US3903950A

US3903950A - Sandwich structure mold

Info

Publication number: US3903950A
Application number: US427732A
Authority: US
Inventors: Nicholas G Lirones
Original assignee: Howmet Corp
Current assignee: Howmet Corp
Priority date: 1973-12-26
Filing date: 1973-12-26
Publication date: 1975-09-09
Anticipated expiration: 1992-09-09
Also published as: DE2459088A1; IT1026158B; SE420799B; BE823853A; GB1486326A; SE7416046L; FR2255977A1; DE2459088B2; CA1039925A; FR2255977B1; JPS5223883B2; JPS5097519A

Abstract

A mold and fabrication method is disclosed for use in molding precision cast parts. In cross-section the mold has inner and outer layers, the essential ingredients of which include a ceramic refractory material and an inorganic binder. Intermediate layers are disposed between the inner and outer layers and employ an organic fugitive binder. When the mold is heated prior to receiving molten metal, the fugitive binder is volatilized, leaving the intermediate layer unbound. After the parts have been cast the mold is easily collapsed for removal of the casting without damage.

Description

Sept. 9, 1975 SANDWICH STRUCTURE MOLD Inventor: Nicholas G. Lirones, North Muskegon, Mich.

Primary Examiner-Francis S. Husar AXSLVIIUII ExaminerCarl ROWOld Assignee: Howme' 'P GrcenWlChr Armrmy Agent, or F1'rmMcDougall, Hersh & Scott Conn.

[57] ABSTRACT A mold and fabrication method is disclosed for use in [22] Filed: Dec. 26, 1973 [2]] Appl No.: 427,732

molding precision cast parts In cross-section the mold 1 has inner and outer layers, the essential ingredients of [52] U.S. 164/26; l06/38i35; 164/36 [51] 822C [/08; 822C l/lO which include a ceramic refractory material and an inorganic binderv Intermediate layers are disposed between the inner and outer layers and em ganic Int.

ploy an orfugitive binder. When the mold is heated prior to receiving molten metal, the fugitive binder is volatilized, leaving the intermediate layer unbound. After the parts have been cast the mold is easily collapsed a] of the casting without damage.

[56] References Cited UNITED STATES PATENTS for remov lO6/389 X |06/38.3 X 26 5 16 Claims, 2 Drawing Figures Opcrhall et al.

INNER LAYER INTERMEDIATE LAYER l4- PATENTEDSEP ems FIG, 2

LPATTERNW L PATTERN I swcco I @PEAfl mNER LAYER INTERMEDIATE.

PRE

ECOND DIP COAT FIRST DIPCOAT PRE HEAT POUR METAL REMOVE MOLD BACKGROUND OF INVENTION This invention relates to the art of precision casting and to materials employed in the practice thereof. More particularly, the invention relates to a casting process and to compositions and methods used in the preparation of molds.

A basic process employed in casting precision parts is known as the lost wax process. This process is partic ularly adapted to producing precision castings for the production of turbine blades, vanes, as well as other complex parts. The conventional lost wax process consists of producing disposable patterns formed of wax, plastic or other suitable material having the desired shape. If necessary, these patterns are assembled into clusters to form a complete assembly. The patterns are then dipped into a ceramic slurry dipcoat containing a colloidal silica or other inorganic binder. While the pattern is wet from the dipcoat. it is stuccoed with granular refractory particles and then dried. This forms a coating on the patterns and by repeating the dipping, stuccoing and drying steps, a mold of a desired thickness can be formed about the pattern.

The pattern is then removed from the mold and the mold is heated to a temperature whereby the silica binder cures and strongly bonds the ceramic dipcoating and granular stucco into a monolithic mass. After inspection, the mold is preheated to a high temperature suitable for receiving molten metal.

After pouring the metal, the casting is allowed to cool to a solidified state. At this stage, the mold strength can be higher than the metal strength, often resulting in hot tears on the casting. After cooling, the mold is broken away from the casting by mechanical or pneumatic vibration, salt bathing, sand blasting or other techniques.

The above described techniques for separation of the casting from the mold have several disadvantages. The high amplitude mechanical vibration necessary to break the mold can cause cracking of the casting. Salt bathing techniques and sand blasting are excessively time consuming and also detrimental to the casting surface in that intergranular attack and surface erosion occurs.

It is accordingly an object of the present invention to produce and provide a method for producing new and improved molds for use in precision casting.

More specifically, it is an object of the present invention to produce a mold which is of sufficiently high strength and stability to enable casting materials to be poured directly therein for molding. and yet which can be easily broken away from the casting without damage thereto.

It is another object of the present invention to pro vide a mold in which the molded products can be easily and efficiently separated from the mold.

It is a further object of the present invention to provide a mold in which at least some ofthe materials used therein can be reclaimed and re-used.

It is still a further object of the present invention to provide a sandwich structured mold which eliminates casting hot tears" and knock-out cracks.

Other objects of the present invention will become apparent from the concluding portion of the specificatron.

SUMMARY OF THE INVENTION A mold for casting precision parts is formed by dipping a wax or plastic pattern into a dipcoat of a first composition including finely divided refractory materials and an inorganic binder, such as a silica binder. While the pattern is still wet, it is stuccoed with granular refractory particles, and then dried. This sequence is repeated as desired to build up an inner layer ofa desired thickness. The pattern and inner layer are then dipped into a second dipcoat, the principal constituents of which are finely divided refractory material and an organic fugitive binder. After stuccoing, this coating is dried. This intermediate layer may be built up as desired by repeating the sequence.

Finally, an outer layer is formed over the intermediate layer and is composed somewhat of the same inorganic constituents as the inner layer, thereby forming a mold which is sandwich structured. When the mold is preheated prior to metal pouring (after the pattern has been removed), the organic fugitive binder of the intermediate layer is substantially eliminated by burning and/or volatilization. This leave the materials of the intermediate layer without binder sandwiched between the inner and outer layers.

After molding, the sandwich mold is easily removed from the casting by collapsing the outer layer onto the intermediate layer and removing the thin inner layer. This mold avoids hot tears as well as cracking in the casting due to the force heretofore required for mold removal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic sectional view through a pattern having a mold formed thereon in accordance with the practice of this invention; and

FIG. 2 is a flow diagram of the process embodying the practice of this invention.

DETAILED DESCRIPTION The concepts of this invention are embodied in two phases. a mold phase" which includes the compositions employed in the manufacture of the mold and the method for the production thereof, and the use phase wherein the formed mold is employed in the process of molding compositions and materials.

The mold phase will be described with reference to compositions employed and the methods of manufacture in a representative process illustrating the practice of this phase of the invention.

In the following description, the term pattern will be used interchangeably with cluster to refer to a wax or plastic pattern 10 or a cluster formed of a multiplicity of such individual patterns.

The pattern 10 is formed of conventional materials disposable by heat or chemicals. If the mold is to be formed about more than one pattern, the plurality of patterns are connected by runners for communication with a pouring spout to form a completed cluster, as described in the issued US. patent to Lirones. US. Pat. No. 3.266,IU6. Where the cluster is to be repeatedly dipped into slurry. identified as a dipcoat, it is desirable to provide a hanger rod for carrying the cluster and for suspending the cluster for drying and the like.

First Dipcoating Composition 8000 cc. colloidal silica (30% grade) (specific gravity 165 pounds zircon (99% through 325 mesh) 6150 cc. water 1 grams sodium fluoride Application of First Dipcoat Composition The pattern or cluster is first inspected to remove dirt, flakes and other objects which may have adhered to the surfaces of the pattern and which, if allowed to remain, would impair the preparation of a good mold and lead to an unacceptable casting. The cleaned clus ter is immersed into the stirred dipcoat composition to cover all of the surfaces of the cluster. To promote the elimination of air pockets, it is desirable to rotate the cluster while immersing in the dipcoat composition. Alternately, the dipcoat composition can be applied by spraying the dipcoat composition onto the surfaces of the pattern.

When fully wet, the pattern or cluster is suspended to drain excess dipcoat composition. During drainage, the cluster can be inspected to detect air pockets which can be eliminated by addressing a stream of air onto the uncoated portions and thereafter allowing the slurry of the dipcoat composition to flow onto the uncovered area. While the cluster is being drained, it should be held in different spatial planes designed to achieve uniform coating on all surfaces. In general, drainage should be completed within a few minutes but, in any event, in less time than would allow the dipcoat composition to dry whereby the surface would not retain stucco, as will be described.

First Stucco Coat Stucco combination-Alundum (100% through 50 mesh with less than 3% through 100 meshbetter than 90% between 60 and 80 mesh).

Application of Stucco Coat After the uniformity of coating has been achieved in the first dipcoat and dripping from the patterns has bccome minimized. the stucco is sprinkled onto the wet cluster substantially uniformly to cover the wet surfaces with a layer of the stucco while, at the same time, minimizing flow of the dipcoat whereby nonuniformities might otherwise develop. In practice, the stucco particles will be rained down from above through a screening member constantly being fed by a vibratory feeder to remove foreign matter from the Alundum particles, while the particles are sprinkled over an area to give more uniform and complete coverage. The stucco will adhere to the wet coating of the slurry and will become partially embedded in the slurry to become integrated with the coating formed on the cluster of wax patterns.

By repeating the above steps, after intermediate drying, the first dipcoat composition and stucco are built up onto the pattern to form an inner layer l2 of a desired thickness. For the purposes of casting molten metal products of normal weight, it is desirable to provide for more than one dipcoat-stuccoing cycle. After sufficient drying, the pattern is then emersed into a second dipcoat to form an intermediate layer 14.

Second Dipcoating Composition Example A. -325 Mesh Alumina Flour 3500 Gr Polyvinyl Alcohol (7.2664 aqueous Solution) 1033 Gr Potato Starch (3% Water Solution) 938 Gr Anionic Wetting Agent 4" cc Example B. 325 Mesh Alumina Flour 3500 Gr Polyvinyl Alcohol (726% Aqueous Solution) 2000 Gr Anionic Wetting Agent 42 cc Example C- 400 Mesh Zircon Flour 4040 Gr Polyvinyl Alcohol (7.26% Aqueous Solution) 1033 Gr Potato Starch (3')? Water Solution) 938 Gr Anionic Wetting Agent 42 cc Anti-foam Colloid 21 cc Example D.- -400 Mesh Zircon Flour 4040 Gr Polyvinyl Alcohol (7.26% Aqueous Solution) I975 Gr. Anionic Wetting Agent 42 cc Anti-foam Colloid 21 cc Example E.

325 Mesh Alumina Flour 122.7 Lbs (Aquadag) Colloidal Graphite (22% Solids) 10.7 Lbs Water (Distilled) 26.3 Lbs Anionic Wetting Agent 418.0 cc Gum 'lragacanth Solution 456.0 cc Example F.

325 Zircon Flour 33.0 Lbs (Aquadag) Colloidal Graphite [22% Solids in Water) 8.8 Lbs Water (Distilled) 1 1.0 Lbs Anionic Wetting Agent 200.0 cc Gum Tragacanth Solution 300.0 cc Example G.

325 Mesh Zircon Flour 160.0 Lbs Latex 280.0 cc Water 14000 cc Gum Tragacanth Solution 3000 cc Anionic Wetting Agent 4000 cc ()ctyl Alcohol 49.0 cc

ln examples E, F, and G the gum tragacanth solution has the following compositions:

Gum Tragacanth Powder 445 Gr Sodium Benzoate S Gr Water 20,000 Gr As the anionic wetting agent, use can be made of sodium heptadecyl sulphate, such as Tergitol Anionic No. 7 of Union Carbide Corporation.

Representative of the antifoam colloids which may be used in the above formulations is Antifoam Colloid No. SXlB produced by Colloids, lnc., Newark, NJ.

Application of Second Dipcoat Composition The second dipcoat composition is applied to the coated pattern in a manner similar to that of the first dipcoat composition. A significant difference between the dipcoat compositions is in the selection of the binder. The first dipcoat composition employs a colloidal silica binder. The second dipcoat composition preferably utilizes an organic fugitive binder as, for example, Polyvinyl alcohol. By organic fugitive binder it is intended to describe an organic compound which is ca pablc of being volatilized and/or burned out by heating or otherwise. By employing an organic fugituve binder it is possible to form a second intermediate layer about the pattern. and at a subsequent time drive off the binder to leave the intermediate layer closely packed but unbound.

In the preferred practice of this invention, it is desired, although not essential. to precede the emersion of the coated pattern into the first and second dipcoat compositions with a pre-wetting step in which the prewetting composition employs substantially the same formulation as the dipcoat composition, with the exception that a lower viscosity is employed. The prewetting composition includes additional amounts of water sufficient to reduce the total solids to about 25 to 75 percent of the total solids in the dipcoat composition. Thus, the coated pattern, having the inner layer 12 dried thereon. is submerged into a pre-wetting composition more completely to penetrate and wet out the coated surface prior to being immersed in the second dipcoat composition. Immersion into the second dipcoat composition follows immediately after the pre-wet immersion.

After the immersion in the second dipcoat composition, the pattern is again allowed to drain and is stuccoed and dried. The steps of pre-wetting, dipcoating, stuccoing and drying are repeated one or more times, as necessary, to build up the intermediate layer 14 to a desired thickness. The stucco employed for the intermediate layer may be the same as that employed for the inner layer.

For special applications and particularly where large molds are utilized, it is desirable to add a very small percentage, on the order of 0. l 25 percent by weight, of colloidal silica binder to the second dipcoat composition which also contains the organic fugitive hinder as previously discussed. The presence of the colloidal silica binder prevents the intermediate layer from becoming completely powdery after the fugitive binder is driven off as will be described. The small amount of silica binder added to the second dipcoat does not in crease the strength or increase the task of removal but permits patching and repair of the mold should such operations be necessary. The following example illustrates the use of a small amount of silica binder in the second dipcoat:

Example H.

Zircon Flour [54 Lbs Antifoam Colloid 500 cc Anionic Wetting Agent 720 cc Colloidal Silica (309i Sol.) 242 cc Polyvinyl Alcohol (726% Sol.) 30.0 Lbs Water i243 Lbs Third Dipcoat Composition and Application The composition of the third dipcoat. which forms the outer layer 16 of the mold can, but need not be, the same as the composition of the first dipcoat. It is applied and stuccoed in a manner similar to that of the first dipcoat. However, usually a coarser stucco is applied. When the outer layer l6 has been built up to a desired thickness the mold is subjected to a final drying cycle.

As an example of the number of dipcoats required to form each of the three layers of the sandwich structure for an average weight casting, two dipcoatings have been employed to form the inner layer 12, three dipcoatings have been employed to form the intermediate layer 14, followed by two dipcoats in the first dipcoat composition to form the outer layer 16. For heavier castings or for large molds, the thickness of the layers may be increased as necessary.

After final drying, the completed mold is heated to remove the pattern contained therein. and leave a mold cavity in which the material to be cast is poured. Pattern removal, hereinafter referred to as dewaxing, can be achieved in a number of ways, as disclosed in US. Pat. No. 3,266,l06. Usually dewaxing the mold is accomplished by firing it to a high temperature. During such'heating, the colloidal silica binder in the inner and

outer layers

12 and 16 strongly bonds the ceramic dipcoat and granular stucco into a monolithic mass. Simultaneously, the organic fugitive binder in the intermediate layer 14 is volatilized and/or burned out leaving the materials making up the intermediate layer unbonded and sandwiched between the inner and outer bonded layers.

Alternatively, the sandwich structure mold of the present invention can be dewaxed at a lower tempera ture on the order of 200 to 400 F. This dewaxing method is possible due to the high green strength achieved in the present sandwich structure mold.

lf the sandwich mold is dewaxed at a low temperature. then it will be preheated to a high temperature prior to use, as indicated in H0. 2, for burning out the mold, i.e., removing any small traces of remaining wax, for burning out or volatilizing the organic binder used for the intermediate layer and for curing the silica binder. Preheating is usually carried out at temperatures in the range of l,000 to 2,850" F.

The sandwich structure mold, after being preheated. is ready for pouringthe molten metal in the mold cavity. During the preheating and pouring of the metal, the sandwich structure mold does not gain sufficient additional strength to cause excessive hardening of the mold. Another advantage of a sandwich structure mold is that, during the cooling process, the relatively thin inner layer 12 is able to collapse, thereby eliminating the possibility. of casting hot tears.

After the casting has cooled completely. the mold is removed. lts construction permits the use of low amplitude mechanical vibration to crack the thin outer layer 16, which breaks off easily. The intermediate layer, being unbound, falls away from the mold and, if desired, may be reclaimed for re-use. The portions of the inner layer are similarly easily broken away.

The castings resulting from this process are relatively free of mold materials, hot tears, and knock-out cracks. Also, sand blasting for final cleaning is minimized.

While I have shown and described an embodiment of this invention in some detail, it will be understood that this description and illustration are offered merely by way of example, and that the invention is to be limited in scope only by the appended claims.

I claim:

I. A method of producing a recyclable sandwich structure mold about a disposable pattern which is rc moved to define a mold cavity comprising the steps of:

a. providing a homogenous first layer by wetting the surface of the pattern with a first dipcoat composition. the essential solids of which consist of finely divided refractory material. including a high strength inorganic binder which is chemically inert to metal to be molded in said cavity;

h. covering the surface of the pattern, while wet with the first dipcoat. with a granular refractory stucco;

c. repeating, at least once, steps (a) and (b) with intervening drying to form a first layer of said mold;

d. wetting said first layer with a second dipcoat composition, the essential ingredients including finely divided refractory material and an organic fugitive binder;

e. covering the surface of the first layer while wet with the second dipcoat with a granular refractory stucco;

f. repeating, at least once, steps (d) and (c) with in tervening drying to form an intermediate layer of said mold; and

g. forming an outer layer over said intermediate layer in the manner recited in steps (a). (b) and (c). to thereby produce a three layer sandwich structure mold.

2. The method of claim 1 further including the step of volatilizing the organic fugitive binder to leave the intermediate layer unbound and sandwiched between said inner and outer layers.

3. The method of claim I wherein the organic fugitive binder is polyvinyl alcohol.

4. The method of claim I wherein the organic fugi' tive binder is one of the group comprising vegetable gums. starches and polymeric latices.

5. The method of claim I, further including the steps of a. removing said pattern; and

b. heating said mold to a temperature sufficient to volatilize said organic fugitive binder to leave said intermediate layer unbound.

6. The method of claim 1 further including the steps of heating said mold to remove said disposable pattern, to volatilize said organic fugitive binder in said intermediate layer, and to fuse said inorganic binder in said inner and outer layers.

7. The method according to claim 5 wherein the step of removing the pattern is accomplished at a temperature within the range of 200 to 400 F., and the step of volatilizing the organic fugitive binder is accomplished at a temperature within the range of l,00()F. to 2,850 F.

8. A sandwich structure recyclable mold for precision casting having a wall structure in cross-section comprising: i

a. a homogenous relatively thin inner layer consisting essentially of finely divided refractory material, including a high strength inorganic binder which is chemically inert to metals to be molded and granular refractory stucco;

b. an intermediate layer consisting essentially of finely divided refractory material, refractory stucco and an organic fugitive binder;

c. an outer layer identical in composition to said inner layer.

9. The mold of claim 8 wherein prior to use the mold 8 is subjected to heating sufficient to volatilize said organic fugitive binder, to thereby leave the intermediate layer refractory material and stucco unbound and sandwiched between the bound inner and outer layers.

10. The mold of claim 8 wherein the'inner and outer layer refractory materials and stucco essentially comprise ceramic flour and binder and ceramicstucco.

11. The mold of claim 8 wherein the intermediate layer refractory materials essentially comprise alumina or zircon flour. 1

12. The mold of claim 8 wherein the organic fugitive binder is selected from the group comprising vegetable gums, starches, polymeric latices and polyvinyl alcohol.

13. A method of precision casting materials, such as metal alloys, comprising the steps of:

a. producing a sandwich structure mold about a disposable pattern, said mold having homogenous inner and outer layers consisting essentially of finely divided refractory material, including a high strength inorganic binder chemically inert. to said alloys and granular refractory stucco, and an intermediate layer consisting essentially of finely divided refractory material, refractory stucco, and an organic fugitive binder;

b. removing said disposable pattern from said mold to provide a mold cavity;

c. firing said mold to a temperature sufficient to volatilize said organic fugitive binder to thereby leave said intermediate layer unbound and sandwiched between the bound inner and outer layers;

d. pouring a molten metal alloy into said mold cavity;

e. cooling the mold; and

f. removing the mold from the cast metal alloy.

14. The method of claim 13 wherein the disposable pattern is formed of wax and steps (b) and (c) are accomplished simultaneously by the heating of said mold to a temperature within the range of l,000 to 2,850 F.

15. The method of claim 13 wherein step (f) includes the sub-steps of:

a. breaking the outer layer with low amplitude mechanical vibration;

b. permitting the unbound intermediate layer to fall away from the mold and collecting for re-use;

c. removing the inner layer.

16. A sandwich structure mold according to claim 9 wherein said intermediate layer also includes approximately 0.125 percent by weight of an inorganic binder for preventing said intermediate layer from becoming completely unbound when said organic fugitive binder is volatilized.

Claims

1. A method of producing a recyclable sandwich structure mold about a disposable pattern which is removed to define a mold cavity comprising the steps of: a. providing a homogenous first layer by wetting the surface of the pattern with a first dipcoat composition, the essential solids of which consist of finely diviDed refractory material, including a high strength inorganic binder which is chemically inert to metal to be molded in said cavity; b. covering the surface of the pattern, while wet with the first dipcoat, with a granular refractory stucco; c. repeating, at least once, steps (a) and (b) with intervening drying to form a first layer of said mold; d. wetting said first layer with a second dipcoat composition, the essential ingredients including finely divided refractory material and an organic fugitive binder; e. covering the surface of the first layer while wet with the second dipcoat with a granular refractory stucco; f. repeating, at least once, steps (d) and (e) with intervening drying to form an intermediate layer of said mold; and g. forming an outer layer over said intermediate layer in the manner recited in steps (a), (b) and (c), to thereby produce a three layer sandwich structure mold.

3. The method of claim 1 wherein the organic fugitive binder is polyvinyl alcohol.

4. The method of claim 1 wherein the organic fugitive binder is one of the group comprising vegetable gums, starches and polymeric latices.

5. The method of claim 1, further including the steps of a. removing said pattern; and b. heating said mold to a temperature sufficient to volatilize said organic fugitive binder to leave said intermediate layer unbound.

7. The method according to claim 5 wherein the step of removing the pattern is accomplished at a temperature within the range of 200* to 400* F., and the step of volatilizing the organic fugitive binder is accomplished at a temperature within the range of 1,000*F. to 2,850* F.

8. A SANDWICH STRUCTURE RECYCLABLE MOLD FOR PRECISION CASTING HAVING A WALL STRUCTURE IN CROSS-SECTION COMPRISING: A. A HOMOGENOUS RELATIVELY THIN INNER LAYER CONSISTING ESSENTIALLY OF FINELY DIVIDED REFRACTORY MATERIAL, INCLUDING A HIGH STRENGTH INORGANIC BINDER WHICH IS CHEMICALLY INERT TO METALS TO BE MOLDED AND GRANULAR REFRACTORY STUCCO, B. AN INTERMEDIATE LAYER CONSISTING ESSENTIALLY OF FINELY DIVIDED REFRACTORY MATERIAL, REFRACTORY STUCCO AND AN ORGANIC FUNGITIVE BINDER, C. AN OUTER LAYER IDENTICAL IN COMPOSITION TO SAID INNER LAYER.

9. The mold of claim 8 wherein prior to use the mold is subjected to heating sufficient to volatilize said organic fugitive binder, to thereby leave the intermediate layer refractory material and stucco unbound and sandwiched between the bound inner and outer layers.

10. The mold of claim 8 wherein the inner and outer layer refractory materials and stucco essentially comprise ceramic flour and binder and ceramic stucco.

11. The mold of claim 8 wherein the intermediate layer refractory materials essentially comprise alumina or zircon flour.

13. A method of precision casting materials, such as metal alloys, comprising the steps of: a. producing a sandwich structure mold about a disposable pattern, said mold having homogenous inner and outer layers consisting essentially of finely divided refractory material, including a high strength inorganic binder chemically inert to said alloys and granular refractory stucco, and an intermediate layer consisting essentially of finely divided refractory material, refractory stucco, and an organic fugitive binder; b. removing said disposable pattern from said mold to proVide a mold cavity; c. firing said mold to a temperature sufficient to volatilize said organic fugitive binder to thereby leave said intermediate layer unbound and sandwiched between the bound inner and outer layers; d. pouring a molten metal alloy into said mold cavity; e. cooling the mold; and f. removing the mold from the cast metal alloy.

14. The method of claim 13 wherein the disposable pattern is formed of wax and steps (b) and (c) are accomplished simultaneously by the heating of said mold to a temperature within the range of 1,000* to 2,850* F.

15. The method of claim 13 wherein step (f) includes the sub-steps of: a. breaking the outer layer with low amplitude mechanical vibration; b. permitting the unbound intermediate layer to fall away from the mold and collecting for re-use; c. removing the inner layer.