Improvements relating to investment casting

Abstract

A device for facilitating drainage of pattern material from a mould comprises a blow-off portion 4 having a stem defining an air channel and a flange bounding an opening of the air channel, a backing

Description

TECHNICAL FIELD This invention relates to wax drainage from a mould. In particular, though not exclusively, this invention relates to a device and a method for aiding drainage of pattern material from a mould, for example an investment casting mould. BACKGROUND Investment casting is one of the oldest known forming techniques. Investment casting is so named because the process invests (surrounds) a pattern, for example made of wax, with material to make a mould (or "investment"). The pattern is removed from the mould and, when a molten substance is cast into the mould, a replica of the pattern is formed. Materials that can be cast include stainless steel alloys, brass, aluminium, carbon steel and glass. Advantageously, the mould material may be a refractory / ceramic material. Due to the hardness of refractory materials, investment casting can produce products with exceptional surface qualities, which can reduce the need for secondary machine processes. Patterns may be made from a range of materials that can be mobilised, for example by melting or vapourisation, before a molten substance is cast. Moulds are formed with an opening for pouring in casting material. This opening can be used for pattern materials to exit the mould. For example, a mould may be placed in a furnace or autoclave with the opening facing downwards, to encourage pattern material, for example wax, to exit the mould. However, mould failures can occur on pattern removal. For example, pattern materials often have a thermal expansion coefficient that is greater than the investment material surrounding it - as the pattern material is heated it expands and introduces stress. Furthermore, particularly when casting high performance parts, residual pattern material in the mould can negatively affect quality. For example, an ash content of the pattern material may remain if such material is contacted with casting material, undermining the surface quality of the cast part as a result. The above problems are exacerbated in more intricate or larger patterns and moulds, as are often desired in modern manufacturing. In certain situations, holes may be drilled into the mould before heating to help remove pattern material. However, this can itself risk introducing uncontrolled stresses, cracks and debris. There remains a need in the art to facilitate drainage of pattern material from moulds. SUMMARY OF THE INVENTION From a first aspect, the invention provides a device for facilitating drainage of a pattern material from a mould, the device comprising: a blow-off portion comprising a stem defining an air channel and a flange bounding an opening of the air channel; a backing for abutting 5 the flange of the blow-off portion in an assembled configuration of the device; and a seal for sealing together the backing and the flange in the assembled configuration. The seal is breakable. Sealing the backing against the blow-off portion allows the blow-off portion to be blown-off in use by introducing pressurised fluid (e.g. air) into the air channel. Where the device has been incorporated into a mould the device can thus be utilised to form 10 an opening in the mould by blowing off the blow-off portion, without any need for drilling. The presence of the backing ensures that blow-off can be achieved consistently by providing control of the substrate against which blow-off forces act. This in turn helps to ensure that openings in moulds are formed predictably without unwanted damage. Such predictability is important since mould formation is typically irreversible. Advantageously, the device may be in the assembled configuration with the backing abutting the flange and the seal sealing together the backing and the flange. This readies it for use to facilitate drainage of a pattern material from a mould. To maximise blow-off force, the seal may seal together the backing and the flange in a substantially airtight manner. To help control blow-off force, the flange and the backing may be shaped to define a void therebetween in the assembled configuration, with the seal arranged to seal the void. The void may advantageously be symmetrical about a central axis of the air channel. Conveniently, the blow-off portion and the backing may be co-operable in an interference fit in the assembled configuration. This can aid assembly, for example by helping to hold together the blow-off portion and backing for application of the seal. Optionally, in the assembled configuration, a boundary of the flange may lie within a boundary of the backing. This can help ensure that the blow-off flange abuts against the backing in its entirety, aiding control of blow-off force. Advantageously, in the assembled configuration, boundaries of the flange and the backing may be aligned at an edge, with the seal sealing the edge. Such alignment facilitates the formation of a seal, in particular an airtight seal. Suitably, the flange and backing may comprise complementary bevelled boundaries that converge at the edge. Conveniently, the flange and backing may be circular in plan. However other shapes are readily conceivable. For example, square, oblong or oval shapes could also be used. 2 Suitably, the flange and backing may comprise an injection moulded plastics material. Injection moulding provides a convenient and cost effective method for manufacturing the flange and backing at scale. Suitable plastics materials are known in the art and include, for example, nylon, polyethylene (high or low density) and polypropylene. Advantageously, the backing may comprise a base and a projection extending from the base for co-operating with the air channel of the blow-off portion to locate the backing with respect to the blow-off portion. Suitably, the projection may be co-operable in an interference fit with the air channel. This facilitates accurate assembly of the device. Conveniently, the projection may be at a central axis of the base. This helps with consistent distribution of blow-off force. In some embodiments, the projection is cross- or star-shaped in section. The backing may comprise a base having curvature for defining a void between the flange and the backing in the assembled configuration. To help control stresses on blow-off, the seal may comprise a tapered edge. Suitably, the seal may comprise an anchor portion on a reverse side of the backing that is opposed to the blow-off portion. Such an anchor portion can aid stability and attachment of the device to a pattern. The anchor portion may comprise an anchor projection. The anchor projection may be aligned with the stem of the blow-off portion, extending in an opposed direction thereto. Conveniently, the seal may comprise a pattern material. This can facilitate attachment of the seal and device as a whole to a pattern. The pattern material may comprise a wax. The stem of the blow-off portion may comprise a blind end opposed to the opening and flange. The stem may comprise a thinned cut-off portion to facilitate cutting open the blind end. Advantageously, the stem may be shaped such that the air channel widens towards the flange. This can help with achieving an interference fit with the backing. The dimensions of the device and its components may be chosen freely, consistent with a desired size of opening to be formed. However, a relatively compact size can facilitate predictable operation. Suitably, the flange of the blow-off portion and / or the backing may have a maximum diameter or diagonal in the range of from 10 mm to 250 mm, for example in the range of from 25 mm to 100 mm, such as in the range of from 30 mm to 70 mm. Optionally, the stem of the blow-off portion may have a length in the range of from 10 mm to 100 mm, for example in the range of from 15 mm to 50 mm. Advantageously, the seal may comprise an anchor portion with an anchor projection having a length in the range of from 5 mm to 100 mm, for example in the range of from 10 mm 20 mm. From a second aspect, the invention provides a method of facilitating drainage of a pattern material from a mould, the method comprising: affixing a device according to any aspect or embodiment of the invention in the assembled configuration to a pattern comprising pattern material; forming a mould around the pattern and the device; blowing air into the air channel of the blow-off portion of the device to eject the blow-off portion and thereby form an opening in the mould; and optionally draining pattern material through the opening. Suitably, forming the mould may comprise forming an investment casting mould comprising a refractory material. The pattern material may comprise a wax. The mould may comprise a ceramic mould. Advantageously, the pattern may be for a high-precision part. Suitably, the pattern may be for a part to be formed of a titanium alloy. Optionally, the pattern may be for an industrial or aerospace component. In some embodiments, the pattern may be for a turbine blade. Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", mean "including but not limited to", and do not exclude other components, integers or steps. Moreover the singular encompasses the plural unless the context otherwise requires: in particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. Preferred features of each aspect of the invention may be as described in connection with any of the other aspects. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figures 1A, IB, and IC are side, top, and bottom views respectively of a device in accordance with an embodiment of the invention; Figure ID is a sectional view of the device of Figures 1A to IC, taken along line A-A indicated in Figure IC; Figures 2A and 2B are top and side views respectively of a blow-off portion of the device of Figures 1A to IC; 5 Figure 2C is a sectional view of the blow-off portion of Figures 2A and 2B, taken along line C-C of Figure 2A; Figures 3A and 3B are top and side views respectively of a backing of the device of Figures 1A to IC; Figure 3C is a sectional view of the backing of Figures 3A and 3B, taken along line D-10 D of Figure 3A; Figures 4A and 4B are top and side views respectively of a seal of the device of Figures 1A to IC; Figure 4C is a sectional view of the seal of Figures 4A and 4B, taken along line E-E of Figure 4A; and 15 Figure 5A to 5C are sequential schematic sectional views of the device of Figures 1A to IC attached to a wax pattern with a ceramic mould formed thereon, illustrating blow-off of the blow-off portion of the device. DETAILED DESCRIPTION With reference to Figures 1A to ID, a device 2 for facilitating drainage of pattern material comprises a blow-off portion 4, a backing 6 and a seal 8. The device 2 is shown in an assembled configuration in Figures 1A to ID, with the seal 8 sealing together the blow-off portion 4 and the backing 6. The blow-off portion 4 and the backing 6 are formed of an injection moulded plastics material, while the seal 8 is formed of a pattern wax. Referring now also to Figures 2A to 2C, the blow-off portion 4 comprises a stem 10 defining an air channel 12 and a flange 14 bounding an opening 16 of the air channel 12. The stem 10 and flange 14 are integrally formed. The stem 10 is shaped such that the air channel 12 widens slightly towards the flange 14. This facilitates engagement of the blow-off portion 4 with the backing 6 in an interference fit, as will be described. The stem 10 comprises a blind end 18 opposed to the opening 16 and flange 14 and a thinned cut-off portion 20 to facilitate cutting open the blind end 18 in use, as will be 5 described. The thinned cut-off portion 20 is defined by a shoulder 22 and a reduced wall thickness of the stem 10. With additional reference to Figures 3A to 3C, the backing 6 is shaped for abutting the flange 14 of the blow-off portion 10 to define a void 24 sealed by the seal 8. The backing 6 comprises a base 26 and a projection 28 extending from the base 26 for co-operating with the air channel 12 of the blow-off portion 4 to locate the backing 6 with respect to the blowoff portion 4. The projection 28 is cross-shaped in section and is located at a central axis of the base 26. The projection 28 is sized to be co-operable in an interference fit with the air channel 12 in the assembled configuration. The base 26 of the backing 6 has a curvature for defining the void 24 between the flange 14 of the blow-off portion 4 and the backing 6 in the assembled configuration. The blow-off portion 4 and the backing are circular in plan. In the assembled configuration a boundary 30 of the flange 14 lies within a boundary 32 of the backing 6. The boundaries 30, 32 of the flange 14 and the backing 6 are aligned at an edge 34. The flange 14 of the blowoff portion 4 and the backing comprise complementary bevelled boundaries 30, 32 that converge at the edge 34. Referring now also to Figures 4A to 4C, the seal 8 seals together the backing 6 and the flange 14 of the blow-off portion 4 in the assembled configuration. The seal 8 runs around the edge 34 defined by the aligned blow-off portion 4 and backing 6 to make the void 24 defined between the flange 14 and the backing 6 substantially airtight, unless seal 8 is broken. The seal 8 comprises a tapered annular edge 36. This helps to concentrate and direct stresses created upon breakage of the seal 8, as will be described. On a reverse side of the seal 8, on an opposed side of the backing to the blow-off portion 4 in the assembled configuration, the seal 8 comprises an anchor portion 38. This provides additional stability to the seal 8 and simplifies affixing the device to a wax pattern, as will be described. The anchor portion 38 comprises an anchor projection 40 aligned with the stem 10 of the blow-off portion 4, extending in an opposed direction thereto in the assembled configuration. Referring now to Figures 5A to 5C, the device 2 is affixed to a wax pattern 42 by heating the anchor projection 40 of the seal 8, which is also made of pattern wax, to soften. The softened wax is then used to secure the device 2 to the wax pattern 40. After the device is affixed, the wax pattern 42 is surrounded by a mould 44 ("investment") in accordance with a conventional investment casting process. Broadly, this might involve repeated coating, stuccoing, and hardening of a refractory material. Details of such processes and suitable materials, typically refractory, are known in the art. Common refractory materials used to create the investments are: silica, zircon, various aluminium silicates, and alumina. The binders used to hold the refractory material in place include: ethyl silicate (alcohol-based and chemically set), colloidal silica (water-based, also known as silica sol, set by drying), sodium silicate, and a hybrid of these controlled for pH and viscosity. The mould 44 is formed with an opening (not shown) for pouring in casting material. The device is also covered by the moulding process. Conveniently, that pattern with the device 2 attached can be dipped to apply the mould material. Referring now to Figures 5B and 5C, once the mould 44 is formed and before it is fired, the device 2 can be actuated to form an opening 46 in the mould. This is achieved by cutting off the cut-off portion 20 of the stem 10 to gain access to the air channel 12, and blowing air into the air 12 channel under pressure, e.g. factory line pressure of about 8 bar. As the blow-off portion 4 is sealed against the backing 6 by the seal 8, the air pressure within the void 24 forces the blow-off 4 portion away from the backing 6. This in turn causes the blow-off portion 4 of the device to be ejected together with part of the mould 44 and thereby form the opening 46 in the mould. The tapered edge 36 of the seal encourages a clean break of the mould 44. The opening can be used to drain the pattern wax 42 from the mould 44. While the opening may initially be covered by part of the device 2, this is readily removable together with the pattern wax 42. As the break in the mould 44 is on an appended part 48 of the mould 44 formed around the device 2, it does not detrimentally affect subsequent moulding of the desired pattern shape using the mould 44. Furthermore, the wax 42 within the mould 44 remains undisturbed and debris therefore does not enter the inside of the mould 44. Importantly, the device 2 can be used to create openings where desired to facilitate efficient and complete removal of wax even when using intricate or large patterns. It should be appreciated that the above embodiment is provided for illustration only and numerous modifications could be made without departing from the scope of the invention. For example, different shapes, materials, or dimensions could be used for the device. 27 05 25

Claims

1. A device for facilitating drainage of a pattern material from a mould, the device comprising:a blow-off portion comprising a stem defining an air channel and a flange bounding an opening of the air channel;a backing for abutting the flange of the blow-off portion in an assembled configuration of the device; anda breakable seal for sealing together the backing and the flange in the assembled configuration.

2. The device of claim 1, wherein the device is in the assembled configuration with the backing abutting the flange and the seal sealing together the backing and the flange.

3. The device of claim 1 or claim 2, wherein the flange and the backing are shaped to define a void therebetween in the assembled configuration and the seal is arranged to seal the void.

4. The device of any preceding claim, wherein the blow-off portion and the backing are cooperable in an interference fit in the assembled configuration.

5. The device of any preceding claim, wherein in the assembled configuration a boundary of the flange lies within a boundary of the backing.

6. The device of any preceding claim, wherein in the assembled configuration boundaries of the flange and the backing are aligned at an edge and the seal seals the edge.

7. The device of claim 6, wherein the flange and backing comprise complementary bevelled boundaries that converge at the edge.

8. The device of any preceding claim, wherein the flange and backing are circular in plan.

9. The device of any preceding claim, wherein the flange and backing comprise an injectionmoulded plastics material.

10. The device of any preceding claim, wherein the backing comprises a base and a projection extending from the base for co-operating with the air channel of the blow-off portion to locate the backing with respect to the blow-off portion.

11. The device of claim 10, wherein the projection is co-operable in an interference fit with the air channel.

12. The device of claim 10 or claim 11, wherein the projection is at a central axis of the base.27 05 2513. The device of any one of claims 10 to 12, wherein the projection is cross- or star-shaped in section.

14. The device of any preceding claim, wherein the backing comprises a base having curvature for defining a void between the flange and the backing in the assembled configuration.

15. The device of any preceding claim, wherein the seal comprises a tapered edge.

16. The device of any preceding claim, wherein the seal comprises an anchor portion on areverse side of the backing that is opposed to the blow-off portion.

17. The device of claim 16, wherein the anchor portion comprises an anchor projection.

18. The device of claim 17, wherein the anchor projection is aligned with the stem of the blowoff portion, extending in an opposed direction thereto.

19. The device of any preceding claim, wherein the seal comprises a pattern material.

20. The device of any preceding claim, wherein the stem comprises a blind end opposed to the opening and flange.

21. The device of claim 20, wherein the stem comprises a thinned cut-off portion to facilitate cutting open the blind end.

22. The device of any preceding claim, wherein the stem is shaped such that the air channel widens towards the flange.

23. A method of facilitating drainage of a pattern material from a mould, the method comprising:affixing a device according to any preceding claim in the assembled configuration to a pattern comprising pattern material;forming a mould around the pattern and the device;blowing air into the air channel of the blow-off portion of the device to force the blow-off portion away from the backing, and thereby ejecting the blow-off portion and forming an opening in the mould; and optionallydraining pattern material through the opening.

24. The method of claim 23, wherein forming the mould comprises forming an investment casting mould comprising a refractory material.

25. The device or method of any preceding claim, wherein the pattern material comprises a wax and the mould comprises a ceramic mould.27 05 25

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