05/224328

11/06/1973

02/07/1972

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General Motors Corporation (Detroit, MI)

164/35

B22C9/04; B22C9/04

164/34,35,338M

3512571

CRYOGENIC FORMATION OF REFRACTORY MOLDS AND OTHER FOUNDRY ARTICLES

May 1970

Phelps

Overholser, Spencer J.

Roethel, John E.

It is claimed

1. The method of removing a wax pattern from a shell investment mold formed thereon, said mold having a wall thin enough to allow rapid transfer of heat therethrough and not sufficiently thick to resist breakage from the force which would result from the expansion of said wax pattern upon slow heating thereof, which comprises freezing said mold and pattern contained therein to a temperature of at least -80° F to create a gap between the mold and the pattern, and then immediately heating the mold and pattern in a manner such that the heat rapidly penetrates the mold wall and initially melts a surface layer of the wax pattern which expands into said gap so that the expansion forces are not sufficient to damage the mold and then completely melts the wax pattern to remove it from the mold.

2. The method of removing a wax pattern from a shell investment mold formed thereon, said mold having a wall thin enough to allow rapid transfer of heat therethrough and not sufficiently thick to resist breakage from the force which would result from the expansion of said wax pattern upon slow heating thereof, which comprises freezing said mold and pattern contained therein to a temperature of about -80° F to about -100° F to create a gap between the mold and the pattern of about 0.002 inch and then immediately heating the mold and pattern in a manner such that the heat rapidly penetrates the mold wall and initially melts a surface layer of the wax pattern which expands into said gap so that the expansion forces are not sufficient to damage the mold and then completely melts the wax pattern to remove it from the mold.

3. The method of removing a wax pattern from a shell investment mold formed thereon, said mold having a wall thin enough to allow rapid transfer of heat therethrough and not sufficiently thick to resist breakage from the force which would result from the expansion of said wax pattern upon slow heating thereof, which comprises freezing said mold and pattern contained therein to a temperature of about -80° F to about -100° F to create a gap between the mold and the pattern of about 0.002 inch and then immediately heating the mold and pattern in a steam autoclave maintained at about 230° F at about 80 psi gauge so that the heat rapidly penetrates the mold wall and initially melts a surface layer of the wax pattern which expands into said gap so that the expansion forces are not sufficient to damage the mold and then completely melts the wax pattern to remove it from the mold.

BACKGROUND OF THE INVENTION

The invention herein described was made in the course of work under contract or subcontract thereunder with the United States Air Force.

This invention relates to the art of precision casting of metal articles and particularly to a method for removing patterns from shell or thin walled one-piece molds of ceramic refractory materials.

As is well known a major problem in the art of making thin walled investment molds is the task of removing the wax pattern from the mold prior to firing or sintering it without cracking or otherwise damaging the mold. The problem results from the fact that when the pattern is removed by melting it the wax undergoes a marked expansion before it liquifies so that the wax pattern exerts very high pressures against the mold walls sufficient to crack or otherwise damage them. Various techniques have been developed for dealing with this problem. One such method proposes to make a thinly invested shell mold by utilizing frozen mercury for the pattern which has very small thermal coating of expansion near its melting point so that it can be melted and removed without damaging the mold. However, the use of a frozen mercury pattern requires carrying the various coating steps for making the mold at a temperature below -39° C and requires the use of a non-aqueous slip or one having a suitably low freezing point. Such temperature limitations cause the process to be relatively expensive and difficult to perform.

Another method which has been proposed involves dipping the pattern and mold into a liquid maintained at a temperature equal to or above the melting point of the pattern material. This causes the hot liquid to be forced under pressure into the porous walls of the mold to thereby contact the pattern at the interface between the pattern and the mold which softens the pattern material at its surface before the body of the pattern has been heated high enough for it to exert expansion stresses on the mold sufficient to break it. This method, however, has shortcomings in that the liquid tends to weaken the mold structure and causes undesirable discontinuities in the casting surface of the mold.

In another method it has been proposed that the mold and pattern be subjected to solvent vapors for the wax pattern material. This method has disadvantages in that such vapors are usually toxic and combustible.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method for making refractory shell molds of the type having walls thin enough to allow rapid transfer of heat therethrough and not sufficiently thick to resist breakage thereof from the force which would result from expanison of a wax pattern contained therein by which the wax pattern is removed from the mold prior to firing or sintering simply and efficiently without damage to the mold. A more specific object is to provide a method of making refractory shell molds in which the mold and the wax pattern contained therein are frozen whereby the pattern contracts sufficiently more than the mold to provide a sufficient gap therebetween so that when the mold and pattern assembly is inserted in a heated environment substantially above the melting temperature of the wax the heat penetrates rapidly through the mold to cause the mold to expand more rapidly than the pattern and meet the surface layer of the pattern, and the gap between the pattern and mold is sufficient to eliminate any internal pressure build-up due to wax expansion which would damage the mold.

These and other objects are carried out by forming refractory shell molds over the wax pattern by any suitable method as, for example, sequentially dipping the pattern a plurality of times in suitable ceramic shell dip slurries with intermediate drying of the dip coat, then placing the dried molds in a cold box maintained at about -80° F to about -100° F for a time sufficient to cause the mold and pattern to approach the box temperature, and then removing the molds from the cold box and placing them directly in a heated environment operative to cause heat to penetrate rapidly through the mold for a time sufficient to initially melt a surface layer of the wax pattern and then to completely remove the wax pattern by melting it.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method of this invention includes first forming a suitable ceramic or refractory shell mold about a heat destructible pattern. The refractory mold material may be any suitable composition applied by any suitable method and the pattern may conveniently be formed of wax. A suitable mold composition and method of application is described hereinafter.

Three different ceramic dip slurries are prepared. The first or prime dip consists of 200 lbs. of milled zircon (zirconium silicate) mixed with 48 lbs. of a 30 percent by weight aqueous solution of colloidal silica and 30 cc's of a wetting agent such as an alkyl aryl sodium sulphonate. A second dip is prepared consisting of 200 lbs. of milled zircon with 38 lbs. of the 30 percent aqueous colloidal silica. A third or backup slurry is prepared consisting of 150 lbs. of fused silica mixed with 78 lbs. of the 30 percent colloidal silica. These dips are maintained at a temperature of 75° - 85° F and the humidity environment of 60 percent. The refractory coating is applied in a sequence of steps by which the pattern is alternately dipped in a slurry bath and dried in a steam autoclave under controlled temperature and humidity conditions.

First, a wax pattern is exposed to an environment having a dry bulb temperature of 84° F and a wet bulb temperature of 73° F to condition the pattern.

Second, the pattern is dipped in the first slurry dip and stuccoed in a fluidized bed of 80 mesh zircon and dried for about 15 minutes in an autoclave maintained at 84° F dry bulb temperature and 73° F wet bulb temperature.

Third, the pattern is dipped in the second bath, stuccoed in the 80 mesh zircon fluidized bed and dried in the autoclave adjusted to a 90° F dry bulb temperature and a 74° F wet bulb temperature.

Fourth, the pattern is dipped in the third or back-up dip, stuccoed in a 50 mesh silica fluidized bed and dried in the autoclave adjusted to a 96° F dry bulb temperature and a 77° F wet bulb temperature.

Fifth, the pattern is dipped in the third back-up slurry, stuccoed in the 50 mesh silica fluidized bed and dried for about 15 minutes in the autoclave adjusted to a 100° F dry bulb temperature and a 75° F wet bulb temperature.

Sixth, the pattern is dipped in the back-up slurry, stuccoed in the 50 mesh silica fluidized bed and dried in the autoclave adjusted to a 106° F dry bulb temperature and a 72° F wet bulb temperature.

Seventh, the pattern is dipped in the back-up slurry, stuccoed in the 50 mesh silica fluidized bed and dried for about 15 minutes in the autoclave adjusted to the 106° F dry bulb temperature and 72° F wet bulb temperature.

Eight, the pattern is dipped in the back-up slurry and allowed to dry for about 15 minutes in the autoclave at the 106° F dry bulb temperature and 72° F wet bulb temperature.

The resulting shell mold and pattern is then placed in a cold box maintained at about -100° F and held therein until the pattern and mold reaches a temperature of -100° F which is a period of about 20 minutes. Under this temperature a gap of about 0.002 inch is formed between the pattern and the mold. The pattern and mold are then removed from the cold box and immediately placed in an autoclave maintained at a temperature of 230° F with a steam pressure of 80 psi gauge. In the autoclave the heat readily penetrates the thin shell mold and melts a surface layer of the wax pattern before the pattern as a whole warms up sufficiently. The expansion due to the melting of the surface layer is taken up by the gap so that the wax pattern is melted out without any damage to the mold. Thereafter the mold is fired in a gas fired furnace at about 1800° F for about 30 minutes to produce a mold suitable for use in casting metals. Firing temperatures of 1600° F to 2000° F are useable and temperatures of 1800° F to 2000° F are preferred for best results.

It has been found that a temperature of -80° F or less will produce a gap between the pattern and the mold in the vicinity of .002 inch which is sufficient to prevent the melting surface layer of the wax pattern from subjecting the mold to damaging pressures. Temperatures in the vicinity of -80° F may be obtained in the cold box by the use of dry ice and acetone as the freezing medium. Preferably temperatures of -100° F are used which are obtainable by the use of liquid nitrogen. Temperatures below -100° F do not appear advantageous. Moreover, temperatures significantly less than -100° F are not desired since deleterious effects such as cracking of the mold may be experienced.

Alternatively after the freezing step described above, the wax pattern may be removed by placing the mold and wax assembly directly in a gas fired furnace at about 1600° F. The high heat is considered necessary to obtain a fast penentration of the heat through the mold to the wax surface so that a surface layer of the wax is melted before the wax pattern as a whole is heated appreciably. Although the mold and pattern assembly could be fired directly after freezing at the usual firing temperatures, this is not a preferred procedure because the wax tends to burn and may have adverse effects on the mold surface. The frozen mold and wax pattern assembly may also be immersed in a wax bath maintained at about 450° F to remove the wax pattern. However, this method is not preferred because of residual wax left on the mold surfaces.

It will be apparent to those skilled in the art that various refractory shell mold compositions and various methods of application to a wax pattern may be used and various heating methods may be employed to remove the wax after freezing within the scope of this invention.