US2892227A

US2892227A - Metal casting process and elements and compositions employed in same

Info

Publication number: US2892227A
Application number: US558578A
Authority: US
Inventors: Operhall Theodore
Original assignee: DERALD H RUTTENBERG
Current assignee: DERALD H RUTTENBERG; Howmet Turbine Components Corp
Priority date: 1956-01-11
Filing date: 1956-01-11
Publication date: 1959-06-30
Anticipated expiration: 1976-06-30

Description

June 30, 1959 T, QPERHALL 2,892,227

METAL CASTING PROCESS AND ELEMENTS AND COMPOSITIONS EMPLOYED IN SAME Filed; Jan. 11, 1956 2 Sheets-Sheet l svn'm fic kism I N RM mm FBLDSPAIZ .51 um IT REFRACTORYBINDE l-REFRACTORY POWDER 2| comrr: 80 -96 ao l nres R J mfim'rz 7;

eumvmo Mm. (325 MESH) sowsur TO RECOVERY umr DRIER PIELLETIZER SHELL. Mow DRY POWDER BURMOFF VAPOR BAKme ovsu OXYGE soo'-24oo"l= BURMOFF INTERIM Bmusz Mow SHELL ASSEMBLY Y MOLTEN METAL CAST m MOLD sum 800F 24 ooF J/zeodore Operh 011' (It-Zornays June 1959 T. OPERHALL 2,89

METAL CASTING PROCESS AND AND COMPOSITIONS EMPLOYED Filed Jan. 11, 1956 2 Sheets-Sheet 2 Fla-"C 2 'IQQ'QJIJIAIII I Etc-k5 INVENTOR. J/zeoaore Operlzcrll zQq'nup, 466

diiarrzays United States Patent Theodore OperhalLWhitehall, Mich., assignor to Derald H. Rutteube'rg, Chicago, 111., as trustee Application January 11, 1956, Serial No. 558,578 18 Claims. (Cl. 2Q193) This invention relates to the art of metal founding and it relates more particularly to a process whereby castings can be economically and efliciently produced in volume to accurately controlled dimensions and in complicated shapes and it is a general object of this invention to'provide a casting process of the type described and elements for use in the practice of same.

A method for the fabrication of castings at low cost to accurately controlled dimension and complicated shapes has been the subject of extensive and continuous research; Many processes are known, such as green sand casting, die casting, investment casting and the like, but for various reasons, the more important of which is cost, such other processes have not been found to be acceptable for use in the fabrication of many parts, such as represented by turbine blades and the like.

Precision castings can be inexpensively formed in large shapes, in accordance with the practice of this invention, by a process which is somewhat similar tothat of shell molding but in which the processhas been changed in 2,892,227 Patented June 30, 1959 ice has sufficient mass integrity, it can be removed for cure thereby more rapidly to release the pattern for use in the preparation of additional shells. For cure, the shell is heated to a temperature Within the range of about 4S0600 F. at which temperature cure is completed in about 2-10 minutes depending upon the thickness of the shell that has been formed andthe mass thereof. The time of heat treatment must be carefully controlled to avoid break-down of the resinous binder, otherwise the shell deteriorates in strength and in other properties desirable for its use in the fabrication of castings.

The mold sections are aligned by matching the cope and the drag sections and the sections are joined in their aligned relation, as by means of clamps, tapes, glue, or by other means for holding the shell parts together. The joined shells are'then provided with the desired sprues, runners, risers or inserts and arranged singly or else in clusters to form a mold into which the molten metal is poured. In the present practice of shell molding, the metal is poured while the shell and its supporting structure are at room temperature or slightly above. Upon solidification of the metal, the mold is broken away from the flask or other retainer and from the casting. The shells or any parts thereof which adhere to the casting can be easily removed by the usual cleaning operations, such as brushing, sand blasting and the like,

many of its essential details to provide castings which can be formed to more accurate dimension and shape with improved surface smoothness and detail and improved density and uniformity in cross-section even when the casting is formed with a considerable amount of thin wall sections. These characteristics are believed to'be secured in addition to the decrease in the cost of' the product because of economies that can bepracticed in the process and because of steps which enable the process to be mechanized in whole or in part for volume produc tion on a substantially continuous basis.

In the fabrication ,of metal parts by the present process of shell molding, use is made-of sand in finely divided form having a thermosetting resinous binder in an amount ranging from 3-7 percent by weight uniformly distributed therein. Machined patterns dimensioned to correspond to the part to bemolded, taking into considerationthe shrinkage which take place i'n'the molding process, are heated to a temperature sufficient to reduce the resinous binder in the sand mixture to an adhesive or tacky stage. A temperature within the range of 300-500 F. has been employed but slightly higher temperatures have been used to accelerate the rate of production of the molded shell parts. Y.

The dried sand and resin mixture is sprinkled or otherwise applied to the surfaces of the heated pattern. The heat from the pattern reduces the resinous binder to a flowable and adhesive stage to bond the sand particles into a cured or partially, cured shell having a cavity which fairly accurately conforms to the shape of the pattern. An amount of the sand-resin mixture is applied to the surface of the heated mold part in amount sufficient to produce a shell having the desired thickness. Any excess material not converted to an adhesive or cured stage is removed While the cured or partially cured shell remains on the pattern.

The shell can be cured onthepattern or, the shell but it is usually only necessary to remove the gates, risers and runners to complete the processand provide the metal'castings.

In general, shell molding, even as it is presently being practiced, enjoys a-number of advantages over other volumes and to accurate dlmension and compllcated processes of metal founding, especially from the standpoint of the type and properties of the castings produced and from the standpoint of certain economies which may be embodied in the foundry practice. By comparison with other methods of castin as represented by green sand molding, shell molding permits the fabrication of castings having a better surface'finish; the product canbe held to closer dimensional tolerances; thinner sections can be cast in the molded product; a product can be produced which is relatively free of the burningin or burning-on which is characteristic of green sand molding with the result that cleaner castings are produced thereby to minimize the number of finishing operations and to reduce wear on tools employed in finishing the castings.

In the foundry, numerous efficiencies and economies can be practiced in a shell molding process. For example, shells can be produced in large numbers in advance of molding and separate and apart therefrom for storage until use thereby to enable mechanization in part of the molding process; savings can be made in the amount of labor required and the level of labor required for operation in many of the steps of molding thereby materially to reduce the cost of the mold, and the castings that are formed, and greatly to improve the working conditions in the foundry.

While the advantages to be found in shell molding have led to its wide acceptance in the field of metal castings, there remain a number of areas in which improvements can be made further to enhance the acceptability of the processand the products that are formed thereby and it is an object of this invention to make such improvements in the process of shell molding and it isa related object to produce new and improved elements foruse in same.

More specifically, as it is presently being practiced,

a shell molding requires that the pattern be heated up to As a result it has been necessary to make useofypat a temperature sufiiciently high to reduce the resinousbinder to an adhesive stage and at least partially :3.(1- vancethe cure of the resinous binder to a set stage.

terns made of metal capable of being heated to tempermines in excess of 600 F. and to introduce additional heat sufiicient to maintain the molds at the desired temperature for setting the binder in the formation of the shell of sand on the pattern surface. Thus it is another object of this invention to provide a method for produ'cing shells 'used in casting metal parts wherein it be comes possible to form the shells without the necessity to heat the patterns to an elevated temperature and it is a related object to produce a shell which differs in its composition and characteristics from shells heretofore produced in shell casting to enable further modifications improvements in the shell molding process and castings which are produced thereof. f

Referring further to the fabrication of the shell, the flow characteristics desirable in the formation of ashell having good detail is somewhat in conflict with the fine particle size required for the formation of this shell having the highest degree of surface smoothness.- In addition, the time required to sprinkle material uniformly over the pattern coupled with the time required for heat penetration to set the binder through a'sufiicient thickness of the sand layer to form the shell parts limits the number of shells that can beproduced from a given pattern and it further limits the characteristics thereof. Thus it is a further object of this invention to provide a method for use in shell casting in which detail of the highest degree is made available in the shell independently of particle size so that details and surface smoothness can both be secured in the shell that is formed and it is a relatedobject to provide a method of the type described wherein the number of shells produced per unit of time from a mold pattern can be greatly increased and wherein a shell of higher density and strength can be producedto enable the molded shell to stand up during subsequent handling thereby to release the pattern immediately for additional use. p I The conditions existing in the present practice of the shell molding process imposes further restrictions to the adaptability of the process for usein the manufacture of .precision castings-especially where sections of thin wallsin outlying portions of the castings are to be formed or wherein the thin wall sections are of substantial length. One reasonwfor this limitation is believed to reside in the necessity to make use of the shell at about room tem perature for pouring the metal into the mold, otherwise break-down would occurin the binder holding the shell together with the'result that the shell would become unfit for use if the shell were preheated to an elevated temperature for pouring.- When the metal is'poured into thecold shell, skins tend to form at the surfaces in contact with the shell walls with the result that flow into the innermost recesses and thin wall section sometimes becomes blocked to prevent complete filling of the mold and the formation of a casting which is uniform in crosssection. Thus it is a still further object to providev a method which can be adapted for shell moldingin which complete filling of the mold is secured to produce a dense product of more uniform composition in'crosssection and it is a related object to produce and to pro vide a method for producing elements for use in same.

These and other objects and advantages of this in- 'ventio'ri will hereinafter appear and for purposes of illustration, but not of limitation, an embodiment of the in- 'vention is shown in the accompanying drawing in which- Figure 1 is a flow diagram of the shell molding process embodying features of this invention;

Figure 2 is a schematic sectional elevational view of the arrangement of parts in the compression molding of the shell; i I

Figure 3 is a sectional view through a portion of a molded shell; I

Figure 4 is a sectional view through a portion of the shell parts joined in the aligned relationship for use in molding, and a Figure 5 is a perspective view of a multiple shell prepared in accordance with the practice of this invention.

The concepts of this invention are embodied in a process wherein shells of a controlled thickness are dry pressed of materials having excellent heat shock resistance and dimensional stabilityat elevated temperature which thereby enables the shells to be heated to elevated temperatures for use as molds which can be filled with the molten metal to reproduce castings having new and improved characteristics and wherein the steps can be combined in a more efiicient process capable of volume production of parts thereby to expand the field to include castings heretofore manufactured by other more expensive processes or incapable even of being produced by other casting methods.

Briefly described, in the manufacture of the shell, use is made of a refractory material reduced to a finely divided stage, and compounded with an interim binder which becomes effective immediately upon dry compressi'on or cure to impart strength to the shell moldedprodnet. for maintaining the dimensional characteristics 7 and shape of the shell' dnring subsequent handling and which is, compoundedwith a refractorybinder that becomes efiective when heated to an elevated temperature to impart strength and mass integrity to the mol ed shell without loss of dimensional stability to enablethe molten metal to be poured int'o the shell, preferably while the latter is heated to elevated temperature. An amount of the compound is loaded in a mold for compression onto a pattern or die to cause the material to be com} pacted and flow sufficiently to take on every detail ofthe diepart and form a shell which is held together by the interim binder which ,is 'immediately active, with or without heating, to provide cold strength to the product that is formed. The shell is removed from the dierpart and then subjected to a heat treatment at a temperature snfiicient toractivate the refractory binder and eliminate substantially all of the volatilizable materials in the shell but without heating to a condition which would cause glass formation or vitrification of the binder in amounts sufficient to destroy the permeability of the shell, its shape or dimensional characteristics. 7

The shell or compacts having good heat shock resistance and strength is matched with the other section or sections of similarly formed shell parts either before the described heat treatment or afterwards to provide a composite mold which can be maintained at anelevated temperature for pouring the metal therein to form the casting thereby to increase the fiowability of the metal over a greater period of time sufficiently completely to fill the fold and form a dense product which conforms more exactly with the mold and which acquires a relatively high degree of finish on the surfaces thereof substantially to produce a finished product. When the shell parts are heat treated in the assembled relation to form the mold, the heat treatment serves in addition to join the parts one to the other. If combined afterward, a binder or other means will be required to hold the aligned shell compacts in mold' formation. When employed at elevated temperature, the shells joined by heat treatment can be used directly at or about the temperature of heat treatment for pouring the metal in casting, otherwise it is necessary to reheat the assembled compacts for casting.

The following example is given by way of illustration of the details of the process employed in the practice of this invention.

Example 1 Composition for use in'v shell manufacture:

parts by weight calcined alumina 3 parts by weight'feldspar 2 parts by weigh t'whiting 5 parts by weight paraffin v 95 parts by weight trichloroethylenea rgcedurarlhewax component is heated to a molten s',tate for solution in the trichloroethylene by addition slowly with stirring tothe molten wax. The solution of .wax in trichloroethyle ne" is added to a mixture of the powders to form a slurry which is introduced into a ball mill for reducing the particles to about 325 mesh or less. QUsually about 24 hours or more of milling is required. i

The solvent phase is removed from the milled product, as by evaporation in open pans, with or without the addition of heat, and preferably-with agitationto prevent segregationbetween the solvent phase and the solid particles, otherwise the particles will not be uniformly coated with the wax binder. The dry product is pulverized preferably before all of the solvent'has been removed since the pulverization of the mass is effective with less work when a small amount (about 3-10 percent by weight) ofsolvent remains. The dry product need not .be reduced back to 325 mesh. In fact, it is desirable to compress theproduct into pellets which can be fed more easily by mechanical means for loading onto the dies for molding. Ultimately the product goes to substantial dryness containing not more,than l2 percent by weight of moisture per solid. J

Shell formation-The, shell is formed by compressing a measured quantity 'ofthe dry powder 12 onto the surface ofdie parts 14 housed within a cavity 16 having a ram 18 aligned therewith for shifting movement between normal or retracted position and molding position' in a cycle of operation; The die part is preferably formed of metal having a highly finished surface to permit easier release of the shell molded thereon and to provide a better finish on the shell surface, which finish is reflected in the smoothness of the surface of the casting that is formed therewith.

An amount of powder is loaded into the mold cavity 'to give a compacting ratio. of about l":3 unde r pressures which may range from LOGO-20,000 pounds per square inch as the ram is activated from normal'to molding position and back to form the shell having a wall thickness in the range of /2 inch. ,1 I

,The film of wax coating each of the particles of alur'nina operates upon compression to bond the particles together and form a good. compact having sufficient strength to resist break-down or. deformation of the moldedshell under normal handling. The shell 10 is removed from the mold cavity to release the die for use inthe preparation of additional shells.

The shell 10, alone or in combination with the mating shell 20 held in aligned relation therewith, is then subjected to a heat treatment at a temperature within the range of about 750-2000 F. for a time sufiicient to reach bisque conditions wherein the refractory binder component is activated for bonding without glass formation or'vitrification in amounts to cause shrinkage or material loss. in the permeability of the shell product. 'Under these conditions, the organic binder which functions in the interim to maintain the shell to the desired mold shape and any other volatilizable material is distilled or burned out of the shell. Such distillation or burning is carried out under. oxidizing conditions to achieve sub 'stantially complete removal fromthe shell. Ordinarily such conditions .can" be provided by the natural drafts of air'from the outside atmosphere throughopenings in the oven, kiln, or other furnace through which the shells are advanced for firing in a batch or in a continuous process. When fired at a temperature of 1900. F., from 2-10 hours are required properly to heat treat'the shell, depending upon the wall thickness of the shell and the iriass of material which is being heat treated.

f The pair of shells 10 and 20, joined by or after heat treatment, are arranged in a suitable mold, alone or with other shells in a cluster 22., with suitable gates, sprues, risers; inserts and runners, for pouring the metal into the shell for solidification toform the desired casting. The steps of pouring, cooling and removing the .castings and cleaning are substantially similar to those which are employed conventional shell molding with the exception that in the preferred practice of this invention, the shells are maintained at a desired elevated temperature or else heated to an elevated temperature, as previously described, for pouring the metal into the shellsin casting .with the result that the metal is .able to flow freely into and through the .thin sections and entrant portions substantially completely to fill the shells in forming a solid and. dense casting having the surface smoothness of the shell walls and the details of the die parts thereby to produce a product of high quality which requires little by way of an additional finishing operation to separate the sand from the surface thereof when broken away from the mold.

The principal component of the shell molding composition comprises a refractory which can berepresented by the calcined alumina of Example 1 or other materials such as silica, zirconium silicates, beryl ores, thoria, zirconite, kyanite, mullite, and silmonite, and other highly reactive oxides and silicates and oresof refractory metals, taken alone. orin combination with each other. More broadly defined, it has been found that, as the refractory component, use can be made of refractory materials which can be identified by their angle of repose on a glass plate as included within the range of 10-30 degrees. The refractory component usually present in amounts ranging from 96 percent by weight ofthe shell molding compound should be selected to have a fusionpoint which is higher than the temperature of the metal being poured to prevent break-down of the molded shell. By way of illustration, it is intended to make use of refractory materials which, in the combina tions employed, have a fusion point above pyrometric cone equivalent 32. In the event that the refractory materials contain combined water or other volatilizable components, it is important to calcine or otherwise treat the materials to drive off the volatilizable material before molding the shell, otheiwise the volatilized materials will be driven off during firing of theshell with resultant shrinkage and possible break-down of the shellior else the volatilizable material will be released when the molten metal at still. higher temperature is poured into the shell with a further break-down or volatilization which may result in the formation of a productof unacceptable quality. i

In the foregoing example, use is made of refractory materials reduced in particle size to less than 325 mesh. The size of the particles is not critical to the process but it is preferred to make use of particles of small dimension for the purpose of securing a molded shell which has fiow sufficient to take on the exact details ofthe die part and for securing maximum smoothness on the surface of the molded shell. Particles of larger dimension can be used but it is undesirable to make use of particles having more than 50 percent larger than mesh and in which all does not pass through '40 mesh.

As the'interirn binder, use can be made of other natural and synthetic waxes, such as carnauba wax, bees wax, petroleum wax, wax polyhydric alcohols, organosilicon polymers and the like; petroleum distillates and coaltar residues; asphalts, rosins and tars; natural resins and gums, such as gum elemi, gum tra-gacanth,- acacia gum, karaya, coumarone and indene resins and the like; synthetic resins of the thermosetting or thermoplastic type, suc h as phenol. formaldehyde, ureaformaldehyde, resorcinol formaldehyde, polyester resin and the like th ermosetting resins or polyvinyl'alcohol,polyvinyl acetate, polyvinyl butyrate, polyethyl acrylate, polybutyl methacrylate, polystyrene of low molecular weight, cellulose propionate-butyrate, butyl cellulose, polyethylene, polybutylene and the like thermoplastic resins; carbohydrates, proteins such as casein, zein, alginates, albumens, gelatins, glues, starches, or use can be made of inorganic binder systems such as'borax, sodium silicate, sodi- 7 umftetraborate, feldspar, "aluminum phosphates and the like. It is preferred to make use of one or more of the organic binders, preferably the paraflin o r petroleum waxes, since the binder component represents one ofthe major costs of the materials used in the molded shell. W hen use is made of a thermosettingresin, the resinous component in the shell may be, advanced to a set sta'ge upon molding as by means of heated molds, or by heat otherwise introduced or catalysts with the result that set conditions can be achieved without the use of excessive pressure and even by contact molding. V

It is desirable to make use of an amount of interim binder which is capable of efiectin'g the desired bonding relationship for maintaining the mass integrity and dimension of the molded shells separate from the die parts. It is undesirable, however, to make use of an amount of binder which will lead to excessive shrinkage or deterioration of the molded shell when the binder is burned or distilled out of the shell during the subsequent firing step. Best results are secured by the use of a binder in amounts ranging from 3-7 percent by weight of the molding compound. As little as 2 percent binder ean'be used in some systems and as much as 10 percent binder can be used with others but it is undesirable to make use of a molding compound containing more than 12 percent by weight of binder unless the binder -is removed during the subsequent firing or volatilized at the temperature of the metal being poured. V I

To achieve uniform distribution of the binder in the shell molding compound, it is best to incorporate the binder in solution in a diluent whereby the particles of refractory materials can be wet throughout with the solution for coating each of the particles with a thin layer of the binder. To prevent non-uniformity in distribution, it is desirable to maintain the particles in uniform mixture with the solution, especially during the steps for eliminatingthe diluent, as by maintaining the materials in a constant stage of agitation. Other than from the standpoint of cost, it is preferred to make use of a solvent system which can be eliminated by vaporization from open pans to about room temperature to achieve substantially complete drying. The high cost solvents can be recovered by various conventional means. From the standpoint of safety and cost, however, it is preferred to make use of aqueous systems containing the water soluble binders in solution. Where the binder can be reduced to sufiiciently fine particle size for distribution, use can be made of aqueous emulsions or dispersions but it is desirable to avoid application of the binder as large particles since the molded shell will have insulficient binder in certain areas while excessive binder will be present in others to raise ditficulties in firing and use.

As the refractory binder, use can be made of such materials as glass frit, feldspanborax, borates, sodium tetr'aborate and the like materials having a pyrometric cone equivalent below 2500 F. for activation of the binder phase to an extent which would not otherwise cause deformation or shrinkage of the molded shell during heat treatment. Further tominimize the dangers of shrinkage and distortion, it is preferred to make use of a minimum concentration ;of binder sufiicient to 'give the desired heat shock characteristics without dimensional change. Best results are secured in the use of the refractory binder in an amount ranging from 3-7 percent by weight of the molding compound. As little as 2 percent and as much as 1.0 percent by weight of the refractory binder may be used. More than 10 percent by weight of the refractory binder-can be employed where the refractory materials present in combination therewith are of the more refractory type whereby the combination can give the desired thermal characteristics. 'For example, '10 percent by weight feldspar and up to '12 percentby Weight feldspar may be used when thoria com prises the refractory component. I,

"The components can 'be combined 'by conventional .stit'uents thereof.

mean. In the event that the refractory materials are of 'the de siredparticle size, further 1"edi '1ctio n will be' 'n nnecessary, Otherwise size reduction can be.. achieved by variousmeans, such. as by the use of a ball mill, roller mill,grinder'gr the like.

The llb i ga q he e mpl f. art sian which may 'be employed in the preparation of molding P t by. weight silica 5 parts byweight glass frit 100 parts by weight water In the above formulation, the polyvinyl "alcohol is .dissolved in the water and the other materials are then added thereto and the particles are reduced to less than "325 meshby ball milling or by milling between rolls;

Example 3 40 parts by weight silica 50 parts by weight calcined alumina 10 parts by weight feldspar 4 parts by weight rosin parts by weight ethyl alcohol V The rosin is dissolved the. ethyl alcohol and the other materials are incorporated therewith and ground to less than .325 mesh.

Example 4 :90 parts by weight periclase ores (less than mesh) 10 parts by weight feldspar (less than 100 mesh) 5parts by weight glass frit '5 parts by weight phenol formaldehyde resin in water soluble A stage 5 parts by weight sodium tetraborate (less than 100 mesh) 90 parts by weight water The phenol formaldehyde resin is dissolved in water and the other materials aremixed into the solution and then dried to form the desired .pellets for molding without any previous grinding. I In the above formulations, many of the refractory ma.- terials normally containing combined water are calcined .prior to incorporation intothe composition to remove the combined water and other of the volatilizable con- Other materials such as pigments and fillers may be incorporated to color the compound but the amount of such fillers an'd pigments should not exceed 5 percent by weight of. the compound. I

When more, than 2 percent by weight and preferably when more than 3 percent by weight of the wax or other organic interim binder is present in the dried moldingcornponnd, sufiicientfiow is often available to enable the shellto be molded by practically any of the conventionalmoldingprocesses, such as by injection mold ing, transfer molding, extrusion or the like, into a cavity containing the pattern as the .die and it will be understood that the term molding as used herein is meant to include such other process for molding. Use can be made of compression molding processes wherein the die part forms one mold surface and the dried molding com pound is compressed thereon by means of a ram or other -body to cause the material to flow into intimate contacting relation with the surfaces of the die and form the shell compact. When use is made of cornpression molding, it is preferred to employ a resilient member such as a rubber diaphragm actuated by by dranlic means or a rubber block 24 located on the end of the rain for compression so that the molding surface can be deformed to correspond to the contour of the die to' 'itfipreve the finish of the shell that is formed and escaped thereby to produce a shell which corresponds more exactly with the pattern and is of substantially uniform thickness and density throughout. As a result, a more desirable molded shell can be secured with less molding compound.

In the process described, it is unnecessary to make use of heated patterns, unless use is made of a thermo-setti'ng binder to be advanced by heat to a set stage, since the interim binder becomes effective generally upon intimate contact with the particles upon compression to form the compact. Thus use can be made of patterns or die parts formed of materials other than metal without limitation as to the melting point of the material, such as of plastics, ceramics, wood and the like. When formed of metal, use can be made of low melting point alloys or metals which permit the die parts or patterns to be manufactured inexpensively on a mass production basis, such as by injection molding of aluminum, contact molding or the like, or from a pattern or from the product itself for use of the die part in the fabrication of large numbers of expendable shells or compacts.

By way of modification, the surfaces of the die may be treated in advance of shell molding with a parting compound, such as an organo-silicon fluid or powder to enhance the release of the molded shell. When used, it is unnecessary to lubricate or treat the die surface between each molding operation because the parting compound is effective over several molding cycles. Heated mold dies may be used if desired as when a binder is employed that is more effective at elevated temperatures, but the process forming the subject matter of this invention is not reliant upon the conversion of the binder to a set condition as in the technique of shell molding in the manner heretofore employed since the binder is subject to elimination during the subsequent firing operation. Mere compression sufficient to cause bonding the compact together for removal and firing is all that is required whereby production of shells at a high rate from a single mold becomes posible.

The wall thickness of the molded shell is not critical. It will be sufficient if the shell walls embody enough strength to resist break-down under the conditions existing under firingand use. A wall thickness of about inch appears to be the minimum and it is preferred to form the shell with a wall thickness of about /2 inch. Larger thickness can be used but little, if any, advantage is derived therefrom unless the shell is designed to contain an exceptionally large volume of cast metal and unless the shell is to be used without backing or support.

By comparison with shells formed by the present processes employed for shell molding, the compacts prepared in accordance with the practice of this invention are relatively impermeable. Thus it is desirable to effect substantially complete removal of the volatile materials during the subsequent firing operation and it is also desirable to avoid excessive glass formation vor vitrification of the refractory materials since such vitrification would operate not only further to reduce the permeability of the shell but would also cause rearrangement of the materials to the effect that uncontrolled shrinkage or deformation would occur. The molded shell may be defined as a semi-permeable structure having sufiicient permeability for the amount of vapors and gases generated to escape therefrom. In the event that greater permeability is desired,'the molding compound can be formulated to contain an amount, such as 10 percent by volume, of

a is

sufiicient to volatilize off the interim .orgaiiicbinder. and I to sinter or reduce the refractory binder toa bisquQQQndition and as its maximum 2. temperature less thari that wherein the ceramic binder is reduced to a liquid phase for vitrification.

In general, the temperatures will lie within thelrange of 7502400 F. and preferably within the range ,of 15002000 F. If the molded shell is heated to a temperature above the distillation temperature or above the thermal break-down temperature (about 500 F. for most organic materials), the organic components of the interim binder begin to come off and burn until complete removal is efiected, generally under oxidizing conditions. @As long as the binder comprises less than 10 percent by weight of the molded shell, the binder can be removed without noticeable dimensional change of the product. If the temperature rises to the bisque range forthe refractory binder, the interim binder and any other volatilizable materials are eliminated to produce a coinposite shell formed essentially of ceramic materials having good heat shock resistance and good dimensional stability at elevated temperatures so that the shells can be used hot in the subsequent metal pouring operation.

The shells can be fired on setter tiles or they can be stacked or else retained in flasks with loose aggregate or sand around them for support. Reaction at a temperature Within the bisque range for sintering can be carried out over an endless period of time but it is undesirable to heat the shells for an excessive amount of time over and above that necessary for the development of the. desired bisque condition (about 1-l0 hours depending upon the temperature of the materials).

Some vitrification can be tolerated when insufficient to cause dimensional change or insuificient to render the molded shell impermeable to the passage of gases. 'I As previously pointed out, the molded shell can be semipermeable by comparison with the shells heretofore produced for shell molding since the shell is sufiiciently permeable to take care of the small amount of volatiles that can be formed when the metal is poured. Furthermore, gases and vapors can escape through the parting lines of the joined shells and, for such purposes, it is best to form the shells with the parting lines passing through the re mote re-entrant portions. I

The shell parts can be fired separately, as described, for subsequent joining in the aligned relationship by means of suitable binders, clamps, or loose aggregate. In the preferred practice of this invention, the shell parts are aligned in advance of firing to efiect a bonding relationship sufiicient to hold the parts together as a.result of the firing operation. When fired in the assembled relation the molded shell parts can be joined by a suitable adhesive or a refractory or organic cement or they can be held together during the firing operation by means of clamps, bolts or the like or by supporting the parts in the aligned relationship by embedding or backing the parts with a suitable aggregate such as loose sand. When fired separately, the parts can be joined in the aligned relation with a suitable refractory cement applied in small amounts to the joining edges or they may be held in the aligned relation by support in the flasks with loose aggregate. I When the molded shells are fired in the'aligned relation for pouring, the heated molds at about the tempera ture for pouring can be withdrawn from the prefiring furnace and immediately transferred to the pouring floor for pouring of the molten metal into the cavity. When the shells are separately fired or when the shells have been allowed to cool down to lower temperature, they can'be heated up in advance of pouring whereby the molten metal can be poured into the cavities while the shells are at an elevated temperature. It will be understood that while the shells are maintained at an elevated temperature for pouring in the preferred practice, the metal may be poured into molds formed of the shells at lower temperature down to about room temperature.

closely approaching that of the molten metal ,or below the temperature offusion or vitrification of the refractory binder, or,1i'n the alternative, use can be made'of the molded shells at lower temperatures down to 'roorn tem- 'perature when'the metal 'is poured into .the c'avity. 'In "general, when the molds are'heate'd to elevated 'tempera- 'ture, the metal can be poured at a temperature which is about 100 200 'F.'below the'temperature for pouring in otherlca'sting processes, or about 3000 3400 F..for most of thesteels'and special alloyslusedinprecision casting. Since the'me'thod of melting and, pouring of therrietal does not differ materially from. techniques heretofore employed'in shell molding, detailed description thereof will not be made. Since the recovery of the castings'from the flasks or molds corresponds to conventional practice,' de- "scription thereof will .beomittedherein'except for there- 'jminderthat'the process becomes'much easier by reason jof 'the'iniprovementin the characteristics of the castings that areforme'd and their relationship with the shells.

'One of the important concepts of this invention resides in the preparation of the shells for use at an elevated temperature'in metal pouring with the result that flow into the remotest recesses and into re-entrant portions ispossible and flow through portions of small cross-section is available substantially completely to fill the mold. The

use of mold shells at elevated temperature in metalpouring also appears materially to eliminate vapor phases which often form and cling to the fine particles forming the surfaces of the mold cavity and it prevents-the formation of vapors which tend to cling to the surfaces of the poured metal and cause imperfections in the castingsthat "are formed. From these and for various other conditions traceable to the use of molded shells heated to an elevated temperature and from which volatilizable materials have been removed in a previous heat treatment, it has become possible to produce precision castings in large volume and at low cost by a casting technique.

The metal can be poured in a single mold or -in 'a cluster of molds assembled with a common sprue and runners that can'be easily' removed from the castp'rodu ct. The shells in clusters can be stacked vertically about a center runner for lateral flow-of the molten metal into the molds or they can be arranged in clusters for vertical flow ofthe metal into the molds. The smoothness of the surfaces of the castings permits separation from the shells and Other supporting material without difiiculty and Without the use of special equipment. The product that is formed requires little, if any, finishing on the surface or machining operations and the castings embody such detail that use can be made of the product in almost ,as cast condition. I

Itwill be apparent from the foregoing that I-have made .material changes in the present shell molding process whereby the process now becomes capable of production in large volume, at low cost,.and in good yield of castings having good dimensional tolerance, good surfacefinish,

improved density, and uniformity of composition in cross-section. I 1

Some of the principal advantages of this invention are to be found in the fact that once suitable dies have been formed to incorporate the configuration desired for the product, shells can be produced at a rapid-rate from the die parts. No expenda'ble patterns are required and the type of equipment required for dry pressing ordry molding of the shells is well known and readily available in the plastics or ceramics art. 7

Further, the eliminationof vaporizable materials from the molded shells having good heatshock resistance enables the molten metal tobe poured into the shells-while heated ,to an elevated temperature whereby thegeneration of gases and vapors is substantially completely eliminated 'by comparison with the shell molding process heretofore employed wherein the molten metal is. poured into shells which still contain their resinous binder and other volatilizable materials whichare released in part during the casting process to interfere with the quality Ofhhfi casting produced andv to. require the use of highly permeable shells jthrough which the gases and vapors can escape. Such highly permeable shells are incapable of providing the 's'urface'finish and the detail available from molds employed in accordance with the teachings of this invention and the volatiles released at different temperatures from the molded shells cause imperfections to exist in the molded product which not only decreases the yield of acceptable castings but also increases the cost of the finishin'g operations of thecastings which are acceptable.

It will be understood that numerous changes may be made in the details of the formulations 'and operations [a dry composition in molding relation in a die pattern having a mold surface corresponding to a part of the product to be cast and wherein the molding composition isformed of the combination ofa refractory material in finely divided form, a pressure-sensitive binder which operates as an interim binder to'impart shape and integrity to the dry molded product, and a refractory binder which becomes efiective'when heated to a temperature within the range of 800-2400 F., removing the dry molded 'product'from the die, and then heating the molded shell part to a temperature Within the range of 800-2400 F. to drive off the interim binder and other volatilizable material from the molded product and to activate the refractory binder to produce a bonded shell part formed of "inorganic materials having good heat shock resistance.

2. The method of metal casting as claimed in claim 1 in which the refractory materials have been calcined in advance of molding to drive off volatilizable material and combined water.

3. The method of metal casting as claimed in claim -1 in which the refractory material is present in an amount ranging from -96 percent by weight of the molding composition.

4. The method of metal casting as claimed in claim 1 in which the interim .binder is present in an amount within the range of2l2 percent by Weight of the dry molding composition.

5. The method of metal casting as claimed in claim 1 in which the interim binder is an organic binder which is burned out-of the shell during the heat treating step.

6. The method of metal casting as claimed in claim 5 in which the heating treating step is carried out under oxidizing conditions cleanly to lburn the organic material from the molded shell.

7. The method of metal casting as claimed in claim 5 in which the organic binder is a wax present in an amount ranging from 2-12 percent by weight.

8. The method of metal casting as claimed in claim 1 in which the dry molding step is carried out to place the molding material under a pressure of LOW-20,000 pounds per square inch.

9. The method of metal casting as claimed in claim 1 in which the time and temperature of heat treatment of the dry molded shell is sufficient to drive oti the volatilizable material in the dry molded shell and to convert the refractory binder to a bisque condition.

10. The method of metalcasting as claimed in claim 1 in which the materials are present in the molding composition in the amounts ranging from 3-7 percent by weight of the interim binder, 3-7 percent by weight of the refractory binder, with the remainder formed of a calcined refractory material in finely divided form.

ii. In the art of metal casting wherein use is made of Shells into which the molten metal is poured, the steps which comprise forming a shell solely by the application of pressure by compressing a dry composition in molding relation in a die pattern having a mold surface corresponding to a part of the mold product to be cast and wherein the mold composition is formed of a combination of a refractory material in finely divided form having a fusion temperature above the temperature of the molten metal, a pressure-sensitive binder which operates as an interim binder to impart strength and integrity to the dry molded product, and a refractory binder which becomes effective as a binder to impart heat shock resistance and strength to the dry molded product, removing the dry molded product from the die, heating the molded shell to a temperature within the range of 800900 F. for a time sufficient to drive off the interim binder and to activate the refractory binder and also to drive off volatilizable materials present in the shell parts, joining corresponding shell parts in alignment to form the shell mold, and then pouring the molten metal into the formed shall molds while the shells are at an elevated temperature.

12. The method as claimed in claim 11 in which the shell parts are joined in their assembled relation subsequent to heat treatment and which includes the additional step of heating the joined shells to an elevated temperature above 800 F. prior to pouring the metal therein for casting.

13. In the method of metal casting wherein use is made of a shell into which the molten metal is poured, the improvement which comprises the preparation of a dry shell molding composition in which the molding composition is formed of a pressure-sensitive binder effective upon compression to impart strength and integrity to the molded product, a refractory binder which becomes effective to impart strength and integrity to the molded product when the molded product is heated to a temperature within the range of 800-l900 F. but in which the refractory binder is selected of a material which provides a minimum amount of liquefaction at the temperature of activation, and a refractory material in finely divided form having a fusion temperature above the temperature of the molten metal and in which the interim binder consists essentially of wax present in an amount ranging from 3-7 percent by weight and in which the refractory binder is present in an amount ranging from 3-7 percent by weight, while the remainder is formed of the refractory materials, the steps of dissolving the interim binder in a volatilizable liquid, mixing the solution of the interim binder with the particles of the refractory binder and the refractory material, driving off the diluent to dry the product, and mixing the composition to distribute the liquid prior to removing of the solvent uniformly to coat the refractory particles.

14. The method as claimed in claim 13 in which the refractory material is calcined prior to admixture to drive ofi combined water and other volatilizable substances.

15. The method as claimed in claim 13 which includes the additional step of grinding the mixture prior to driving off the liquid to reduce the particles to less than 20 mesh.

16. The method as claimed in claim 13 in which the refractory material is present in an amount ranging from 14 8096 percent by weight and in which the interim binder is present in an amount ranging from 210 percent by weight, and the refractory binder is present in an amount ranging from 2l2 percent by weight.

17. In the art of metal casting wherein use is made of a shell into which the molten metal is poured, the steps which comprise forming a shell of the combination of materials including a refractory material from which all volatilizable substances have been removed and which has a fusion point above the temperature of the metal to be poured, a refractory binder which is activated to impart mass integrity and strength to the molded shell at a temperature Within the range of 8002400 F. and an organic interim binder which is effective upon compression to impart mass integrity and strength to the molded shell and wherein the materials are present in the ratio of 2l2 parts by weight of the interim binder, 2l2 parts by weight of the refractory binder, with the remainder being formed of the refractory material, including the steps of uniformly distributing the binder with the ceramic material, drying the composition, pressure molding the dry composition in a die pattern having a mold surface corresponding to a part of the product to be cast, removing the dry molded product from the die, heating the molded shell to a temperature within the range of 800-2400 F. to drive off the volatilizable materials and interim binder and to activate the refractory binder for producing a shell having good heat shock resistance, assembling the shells in an aligned relation to form a shell mold, and then pouring the metal into the shell mold while the latter is heated to an elevated temperature.

18. A composition for use in the dry molding of shells in shell casting in which the molding composition consists essentially of a pressure-sensitive organic binder consisting essentially of wax present in an amount ranging from 3-7 percent by weight of the composition, a refractory binder which is converted to a bisque condition at a temperature within the range of 800-2400 F. with a minimum amount of fusion and present in an amount ranging from 3-7 percent by weight of the composition, and a refractory material in finely divided form having a fusion temperature in excess of the temperature of the molten metal to be poured.

Claims

1. IN THE ART OF METAL CASTING WHEREIN USE IS MADE OF A SHELL INTO WHICH THE MOLTEN METAL IS POURED, THE IMPROVEMENT WHICH COMPRISES THE STEPS OF FORMING THE SHELL SOLELY BY THE APPLICATION OF PRESSURE BY COMPRESSING A DRY COMPOSITION IN M OLDING RELATION IN A DIE PATTERN HAVING A MOLD SURFACE CORRESPONDING TO A PART OF THE PRODUCT TO BE CAST AND WHEREIN THE MOLDING COMPOSITION IS FORMED OF THE COMBINATION OF A REFRACTORY MATERIAL IN FINELY DIVIDED FORM, A PRESSURE-SENSITIVE BINDER WHICH OPERATES AS AN INTERIM BINDER TO IMPART SHAPE AND INTEGRITY TGO THE DRY MOLDED PRODUCT, AND A REFRACTORY BINDER WHICH BECOMES EFFECTIVE WHEN HEATED TO A TEMPERATURE WITHIN THE RANGE OF 800-2400* F., REMOVING THE DRY MOLDED PRODUCT FROM THE DIE, AND THEN HEATING THE MOLDED SHELL PART TO A TEMPERATURE WITHIN THE RANGE OF 800-2400* F. TO DRIVE OFF THE INTERIM BINDER AND OTHER VOLATILIZABLE MATERIAL FROM THE MOLDED PRODUCT AND TO ACTIVATE THE REFRACTORY BINDER TO PRODUCE A BONDED SHELL PART FORMED OF INORGANIC MATERIALS HAVING GOOD HEAT SHOCK RESISTANCE