INVESTMENT CASTING COMPOSITIONS

Abstract

A composition which comprises a soluble or partially soluble phosphate, a refractory material and less than 1% of an oxide or hydroxide of magnesium or calcium. The composition may be mixed with water to form an investment casting slurry into which a wax pattern may be dipped. Slurry coated onto the pattern may be set by applying a stucco composition which comprises an oxide or hydroxide of magnesium or calcium. Coats of set slurry may be built up on the pattern to form an investment casting shell.

Claims

1. A method of producing an investment casting shell comprising the steps of; providing a slurry comprising water, a refractory material and a soluble or partially soluble phosphate, dipping a pattern in the slurry to form a coat of slurry on the pattern, applying a stucco composition which comprises magnesium oxide, calcium oxide, magnesium hydroxide or calcium hydroxide to the slurry coated on the pattern, and allowing the slurry and the stucco composition to set to form a shell.

2. An investment casting composition comprising a soluble or partially soluble phosphate, a refractory material and less than 1 wt % of an oxide or hydroxide of magnesium or calcium.

3. An investment casting composition according to claim 2, wherein the composition further comprises insoluble fibres.

4. An investment casting composition according to claim 2, which comprises less than 0.4 wt %, 0.37 wt % or 0.3 wt % and preferably 0.2 wt % or less of an oxide or hydroxide of magnesium or calcium.

5. An investment casting composition according to claim 2, wherein the soluble or partially soluble phosphate comprises mono ammonium phosphate, mono sodium phosphate, mono potassium phosphate or mixtures thereof.

6. An investment casting composition according to claim 2, wherein the soluble or partially soluble phosphate comprises disodium phosphate, diammonium phosphate, dipotassium phosphate, or mixtures thereof.

7. An investment casting slurry comprising an investment casting composition according to claim 2 and water.

8. An investment casting slurry according to claim 7, wherein the viscosity of the slurry is 10-50, and preferably 20-40 on a B4 cup.

9. A method according to claim 1, wherein the slurry is according to claim 7.

10. A method according to claim 1, wherein the D.sub.50 of the stucco composition is between 0.25 mm and 1 mm.

11. A method according to claim 1, further comprising the step of heating the shell to melt the pattern.

12. A method according to claim 1, further comprising the step of firing the shell at 700-900 C.

13. A method of producing an investment casting shell according to claim 12 further comprising the step of delivering molten metal into the fired shell.

14. A method according to claim 13, further comprising the steps of allowing the molten metal to solidify and removing the shell to provide a cast part, and optionally finishing the cast part.

Description

EXAMPLE 1

[0047] 1000 g molochite, 125 g MAP, 125 g MSP and 15 g of nylon fibre (3.3 dctx by 1.6 mm long) were mixed to form a stable dry mix. The dry mix was then mixed with water to produce a stable slurry at a viscosity of between 30/32 on a B4 cup. This slurry was used to build up a shell formed of 6 coats.

EXAMPLE 2

[0048] 1000 g of 200 mesh zircon flour, 300 g MSP and 25 g cut nylon fibre (3.3 dctx 1 mm long) were mixed to form a dry mix. Water was added to the dry mix to produce a slurry having a viscosity of 40 on a B4 cup. This mixture is particularly useful as a face coat, i.e. the first coat of a shell, due to the presence of Zircon flour and the associated wetting properties.

EXAMPLE 3

[0049] 500 g of fused silica 200 mesh, 500 g zircon 200 mesh, 250 g MSP and 25 g polyester fibre (3.3 dctx 0.25/1 mm long) were mixed to form a dry mix. 450 cc of water was added to the dry mix to form a slurry having a viscosity of 40 on a B4 cup. This slurry can be used for first coat or backup coats.

EXAMPLE 4

[0050] A 25 kg dry mix was prepared. 6 kg MSP, 19 kg of molochite and 300 g of nylon fibre (3.3 dctx by 1.5 mm long) were mixed. In use, this dry mix was added to 13 l of water to form a slurry. The viscosity was between 30 and 32 on a B4 cup. This composition provides very strong shells after firing. This is because the sodium product of the ion exchange reaction has a low melting point and therefore helps to sinter the shell. This makes this composition well suited for nonferrous metals which have lower casting temperatures, but unsuitable in some ferrous applications which require higher casting temperatures (as the sodium product would melt completely). A shell using this composition can be made ready for metal casting in as little as 4 hours.

EXAMPLE 5

[0051] A dry mix and slurry was prepared as in example 4, but using 3 kg of MSP and 3 kg of MAP, instead of the 6 kg of MSP.

EXAMPLE 6

[0052] A dry mix and slurry was prepared as in example 4, but using 6 kg of MAP, instead of the 6 kg of MSP. This composition produces a weak shell when fired at 800 C. but is useful for speciality steels which require high temperature fired shells.

EXAMPLE 6A

[0053] A dry mix and slurry was prepared as in example 6, but using 6 kg of mono Magnesium phosphate powder, instead of the 6 kg of MSP. This composition produces a weak shell when fired at 800 C. but is useful for speciality steels which require high temperature fired shells.

EXAMPLE 6B

[0054] A dry mix and slurry was prepared as in example 6A, but using 6 kg mono aluminium phosphate powder, instead of the 6 kg of mono Magnesium phosphate powder.

EXAMPLE 7

[0055] A dry mix and slurry was prepared as in example 4, but using 6 kg MKP as an alternative to MAP. Some examples of the use of liquid phosphates are as follows in examples 8 and 9:

EXAMPLE 8

[0056] Clinochem P450 (proprietary solution of magnesium aluminium phosphate mix available from Clinochem Limited of Dunedin High Elms Road, Downe, Orpington, Kent, BR6 7JN), or Brenntag FFB80 or Albrite Magolop was diluted with water in the ratio of one part by volume of phosphate to one part by volume of water. Fibre was added at 20 mg per litre of prepared solution together with molochite 200 mesh to give the requisite viscosity. This makes a very strong shell but requires higher firing than the normally accepted firing temperatures. An alternative to high temperature firing is to add a sintering aid such as bentonite or fumed silica or calgon.

EXAMPLE 9

[0057] Clinochem P8 (polyammonium phosphate) or Albrtite 36AZ is treated in the same way as in example 8. This example generates significant quantities of NH.sub.3 while gelling which in production might limit its application.

EXAMPLE 10

[0058] An example of dry phosphate bond:

To prepare a dry binder blend mix, 500 g of MSP and 500 g of MAP were mixed with 75 g of polyester fibres (3.3 dctx by 1.5 mm long). To this dry blend water and refractory are mixed as described above.

Experiment 11

[0059] An investment casting slurry was prepared using a mono sodium phosphate (MSP). This preparation used mono sodium phosphate as the bonding component for the shell. The slurry was made by combining 80 g of MSP crystals with 320 g of calcined china clay (Molochite 200) and mixing this into warm water of approximately 20 C. to give a smooth slurry of viscosity 30 seconds on a B4 cup. The slurry was left overnight to de-air and allow the phosphate to dissolve.

[0060] Surprisingly on the following day it was found that the slurry was readily remixed to homogeneity. After retesting the viscosity the slurry was used to make a shell in the following manner.

[0061] A wax pattern was dipped into the slurry (as in normal practice) for 10 seconds and slowly removed to allow excess to drain off the pattern thus leaving a thin film of slurry. An even coating of sintered magnesite stucco (that is to say of granular composition with a particle size of 0.25 to 0.7 mm) was applied. This granular composition was surprisingly effective in causing a gel of the residual slurry upon the wax pattern such that with no further treatment (heating or drying) a further dipping could be carried out within 15 minutes at 20 C. The procedure was repeated so that after one hour the wax pattern had four coats of set ceramic. A further three coats were added using a coarser fraction of sintered magnesite stucco (0.7 to 1.2 mms) to encourage a more rapid increase of shell thickness and thus the strength.

[0062] A shell of seven coats was made within two hours with no drying or heating required. The shell so formed had a shell strength similar to a shell made with a silica sol binder which would have required a substantially longer time for drying. The phosphate shells so made were dewaxed immediately and ready to be fired to remove residual wax and the water. The firing called for careful heating to prevent cracking. The shells were heated to 850 C. as is normal practice. The sodium phosphate glass assisting the sintering of the shells. The quantity of sodium phosphate would, in some cases, be detrimental to steel casting.

[0063] The remaining slurry remained entirely stable for in excess of 5 weeks with no requirement to make adjustments to the slurry for pH. Evaporation of water was avoided by keeping the slurry in a sealed container. It was necessary to remix in order to ensure homogeneity before subsequent use.

Experiment 12

[0064] Carried out as Experiment 11 except that mono ammonium phosphate (MAP) was used instead of MSP. It was found that after firing at 850 C. the shell was very fragile and extreme care required to avoid collapse of the shell. A further shell was made but fired at 1250 C. This shell showed good strength after firing and was used to make a casting. This high firing temperature is not that normally used in the investment casting industry.

Experiment 13

[0065] Carried out as in Experiment 11 except that 80 g in a 50/50 mixture of MAP and MSP was mixed with 320 g of Molochite 200. This combination gave a satisfactory steel casting. The combination of these two phosphates in this particular ratio is not necessarily an optimum.

Experiment 14

[0066] Carried out as in Experiment 13 except that the combination used was MSP and fine particle, slightly soluble, mono magnesium phosphate. This behaved in a very similar manner to Experiment 13.

Experiment 15

[0067] 1000 g of Molochite 200, 125 g of MAP and 125 g of MSP were dry blended and stored in a dry place (sealed container). 1000 ml of water were blended with nylon fibres of 3.3 dtx by 0.5 mm length. It was found that the dry constituents required 450 ml of the liquid to arrive at a viscosity of 30 seconds on a B4 cup. After mixing, as in Experiments 11-14, the preparation of the shell was the same. However it was found that only four coats were required to achieve the same shell thickness as Experiments 11-14. The shells were more robust and it was found that a shell so made could be dewaxed and then immediately fired by plunging the shell into a preheated furnace at 850 C. Surprisingly the shells were found to resist this thermal shock. The shells so formed were readily knocked out after casting the metal. Each of these features aids the reduction of costs of making a casting.

Statements of Invention

[0068] 1. An investment casting composition comprising a soluble or partially soluble phosphate, a refractory material and less than 1 wt % of an oxide or hydroxide of magnesium or calcium.
2. An investment casting composition according to statement 1 wherein the composition further comprises insoluble fibres.
3. An investment casting composition according to either preceding statement, which comprises less than 0.4 wt %, 0.37 wt % or 0.3 wt % and preferably 0.2 wt % or less of an oxide or hydroxide of magnesium or calcium.
4. An investment casting composition according to any preceding statement wherein the soluble or partially soluble phosphate comprises mono ammonium phosphate.
5. An investment casting composition according to any preceding statement wherein the soluble or partially soluble phosphate comprises mono sodium phosphate.
6. An investment casting composition according to any preceding statement wherein the soluble or partially soluble phosphate comprises mono potassium phosphate.
7. An investment casting composition according to any preceding statement wherein the soluble or partially soluble phosphate comprises a mixture of two or more of; mono potassium phosphate, mono sodium phosphate and mono ammonium phosphate.
8. An investment casting composition according to any preceding statement wherein the soluble or partially soluble phosphate comprises disodium phosphate, diammonium phosphate, dipotassium phosphate, or mixtures thereof.
9. An investment casting composition according to any preceding statement, wherein the refractory material comprises molochite 200.
10. An investment casting slurry comprising an investment casting composition according to any preceding statement and water.
11. An investment casting slurry according to statement 10 wherein the viscosity of the slurry is 10-50, and preferably 20-40 on a B4 cup.
12. A method of producing an investment casting shell comprising the steps of;

[0069] providing a slurry comprising water, a refractory material and a soluble or partially soluble phosphate,

[0070] dipping a pattern in the slurry to form a coat of slurry on the pattern,

[0071] applying a stucco composition which comprises magnesium oxide, calcium oxide, magnesium hydroxide or calcium hydroxide to the slurry coated on the pattern, and

[0072] allowing the slurry and the stucco composition to set to form a shell.

13. A method according to statement 12 wherein the slurry is according to either of statements 10 or 11.
14. A method according to either of statements 12 or 13 wherein the D.sub.50 of the stucco composition is between 0.25 mm and 1 mm.
15. A method according to any of statements 12-14 in which the steps of dipping the pattern, applying the stucco composition and allowing the slurry to set to form a shell are repeated.
16. A method according to any of statements 12-15 further comprising the step of heating the shell to melt the pattern.
17. A method according to any of statements 12-16 further comprising the step of firing the shell at 700-900 C.
18. A method of investment casting comprising the step of delivering molten metal into a fired shell obtainable by the method of statement 17.
19. A method according to statement 18 further comprising the steps of allowing the molten metal to solidify and removing the shell to provide a cast part, and optionally finishing the cast part.
20. An investment casting shell comprising a refractory material and calcium or magnesium phosphate.
21. An investment casting shell according to statement 20 wherein the shell is obtainable by the method of any of statements 12-17.