US3321005A - Method of making shell molds for casting reactive metals - Google Patents

Description

United States Patent 3,321,005 METHOD OF MAKEN G SHELL MOLDS FOR CASTlNG REACTIVE METALS Nick G. Lirones, North Muskegon, Mich., assignor to Howrnet Corporation, a corporation of Delaware No Drawing. Filed Apr. 19, 1965, Ser. No. 449,294 20 Claims. (Cl. 164-26) This invention relates to the preparation of molds in which shaped products may be cast of metals such as beryllium, titanium, zirconium, hafnium, molybdenum, tungsten, uranium, and the like refractory metals, reactive heavy metals and metals of group IV-b of the periodic system.

The invention will be described with reference to the preparation and use of a mold formed about a heat or otherwise disposable pattern by repeatedly wetting with a dip coat composition and stuccoing to build up a monolithic structure from which the pattern is removed to leave a corresponding mold cavity into which molten metal may be directly cast to produce a metal product having a shape corresponding to the cavity left by the removed pattern. It will be understood that the concepts of this invention Will have application also to the mold that is formed by investment of the monolithic structure in a ceramic body for support, in accordance with conventional investment casting procedures, but it is preferred to make use of the monolithic structure without investment when suflicient strength can be embodied in the walls of the monolithic structure to enable the molten metal to be poured directly therein without support.

As described in the patents of Collins No. 2,380,945; Feagin et al. No. 2,439,207, No. 2,441,695 and No. 2,521,839, and Operhall et al. No. 2,961,751, such monolithic mold structures are produced by immersion of a cluster of wax patterns into a dip coat composition formulated of an inorganic binder, such as colloidal silica, and a ceramic flour such as is formed of silica, zircon, alumina, fused quartz and the like. While the pattern is wet with the dip coat composition, application is made of a stucco, such as larger particles of silica, zircon, alundum, fused quartz and the like ceramic materials, by sprinkling the stucco onto the pattern wet with the dip coat composition whereby a layer of the stucco is retained on the wet surface. This procedure of wetting with the dip coat composition and stuccoing is repeated for a number of times with intermediate drying until a composite layer of the desired thickness and strength has been built up about the cluster formed of a number of the patterns of wax or other preferably heat disposable material.

Thereafter, the assembly is heated to a temperature above the melting point temperature of the wax to remove the pattern and the remaining mold, having a mold cavity corresponding in shape to the displaced pattern, is fired at an elevated temperature in the range of l5002300' F. to cure the mold.

While molds of the type described are suitable for use in casting shaped products of steels and alloys of steels including steels alloyed with nickel, cobalt and the like, they have been found to be unsuitable for use in the casting of shaped products from such high melting point metals as the refractory metals or reactive heavy metals such as titanium, zirconium, uranium, hafnium, beryllium, or other metals of the group IV-b series. One of the major difficulties stems from the reactions that take place at the elevated temperature of the molten metal between the molten metals and the silicon oxides or ceramic components making up the mold. This results not only in an imperfect casting or a casting in which the reaction which takes place at the surface produces an imperfect casting but the reactions taking place operate also to ice modify the chemistry of the cast product and to produce an unsound casting.

In the copending application Ser. No. 407,256, filed Oct. 28, 1964, description is made of a procedure for protection of the ceramic materials in the mold for the purpose of minimizing reaction with the molten metals. For this purpose, colloidal graphite is incorporated as a component in the dip coat composition to coat the individual ceramic particles with the non-reactive colloidal graphite. While some improvement is derived from the described combination of colloidal graphite with the ceramic particles of the dip coat composition, a continuous protective surface is not made available whereby undesirable reactions are still capable of taking place.

It is an object of this invention to produce and to provide a method for producing ceramic molds of the type described wherein the formed mold can successfully be employed for the casting of metals and alloys of the types heretofore described including beryllium, titanium, zirconium, hafnium, molybdenum, tungsten, uranium, and the like refractory metals, heavy reactive metals and metals of group IV-b of the periodic system. It is a related object to provide a method for the casting of shaped products of such metals in molds of the type described.

It has been found that a mold formed of ceramic material of the types described can be used in the production of shaped products of metals of the types described and alloys thereof when the individual particles of the ceramic materials making up the mold portion immediately surrounding the mold cavity, and preferably the entire crosssection of the mold walls, are provided with a substantially continuous coating of an inert carbonaceous material formed in situ in the mold by thermal decomposition of an organic material and preferably by thermal decomposition of an organic resinous material such as a phenol aldehyde resin, cresol aldehyde resin, furfural aldehyde resin, resorcinol aldehyde resin, acrylic acid ester polymer, and the like.

In practice, the thermally reducible organic resinous material can be incorporated into the mold by formulation of the resinous material as a part of the dip coat composition whereby such organic resinous component will become uniformly distributed substantially throughout the cross-section of the mold wall that is formed about the disposable pattern. Instead, the organic resinous component can be introduced into the mold Walls after removal of the disposable pattern by impregnation of the resulting mold with a dilute solution or dispersion of the organic resinous component in a suitable solvent or aqueous medium. When incorporated by impregnation of the mold, it is preferred to effect such impregnation from the inside out by introducing the impregnating composition into the formed mold cavity so that the maximum amount of impregnation will be achieved in the portions of the mold wall immediately surrounding the mold cavity. It will be understood that a combination including the formulation of the clip coat composition to contain the heat decomposable organic resinous component plus impregnation of the formed mold with a solution or dispersion of an organic resinous component can be employed for more effective and efiicient coverage of the ceramic materials making up the mold.

When the mold is thereafter hated to cure the mold or to fire the mold, if such heating is effected while the mold is in a non oxidizing or inert atmosphere, then instead of burning out the organic resinous component, the resinous component will thermally break down to a tenacious carbonaceous residue of an inert form of carbon or carbonaceous material which will effectively coat the inorganic or ceramic particles in at least the portions of the mold adjacent the mold cavity. This will result in a substantially continuous covering which will protect the ceramic or inorganic materials to enable the described molten metals or alloys to be poured directly into the mold cavity without reaction with components making up the mold.

When the organic resinous component is incorporated to form a part of the dip coat composition, the organic resinous component can be embodied in the dip coat composition in addition to the conventional ceramic binder, such as colloidal silica. Instead, the inorganic binder component can be eliminated whereby the organic resinous material will function in the dip coat composition as an interim binder for the buildup of the layers of ceramic flour and stucco. When formulated in combination with a conventional binder, the dip coat composition can be formulated to contain the organic resinous component in an amount within the range of 40% by weight and preferably in an amount within the range of 25% by weight. When formulated into the dip coat composition without a conventional ceramic binder, the amount of organic resinous component is preferably increased to within the range of 10-40% by weight and preferably an amount within the range of -30% by weight of the dip coat composition.

It is preferred to incorporate the organic resinous component in a dissolved state so as to form a coating about the ceramic particles upon drying. For such purpose, use can be made of a diluent in the form of a conventional aqueous medium when the resinous component is in a water soluble or A stage. In the event that the organic resinous component is water insoluble, the dip coat composition can be formulated with an organic solvent in which the resinous material is soluble or the organic resinous component can be incorporated as a dispersed phase in the aqueous medium to enable formulation of the dip coat composition as an aqueous system.

When the organic resinous material is incorporated by way of impregnation of an already formed mold, it is desirable to make use of the organic resinous material in a dilute solution in an organic solvent or a dilute dispersion in an aqueous medium. The amount of the organic resinous component formulated into the impregnating composition will depend somewhat on the character and the molecular weight of the resinous system. As a general rule, a satisfactory impregnating composition can be formulated with the organic resinous component present in an amount within the range of 1-80% by weight and preferably 2-10% by weight. It will be understood that when a larger amount of resinous material is required in the formed mold, the impregnation process can be repeated one or more times.

The process of impregnation is often preferred as the means for the incorporation of the organic resinous component since such impregnation will operate not only to coat the ceramic flour of the dip coat composition but also the stucco which has been ap lied. Equivalent results can be achieved by the formulation of the dip coat composition to contain the organic resinous material, as previously described, and wherein the stucco has been precoated with an organic resinous material, as by wash coating with a solution thereof and which will hereinafter be referred to as resinous precoated ceramic stucco.

Having described the basic concepts of this invention, illustration will be made first of the system wherein the organic resinous material is incorporated to form a component of the dip coat composition.

A cluster of wax patterns is formed in the conventional manner by attachment of the patterns to runners, also formed of wax or other heat disposable materials, and to a funnel or cup which communicates with the runners to define the opening in the mold through which the molten metal is poured for filling the mold cavity. Instead of wax, the patterns, runners and/or funnel can be formed or plastic or other heat disposable materials, or of materials which can be disposed of by chemical solution, sublimation or by mere increase to room temperature, as in the frozen mercury process.

The cluster should be inspected for the removal of dirt, flakes or other materials which might have adhered to the surfaces of the patterns and which, if allowed to remain, would reproduce and provide imperfections in the surfaces of the metal casting.

Example 1 Dip coat composition (with ceramic binder): Colloidal silica (30% grade, 1.198 specific gravity) cc 8000 Zircon (99% through 325 mesh) pounds 165 Water cc 6150 Water soluble A-stage phenol aldehyde resin pounds 30 Sodium fluoride grams 110 Example 2 Dip coat composition (without ceramic binder):

Percent by wt. Liquid phenol formaldehyde resin (Catalin 136) 27 Isopropyl alcohol 35 Silica flour (-325 mesh) 38 The cleaned cluster is immersed in the slurry c0mpris ing the dip coat composition to wet all of the surfaces with the exception of the lip of the pouring spout. Instead of dipping, the surfaces can be wet with the slurry of the dip coat composition by other coating techniques, such as by flow coating, spray coating and the like.

While the cluster is still wet with the dip coat composition, a finely divided, dry stucco is sprinkled over the surfaces whereby some of the stucco will adhere to the wet coating of the dip coat composition and be retained on the surfaces of the pattern for integration with the slurry of the dip coat composition to form a composite layer therewith about the cluster. The stucco can also be applied onto the wet surfaces of the cluster by means of a moving bed into which the cluster is immersed substantially completely to coat the wet surfaces of the cluster with the stucco material. As the stucco, use can be made of alumina, zircon, silicate, fused silica, with or without resinous pretreatment, and in which the stucco is dimensioned, at least in the first few coats, to be less than 50 mesh but more than mesh. The cycle of wetting with the dip coat composition and stuccoing is repeated a number of times, with intermediate drying between each cycle, until -a composite layer of the desired thickness has been built up about the walls of the cluster including the patterns and parts. A mold having a wall thickness built up to about A to /2 inch usually provides strength suflicient for investment or to enable the molten metal to be poured directly into the mold for the production of shaped metal products of normal weight. The buildup of walls of greater thickness can be produced for use in the molding of larger and heavy castings. Usually, a wall thickness of A to /2 inch can be achieved with from five to ten cycles of dip coating, stuccoing, and intermediate drying.

While it is preferred to make use of dip coat compositions of the same formulation for all cycles thereby uniformly to distribute the organic resinous material throughout the cross-section of the mold wall, it will be sufficient if only the inner one-half of the wall cross section is formed of the dip coat composition containing the organic resinous component while the remainder of the built-up wall is formed of a conventional dip coat composition formulated with ceramic materials.

'After drying, the composite structure is dewaxed by inverting the mold and by heating to a temperature above the melting point temperature for the wax and preferably to a temperature above 400 F. but below the temperature at which the organic resinous material will be burned out of the mold. Dewaxing can be carried out by other processes such as hot sand dewaxing wherein the mold is surrounded with sand preheated to a temperature within the range of 400-800 F. or in which the hot sand is poured over the mold; steam dewaxing wherein the composite structure is housed in an autoclave or else steam at high pressure is introduced onto the mold.

The dewaxing step can be carried out as a separate operation in the manner described but it is preferred to combine the dewaxing step with the subsequent step of curing the mold wherein the mold, with the wax pattern or after the wax pattern has been removed by one or the other of the dewaxing processes, is introduced into a zone heated to a temperature above 800 F. and preferably to a temperature within the range of 1000- 2300 F. while the atmosphere in the zone is maintained as an inert or non-oxidizing or reducing atmosphere, as by the use of an inert gas such as argon, nitrogen, carbon dioxide and the like. Under such conditions, the mold is cured and the organic resinous component is thermally decomposed to a stable form of carbon or carbonaceous decomposition product which effectively coats the adjacent ceramic materials with a protective coating that blocks reaction between the ceramic and hot molten metals poured into the cavity of the mold. The desired cure and thermal breakdown of the organic resinous material can usually be achieved at the there is no harm in heating for a longer period of time to insure complete stabilization of the materials making up the mold.

It has been found that the carbonaceous decomposition product that is formed is of the type which either swells or otherwise operates to fill up interstices of the mold since the formed mold is less pervious to the penetration of the molten metal poured into the mold cavity while still providing microporous openings through which the mold can breathe thereby to block reaction between .the metal and the materials of the mold while at the same time providing a more uniform smooth surface at the interface for the production of a molded product which conforms more exactly with the original shape of the mold cavity and with less surface imperfection. Thus there is produced an acceptable product of such metals which have heretofore been incapable of being processed in ceramic molds.

Description will now be made of the alternative process wherein the organic resinous material is incorporated after the ceramic mold has been formed and dewaxed. For this purpose, the conventional processes which make use of conventional ceramic materials as described in the aforementioned patents can be used up to the point that the formed ceramic mold is dewaxed or dewaxed and cured.

Thereafter, in accordance with the concepts of this invention, a solution of .5% to 25% by weight furfural formaldehyde resin or a solution of .5% to 80% by Weight of a phenol or cresol formaldehyde resin in propanol is poured into the mold cavity for flow as by impregnation into the interstices of the porous mold. The step of impregnation can be repeated one or more times if a higher concentration of the organic resinous component is desired in the mold walls. Usually the step of pour ing the impregnating composition into the mold is followed by removal of the material that remains and by drying before subsequent re-introduction of impregnating composition for increasing the amount of material introduced into the mold walls. The dried resinous material coats the ceramic flour and stucco making up the ad jacent portions of the mold walls.

Thereafter, the mold is fired in the manner previously described for curing in an inert or non-oxidizing atmosphere to effect the thermal decomposition of the impregnating organic resinous material for reduction thereof to temperature described in from 15 to minutes but the inert carbonaceous decomposition product coating the ceramic particles of the mold and filling the interstices of the mold to block penetration of molten metal poured into the mold cavity during the subsequent. steps of metal casting.

It will be understood that both of the prevously de scribed systems may be combined to produce a mold having the organic resinous component incorporated into the walls of the mold by way of a dip coat composition and in which additional organic resinous material is incorporated, at least in the inner portions of the mold about the mold cavity, by the subsequent process of impregnation.

It is believed that the stabilized thermal decomposition product formed of the organic resinous component is capable also of the characteristics of a binder in the cured product since a mold of the desired strength results even though no ceramic binder is present in the dip coat composition.

Although not equivalent to the described easily thermally decomposable organic resinous material in the described formulations, other organic materials which are easily thermally decomposable to a stabilized carbon or carbonaceous reaction product can be employed as the organic component in the amounts described for the organic resinous material in the preceding examples. Such other materials which can be substituted for the organic resinous materials in equivalent amounts in the foregoing examples include natural resins and gums, such as copal resin, ester gum, gum tragacanth, gum arabic, terpene resins, coumarone indene resins and the like; sugars, carbohydrates and starches such as casein, albumen, algins and the like; coal tar and petroleum resins and the like.

Molten metal can be poured directly :into the mold cavity for the fabrication of molded products since the mold possesses sufiicient strength and has sufficient mass integrity to enable the motlen metal to be poured directly into the mold. While preheating is not essential, it is desirable to preheat the mold prior to metal pouring. If the graphitized mold is to be preheated to a temperature above 800 F, it is desirable either to effect such preheat under vacuum conditions or in an inert or non-oxidizing atmosphere, as in an atmosphere of argon, nitrogen or carbon monoxide or carbon dioxide, otherwise the carbonaceous product will burn from the mold. Since the described refractory metals, reactive heavy metals or metals of the group IV-b have a melting point far in excess of 800 B, it is desirable to achieve metal pouring by vacuum casting techniques wherein the mold is enclosed within a vacuum chamber which communicates with a melting furnace whereby a vacuum can be drawn to evacuate the chamber and the mold prior to metal poun'ng. The mold and metal cast therein are preferably maintained under vacuum conditions until the poured metal has solidified or cooled to a temperature below 800 F. Thereafter, the assembly can be removed from the vacuum chamber for further processing.

To assist in filling the mold, centrifugal casting techniques can be employed in combination with metal pouring. By way of specific illustration, description will be made of the use of a mold embodying the features of this invention in the preparation of a cast metal product of titanium, it being understood that others of the heretofore described refractory metals, heavy reactive metals or metals of group lV-b and alloys thereof may be similarly processed.

In practice, the mold is housed in a vacuum chamber on the underside of a melting furnace and the molten titanium is poured under vacuum into the mold, with or without preheating the mold. When preheating is employed, it is desirable to preheat the mold while under vacuum conditions in which the preheat may be to a temperature up to 800 F. although preheating to higher temperatures is preferred.

The poured metal is allowed to cool in the vacuum chamber or under a protective inert or non-oxidizing atmosphere to a temperature below that at which oxidation of the carbonaceous material can take place before removal :of the cast metal product from the protective atmosphere of the mold. The cast metal product can be removed by conventional techniques such as by impacting and shaking to disintegrate the mold and free the casting.

It will be apparent from the foregoing that I have provided a new and novel mold system which makes use of ceramic materials and in which shaped products can be successfully produced of such metals as titanium, zirconium, uranium, hafnium, beryllium, or other metals of the group IV-b of the period system.

As used herein, the terms ceramic flour and ceramic stucco are intended to include flour and stucco formed of ceramic materials such as silica, fused glass, fused quartz, Zirconium silicates, ores such as beryl ores, thoria, zirconite, kyanite, mullite and sillamanite, and oxides of the types previously described including zircon and alumina.

It will be understood that changes may be made in the details of formulation, fabrication and construction of the mold without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. In the method of producing a mold for use in casting shaped products of refractory metals, heavy reactive metals, metals of group IV-b of the periodic system and alloys thereof in which use is made of a disposable pattern of the shaped product, the steps of wetting the surfaces of the pattern with a fluid composition the solids of which consist essentially of the combination of a ceramic flour and an organic resinous material which is easily thermally decomposable in a non-oxidizing atmosphere to :a stable form of a carbonaceous decomposition product, :applying a ceramic stucco to the surfaces of the pattern while wet with the dip coat composition whereby some of the stucco is retained on the Wet surfaces, repeating the applications of dip coat composition and stucco for a number of cycles to form a composite layer of the dip coat solids and stucco on the surfaces of the pattern, treating the composite to effect removal of the pattern and provide a mold having a mold cavity corresponding to the removed pattern, heating the mold to a temperature in excess of 800 F. in a non-oxidizing atmosphere to cure the mold and thermally decompose the organic binder component in situ in the mold to produce a mold in which the ceramic particles in at least the wall portions about the mold cavity are protected by the carbonaceous thermal decomposition product.

2. The method as claimed in claim 1 in which the organic resinous binder is selected from the group consisting of phenol aldehyde resin, cresol aldehyde resin, resorcinol aldehyde resin, and furfural aldehyde resin.

3. The method as claimed in claim 1 in which the organic resinous component is present in the dip coat composition in an amount within the range of 540% by weight.

4. The method as claimed in claim 1 in which the stucco is precoated with an organic thermally decomposable material.

5. The method as claimed in claim 1 in which the composite is heated to a temperature within the range of 1000-2300 F. in the non-oxidizing atmosphere.

6. In the method of producing a mold for use in casting shaped products of refractory metals, heavy reactive metals, metals of the group IVb of the periodic system and alloys thereof in which use is made of a heat disposable pattern of the shaped product, the steps of wetting the surfaces of the pattern with a fluid composition the solids of which consist essentially of the combination of a ceramic flour and an organic material which is easily thermally decomposable in a non-oxidizing atmosphere to a stable form of carbon, applying a ceramic stucco to the surfaces of the pattern while wet with the dip coat composition whereby some of the stucco is retained on the wet surfaces, repeating the applications of dip coat composition and stucco for a number of cycles with intermittent drying whereby a composite layer of the dip coat solids and stucco is built up on the surface of the pattern, heating the composite to a temperature above the melting point temperature of the material of which the pattern is formed to effect removal of the pattern and heating the mold to a temperature in excess of 800 F. in a nonoxidizing atmosphere to cure the mold and thermally decompose the organic component in situ in the mold to produce a mold in which the ceramic particles in at least the wall portions about the mold cavity are protected with the carbonaceous thermal decomposition product.

7. The method as claimed in claim 6 in which the organic component of the dip coat composition is selected from the group consisting of a synthetic resin, a natural resin, a gum, starch, a protein, and a carbohydrate.

8. In the method of producing a mold for use in casting shaped products of refractory metals, heavy reactive metals, metals of group IV-b of the periodic system, and alloys thereof in which use is made of a disposable pattern of the shaped product, the steps of wetting the surfaces of the pattern with a fluid composition the solids of which consist essentially of the combination of a ceramic tbinder, a ceramic flour and an organic resinous material which is easily thermally decomposable in a non-oxidizing atmosphere to a stable form of carbonaceous decomposition product, applying a ceramic stucco to the surfaces of the pattern while wet 'with the dip coat composition whereby some of the stucco is retained on the wet surfaces, repeating the applications of dip coat composition and stucco for a number of cycles to form a composite layer of the dip coat solids and stucco on the surfaces of the pattern, treating the composite to effect removal of the pattern and provide a mold having a mold cavity corresponding to the removed pattern, heating the mold to a temperature in excess of 800 F. in a non-oxidizing atmosphere to cure the mold and thermally decompose the organic binder component in situ in the mold to produce a mold in which the ceramic particles in at least the wall portions about the mold cavity are protected by the carbonaceous thermal decomposition product.

9. The method as claimed in claim 8 in which the organic resinous component is selected from the group consisting of a phenol aldehyde resin, a cresol aldehyde resin, a resorcinol aldehyde resin, and a furfural aldehyde resin.

10. The method as claimed in claim 8 in which the organic resinous component is present in an amount within the range of 540'% by weight of the dip coat composition.

11. The method as claimed in claim 8 in which the mold is heated to a temperature within the range of 10002300 F. in a non-oxidizing atmosphere.

12. In the method of producing a mold for use in casting shaped products of refractory metals, heavy reactive metals, metals of group IV-b of the periodic system, and alloys thereof in which use is made of a heat disposable pattern of the shaped product, the steps of wetting the surfaces of the pattern with a fluid composition the solids of which consist essentially of the combination of a ceramic binder, a ceramic flour and an organic material which is easily thermally decomposable in a non-oxidizing atmosphere to a stable form of carbonaceous decomposition pro-duct and which is present in the dip coat composition in an amount within the range of 540% by weight, applying a ceramic stucco to the surfaces of the pattern while wet with the dip coat composition whereby some of the stucco is retained on the wet surfaces, repeating the applications of dip coat composition and stucco for a number of cycles, with intermediate drying, whereby a composite layer of the dip coat solid and stucco is built up on the surfaces of the pattern, heating the composite to a temperature above the temperature for disposal of the pattern whereby the pattern is removed from the mold, heating the mold to a temperature within the range of l0002300 F. in a nonoxidizing atmosphere to cure the mold and thermally decompose the organic component in situ in the mold to produce a mold in which the ceramic particles in at least the wall portions about the mold cavity are provided with a protective coating of the carbonaceous thermal decomposition product.

13. In the method of producing a mold for use in casting shaped products of refractory metals, heavy reactive metals, metals of group IV-b of the periodic system, and alloys thereof in which the mold is formed of a composite of a dip coat composition the solids of which consist essentially of a ceramic flour and a ceramic binder and a stucco of ceramic particles, the steps of impregnating the mold with a dilute fluid composition containing an organic compound which is easily thermally decomposable to a stable carbonaceous decomposition product, firing the impregnated mold in a non-oxidizing atmosphere to a temperature above the thermal decomposition temperature for the organic material thermally to decompose the organic material in situ in the impregnated mold.

14. The method as claimed in claim 13 in which the step of impregnation is repeated a number of times with intermediate drying.

15. The method as claimed in claim 13 in which the impregnated mold is fired to a temperature Within the range of 1000-2300 F.

16. The method as claimed in claim 13 in which the organic material is an organic resinous polymer,

17. The method as claimed in claim 16 in which the organic resinous polymer is selected from the group consisting of a phenol aldehyde resin, a cresol aldehyde resin,

a resorcinol aldehyde resin, and a furfural aldehyde resin.

18. The method as claimed in claim 1 which includes the step of impregnating the mold subsequent to pattern removal but prior to curing with a fluid composition containing an organic compound which is thermally decomposab-le to a stable carbonaceous decomposition product. 19. The method as claimed in claim 18 in which the organic material in the impregnating composition comprises an organic resinous polymeric material.

20. The method as claimed in claim 8 which includes the step of impregnating the mold subsequent to pattern removal but prior to curing with a fluid composition containing an organic resinous material.

References Cited by the Examiner UNITED STATES PATENTS 20 2,683,296 7/1954 Drumm et al 22-193 2,886,869 5/1959 Webb et al. 22196 2,961,751 11/1960 Operhall et al 22-193 2,991,267 7/1961 Bean 22-193 X 25 3,005,244 10/1961 Erdle et al. 22-196 3,042,541 7/1962 Kaplan 10638.22X

I. SPENCER OVERHOLSER, Primary Examiner,

E. MAR, Assistant Examiner, 3Q

Claims (1)

13. IN THE METHOD OF PRODUCING A MOLD FOR USE IN CASTING SHAPED PRODUCTS OF REFRACTORY METALS, HEAVY REACTIVE METALS, METALS OF GROUP IV-B OF THE PERIODIC SYSTEM, AND ALLOYS THEREOF IN WHICH THE MOLD IS FORMED OF A COMPOSITE OF A DIP COAT COMPOSITION THE SOLIDS OF WHICH CONSIST ESSENTIALLY OF A CERAMIC FLOUR AND A CERAMIC BINDER AND A STUCCO OF CERAMIC PARTICLES, THE STEPS OF IMPREGNATING THE MOLD WITH A DILUTE FLUID COMPOSITION CONTAINING AN ORGANIC COMPOUND WHICH IS EASILY THERMALLY DECOMPOSABLE TO A STABLE CARBONACEOUS DECOMPOSITION PRODUCT, FIRING THE IMPREGNATED MOLD IN A NON-OXIDIZING ATMOSPHERE TO A TEMPERATURE ABOVE THE THERMAL DECOMPOSITION TEMPERATURE FOR THE ORGANIC MATERIAL THERMALLY TO DECOMPOSE THE ORGANIC MATERIAL IN SITU IN THE IMPREGNATED MOLD.