US2797457A - Method of joining shell molds - Google Patents

Description

July 2, 1957 l. R. KRAMER METHOD OF JOINING SHELL MOLDS Filed June 29, 1954 I/ew/v 1?. Aka/wee BY w MET HDD F JOINING SHELL MOLDS Irvin R. Kramer, Scarsdale, N. Y., assignor to Mercast Corporation, New York, N. Y., a corporation of Betaware Application June 29, 1954 Serial No. 440,128

7 Claims. (Cl. 22- -193) This invention relates to shell molds for casting metal parts and more particularly to composite shell molds of the type which have found wide commercial use in recent years. Heretofore, such composite or sectional shell molds have been produced by the methods of the type described in the publication Fiat Final Report No. 1168, dated May 30, 1947, by W. W. McCulloch which was distributed by the Department of Commerce and in the recent articles Shell molding published in Modern Pattern Making of June and July 1954. In all such prior art sectional shell molds, the individual mold sections are formed of refractory particles, which constitute about 92% to 94% of its content and are usually bound by thermo-setting organic resin which is set by heating to about 300 C. However, such prior art sectional shell molds have serious limitations. When casting hightemperature molten metal, the resin binder content at the interface between the hot cast metal and the mold cavity is decomposed into gaseous products which contaminate the casting and often result in defective castings. it is also impossible to preheat such shell molds to high temperatures above 350 C., as is often required to assure that the cast metal shall remain molten until it reaches all narrow mold cavity spaces used for casting special critically dimensioned parts. In addition, it is impossible to make large size castings with such prior art shell molds, because the heat capacity of a large mass of molten metal cast into the mold cavity, causes disintegration'of the resin content of the mold walls bound thereby before the cast molten metal fills the mold cavity. Furthermore, it is impossible to make castings with close dimensional tolerances, such as plus or minus .005 inch per inch, with known shell mold sections which have to be joined to each other by clamps, staples, and/ or adhesives.

Throughout the specification and claims, all propor tions are given in weight, unless otherwise specifically stated in each instance. 1

The present invention is based on a discovery which makes it possible to overcome the foregoing limitations encountered with prior art sectional shell molds, and to provide much superior shell mold sections which may be joined into an integral thin-Wall porous composite mold structure for use in casting critical metal parts with tolerances as high as .005 inch per inch. More particularly, the invention is based on the discovery that by forming the individual shell mold sections with an investment matter which combines a predominant amount of refractory particles with a critical amount of an organic thermo-setting resin binder and also a critical amount of an inorganic high temperature binder which is rendered effective-after first forming the individual selfsupporting mold sections by heat treatments which set the resin binder-by assembling the mold sections into the composite complete mold and heating the assembly to between 500 to 1300 C. and thereby causing the high temperature binder to bind the refractory particles of the individual mold sections and also along the mating atent Q Patented July 2, 1957 surfaces of the individual mold sections, there is obtained an integral composite porous shell mold which isin many respects superior to prior art shell molds and overcomes their serious practical limitations.

The foregoing and other objects of the invention will be best understood from the following description of an exemplification thereof, reference being had to the accompanying drawing wherein Fig. 1 is a cross-sectional view of a bell-like end wall of an electric motor representing an example of a casting made by a composite shell mold of the invention;

Fig. 2 is a cross-sectional View of the composite shell mold used in making the casting of Fig. 1.

Fig. 3 is the upper section of the composite shell mold of Fig. 2 as it has been formed on a metallic pattern section;

v Fig. 4 is the bottom section of the composite shell mold of Fig. 2 as it is formed on a metallic pattern section in accordance with the principles exemplifying the invention.

Prior art composite or sectional shell molds, as known heretofore, are produced with various minor modifications and adaptations by the following process. A pattern of the object to be cast is made in the form of two or more metallic pattern sections for forming on the several pattern sections corresponding mating shell mold sections shaped so that when they are assembled in abutting relation along their mating junction surfaces they will define the mold cavity of the object to be cast. In most cases, the individual prior art shell mold sections are formed. by mixing high grade silica sand with a thermosetting resin binder usually in proportions of about 92% to 94% silica sand and 6 to 8% of the resin binder. The silica sand is usually of mesh to 200 mesh particle size, and care is taken that the silica sand shall be free of clay, metal oxides, moisture and organic matter. The resin, which consists in most cases ofa phenol formaldehyde resin is in fine particle form, such as -200 mesh to -,325 mesh particle size, and is thoroughly mixed with the sand.

The sectional metal patterns are heated to about 250 to 400 C. and the sand-resin powder mixture is then dumped or sifted on the exposed surface of each individual heated sectional pattern. As the sand resin powder mixture comes in contact with the heated pattern, the heated binder content becomes set and binds the sand particles into a shell layer conforming to the shape of the pattern. The excess of the dumped sand-resin powder, which remains loose, is removed by inverting the pattern section with the shell layer formed thereon in such manner as to allow the excess non-adhering investment mixture to drop off. Before dumping the investment pattern mixture on the heated powder section, the exposed surface is first coated with a thin film of a parting agent, such as silicone resin diluted in a suitable solvent which on evaporation of the solvent leaves a minute stratum of silicone resin coating on the pattern surface. The sectional metal patterns with the shell mold sections so formed thereon are then placed in an oven and further heated at'about 300 C. for 2 to 3 minutes to cause all of the thermosetting resin binder content of the shell mold sections to become fully set and bind the sand particles into strong hard self supporting shell mold sections.

Thereafter, the individual shell mold sections are removed from their pattern sections, and the thermally set hard complementary pattern sections are assembled with their mating boundary edge surface in abutting relation to provide a composite mold having a mold cavity of the desired object to be cast. The abutting mating edge regions of the mold sections, which are usually foamed with flange extensions, are joined to each other with adhesive staples, pinch clamps or similar devices. In the case of small light castings, the assembled mold sections are usually held together in a frame with clamps only during the casting process. In case of large castings the assembled shell mold sections are usually backed up with metal shot or gravel held in position Within the flask When casting the metal. Such prior art sectional shell molds'have serious limitations. Thehigh. temperature of the molten metal cast into the mold cavity causes the resin of the shell mold at the interface with the cast metal to decompose and form gaseous products which cause contamination and defects in the casting. Further, in many castings it is necessary to preheat the mold to high temperatures in order to fill out thin cavity spaces of the mold by the metal poured into it. Since such known shell molds cannot be heated to a high temperature above about 400 C. it is impossible to use them for making casting with thin wall sections or in general, castings which require heating of the mold to a high temperature in excess of 400 in order to assure that the molten metal entering the mold cavity will reach all narrow or more or less obstructed parts of the mold cavity.

Another limitation of the known sectional shell molds of the type described above, is the fact that they impose a limitation on the size of the casting. When a large mass of high-temperature molten metal is cast into the relatively large cavity of such prior'art shell molds, the resin binder content of such mold is heated by the large mass of the molten metal to a sufiiciently high temperature to cause decomposition of at least some of the resin content before the molten metal actually fills all sections of the mold cavity. As a result, large castings made with such prior-art shell molds are frequently defective.

Another limitation is the inability to hold close dimensional tolerances suchas plus or minus .005 inch per inch when the prior art mold sections are joined together by clamps, staples and/or adhesives as heretofore practiced.

The present invention is based on the discovery that the lirnitationsheretofore encountered with the available sectional shell molds are overcome and much superior composite porous thin shell molds are obtained if the individual mold sections are formed with an investment material composition consisting predominantly of fine refractory particles which are combined with only a small criticalamount of a thermosetting resinous binder and also with a critical small amount of a high temperature inorganic binder which becomes effective on heating the shell mold material to a temperature in the range between about 500 and 13.00 C., the heating of the shell mold sections to such high temperature being carriedout while the complementary shell mold sections arev joined along their mating surfacesinto the composite shell mold with the desired mold cavity.

The invention isgalso based on the discovery, that when the assembled mating complementary shell mold sections made with an investment composition material of the invention is heated to render the inorganic high-temperature binder effective in binding the refractory particles of theindividual mold sections, the high-temperature binder content will also become effective in joining the abutting shell mold sections along their mating junction surfacesinto an integral shell mold structure. Although the junction along the mating surfaces between the individual mold sections of such integral composite mold is by itself not very strong, ity is sufliciently strong to assure that the shell mold sections are maintained in their desirable critical spacing relation for making it possible to cast into the mold cavity castings with relatively high tolerances such as .005 inch per inch.

Suitable refractory particle material for making composite shell molds of the invention are zirconium silicate, unstabilized zirconia and also stabilized zireonia. Also, beryllium oxide, aluminum oxide silicon oxide, chromite, magnesium oxide, aluminum silicate, alumina, ground quartz, flint, silicon, carbide and mixtures of the foregoing materials are suitable. in commercial practice, very good results are obtained by using zirconium silicate particles as the refractory material.

Suitable thermosetting resin additions for use in making with such refractory particles composite shell molds of the invention are phenol formaldehyde condensation products in its non-condensed A stage or condensed to its intermediate or B stage. Also, urea formaldehyde, melamine formaldehyde, alkyd and polyester resins and phenolic furfural may be used.

Suitable high-temperature binder ingredients for admixing to the refractory particles and resin content of the investment composition used in forming shell molds 'of the invention are alkali metal fluorides, and mixtures of an alkali metal fluoride with a boron compound such as boric acid, boric oxide, alkali metal b'orates, alkali metal silicates, or alkali metal aluminates. Also ammonia phosphate in small particle size and compounds which on heating form phosphoric acid may be used. Also, calcium sulphate, zirconium oxychloride, magnesium oxychl'oride, boric acid and mixtures of the foregoing compounds may be used. Instead of alkali metal compounds of the foregoing type corresponding metal compounds may be used. However, alkali metal compounds are more desirable because they bring about the binding action of the refractory particles into a self-supporting porous shell mold by heating at lower temperatures than in the case of non-alkali metal compound binders.

According to a further phase of the invention, the solid investment material ingredients of the type described above are combined, as by mixing, with a small amount of a liquid solvent which is a solvent for the thermosetting resin ingredients and forms therewith a coating liquid which has the property of wetting the refractory particles in such a way that the coating liquid with its dissolved thermosetting resin is caused to envelope and cover with minute liquid coating strata. the individual refractory particles. The fine particles of the high temperature inorganic binder are thoroughly dis persed in and admixed with the liquid coating solvent before it is added to and mixed with the refractory particles.

Suitable liquid solvents for use in making up such liquid containing investment compositions are solvents such as acetone, ethyl alcohol, methyl ethyl ketone, ethyl acetate and like solvents in which the selected thermosetting resin is soluble.

For instance, phenol formaldehyde resin in the A stage is soluble in acetone as well as ethyl alcohol whereas phenol formaldehyde resin in its B stage is soluble in acetone.

The investment compositions of the invention for. forming the individual mold, sections of the composite shell mold of the invention which do not contain admixed thereto any liquid solvent are made up with refractory particle material having a particlesize of --l50 mesh or somewhat largerup to about =l50 mesh. The critical range of the proportions over which the resin binder addition to the refractory particles may be varied is between about /2 up to about 3% of thesolid ingredients of the composition applied to form the shell mold. For best results, the resin binder addition should not exceed about 1 and should be at most about 2% of the solid shell mold material ingredients. The critical range of proportions over which the high temperature binder addition may be variedis between /2% to 5% of the solid investment material composition ingredients. Very good results are obtained with high temperature binder content of about 2% in the refractory particle'composition applied to form the shell mold. In case of high temperature binders such as ammonium phosphate in small particle size, the critical binder content may vary between about 1% and up to 5% of the solid investment composition material. For best results, the ammonium phosphate particle binder content should be about 3% to 4%, such as 3.5% of the solid investment material ingredients.

The advantages of the use of a liquid solvent addition to the investment material composition applied to form composite shell molds of the invention will be now explained. All heretofore available processes and investment compositions suitable for making sectional shell molds had to be made up with refractory particles of relatively large particle size. In other words, it was impossible to make prior art sectional shell molds with refractory particles having fine particle size, such as 325 mesh, required in order to provide such shell molds with a smooth cavity surface that would yield a metal casting with a fine surface finish. Thus, if an attempt is made to form in accordance with prior art practice a sectional shell mold with an investment composition containing predominantly refractory particles of 325 mesh particle size, the thermosetting resin content of the investment composition would have to consist of at least 25% of the solid composition ingredients in order to provide the binding action for the fine refractory particles. With such high proportions of the thermosetting resin in the shell mold, excessive decomposition of the resin content occurs when hot cast molten metal comes in contact with the mold cavity surface with the result that the cast metal article becomes contaminated with the gases and defective.

The liquid containing investment composition is applied to the surface of the heated sectional metal pattern in any of a number of ways. By way of example, in one arrangement, the investment composition is dumped on the exposed surface of the heated metal pattern in the conventional way as used in making prior art sectional shell molds out of a dry mixture of silica sand and the thermosetting resin. In another arrangement, the liquid containing investment composition is deposited on the exposed surface of the heated metal pattern by means of a conventional core blowing machine, with the investment composition delivered as from a hopper to a duct wherein a blast of air carries the investment composition to and deposits it on the exposed surface of the heated sectional metal pattern.

There will now be described, by way of example, practical ways for making composite shell molds in accordance with the principles of the invention.

Fig. 1 shows, by way of example, a metal casting of a bell-like end wall 171 of an electric motor made with a composite shell mold of the invention shown in Fig. 2. The composite shell mold 10, shown in cross-section in Fig. 2, comprises a shell mold section 12 and a mating shell mold section 13 which are joined to each other along mating junction surfaces 14, 141, 15 to provide between them a mold cavity, including the mold cavity spaces 16, 17, for casting therein the metal member 11 of Fig. l. The shell mold sections 12 and 13 have relatively small wall thickness, such as about /8 to A of an inch depending on the size of the casting to be produced. However, large castings made with shell molds of the invention need not have a wall thickness greater than about A to of an inch.

The several mating shell mold sections, which may be larger in number and are shown in the present example, as consisting of only two shell mold sections, are formed on separate metal patterns such as indicated by way of example in Figs. 3 and 4. Fig. 3 shows a pattern section 21 of metal with an exposed pattern surface 22 on which a shell mold section 12 has been formed. Fig. 4 shows the other pattern section. 31 of metal with an exposed pattern surface 32 on which pattern section 13 has been formed. The two metal patterns 21, 31 are formed with pattern surface shaped portions for forming thereon the flange portions 18, 19 of the two mating mold sections 12, 13, so that the mold flange 18 has a keyhole shaped to register with and receive an aligning key projection 19-1 of the mating shell mold section-13 when the two mold sections 12, 13 are assembled into the composite shell mold, such as shown in Fig. 2.

By way of example, the following procedure is followed in making composite shell molds of the invention of Fig. 2 out of shell mold sections formed with metal patterns such as shown in Figs. 3 and 4. The refractory particles are mixed with about 1% of the thermosetting resin binder and about 1% ofthe high temperature binder. The metal pattern is' first coated with a parting coating consisting of a silicone resin dissolved in a suitable 801-. vent so that when the metal patterns such as '21 or 31 are heated to about 300 C.,there will remain on the exposed pattern surfaces 22 and 32, respectively, a thin silicone resin stratum which will permit ready parting or separation of the solidified shell mold section from the patterns 21, 31 respectively. 0

After heating the individual sectional metal patterns, such as 21, 31 to about 300 to 320 C., the mixture of the investment composition consisting of the refractory particles with the small resin binder and inorganic binder contents are dumped or sifted on the exposed heated pattern surfaces 22, 32 of the individual met-a1 patterns. As the investment powder composition comes in contact with the heated pattern sections, their thermosetting binder content becomes set to bind together the refractory panti cles into a thin shell layer conforming to the shape of the pattern. Good results are obtained with the refractory particles of 125 mesh size combined with 1% of the thermosetting resin content and 1% of the high temperature binder content. For refractory particles of l mesh size, good results are obtained with a thermosetting binder content of about 2 to 3% and a high tem perature binder content of 1% to 2%.

After the dumped or deposited investment composition is thus heated by the hot pattern section to bind the refnactory particles into shell layer sections about A; to A of an inch in thickness, the pattern sections, such as 21, 31, withthe shell mold layers formed thereon are in-. verted for dropping off the excess not adhering investment composition ingredients. Thereafter, the individual pattern sections, such as 21, 31, with the respective shell mold sections 12, 13 formed thereon are placed in an oven and heated for 2 to 3 minutes at about 300 C., to com-.

plete the setting of the thermosetting resin content of the shell layer sections, such as 12, 13. The shell layer seccomposite shell mold such as seen in Fig. 2. The so assembled shell mold sections, such as 12, 13, are then heated as in an oven to a temperature between about 1000 to 1200 C., forl to 3 hours, usually about 2 hours, and thereby causing the high temperature binder content to become elfective in binding the refractory particles of the several shell mold sections into hard strong self-supporting shell molds.

The heating of the assembled shell mold section to a temperature between 1000 *C. and 1200 C. renders the high-temperature binder content of the individual mold sections to become effective in binding their refractory particles into a self-supporting shell mold section, and also causes the high-temperature binder content of the several mold sections to join them along their mating junction surfaces, such as junction surfaces 14, 14-1, 15, of Fig. 2, into an integral self-supporting shell mold structure defining the mold cavity of the object to be cast. Although not shown, in Figs. 2, 3, and 4, the pattern sections are so shaped that the resulting shell mold section and the composite shell mold such as shown in Fig. 2, is also provided with a conventional gate for delivering into the mold cavity the cast molten metal.

As the composite shell mold assembly of Fig. 2 is thus heated at 800 to 1200 C. for rendering the high temperatu're binder effective in joining the refractory particles of the s'everal shell rnold section's into an-integral com posite shell mold, the thermosetting resin content is do composed and expelled from the walls of-the composite shell mold, thereby rendering the shell mold structure highly porous and permitting free how of hot cast metal into the mold cavity withoutthe necessity of any vents or risers. -In cases where the high temperature binder content, such as ammonium phosphate particlesybecomes effective in producing the desired binding action at tern peratu'res such as 500 C., the assembly of the shell mold sections need to be heated only at such lower temperatures for l to 3 hours for causing the refractory particles of the assembled shell mold sections to be bound to each other into an integral shell mold structure and for decomposing the resin binder content. However, in most cases, the composite shell mold is heated to a high temperature of-about 1000 C.-or higher before the cast metal is poured into the mold cavity in order to assure that the molten-metal will enter all crevices of the mold cavity before solidifying when it comes in contact with cavity wall surfaces.

Below are given specific examples of compositions suitable for forming shell molds of the invention compositions without the liquid solvent addition.

Example I-A Ammonium phosphate, 325 mesh particle size 3.5

Below are given examples of compositions for forming shell. molds of the invention which composition also containsa solvent for the thermosetting resin ingredients.

Example 2-'A Percent Zirconium silicate, 325 m'e's'hparticle size '98 Phenol formaldehyde condensation product; 1 Sodium fiuoride .75 Boric acid .25

For each l gr ams of these-combined ingredients there is added milliliters (mL) of acetone.

Example 2-B Percent Zirconium silicate 325 mesh particle size 95.5 Phenol formaldehyde condensation vproducts 1 Ammonium phosphate particles -325 mesh particle size 3.5

For each 100 grams of these ingredients, there is added 5 ml. of acetone.

In preparing the compositions of Examples 2--A and 2-'B the resin bin der is dissolved in the acetone and the high temperature binder is dispersed in theliquid solution before mixing it 'with the refractory particles.

It will be apparent to those skilled in the art that the novel principles of the invention disclosed herein in connection with specific exemplifications thereof will suggest various other modifications andappli'cations of the same. It is accordingly desired that in construing the breadth of the appended claims they shall not'be limited to the specific exemplifications of the invention described herein.

The liquid solvent for forming compositions of the type given in Examples l-B, 2- -B should be resent in an amountl'sufiicientt'ocause the resinc'ontent to become sticky audit does not have to dissolve the entire-resin content. Good results are obtained by using a liquid content between -2 ml. to 25 ml. for each l00'grams of the 'dry composition ingredients.

I claim: 7

l. The process of making a composite shell mold which comprises preparing an'investment composition containing predominantly refractory particles with atmost about 3% of a thermosetting resin content and at most about 5% 'of a high temperature binder content, depositing a layer of said investment composition on a plurality of heated metal patterns maintained at a temperature between about 200 and 500 C. to cause the deposited resin content to bind together at least a thickness of said layers i'nto bound shell layer sections which when assembled along mating junction surfaces will define a mold cavity of a metal object to be cast, removing any excess of deposited composition content not adhering to the individual. layer sections so formed, thereafter assembling the individual shell layer sections and joining them along their mating junction surfaces into a composite shell mold structure, thereafter heating the assembled composite shell mold structure to a temperature in excess of 500 C. for causing said high temperature binder content to become elfective and causing the refractory particles of said individual shell layer sections to become bound to each other and also to establish a junction bond along their mating junction surfaces and thereby joining the mating shell layer sections into an integral mold structure.

2. The process as claimed in claim 1, the resin content of the applied investment composition being about /2. to about 3% of the solid composition ingredients applied to form said shell mold sections.

3. The process as claimed in claim 1, the high-temperature binder content of the applied investment composition being about Me to about 5% of the solid composition ingredients applied to form said shell mold sections.

4. The process as claimed in claim 1, the resin content of the applied investment composition being about /2 to about 3% of the solid composition ingredients applied to form said shell mold sections, the high-temperature binder content of the applied investment composition being about /2 to about 5% of the solid composition ingredients applied to form said shell mold sections.

5. The process as claimed in claim 1, the high-temperature binder content of the applied investment composition being about /2% to about 5% of the solid composition ingredients applied to form said shell mold sections, said high-temperature binder consisting of a mixture of an alkali metal fluoride and boric acd.

6. The process as claimed in claim I, the high-temperature binder content of the applied investment composition being about /2% to about 5% of the solid composition ingredients applied to form said shell mold sections, said high-temperature binder consisting of a mixture of an alkali metal fluoride and boric oxide.

7. The process as claimed in claim 1, the high-temperature binder content of the applied investment composition being about /2% to about 5% of the solid composition ingredients applied to form said shell mold sections, said high-temperature binder consisting of an ammonium phosphate of small particle size.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES The Iron Age, June 3, 1954, pages 113415. Foundry, September 1952, pages 108-111.

Claims (1)

1. THE PROCESS OF MAKING A COMPOSITE SHELL MOLD WHICH COMPRISES PREPARING AN INVESTMENT COMPOSITION CONTAINING PREDOMINANTLY REFRACTORY PARTICLES WITH AT MOST ABOUT 3% OF A THERMOSETTING RESIN CONTENT AND AT MOST ABOUT 5% OF A HIGH TEMPERATURE BINDER CONTENT, DEPOSITING A LAYER OF SAID INVESTMENT COMPOSITION ON A PLURALITY OF HEATED METAL PATTERNS MAINTAINED AT A TEMPERATURE BETWEEN ABOUT 200* AND 500*C. TO CAUSE THE DEPOSITED RESIN CONTENT TO BIND TOGETHER AT LEAST A THICKNESS OF SAID LAYERS INTO BOUND SHELL LAYER SECTIONS WHICH WHEN ASSEMBLED ALONG MATING JUNCTION SURFACES WILL DEFINE A MOLD CAVITY OF A METAL OBJECT TO BE CAST, REMOVING ANY EXCESS OF DEPOSITED COMPOSITION CONTENT NOT ADHERING TO THE INDIVIDUAL LAYER SECTIONS SO FORMED, THEREAFTER ASSEMBLING THE INDIVIDUAL SHELL LAYER SECTIONS AND JOINING THEM ALONG THEIR MATING JUNCTION SURFACES INTO A COMPOSITE SHELL MOLD STRUCTURE, THEREAFTER HEATING THE ASSEMBLED COMPOSITE SHELL MOLD STRUCTURE OT A TEMPERATURE IN EXCESS OF 500*C. FOR CAUSING SAID HIGH TEMPERATURE BINDER CONTENT TO BECOME EFFECTIVE AND CAUSING THE REFRACTORY PARTICLES OF SAID INDIVIDUAL SHELL LAYER SECTIONS TO BECOME BOUND TO EACH OTHER AND ALSO TO ESTABLISH A JUNCTION BOND ALONG THEIR MATING JUNCTION SURFACES AND THEREBY JOINING THE MATING SHELL LAYER SECTIONS INTO AN INTEGRAL MOLD STRUCTURE.

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