GB2030065A

GB2030065A - Slip casting

Info

Publication number: GB2030065A
Application number: GB7803716A
Authority: GB
Original assignee: Norton Co
Current assignee: Saint Gobain Abrasives Inc
Priority date: 1978-09-18
Filing date: 1978-09-18
Publication date: 1980-04-02
Also published as: GB2030065B

Abstract

A process of obtaining a casting comprises: (a) solidifying in a porous mold a castable slip comprising refractory particles and liquid, so as to provide a casting in said mold; (b) treating said casting while said casting is in said mold, so as to give or increase green strength of said casting; and (c) weakening at least a portion of said mold, so as to facilitate removal of said mold from said casting having said green strength. Preferably, said mold comprises plaster of paris. <IMAGE>

Description

SPECIFICATION Process of forming silicon carbide bodies In the formation of intricate cast shapes of silicon carbide having delicate, thin, complex, curved surfaces, it is desirable to utilize a mold such as plaster of paris which can be conveniently formed by known processes. In employing such a mold it is desirable to use a slip cast technique as described in Fredriksson United States Patent No. 2,964,823. However, the green product formed by slip casting silicon carbide is relatively fragile and, in a complex mold system, it is extremely difficult to remove the mold without harming the green casting or it can be very costly to make a multi-piece mold. In the present invention this difficulty is overcome in several ways, preferably by providing in the slip casting mix a water miscible resin which can be cured by firing the mold (and the cast shape) to a relatively low temperature.Alternatively the mold and silicon carbide slip casting are heated to a temperature in excess of about 600"C. for a sufficiently long time to oxidize the silicon carbide and thereby substantially increase the green strength of the casting. Thereafter, the mold and the cured casting are subjected to an acid bath which destroys the plaster of paris in the mold and frees the casting for subsequent treatment.

Alternatively, or additionally, the plaster mold may be treated by incorporating in the plaster of paris mold mix 10- 15% (by weight) of an aqueous solution which contains about 1 to 5% (by weight) of an organic polymer and the mold is heated in an oxygen-containing atmosphere to a sufficiently high temperature to burn out a substantial portion of the polymer in the mold and thereby drastically weaken the mold to permit its ready removal from the green casting.

Recrystallization of silicon carbide powder to form high temperature elements goes back at least 75 years to the old United States Patent No. 650,234 to Fitzgerald. A number of patents to Tone (such as United States Patents Nos. 913,324 and 1,013,701) show the siliconization of silicon carbide. Similarly the Hutchins United States Patent No. 1,266,478 shows the in situ formation of silicon carbide in a body by reaction between silicon and carbon incorporated in the body. Fredriksson United States Patent No. 2,964,823 shows the slip-casting of silicon carbide bodies which can then be fired to recrystallization temperature. The Johnson United States Patent No. 3,079,273 shows the formation of objects from a mixture of silicon carbide, graphite and polymeric plastic.The objects are molded from the plastic and joined by part of the same mix and then fired at an elevated temperature to carbonize the plastic and siliconize the carbon contained in the mixture. Alliegro United States Patent No. 3,482,791 describes the slip casting of a silicon carbide refiner plate which is then siliconized to increase its impermeability.

In Schleicher United States Patent No. 2,303,303 there is described a decomposable slip casting mold containing wood flour which can be burned out to weaken the mold.

LeBlanc et al United States Patent No. 3,549,473 describes the addition of organic resin to a ceramic. There is a similar disclosure in the old United States Patent No. 1,450,140 to Danahower. These latter two patents do not relate to slip casting, however.

Referring now to the present invention, in general a castible slip of a biomodal distribution of silicon carbide particles is prepared. The slip is then cast into a plaster of paris mold which can be ultimately destroyed to permit its removal from the green casting. Prior to the removal of the mold from the casting, the green casting is partially strengthened in one of several ways. In one method a curable resin is added to the slip and this curable resin is cured prior to removal from the mold. In another method of strengthening the green casting it is heated, while still in the mold, to temperaures sufficiently high to slightly oxidize the silicon carbide and thereby form a glassy bond which strengthens the green casting.

The mold can be weakened in several ways to permit ready removal from the strengthened green casting. In one method it can be immersed in acid or some other medium which attacks the plaster of paris itself. In still another method of forming a decomposable mold a water soluble polymer may be added to the plaster of paris, this polymer being subsequently burned out of the mold at an elevated temperature. This drastically weakens the mold so it can be brushed away from the green casting.

The plaster of paris is preferably treated, at least on its inner surface which contacts the slip, so as to appreciably slow down the absorption of water from the slip to permit complete filling of the mold, this being particuarly important when very thin sections, such as airfoils of turbine blades and the like, are to be formed. The curable or otherwise settable resin preferably is one which is cured or set at a relatively low temperaure on the order of 200"C. or less and one which, when cured, has sufficient resistance to attack by hydrochloric acid, for example, so as to readily survive the mold-removing bath.

The green product removed from the mold is preferably fired to a sufficiently elevated temperature to recrystallize the silicon carbide and form a relatively rugged structure. This product will have some residual carbon remaining after the firing operation due to the decomposition of the resin binder. Where a fairly dense product is desired, additional carbon can be provided in the interstices of the relatively porous recrystallized silicon carbide structure by utilizing furfuryl alcohol, as described in Heyroth United States Patent No. 2,431,326 or Taylor United States Patent No. 3,205,043.As the final step in the preparation of the product, the carbon-containing porous silicon carbide is fired in a silicon-containing atmosphere at a temperature on the order of 2000"C. to convert all of the contained carbon to silicon carbide and, if desired, to completely impregnate the body with free silicon.

In one preferred embodiment of the invention a set of turbine wheel blades is cast as set forth in the following non-limiting example.

EXAMPLE I Exact wax replicas of the final turbine wheel blades were prepared by suitable molding techniques and these were used as the forms around which plaster of paris molds were then formed. The wax used for the initial bodies was typically a jewelry pattern wax obtained from Freeman Manufacturing Company under the tradename JW 3715-H and was injected at about 66"C. (150"F.), about 0.7 Kg/cm2 (10 p.s.i.). After the plaster of paris was hardened the molds were heated to about 49"C. (120"F.) to melt out the wax, thus leaving a thin coating of wax on all of the interior surfaces of the plaster of paris and appreciably decreasing the absorption of the surfaces for water from the casting slip.

A slip for casting was prepared from the following mixture: fine silicon carbide average particle size 2.5 to 3 microns 5833 grams coarse silicon carbide-200 mesh Tyler, which passes particle sizes less than 74.0 microns 5600 grams water 2500 grams sodium silicate 25 grams The mixture was tumbled in a plastic milling jar with 2.5 centimeter diameter rubber balls for 16 hours. The final slip was prepared by adding to this milled mixture, 698 grams of a liquid resole resin Varcum 5137 (manufactured by Reichhold Chemicals, Inc.) and milling this combination for one hour. The slip was then poured into the plaster of paris molds preferably through a bottom sprue, the slip being allowed to pass completely through each mold into a sprue extending from the top thereof.When it appeared that the molds were completely filled, the slip was put under a gas pressure of about 1.4 Kg/cm2 (20 p.s.i.) and maintained for 45 minutes to make sure that all parts of the molds were completely filled. Thereafter the pressure was removed after the slip had solidified in each mold. The molds and the solidified slips were then heated to 90"C. for 24 hours and 180"C. for 2 hours to cure the phenolic resin. Thereafter the molds containing the cured resin castings were placed in a bath of boiling 50% hyrdochloric acid for 5 minutes. This acid bath attacked the molds so that after removal from the bath and washing with water the remnants of the molds could be easily brushed from the delicate green castings.Thereafter the piece was fired to about 1970"C. for 1 5 minutes to recrystallize the silicon carbide and carbonize the resin.

The blade shapes were then removed from the oven and were treated with a polymerizable furfuryl alcohol based material, by impregnating with a solution of FAPREG P5 (Quaker Oats Co.) for 2 hours at room temperature, cured at 90"C. for 24 hours and 250"C. for 2 hours. This process was repeated twice, leaving the pieces essentially free of void areas.

The pieces were then fired in a furnace having a silicon atmosphere at about 2070"C. for about 30 minutes to convert essentially all of the carbon in the products to silicon carbide. The firing also permitted an additional amount of silicon to be absorbed in the products so that the final products contained ~ 12% elemental silicon.

EXAMPLE 2 A monolithic ceramic turbine rotor was formed by casting a slip essentially the same as that of Example 1, into a plaster of paris mold of a turbine rotor shape. The slip was cast and processed as described above and the mold was removed from the set green shape, by boiling in hydrochloric acid. The subsequent recrystallization and siliconizing step were carried out in the manner of Example 1.

While one preferred method of increasing the density of the product by the use of furfuryl alcohol as described in the Heyroth United States Patent No. 2,431,326 and Taylor United States Patent No. 3,205,043, other methods of precipitating carbon in the recrystallized silicon carbide body can be employed.

A preferred resin for curing in the original slip is described above as being a liquid resole phenolic resin, but solid powdered phenolic resins such as "Varcum" 29346, are equally suitable. Union Carbide Corporation's BRP-5417 and BRP-5980, Borden Chemical Company's 5164, and Ashland Chemical Company's resin 890, are equally suitable. Almost any of the numerous organic polymers commercially available, that cure or at least set to a solid state and are resistant to boiling acid for a short period of time, are operable. Resorcinol-formaldehyde or alkyd resins can be substituted for the phenol-formaldehyde resole utilized in the example above. Polymers such as melamine-formaldehyde, urea-formaldehyde, epoxy, polyester, and the like, may also be used but these are employed with much more difficulty.Some of these materials have a drastic effect on the pH of the slip, so that if manipulation of the pH is not done, the addition of one of these resins, e.g. an amine-epoxy system, will cause the pH to increase so substantially that the viscosity of the slip increases to the point where the slip will no longer cast properly.

Similarly, a lacquer, such as L-18 Clear Lacquer sold by Raffi and Swanson Inc., can be used to coat the interior of the plaster of paris mold and reagents other than hydrochloric can be used to decompose the plaster of paris mold.

In the preceding discussion the plaster of paris mold was dissolved from the slip casting by the use of hydrochloric acid. It is often desirable that the material of the mold be relatively soft to prevent its exerting stresses on the cast piece during the heat treatment of the cast piece to give it additional green strength. In order to obtain such a relatively soft mold a procedure has been developed which involves adding a solution of an organic polymer to the plaster slip just before the mold is cast. In the preferred embodiment of the invention this organic polymer is methylcellulose, sold under the trademark Methocel.

The introduction of methylcellulose causes the formation of very fine bubbles in the plaster of paris mix which subsequently causes porosity in the final plaster casting. Because these bubbles are extremely small and well distributed they form a smooth surfaced, but porous, soft plaster which can be easily decomposed. The methylcellulose serves a second function and that is the suspension of undissolved plaster particles. If one were to use the same plaster to water ratio, without the methylcellulose, as can be used with the methylcellulose the plaster would tend to settle a great deal.

In the course of working with the methylcellulose it was found that the amount of methylcellulose could be changed considerably. When the high concentrations of the methylcellulose are used it can serve as a combustible component in the plaster of paris mix which, when burned out, drastically weakens the plaster of paris mold. An example of the use of methylcellulose is given in the following nonlimiting example: EXAMPLE 3 200 grams of plaster of paris is added to 2 grams of water. This is mixed for 3 minutes and then a pinch of already set plaster which has been ground up is added to cause more rapid setting. Mixing is continued for 9 minutes and thereafter a methylcellulose solution is added for an additional mixing time of 2 minutes. Thereafter the mix is poured to form a plaster of paris mold.The amount of methylcellulose may vary from .1% to 4%, by weight, this amount being added in the form of a methylcellulose solution whose concentration varies from about 1 % to 8% methylcellulose. The upper concentration of methylcellulose is reached when the mold shrinks excessively during drying. In general, 10 to 15% of a 2% methylcellulose solution is quite satisfactory, particularly when the plaster of paris mold is to be decomposed by acid leaching. When the plaster of paris mold is to be decomposed by burning it is preferred that a somewhat higher concentration of the methylcellulose be utilized.

To burn out the polymer, the mold with the silicon carbide slip casting is heated from room temperature to about 650 to 700'C. over a time schedule of 15 C. per hour. After cooling back to room temperature the plaster mold is extremely soft and may be easily brushed from the piece. During this heating cycle the silicon carbide green casting is slightly oxidized to form a glassy phase which substantially increases the strength of the green casting.

While one type of water soluble organic polymer has been described above numerous others may be employed which can be either decomposed by acid or can be burned out to weaken the casting. Examples of such additional organic polymers are: carboxyvinyl polymers of the type sold by B.F. Goodrich under the trademark Carbopol 934, xanthan gums as sold by Kelco Company under the trademark Kelzon or an ammonium alginate as sold by Kelco Company under the trademark Superloid.

Although a plaster of paris mold is preferred, since the technology for its use is highly developed, other types of molds can also be employed, particularly where a water soluble organic polymer is utilized to provide additional porosity.

It should be noted, that although the foregoing description of the invention is presented within the context of forming thin walled, delicate, and complex shapes, this is not to be construed as a limitation. The invention process is also applicable to forming large and small, thin and thick, complex and simple shapes.

Likewise, the examples set out above utilize silicon carbide, but the process can be used with silicon metal powder, for example, which may then be nitrided as taught by E.R.W. May in United States Patent No. 3,819,786 or by N.L. Parr et al in United States Patent No.

3,222,438. The methods described herein are also amenable to forming intricate shapes of other powdered refractory materials such as silicon nitride, aluminum oxide, composites of silicon carbide and silicon nitride, and the like when a curable resin is used to give green strength to the casting. Any refractory compound powder that oxidizes slightly to form a stable glass (e.g. B2O3, SiO2 or TiO2) to strengthen the green casting can be used when the green casting is strengthened by heating in air.

For the purpose of the present specification and the appended claims, the expressions "bimodal mixture" and "biomodal distribution" refer to a mixture of two populations of particle sizes. A population has a distribution defined by the normal distribution curve. When the two populations are mixed, there is a double peak size plotted against frequency.

Claims

1. A process of obtaining a casting, comprising: (a) solidifying in a porous mold a castable slip comprising refractory particles and liquid, so as to provide a casting in said mold; (b) treating said casting while said casting is in said mold, so as to give or increase green strength of said casting; and (c) weakening at least a portion of said mold, so as to facilitate removal of said mold from said casting having said green strength.

2. A process as claimed in claim 1, wherein in step (a) said mold has surface for contacting said castable slip, at least a portion of which surface is adapted to delay absorption of said liquid by said mold.

3. A process as claimed in claim 2, wherein said surface comprises coat material for delaying absorption of said liquid by said mold.

4. A process as claimed in any one of claims 1 to 3, wherein in step (a) the material constituting said mold comprises a proportion of organic polymer.

5. A process as claimed in claim 4, wherein said proportion of organic polymer is 0.1 to 4.0% by weight based on the weight of said mold.

6. A process as claimed in claim 4 or 5, wherein said polymer is adapted to facilitate step (b).

7. A process as claimed in claim 6, wherein said polymer is adapted to facilitate step (c).

8. A process as claimed in claim 7, wherein said polymer is adapted to be burned out from said mold when step (c) comprises heat treating said mold to burn out at least a proportion of that polymer.

9. A process as claimed in any one of claims 6 to 8, wherein said polymer comprises methylcellulose.

10. A process as claimed in any one of claims 6 to 8, wherein said polymer comprises at least one of methylcellulose, caboxyvinyl polymer, xanthan gum, and ammonium alginate.

11. A process as claimed in any one of claims 1 to 10, wherein the material of said mold comprises plaster.

1 2. A process as claimed in claim 11, wherein said plaster comprises plaster of paris.

1 3. A process as claimed in any one of claims 1 to 12, wherein said refractory particles comprise particles of silicon carbide.

14. A process as claimed in any one of claims 1 to 12, wherein said refractory particles comprise particles of at least one of silicon, silicon carbide, aluminum oxide, and composite of silicon carbide and silicon nitride.

1 5. A process as claimed in any one of claims 1 to 14, wherein said refractory particles comprise particles oxidizable to a stable glass.

1 6. A process as claimed in claim 15, wherein said oxidizable particles comprise particles of at least one of B203, SiO2, and TiO2.

1 7. A process as claimed in any one of claims 1 to 16, wherein said refractory particles in said castable slip comprise a mixture of different populations of particle sizes.

18. A process as claimed in claim 17, wherein said refractory particles in said castable slip comprise a mixture having a proportion of refractory particles having an average particle size in the range 2.5 to 3.0 microns; and a proportion of refractory particles having an average size of less that substantially 74.0 microns.

1 9. A process as claimed in claim 18, wherein said refractory particles in said castable slip comprise a mixture having a proportion of refractory particles having an average particle size of substantially 3.0 microns; and a proportion of refractory particles having an average size of less than substantially 74.0 microns.

20. A process as claimed in claim 1 8 or 19, wherein said first proportion is of first silicon carbide particles; and said second proportion is of second silicon carbide particles.

21. A process as claimed in any one of claims 1 to 20, wherein said liquid comprises water.

22. A process as claimed in any one of claims 1 to 21, wherein said castable slip comprises a proportion of organic resin that is curable or stable during step (a) and/or step (b).

23. A process as claimed in claim 22, wherein said proportion of organic resin is 1 to 10% by weight based on the weight of said castable slip.

24. A process as claimed in claim 23, wherein said resin is heat curable or heat settable in step (a) and/or step (b).

25. A process as claimed in claim 23, wherein said resin is heat curable or heat settable at a temperature below at most substantially 200"C.

26. A process as claimed in any one of claims 22 to 25, wherein said resin is adapted to be resistant to step (c) for at least a proportion of the duration of step (c).

27. A process as claimed in any one of claims 22 to 26, wherein said resin comprises phenolic resin.

28. A process of claim 27, wherin said phenolic resin comprises a resole phenolic resin.

29. A process as claimed in any one of claims 22 to 26, wherein said resin comprises at least one of phenolic resin, resorcinal-formaldehyde resin, alkyd resin, melamine-formaldehyde resin, urea-formaldehyde resin, epoxy resin, and polyester resin.

30. A process as claimed in any one of claims 22 to 29, wherein said proportion of organic resin is at least partly miscible in said liquid.

31. A process as claimed in any one of claims 1 to 30, wherein in step (a) said solidifying castable slip or said casting is subjected to consolidation.

32. A process as claimed in claim 31, wherein in step (a) said consolidation is provided by application of pressure to said solidifying castable slip or said casting.

33. A process as claimed in claim 32, wherein said pressure is substantially 1.4 Kg/cm2.

34. A process as claimed in claim 32 or 33, wherein said pressure is provided by a gas atmosphere.

35. A process as claimed in any one of claims 1 to 34, wherein step (b) is carried out for at least a proportion of the duration of step (a).

36. A process as claimed in any one of claims 1 to 34, wherein step (b) is carried out after step (a).

37. A process as claimed in any one of claims 1 to 36, wherein in step (b) said casting is heated.

38. A process as claimed in any one of claims 1 to 37, wherein step (c) is carried out for at least a proportion of the duration of step (b).

39. A process as claimed in any one of claims 1 to 37, wherein step (c) is carried out after step (b).

40. A process as claimed in claim 39, when according to any one of claims 6 to 10.

41. A process as claimed in claim 39, when according to claim 7 or 9, and wherein step (c) comprises acid treating said mold.

42. A process as claimed in claim 41, wherein said acid treating comprises placing at least a pbrtion of said mold into acidic liquid for providing said weakening.

43. A process as claimed in claim 42, when according to claim 12, wherein said acidic liquid comprises hydrochloric acid.

44. A process as claimed in claim 39, when according to claim 8 or 9, and wherein step (c) comprises burning out from said mold at least a proportion of said polymer.

45. A process as claimed in claim 44, wherein in step (c) said mold is heated in an oxygencontaining atmosphere, so as to provide said burning out of polymer.

46. A process as claimed in claim 44 or 45, wherein in step (c) said mold is heated above substantially 600"C.

47. A process as claimed in any one of claims 1 to 46, when according to any one of claims 22 to 30, wherein at least a proportion of said resin is resistant to step (c).

48. A process as claimed in any one of claims 1 to 47, comprising (d) removing said weakened mold so as to free therefrom said casting having said green strength.

49. A process as claimed in claim 48, comprising (e) heating said free casting, so as to recrystallize at least a proportion of said free casting.

50. A process as claimed in claim 49, when said free casting comprises recrystallized silicon carbide.

51. A process as claimed in claim 50, comprising (f) providing additional carbon in interstices of said free casting.

52. A process as claimed in claim 51, wherein step (f) comprises subjecting said free casting to polymerizable furfuryl alcohol based material.

53. A process as claimed in any one of claims 49 to 52, comprising (g) siliconizing said recrystallized casting from step (e) or step (f).

54. A process as claimed in claim 53, wherein step (g) comprises subjecting said recrystallized casting to firing in a silicon atmosphere.

55. A process as claimed in claim 1, substantially as described in Example 1.

56. A process as claimed in claim 1, substantially as described in Example 2.

57. A process as claimed in claim 1, substantially as described in Example 3.

58. A casting obtained after removing a weakened mold therefrom in accordance with any one of claims 1 to 57.

59. A casting as claimed in claim 58, wherein said obtained casting is in the form of an airfoil of a turbine blade.

60. A casting as claimed in claim 58, wherein said obtained casting is in the form of a turbine rotor.

61. A casting as claimed in claim 58, wherein said obtained casting is in the form of a turbine wheel blade.

62. A process for forming refractory shapes including the steps of forming a castable slip from particles of a finely powdered silicon carbide, casting said slip into a plaster of paris mold, heat treating the SiC slip to increase the green strength of the casting, and decomposing the plaster of paris mold to facilitate removal from the strengthened casting, removing said mold to free the green casting, and further heat treating the casting by firing to recrystallization temperature, wherein the improvement comprises: incorporating in said slip from approximately 1 to 10% of a water miscible resin that will set or cure at a temperature below 200 C., and setting or curing said resin by the first mentioned heat treating step before removal of the casting from the mold, thereby imparting substantial strength to the green casting.

63. The process of forming a silicon carbide body having thin delicate sections which comprises forming a castable slip of a bimodal mixture of silicon carbide fine particles, a portion oftsaid particles having an average size on the order of 3 microns and another portion of said particles having an average size of less than substantially 74 microns, casting said slip into a porous plaster mold, said mold having been treated by incorporating in the plaster of paris mold mix .1 to 4% (by weight) of an organic polymer, said polymer being in an aqueous solution, solidifying the slip in the mold, heating the mold and the casting to a sufficiently elevated temperature for a sufficiently long time to substantially increase the strength of the green casting and thereafter decomposing the plaster mold.

64. Process of claim 63 wherein there is incorporated in the silicon carbide slip from approximately 1 to 10% (by weight) of a water miscible resin that will set or cure at a temperature below 200"C., the heating step setting or curing the resin so as to substantially increase the strength of the green casting, thereafter decomposing the mold by dipping it in an acid solution which will not attack the cured resin in the slip casting.

65. The process of claim 63 wherein the mold is heated in an oxygen-containing atmosphere to a sufficiently high temperature to burn out a substantial portion of the polymer in the mold and thereby drastically weaken the mold to permit its ready removal from the green casting.

66. The process of claim 63 wherein silicon carbide slip casting is heated to a temperature in excess of about 600 C. for a sufficiently long time to oxidize the silicon carbide and thereby substantially increase the green strength of the casting, the heating step also serving to oxidize the polymer in the mold to decompose the mold sufficiently so that it can be readily removed from the green casting.

67. A process for forming refractory shapes including the steps of forming a castable slip from particles of a finely powdered refractory material, casting said slip into a casting mold, removing said mold to free the green casting, and heat treating the casting, wherein the improvement comprises: incorporating in said slip from approximately 1 to 10% of a water miscible resin that will set or cure at a temperature below 200"C., setting or curing said resin before removal of the casting from the mold, thereby imparting substantial strength to the green casting, and decomposing the mold to permit its ready removal without damage to the strengthened green casting.

68. The process of claim 67 wherein the mold is plaster of paris and is removed from said casting by decomposition.

69. The process of claim 68 wherein the plaster of paris mold is decomposed by soaking in an acid solution.

70. The process of forming a silicon carbide body having thin delicate sections which comprises forming a castable slip of a bimodal mixture of silicon carbide fine particles, a portion of said particles having an average size on the order of 3 microns and another portion of said particles having an average size of less than substantially 74 microns, the castable mix containing about 1 to 10% of a water miscible resin which is curable at a temperature below -200"C., casting said slip into a porous plaster mold, the surface of said mold having been treated so that its porosity is appreciably less than that of plaster or paris, solidifying the slip in the mold, heating the mold and the casting to a sufficiently elevated temperature for a sufficiently long time to cure the resin in the casting, thereafter decomposing the plaster mold.

71. The process of claim 70 wherein the filled plaster mold is placed under a hydraulic pressure on the order of 1.4 Kg/cm2 to consolidate the slip in the mold.

72. The process of claim 70 wherein the mold is formed of plaster of paris whose internal surface has been lacquered to reduce its permeability to water.

73. The process of claim 70 wherein the resin is a phenolic resin.

74. The process of claim 70 wherein said cured resin is resistant to attack by acid and said plaster mold is decomposed by immersion in an acid solution.