of casting metal comprising taking a pattern of decomposable material (defined as a solid state material capable of being transformed into the gaseous state by heat or combustion, e.g. expanded polystyrene or wood pulp), coating the pattern with a ceramic material, heating the thus coated pattern to fire the ceramic material to provide a rigid shell mould and to cause the pattern to decompose, surrounding the thus formed mould with dry unbonded sand, consolidating the sand to support the mould during casting, pouring liquid metal into the mould, permitting the liquid metal to solidify to provide a casting in the mould, removing the mould and casting from the sand and removing the mould from the casting. <IMAGE>

Description

SPECIFICATION Description of Invention This invention relates to casting metal.

An object of the invention is to provide a new and improved method of casting metal.

According to the invention, we provide a method of casting metal comprising taking a pattern of decomposable material (as herein defined), coating the pattern with a ceramic material, heating the thus coated pattern to fire the ceramic material to provide a rigid shell mould and to cause the pattern to decompose, surrounding the thus formed mould with dry unbonded sand, consolidating the sand to support the mould during casting, pouring liquid metal into the mould, permitting the liquid metal to solidify to provide a casting in the mould, removing the mould and casting from the sand and removing the mould from the casting.

By "decomposable material" we mean a material which, when in a solid state, is sufficiently strong to enable the ceramic material to be formed therearound and which can be, at least substantially completely, transformed to the gaseous state by subjecting the material to heat to cause it to vapourise, and/or to burn, and/or some other heat initiated chemical reaction.

One example of a suitable decomposable material is expanded polystyrene which is decomposed to the gaseous state by burning, but any other decomposable material, which is preferably an expanded material, such as an expanded polymer, may be used, and it is thought that a non-expanded material such as wood pulp may be used.

The pattern may be coated with ceramic material by using a slurry which may be water based or alcohol based.

Examples of water based slurries are: Colloidal silica binder and zircosil flour Colloidal silica binder and fused alumina Colloidal silica binder and silica flour.

Examples of alcohol based slurries are: Ethyl silicate binder and zircosil flour Ethyl silicate binder and an alumino silicate refractory filler such as molochite filler.

After draining, the thus slurry coated pattern may be coated with stucco, for example, by being dipped into a fluidised bed of the stucco or the stucco can be "rained" on.

The stucco may comprise fine particles of zircon.

Thereafter, the thus coated pattern may be allowed to dry for at least four hours after which at least one further cycle of slurry and stucco application may be performed.

The at least one further cycle may be performed using materials of lower refractoriness.

The shell mould may be fired at a temperature suitable for the ceramic material, for example lying in the range 1000"C to 11 000C for at least one hour.

The sand may be consolidated by means of vibration and the sand may be contained within a moulding box.

The method of the present invention permits of rapid and economic production, of shell moulds suitable for mass production without the disadvantages incurred with the conventional wax moulds of investment casting.

The method avoids any problem of gas and carbon pick-up such as is encountered using known methods of casting utilising the full mould technique with expanded polystyrene as the pattern. The method eliminates any risk of sand inclusions which can be a problem with the full mould technique. The method also avoids known disadvantages of conventional sandlshell mould casting since cores are unnecessary, draft angles are reduced and flash is eliminated.

It has been found that 75% of pattern shrinkage occurs within seven days of pattern manufacture.

The remaining 25% occurs slowly over the following twenty one days. As the total pattern shrinkage is of the order of 0.100in/foot, after 75% shrinkage has occurred the remaining shrinkage is well within casting tolerance and thus the problem of pattern shrinkage can be overcome by storing the patterns for seven days after production.

A polyvinyl acetate sealant may be applied to the internal surface of the shell mould to fill any open pores in the surface, thereby improving the surface quality.

The invention will now be described in more detail by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a flow diagram illustrating the steps involved in the example described.

A die of the configuration to provide the desired final shape of the casting is produced and incorporated in a conventional vacuum blow moulding machine.

Polystyrene granules of approximately 0.5 mm diameter and containing pentane are pretreated by heating with steam which reacts with the pentane to increase the size of the granules. The granules are then transferred to storage bins where they are allowed to age for at least four hours. The aging process is significant as it controls the final pattern shrinkage.

After aging, the granules are transferred to the blow moulding machine where they are injected into the die. At the same time, a vacuum is applied inside the die which pulls the granules into the corners of the die. Steam is then injected which causes the granules to fuse together. Thereafter, the resultant expanded polystyrene pattern is water cooled and ejected.

Depending upon the shape of the final casting, the patterns may be moulded in two or more parts. In this case, the parts are bonded together using a suitable contact adhesive.

The pattern is then stored for at least seven days so that at least 75% of the total pattern shrinkage takes place.

The pattern is then coated with ceramic material.

In the present example, this is obtained by immersing the pattern in a slurry comprising a colloidal silica binder with a zircon filler. After removal from the slurry bath, the coated pattern is drained and then dipped into a fluidised bed of fine zircon grannular stucco. After removal from the fluidised bed, the coated pattern is allowed to dry for at least four hours. Thereafter, a secondary coat is applied to give strength and support to the primary coat previously described and in the present example is made from a material of lower refractoriness, although, of course, could be made from the same material, as the primary coat.

The secondary coat is applied by immersing the primary coated pattern in a slurry comprising an ethyl silicate binder and a filler of alumino silicate refractory. In the present example molochite.

After dipping of the pattern into the slurry and draining to give an even cover, it is dipped into a fluidised bed of molchite stucco. The molochite stucco has a grain size falling in the range BSS mesh size -30 to -80. As the slurry is alcohol based a drying period of about one hour is all that is required.

Thereafter, in the present example, three more coats of the ethyl silicate binder with the molochite filler, each followed by dipping into a fluidised bed of molochite stucco having a grain size lying in the range BSS mesh size -16 to -30 are applied.

The completed shell is allowed to dry overnight before the next stage of the process is performed which comprises firing the shell, in the present example in a gas batch oven at 1 0500C for one hour.

The firing operation not only develops high strength in the ceramic material but also removes the polystyrene pattern by burning so that it escapes through the porous ceramic material without damage to the shell mould.

When the shell mould is to be used for casting iron, the shell mould is cooled. When the shell mould is to be used for casting steel or non-ferrous metals, casting is performed with the shell mould hot.

Therefore, after a relevant time interval, which may be up to days or even weeks for casting iron, and is as soon as possible after leaving the oven for steel or non-ferrous metals, the mould is placed in a moulding box and covered with dry unbonded sand.

The sand is consolidated by a vibration table so as to give additional support to the mould during casting.

The metal to be cast is then poured into the mould and after it has cooled, the resulting casting and the mould are removed from the sand and then the ceramic shell mould is removed, for example by using a combination of vibratory hammering and shot-blasting.

Claims (27)

1. A method of casting metal comprising taking a pattern of decomposable material (as herein defined), coating the pattern with a ceramic material, heating the thus coated pattern to fire the ceramic material to provide a rigid shell mould and to cause the pattern to decompose, surrounding the thus formed mould with dry unbonded sand, consolidating the sand to support the mould during casting, pouring liquid metal into the mould, permitting the liquid metal to solidify to provide a casting in the mould, removing the mould and casting from the sand and removing the mould from the casting.

2. A method according to Claim 1 wherein the decomposable material comprises expanded polystyrene which is decomposed to the gaseous state by burning.

3. A method according to Claim 2 wherein the pattern is coated with ceramic material by using a slurry which is water based or alcohol based.

4. A method according to Claim 3 wherein the slurry is water based and comprises colloidal silica binder and zircosil flour or colloidal silica binder and fused alumina or colloidal silica binder and silica flour.

5. A method according to Claim 3 wherein the slurry is alcohol based and comprises ethyl silicate binder and zircosil flour or ethyl silicate binder and an alumino silicate refractory filler.

6. A method according to any one of Claims 3 to 5 wherein the slurry coated pattern is drained and then coated with stucco.

7. A method according to Claim 6 wherein the pattern is coated with stucco by being dipped into a fluidised bed of the stucco.

8. A method according to Claim 6 wherein the pattern is coated with stucco by "raining "the stucco onto the pattern.

9. A method according to any one of Claims 6 to 8 wherein the stucco comprises fine particles of zircon.

10. A method according to any one of Claims 6 to 9 wherein after coating with stucco, the thus coated pattern is allowed to dry for at least four hours.

11. A method according to Claim 10 wherein after said drying for at least four hours, at least one further cycle of slurry and stucco application is performed.

12. A method according to Claim 11 wherein the at least one further cycle is performed using materials of lower refractoriness than were used in the first performed cycle.

13. A method according to any one of the preceding claims wherein the shell mould is fired at a temperature lying in the range 1 0000C to 11 000C for at least one hour.

14. A method according to any one of the preceding claims wherein the sand is consolidated by means of vibration alone.

15. A method according to any one of the preceding claims wherein the sand is contained within a moulding box.

16. A method according to any one of the preceding claims wherein the pattern is stored for a time such that a predetermined amount of shrinkage occurs prior to coating the pattern with said ceramic material.

17. A method according to Claim 16 wherein said predetermined amount of shrinkage is 75%.

18. A method according to any one of the preceding claims wherein a polyvinyl acetate sealant is applied to the internal surface of the shell mould to fill any open pores in the surface, thereby improving the surface quality.

19. A method according to any one of the preceding claims wherein the material comprises iron or steel.

20. A method according to Claim 19 wherein the metal is iron and the mould is cooled.

21. A method according to Claim 19 wherein the metal is steel and the mould is not cooled.

22. A method according to any one of Claims 1 to 18 wherein the metal is a non-ferrous metal.

23. A method according to Claim 22 wherein the mould is not cooled.

24. A method according to any one of the preceding claims wherein the sand is not provided with a bonding agent.

25. A method of casting metal substantially as hereinbefore described with reference to the accompanying drawings.

26. A metal casting when made by a method as claimed in any one of the preceding claims.

27. Any novel feature or novel combination of features disclosed herein andlor shown in the accompanying drawings.