US3010852A - Eliminating patterns from molds - Google Patents

Description

United States Patent 3,010,852 ELllVIlNATEN G PATTERNS FROM MOLDS Charles H. Prange, New York, John I. Leonard, Baldwin, and Henry W. Soller, Dover, N.Y., and Roy C. Feagin, Mountain Lakes, N.J., assignors, by mesne assignments, to Howe SoundCompany, New York, N.Y.,

a corporation of Delaware No Drawing. Filed June 10, 1958, Ser. No. 740,993 14 Claims. (Cl. 134-5) This invention relates to eliminating patterns from molds, and is particularly directed to eliminating patterns of wax, thermoplastic resins, and similar fusible materials of high thermal expansivity from refractory shell molds. The invention provides an improved method for eliminating such patterns by subjecting the mold with pattern therein to very rapid heating, whereby a major part of the pattern material is quickly melted and can be run out of the mold cavity. The residue of pattern material which remains in the mold can then be eliminated by heating in the usual manner in an oxidizing oven atmosphere.

In the manufacture of accurate metal castings, it is customary to make a pattern of the desired casting in wax or similar readily fusible material, and then to form a refractory mold by dip-coating such pattern a number of times with a liquid suspension of a fine-grained refractory which hardens on the pattern. When a refractory shell mold has thus been built up on the pattern, it is only necessary to eliminate the pattern from within it in order to form an accurate mold of the article to be cast.

It is not a simple matter to eliminate the pattern material from the mold, however. One of the main difficulties in the successful utilization of multiple dipped shell molds is that of cracks developing in the shell during pattern elimination. These cracks are caused by the high thermal expansion coeflicient of the pattern material as compared with the ceramic shell material. Normal practice is to place the shell with pattern therein into an oven in which the wax or plastic is melted or burned out and in which the shell mold is heated to proper casting temperature. Then the molten metal is poured into the shell. This method of pattern elimination causes considerable cracking of the shell, particularly in the vicinity of heavy sec-' tions of the pattern material, such as about sprues, feeder bars, and the pouring basin end. These cracks many times require repairs with the dipcoat or with refractory cement. The castings made in such repaired shell molds require considerable finishing at the fin produced and also are not accurate.

Heretofore a variety of rather cumbersome expedients have been employed in the precision casting industry to cope with the problems caused by the high thermal expansivity of wax and other plastic pattern materials. One such expedient, which has been extensively used despite the expense it involves, is to reinforce the coating formed on the pattern (called the primary investment) by surrounding it with a thick reinforcement of cementitious refractory (called the secondary investment). Another expedient, which has enjoyed less practical success because it often is harmful to the mold, is to dissolve the pattern material from the mold by a solvent (either in liquid or vapor form). Another expedient has been to use frozen mercurya material of relatively low thermal expansivity-as the patternmaterial, but this expedient is subject to the difliculties and expense of forming the pattern and the mold at very low temperatures.

The present invention. provides a pattern elimination procedure by which it is possible to eliminate wax and plastic pattern materials from refractory shell molds by a simple melting operation, without cracking and without resort to any of the troublesome or costly expedients heretofore employed. The new method is based on the finding ice that if a mold with pattern therein is subjected to very rapid heating, the bulk of the pattern material will melt quickly without causing the mold shell to crack or otherwise deform. The thus-melted pattern material can be run out from the mold, after which the residue of wax in the mold can be eliminated by relatively slower heating in a hot oven atmosphere.

Accordingly, the method of this invention for eliminating a pattern of readily fusible material having a high 00-" efficient of thermal expansion from a refractory shell mold comprises immersing the mold with pattern therein in a heating medium of high heat capacity at a tempera ture at least 200 F. above the flow point of the pattern material, retaining the mold immersed in such hot medium until at least a major part of the pattern has melted, and discharging the molten pattern material from the shellv mold.

The heating media which have adequately high heat capacity for use in carrying out the method of the inven tion are liquids and granular solids. The liquids'that may be used include both organic and inorganic liquids of high boiling point, and molten readily fusible metals and alloys. The granular solids, that may be used include granular refractory materials, and metals in shot form. Gaseous heating media generally have too low heat ca-' pacity to provide the rapid heating of the pattern in the mold which is necessary to the success of the new method.

The most common and advantageous pattern materials are waxes and thermoplastic materials which have a flow point below 175 F., and generally in the range from F. to F. The preferred method according to this invention for eliminating such pattern materials from a shell mold involves immersing the mold in a heating medium of high heat capacity heated to a temperature above 350 F. (above 400 F. in the case of the highermelting materials). The mold is held immersed in this hot medium until at least a major part of the wax or other pattern material has melted, and the molten pattern material is poured or otherwise discharged from the mold. Then the mold is heated in a hot oxidizing ovenat'mosphere until the residue of pattern materialis substantially completely eliminated.

When the heating medium is a hot liquid, it is usually preferred to set the mold upright (that is, with the pour ing basin uppermost) in the container of hot liquid. When the wax or other pattern material has to a large extent melted, the mold is lifted from the heating me dium container and is tilted to pour out the molten pat tern material. However, when a granular solid is used as the heating medium, the method of the invention contemplates mounting the mold with pattern therein in inverted position (that is, with pouring basin lowermost) in a vessel with a perforated bottom, and then pouring the hot granular solid material, heated usually to above 350 F. and in any event to at least 200 F. above the flow point of the pattern material, about the inverted mold until it is substantially completely buried therein. The mold is held buried in the hot granular material until at least a major part of the wax has melted and flowed from the mold. Then the vessel may be tilted to pour off the hot granular solid, and the mold may be transferred to an oven where it is heated to a high enough temperature to eliminate the residue of pattern material. In carrying out the new method, a refractory shell mold is prepared in amysuitable manner by coatinga wax or. thermoplastic pattern with a succession of coats of re-v fractory slurry and allowing each such coat to harden on' the pattern. When the thickness of the refractory has" been built up to form a mold shell of desired thickness, and when such shell has been properly dried and hardened, it is ready for elimination of the pattern materialby the method of this invention.

If the pattern is to be eliminated by immersion of the mold in a hot liquid, the liquid in a suitable container is first heated to the desired temperature. The liquid may be heated in the container, or it may be heated elsewhere and introduced but into the container. In any event its temperature is brought to at least 200 F. above the flow point of the pattern material. Thus, when a wax or thermoplastic pattern material having a flow point somewhat under 150 F. is used, the liquid should be heated at least to about 350 F. Preferably the liquid is heated to above 400 F., and maywith advantage be heated to above 500-F.

Among the liquids that may be employed are the'tri- (alkylaryl) phosphates sold commercially for use as heat transfer agents. These compounds have high specific heat, low vapor pressure at temperatures in the range from 400 F. to 500 F., high flash point (about 455 F.), high auto-ignition temperature (about 1100 F.), and high flame support temperature (fire point about 665 F J. Other organic liquids that may be used successfully ing only a nubbin of unmelted wax, plus what adheres include diphenylmethane, polyethylene glycol, molten chlorinated aromatics such as chlorinated naphthalene, and silicone oils. Other high-boiling organic liquids also are available and may be used, such as cottonseed oil and other vegetable oils, and mineral oil; but generally it is preferable and safer to employ organic materials of higher flash point. The organic liquid used preferably is not a solvent for the pattern material, in order to avoid contaminating either the pattern material which may be recovered or the heating medium, but this is not necessary to satisfactory operation of the invention.

Another type of liquid that may be used is molten metal, particularly low-melting alloys such as die metal (the eutectic alloy of tin and bismuth which melts at about 280 F.), Woods metal (M.P. 158 F.), the eutectic alloy of bismuth, lead and tin (M.P. 203 F.), and other alloys which can form molten baths at temperatures of about 400 F. A molten metal bath has the advantage that it does not wet the outside of the mold shelland so none adheres to the mold shell when it is removed from the bath. However, care must be taken when using a molten metalbath not to allow any of the bath metal to enter and remain in the mold cavity or it will leadto an imperfect casting.

It is also possible to employ inorganic liquids such as very fusiblemiolten salt mixtures, but the number of such liquids is limited and their use is generally accompanied by'disadvantages sothey are not preferred.

' The shell mold with pattern therein is immersed in whichever of these liquids has been selected and heated. Preferably the mold is immersed upright, so the wax (or other pattern material) that melts will mostly remain inthe mold cavity. However, when using a hotliquid Which is immiscible with the pattern material, and does not either contaminate it objectionably or become objec: tionably contaminated by it, the mold may be immersed in a position such that the molten pattern material can flow out into the container of hot liquid from which it is ultimately recovered. The hot liquid may with advan tage be kept agitated, to insure uniform heat distribution throughout its mass and to assure the most rapid heat transfer from the liquid to the pattern. The mold is kept immersed in the hot liquid until a' major part of the pattern material has melted. Sometimes, especially when the pattern is small, or of thin section, it will become substantially completely melted while the mold is immersed in the hot liquid. More generally, however, it is not necessary to hold the mold in the hot liquid after about half, .more or less, of the pattern material has melted. The time required for melting of a major portion of the pattern to occur depends upon such factors as size of the pattern, thickness of the shell mold, and temperature of the hot liquid, but in a typical case it will be about two to five minutes. Then the mold is withdrawn from the hot liquid, and, if the melted pattern material has not already run out, it is poured out, leavis heated in the usual manner to a high enough tempera ture (preferably in an oxidizing atmosphere) to eliminate all residue of the pattern material from within it and to bring it to casting temperature (typically about 1800" F Any volatile or decomposable organic or other liquid from the heating bath adhering to the outside surface of the mold is eliminated by this same oven heating operation. Instead of heating the mold to casting temperature, the mold may be heated in the oven only hot enough to eliminate all trace of the remaining wax (say to 1100 F.), and may then be cooled to room temperature and stored until required for use, at which time it may be heated to casting temperature. Such of the wax or other pattern material remaining in the mold that melts and flows from the oven may be combined with that poured from the mold after its withdrawal from the hot liquid, or re-use in making patterns. The remainder, plus such of the heating liquid as adhered to the mold, is volatilized or burned ed in the oven.

If the pattern is to be eliminated by immersion of the mold in a hot granular solid, the mold with pattern therein is preferably first placed in a container having a perforated bottom. Advantageously the container is formed of a separate perforated base on which the mold is placed, and an open-ended cylinder which is then set on the base about the mold. The mold is placed in the container in inverted position, that is, with the pouring basin lowermost. If the container bottom is formed of a screen, or if it otherwise is uniformly perforated over its entire area, the mold may be placed in any convenient position in the container; but if the perforations in the container bottom are not closely spaced, then the mold should be placed so that the pouring basin is directly over one or more perforations.

Hot granular solidmaterial then is poured into the container until the mold is substantially completely buried. The granular solid material may be preheated in any desired type of heating furnace or oven; but in accordance with the invention it is preheated to a temperature at least 200 F. above the flow point of the pattern material before it is poured about the mold. The granular solid may be heated, for example, by direct firing in a fuel-fired furnace, or it may be heated indirectly in an oven, muffle or pot. Generally, it should be heated to a temperature above 350 .F., and in most cases it is advantageous to heat it to a temperature in the range from 400 F. to 500 F., or even higher, up to 1000 F. or more.

Any granular solid material capable of being heated to the necessary temperature without fusion or sintering may be employed. Generally, a granular refractory material, such as silica sand, 21 fired refractory fireclay grog, or coarse particles of fused periclase, isvery satisfactory; Another type of granular solid which has been used with considerable success is metal in granular or pelletized form, such as spherical steel shot, and irregularly shaped shot of hardrefractory alloys such as chrome-nickel stainless alloys and cobalt-base alloys. An advantage of metal in shot form is that the conductivity ofheat through a mass of it is considerably more rapid than through a similar mass of non-metallic refractory such as sand or grog. Oxides and anhydrous salts of adequately high melting point, such as sodium chloride or iron oxide, can be used, though they are less preferred than the more refractory materials. I

The mold is kept immersed in the hot granular solid until 'a major part ofthe wax or other fusible pattern material has melted. Such-melting takes place rapidly (generally in a period of from two to five minutes) be- 35 cause of the high rate of heat transfer from the hot granular solid through the relatively thin wall of the shell mold. As it melts, the pattern material drains out through the mold pouring basin and through the perforated container bottom, and may be recovered for reuse in pattern making.

When a major part of the pattern material (for example, from oneto two-thirds of it) has thus been melted and drained from the mold, the container may be tilted to pour off the hot granular solid material, and the mold, with a residue of the pattern material therein, may be taken from the container and inspected for cracks or other defects. If none are found (as will usually be the case in a properly conducted operation), the mold may be transferred to an oven in which it is heated (advantageouslyin the inverted position) until the nubbin of wax it may still contain has melted and run out, and until the residue of wax adhering to the mold walls has been volatilized or burned off and the mold brought to casting temperature.

A particular advantage of using hot granular solid material, instead of a hot liquid, is that it does not wet and adhere to the surface of the mold shell. Upon removal of the mold from the mass of hot granular material, it is completely dry and clean. Another advantage of using hot granularsolid material is that the mold'may be inverted when the hot material is poured about it, so that the pattern material may run out as it melts. However, it is not necessary that the mold be inverted when it is surrounded by the hot granular material. Instead, as in the case of the alternative immersion of the mold in a hot liquid, it may be in the upright position when surrounded by the hot granular solid.

An important characteristic of the method of this invention lies in the use of a heating medium (liquid or solid) of high heat capacity which is preheated to a temperature far above the flow point of the pattern ma- 'terial.

As a result heat is transferred very rapidly from the heating medium through the wall of the shell mold to the pattern material, and the pattern material melts very rapidly. Under such heating conditions a major part of the pattern material becomes so quickly melted that no deformation or cracking of the, mold ordinarily occurs.

Although it is quite possible to melt the entire mass of the pattern material while the mold is held immersed in the hot liquid or granular solid heating medium, it is unnecessary to do so, and generally it is more convenient to withdraw the mold from such heating material when .a major part of the pattern material has become melted. The nubbin of pattern material which remains in the mold, after the melted portion has been run out, is

sufiiciently small in size, relative to the mold cavity, so ,that it can be melted without harm to the mold when the. mold is heated in an oven atmosphere to eliminate the pattern residue adhering to the mold walls.

Following are examples of the method of this invention:

Example 1.A refractory shell mold was formed about a wax pattern of a cluster of nine turbine blades. This mold, after normal hardening and drying, and with the pattern material therein, was immersed in the upright position in a molten bathpf die metal heated to a temperature of 400 F. The mold was held so immersed for a period of three minutes. At the end of this period .a- ,major portion of the wax pattern material had melted. 'The mold was then withdrawn from the molten metal bath and inverted to allow the molten wax to fiow out. The mold was carefully examined andfound to be free from cracks or other defects. T he mold was then heated -to 1100 F. in an oxidizing oven atmosphere to melt the remaining residue of wax and to volatilize and burn out such Wax as would not run out. The mold was cooled to room temperature. Subsequently "it-was: heated ,to 1850 Rand molten refractory cobalt-base alloy was then poured intoit and cooled in the usual manner. Upon breaking the mold away from the casting, the casting was found to be in excellent condition, accurate in size, and free from deformities of any sort.

Example 2.A shell mold was formed on a wax patternof a cluster of nine turbine blades and hardened and dried in the usual manner. The mold with pattern there-in was then immersed, in the upright position, in a container of a commercial tri-(alkylaryl) phosphate heat transfer agent heated to a temperature of 580 F. The mold was held thus immersed for a period of two minutes, at the end of which time a major portion 'of the pat-tern material had melted. The mold was then withdrawn from the hot liquid and inverted to pour out the melted wax. Inspection of the mold at this time showed it to be free from cracks or other deformation. It was then heated in an oxidizing oven atmosphere to a temperature of 1100 F. to meltand burn out the residue of wax remaining within it and to burn off the film of heating-liquid which adhered to its outer surface, and it was then cooled slowly to room temperature. Thecooled mold was found to be inperfect condition. .It was thereafter used for making a refractory alloy casting, and after breaking the mold from the cast metal, the castings were found to beaccurate in dimensions and otherwise in excellent condition.

Example 3.A mold was prepared similarly to that of Examples 1 and 2. It was immersed, with Wax pattern therein, in an agitated bath of polyethylene glycol which was heated to a temperaturebetween 450 F. and 460 F. The mold was held immersed in the hot agitated bath for three minutes, in the upright position, after which it was withdrawn and the molten wax poured out. It was noted that a major part of the wax had melted, though a nubbin of unmelted wax remained. An examination of the mold at this stage showed it free from cracks or other damage. The mold was then heated in an oxidizing o-ven atmosphere to a temperature of 1000 F., to melt the remaining nubbin of wax and to volatilize and burn out the wax residue adhering to the interior of the mold walls, and to burn off the film of heating liquid adhering to the exterior of the mold walls. It was then cooled to room temperature. Further examination at this'stage revealed the mold still to be in perfect condition. Thereafter it was heated to 1850 F., and a refractory alloy casting was made in it. After breaking the mold from the casting, the latter was found to beaccurate and free from imperfections.

Example 4.A refractory shell mold was formed about a wax pattern of a cluster of two turbine blades, and allowed to harden. The mold with pattern therein was then mounted on a perforated support plate, with the pouring basin of the mold overlying the perforation in the plate. The mold was then surrounded by an openended cylindrical steel flask. Steel shot was preheated to a temperature of 315 C. (597 F.), and the hot steel shot was then poured into the flask about the mold.

The mold was held buried in the hot shot for a period of five minutes, during which much of the wax melted and ran from the mold. The flask was then tilted to pour out the hot shot, and the mold was removed. Examination showed the mold to be free from cracks or other de formation. The mold was then placed in an oven in an inverted position, and heated first to a temperature of 360 F., to melt the nubbin of wax which remained ,in it. After as much wax as possible had run from the mold, the oven temperature was raised to 750 F; to burn out the. remaining residue of wax. Then the oven temperature was further increased to 1850 F. to bring the mold to casting temperature. With the mold at this temperature, a refractory cobalt-base .alloy (at a temperature of 3000'F.) waspressure-cast in it. After the casting had cooled and solidified, and the mold had been broken away, the casting was found to be accurate in size and completely free from fins and other defects.

We claim:

l. The method of eliminating a readily fusible pattern of material having a high coefiicient of thermal expansion from a refractory shell mold which comprises immersing the mold with pattern therein in a non-gaseous heating medium of high heat capacity at a temperature at least 200 F. above the flow point of the pattern material, retaining the mold immersed in said hot medium until at least a major part of the pattern has melted, and discharging the molten pattern material from the shell mold. I

2. The method of eliminating a readily fusible pattern of material having a high coefiieient of thermal expansion from a refractory shell mold which comprises immersing the mold with pattern therein in a liquid heated to a temperature at least 200 F. above the flow point of the pattern material, retaining the mold immersed in said hot liquid until at least a major part of the pattern has melted, and discharging the molten pattern material from the shell mold.

3. The method of eliminating a readily fusible pattern of material having a high coefiicient of thermal expansion from a refractory shell mold which comprises immersing the mold with pattern therein in granular solid material heated to a temperature at least 200 F. above the flow point of the pattern material, retaining the mold immersed in said hot granular solid until at least a major part of the pattern has melted, and discharging the molten pattern material from the shell mold.

, 4. The method of eliminating a wax pattern having a flow point below 150 F. from a refractory shell mold which comprises immersing the mold with pattern therein in a non-gaseous heating medium of high heat capacity heated to a temperature above 350 F., holding the mold immersed in said hot medium until at least a major part of the wax has melted, discharging the molten wax from the shell mold, and then heating the mold in a hot oven atmosphere until the wax is substantially completewhich comprises immersing the mold with pattern there- .in in a high-boiling liquid heated to a temperature above 350 F, holding the mold immersed in said hot liquid until at least a major part of the wax has melted, dis- .chargingthe molten wax from the shell mold, and then heating the mold in a hot oven atmosphere until the wax is substantially completely eliminated. i

6. The method of eliminating a wax pattern having a how point below 150 F. from a refractory shell mold which comprises immersing the mold with pattern therein in a molten bath of low melting metal heated to a temperature above 350 F., holding the'mold immersed in said hot molten metal bath until at least a major part of the wax has melted, discharging the molten wax from the shell mold, and then heating the mold in a hot oven atmosphere until the wax is substantially complete- 1y eliminated.

7. The method of eliminating a wax pattern having a new point below 150 F. from a refractory shell mold which comprises immersing the mold with pattern therein in a granular solid heated to a temperature above 350 F., holding the mold immersed in said hot granular solid until at least a major part of the wax has melted, discharging the molten wax from the shell mold, and

then heating the mold in a hot oven atmosphere until the wax is substantially completely eliminated.

of shot until at least a major part of the wax has melted, discharging the molten wax from the shell mold, and then heating the mold in a hot oven atmosphere until the wax is substantially completely eliminated.

9. The method of eliminating a wax pattern having a flow point below 150 F. from a refractory shell mold which comprises mounting the mold with pattern therein in inverted position, pouring granular solid material heated to a temperature above 350 F. about the inverted mold until it is substantially completely buried therein, and holding the shell mold buried in said hot granular solid until at least a major part of the wax has melted and flowed from the mold.

10. The method of eliminating a wax pattern having a flow point below 150 F. from a refractory shell mold which comprises mounting the mold with pattern therein in inverted position, pouring metallic shot heated to a temperature above 350 F. about the inverted mold until it is substantially completely buried therein, and holding the shell mold buried in said hot shot until at least a major part of the wax has melted and flowed from the mold.

'11. The method of eliminating a pattern of fusible material having a high coefiicient of thermal expansion and a flow point below F. from a refractory shell mold which comprises immersing the mold with pattern therein in a high-boiling organic liquid heated to above 400 F, holding the mold immersed in said hot liquid until at least a major part of the pattern has melted, and discharging the molten pattern material from the shell mold.

12. The method of eliminating a pattern of fusible material having a high coeilicient of thermal expansion and a flow point below 175 F. from a refractory shell mold which comprises immersing the mold with pattern therein in a molten bath of low-melting metal heated to 'a temperature above 400 F., holding the mold immersed in the hot molten bath until at least a major part of the pattern has melted, and discharging the molten pattern material from the shell mold.

13. The method of eliminating a pattern of fusible material having a high coefiicient of thermal expansion and a flow point below 175 F. from a refractory shell mold which comprises pouring a granular solid heated to a temperature above 400 F. about the mold with pattern therein, holding the mold in said hot granular solid until at least a major part of the pattern has melted, and discharging the molten pattern material from the shell mold.

14. The method of eliminating a pattern of fusible material having a high coeflicient of thermal expansion and a flow point below 175 F. from a refractory shell mold which comprises mounting the mold with pattern therein in inverted position, pouring granular solid material heated to a temperature above 400 F. about the inverted mold until it is substantially completely buried therein, and holding the shell mold buried in said hot granular solid until at least a major part of the pattern has melted and flowed from the mold.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Cross; Handbook of Petroleum, Asphalt and Natural Gas, page 418. Published as Bulletin No. 25( 1928 revision), Kansas City Testing Laboratory, Kansas City,

Claims (1)

1. THE METHOD OF ELIMINATING A READILY FUSIBLE PATTERN OF MATERIAL HAVING A HIGH COEFFICIENT OF THERMAL EXPANSION FROM A REFRACTORY SHELL MOLD WHICH COMPRISES IMMERSING THE MOLD WITH PATTERN THEREIN IN A NON-GASEOUS HEATING MEDIUM OF HIGH HEAT CAPACITY AT A TEMPERATURE AT LEAST 200*F. ABOVE THE FLOW POINT OF THE PATTERN MATERIAL, RETAINING THE MOLD IMMERSED IN SAID HOT MEDIUM UNTIL AT LEAST A MAJOR PART OF THE PATTERN HAS MELTED, AND DISCHARGING THE MOLTEN PATTERN MATERIAL FROM THE SHELL MOLD.