INVESTMENT POWDER

Abstract

An investment powder which is safer than conventional powders comprising tricalcium phosphate, and being substantially or entirely free of free silica in the respiratory portion yet providing an overall expansion at 750° C. of 1% or higher sufficient to prevent mould cracking during casting. A method of making a casting comprising forming a slurry by mixing a gypsum bonded investment powder with water, pouring the slurry into a stainless steel flask around a low melting point material model, allowing the slurry to set to define a mould, heating the mould to burn out the model and casting material into the mould wherein the stainless steel flask consists of a 400 series martensitic stainless steel.

Claims

1. A gypsum bonded investment powder comprising tricalcium phosphate, and containing less than 1% by weight free silica in the respiratory fraction.

2. The investment powder of claim 1 further comprising plaster.

3. An investment powder of claim 2 which comprises more than 30% to 70% by weight tricalcium phosphate.

4. The investment powder of claim 2 wherein the plaster comprises aridised plaster.

5. The investment powder of claim 1 comprising magnesium oxide and/or one or more low silica minerals.

6. (canceled)

7. The investment powder of claim 1 comprising: 10 to 30% plaster 25 to 75% tricalcium phosphate 10 to 65% magnesium oxide 0 to 25% of one or more low silica minerals 0 to 10% additives

8. The investment powder of claim 7 wherein said low silica minerals are selected from the group consisting of Vermiculite, Nepthaline Cyanite, Kyanite, Chlorites, Feldspar, Mica and Talc.

9. The investment powder of claim 1 comprising as an additive one or more wetting agents, de-foam agents, suspension agents, accelerators or retarders.

10. The investment powder of claim 1 having an overall expansion at 750° C. when formed into an investment casting mould of greater than 0.7%, preferably greater than 1% and more preferably greater than 2%.

11. (canceled)

12. A method of making an investment casting slurry by mixing the investment powder of claim 1 with water.

13. A method of making a casting comprising forming a slurry according to the method of claim 12, pouring the slurry around a low melting point material model, allowing the slurry to set to define a mould, heating the mould to burn out the model and casting material into the mould.

14. Use of a composition as an investment powder, said composition comprising 10 to 30% plaster 25 to 75% tricalcium phosphate 10 to 65% magnesium oxide 0 to 25% of one or more low silica minerals 0 to 10% additives

15. A method of making a casting comprising forming a slurry by mixing a gypsum bonded investment powder with water, pouring the slurry into a stainless steel flask around a low melting point material model, allowing the slurry to set to define a mould, heating the mould to burn out the model and casting material into the mould wherein the stainless steel flask consists of a 400 series martensitic stainless steel.

16. The method of claim 15 wherein said 400 series martensitic stainless steel is 410 stainless steel.

17. The method of claim 15 wherein said investment powder comprises an investment powder comprising plaster and calcium phosphate.

18. The method of claim 17 wherein said investment powder comprises tricalcium phosphate and/or magnesium oxide.

19. (canceled)

20. The method of claim 17 wherein said investment powder further comprises one or more low silica minerals.

21. The method of claim 15 wherein said investment powder comprises aridised plaster.

22. The method of claim 15 wherein said investment powder comprises: 10 to 30% plaster 25 to 75% calcium phosphate 10 to 65% magnesium oxide 0 to 25% of one or more low silica minerals 0 to 10% additives

23. The method of claim 15 wherein said investment powder comprises as an additive one or more wetting agents, de-foam agents, suspension agents, accelerators or retarders.

24. (canceled)

Description

EXAMPLE 1

[0093] The following tests were carried out using an investment powder comprising tricalcium phosphate, aridised beta plaster, and dead burned magnesite in flasks of either 316 or 410 stainless steel.

[0094] 9.8 kg of powder were weighed out and 3.724 litres of water were measured out. This is a mix ratio of 38/100.

[0095] 4 flasks were prepared, two were 9 by 4 inch 316 flasks and two were 7 by 4 inch 410 flasks.

[0096] The powder was added to the water and mixed without vacuum for 30 seconds, the blades were then scraped down and the slurry was mixed under vacuum for 4 minutes. The four flasks were poured in a total of 2.25 minutes and then vacuumed for a further minute.

[0097] After release of the vacuum gloss off occurred at a total of 14.75 minutes with a slurry temperature of 18 C.

[0098] Burn Out Cycle

[0099] The flasks were allowed to bench set for 90 minutes then fired in a furnace using the following burn out cycle—

[0100] Heat at 150 C and hour to 220 C

[0101] Hold at 220 C for 4 hours

[0102] Heat at 150 C an hour to 720 C

[0103] Hold at 720 C for 5 hours

[0104] Cool to casting temperature

[0105] Casting

[0106] All castings were done in silver and quenched at 15 minutes

[0107] Test 1—316 Flask—9 by 4 inch

[0108] Flask temperature 700 C

[0109] Metal temperature 1000 C

[0110] Metal weight 11 oz

[0111] A small amount of flashing was observed in the centre of the tree mainly to one side. 4 pieces were affected.

[0112] Test 2—410 Flask—7 by 4 inch

[0113] Flask temperature 650 C

[0114] Metal temperature 975 C

[0115] Metal weight 9.5 oz

[0116] On this casting there were no faults.

[0117] Test 3—316 Flask—9 by 4 inch

[0118] Flask temperature 500 C

[0119] Metal temperature 1000 C

[0120] Metal weight 17.5 oz

[0121] On this 316 flask tree there was again flashing present in the centre of the tree

[0122] Test 4—410 Flask—7 by 4 inch

[0123] Flask temperature 500 C

[0124] Metal temperature 950 C

[0125] Metal weight 9.5 oz

[0126] This casting appeared perfect with a good surface and quench in the 410 flask.