GB2400066A

GB2400066A - Removal of a fugitive material in investment casting

Info

Publication number: GB2400066A
Application number: GB0307273A
Authority: GB
Inventors: Michael Anderson Brannan
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2003-03-29
Filing date: 2003-03-29
Publication date: 2004-10-06
Also published as: GB0307273D0

Abstract

A method of removing a fugitive material from a mould (22) is disclosed. The method comprises providing a mould (22) having a pattern (16, 20) therein formed from the fugitive material. A supercritical fluid (e.g. carbon dioxide) is supplied to the mould (22) to dissolve the fugitive material. The supercritical fluid is thereafter allowed to be removed from the mould (22). The mould is placed in a pattern removal chamber of an apparatus, which includes means for supplying a supercritical fluid to dissolve the fugitive material. Collection means may be provided to collect the supercritical fluid and the dissolved fugitive material. The supercritical fluid comprises one or more of a list of compounds provided in the text.

Description

REMOVAL OF A FUGITIVE MATERIAL

This invention relates to removal of fugitive pattern material. More part cuiarly, but not exclusively, the invention relates to removal of fugitive pattern material from a mould during an investment casting process.

Investment casting processes are used in the aerospace industry for the precision manufacture of various components, for example turbine or compressor blades.

The process involves the stages of first forming a master pattern, which may incorporate a core, and thereafter producing a metal die from the master pattern. A fugitive material, for example wax, or a wax blend comprising a complex mixture of resin, filler and wax, are then used to form a fugitive pattern from the die.

A plurality of fugitive patterns may then be assembled onto a runner system which is then formed into a mould by being dipped into a slurry of a ceramic material. After formation of the mould, the fugitive material is removed, generally by heating to melt the fugitive material and allowing it to run out of the mould. The cavity remaining in the mould is an exact replica of the original master pattern.

The moulding material is then poured into the cavity in the mould to form the final product. The product is then removed by any of various means known in the art and then finished to the required standard.

The heating of the fugitive material in the mould to remove it can cause problems in cases where the coefficients of expansion between the fugitive material and the mould are different, which can result in damage to the mould. - 2

According to one aspect of this invention, there is provided a method of removing a fugitive material from a mould comprising providing a mould having a pattern therein formed from said fugitive material, and supplying a supercritical fluid to said mould to dissolve and remove the fugitive material.

Preferably, the supercritical fluid comprises one or more of the following: methane, ethylene, hexafluoroethane, trifluoromethane, trifluorochloromethane, carbon dioxide, ethane, sulphur hexafluoride, propylene, propane, ammonia, pentane, isopropanol, methanol, ethanol, isobutanol, benzene, pyridine, water. Preferably, the supercritical fluid comprises carbon dioxide.

The fugitive material may comprise wax, or may comprise a mixture of a resin, a filler and wax. Alternatively, the fugitive material may comprise a polymeric material.

Preferably, the fugitive material comprises a composition comprising a plurality of polymeric materials.

The method may involve arranging the mould in a pattern removal chamber, and the method may further include supplying said supercritical fluid to said pattern removal chamber to dissolve the fugitive material. Collection means may be provided to collect the supercritical fluid and the dissolved fugitive material from the chamber. The collection means may comprise outlet means which may comprise a valve in the chamber through which the supercritical fluid and the dissolved fugitive material can pass.

The step of supplying the supercritical fluid may comprise providing a pump to pump the fluid at pressures above critical pressure For example, where the fluid is carbon dioxide, the supply pressure is preferably greater than 73.8 bar.

The supercritical fluid is preferably maintained at a temperature greater than the critical temperature of the fluid. Where the fluid is carbon dioxide, the temperature Is preferably maintained at greater than 31.1 C. - 3

According to another aspect of this invention, this provides an apparatus for removing a fugitive material from a mould during the casting process, the apparatus comprising means for receiving a mould from a pattern therein formed from said fugitive material and supply means to supply to said receiving means a supercritical fluid to dissolve and remove the fugitive material from the mould.

Preferably, the apparatus further includes means for collecting the supercritical fluid and the dissolved fugitive material. Said collecting means may be a valve in the receiving means.

Preferably, the receiving means comprises a chamber in which the mould is held.

The fluid supply means may comprise a pump capable of pumping fluid at pressures greater than supercritical pressure.

Where the fluid is carbon dioxide, the pump is preferably capable of pumping at greater than 73.8 bar.

Means may be provided to maintain the supercritical fluid and the receiving means at a temperature greater than the critical temperature of the fluid. Where the fluid is carbon dioxide, the temperature maintaining means may be adapted to maintain the temperature at greater than 31.1 C.

An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows the steps in an investment casting process; Figure 2 is a schematic apparatus for removing fugitive material from a casting. - 4

Referring to Figure 1, there is shown the various steps in an investment casting process. These steps are labelled A-J. A die 10 is formed from a master pattern, such that the die defines therein a cavity 12 which is the exact size and shape of the master pattern. In step A, the fugitive material, which may be wax, or wax blend, or a polymeric material, is injected Into the cavity 12 In the die 10 by a suitable injection device 14. Thus, a fugitive pattern 16 is formed in the cavity 12 from the fugitive material. As shown with reference to step B. a plurality of the fugitive patterns 16 are assembled onto a sprue 18 formed of the same fugitive material, to form a sprue assembly 20. In steps C and D, an investment mould is formed around the sprue assembly 20 by repeated dipping of the sprue assembly 20 into a ceramic slurry 22 (step C) and allowing the ceramic material to dry on the sprue assembly (step D). Thus, steps C and D are repeated until a sufficient amount of the ceramic material has built up around the sprue assembly 20 to form a mould 22. After the ceramic material is set, the fugitive material is then removed, as shown in step E. In the preferred embodiment of this invention, the fugitive material is removed by the use of a supercritical fluid, for example carbon dioxide. The supercritical carbon dioxide dissolves the fugitive material so that a cavity 24 remains in the mould 22 which is identical in size and shape to the original master pattern. This is explained in more detail below.

The mould 22 can then be fired as shown in step F. to remove any residual fugitive material and to improve the mechanical properties of the mould 22 for casting.

In step G. a molten metal material 26 may be poured into the cavity 24 in the mould and allowed to cool and solidify, to provide cast components in the form of a metal sprue assembly 28 in the mould 24. The mould material may then be removed as shown in step H. for example by impact, vibration, grit blasting, high pressure water blasting or chemical dissolution, and the cast components 30 can then be removed as shown In step I and then passed to a final finishing stage, in step J for finishing by finishing apparatus 32.

Referring to Figure 2, there is shown apparatus 40 for the removal of the fugitive material during step E described above.

The apparatus 40 comprises a device 42 for supplying a fluid for example carbon dioxide at supercritical temperature and pressure via a pipe 44 to a chamber 46. The device 42 can be a suitable pump. In the case of carbon dioxide, the fluid is pumped at a pressure greater than 73.8 bar, and at a temperature of greater than 31.1 C. The chamber 46 receives the mould 22 containing therein the fugitive pattern 16. In the embodiment shown, only one fugitive pattern 16 is provided, but it will be appreciated that a sprue assembly 22 could be provided. This embodiment of the invention will operate just as well in either case.

The chamber 46 further comprises collection means which may comprise a fluid removal conduit 48 and a valve 50 through which the supercritical fluid and the fugitive material dissolved therein can be collected from the chamber 46 for subsequent reprocessing to recover the constituent materials.

Heating means 52 may also be provided to ensure that the temperature inside the chamber is maintained above the critical temperature for carbon dioxide, namely above 31.1"C.

In operation of the apparatus shown in Figure 2, the mould 22 comprising a moulding material having a fugitive pattern 16 therein is arranged in the chamber 46. The carbon dioxide is pumped into the chamber 46 after it has been sealed. The pumping of the carbon dioxide occurs at a pressure of above the critical pressure of carbon dioxide namely above 73.8 bar. The heating means 52 maintains the temperature within the chamber 46 at above the critical temperature for carbon dioxide, namely above 31 1 C - 6 The supercritical carbon dioxide within the chamber 46 dissolves the fugitive material in the mould 22 and removes it therefrom to leave a cavity 24 therein which is the same size and shape as the master pattern. After a suitable time during which all the fugitive material In the mould 22 has been removed, the valve 50 is opened to allow the solution of the supercritical carbon dioxide and the fugitive material to be removed from the chamber via the conduit 48.

Various modifications can be made without departing from the scope of the invention, for example, the fluid could be another fluid other than carbon dioxide.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon. - 7

Claims

1 A method of removing a fugitive material from a mould, comprising providing a mould having a pattern therein formed from said fugitive material, and supplying a supercritical fluid to said mould to dissolve and remove the fugitive material.

2 A method according to claim 1, wherein the supercritical fluid comprises one or more of the following: methane, ethylene, hexafluoroethane, trifluoromethane, trifluorochloromethane, carbon dioxide, ethane, sulphur hexafluoride, propylene, propane, ammonia, pentane, isopropanol, methanol, ethanol, isobutanol, benzene, pyridine, water.

3 A method according to claim 1 or 2, wherein the supercritical fluid comprises carbon d ioxide.

4 A method according to any preceding claim, wherein the fugitive material comprises wax, or a mixture of a resin, a filler and wax.

A method according to any of claims 1 to 4, wherein the fugitive material comprises a polymeric material.

6 A method according to claim 5, wherein the fugitive material comprises a composition comprising a plurality of polymeric materials.

7 A method according to any preceding claim comprising arranging the mould in a pattern removal chamber and supplying said supercritical fluid to said pattern removal chamber to dissolve the fugitive material. - 8 c

8 A method according to claim 7 comprising providing collection means associated with the chamber to collect the removed fugitive material.

9 A method according to claim 8, wherein said collection means comprising a valve in the chamber through which the supercritical fluid and the dissolved fugitive material canvass.

A method according to any preceding claim, wherein the step of supplying the supercritical fluid comprises providing a pump to pump the fluid at pressures above critical pressure.

11 A method according to any preceding claim comprising heating means to heat the fluid to a supercritical temperature of greater than the critical temperature of the fluid and to maintain said supercritical temperature at least during said removal of the fugitive material.

12 Apparatus for removing a fugitive material from a mould during the casting process, the apparatus comprising means for receiving a mould from a pattern therein formed from said fugitive material and supply means to supply to said receiving means a supercritical fluid to dissolve and remove the fugitive material from the mould.

13 Apparatus according to claim 12, wherein the apparatus further includes means for collecting the supercritical fluid and the dissolved fugitive material, said collecting means comprising a valve in the receiving means.

14 Apparatus according to claim 12 or 13, wherein the receiving means comprises a chamber in which the mould is held.

Apparatus according to any of claims 12 to 14, wherein the fluid supply means comprises a pump capable of pumping fluid at pressures greater than the critical pressure of the fluid.

16 Apparatus according to any of claims 12 to 15, wherein means is provided to maintain the pumped supercritical fluid and the receiving means at a temperature greater than the critical temperature of the fluid.

17 A method substantially as herein described with reference to Figure 2 of the accompanying drawings.

18 Apparatus substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

19 Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.