US2476994A - Process for making molds - Google Patents

Description

July 26, 1949, c. l.. MILTON, JR., ETAL 2,476,994

PROCESS FOR MAKING HOLDS 2 Sheets-Sheet 1 Filed Feb. 2', 1948 alfornmr c. L .'MILToN, JR., Erm. 2,476994 July 26Z 1949.

PROCESS FOR MAKING HOLDS 2 Sheets-Sheet 2 Filed Feb. 2. 1948 -r/ /LNI' [son anotan-6 Patented July 26, 1949 PROCESS FOR MAKING HOLDS Clare L. Milton, Jr., Akron, Ohio, and Carl A. Nielson, Takoma Park, Md., mignon to the United States of America as represented by the Secreta-yo! the Army Y Application February z, 194s, serial No. 5,711

(c1. :zz-19o) (Granted under the m of Meren s. lass, n

governmental purposes, without payment to us of any royalty thereon.

The present invention relates to the precision casting of molds for the casting of complicated and intricate shapes.

Molds and other objects of intricate shape are constructed frequently by a precision casting method. In this method, Wax or rigid combustible plastic patterns oi' the pieces to be cast are invested in a suitable medium, frequently one composed largely of water, a hemihydrate of calcium sulphate, and a brous illler. Following dehydration, the wax or plastic is melted and/or burned out and molten metal is poured into the cavity which is left, often under vacuum, pressure, or centrifugal force.

Flexible patterns are used occasionally, but when high precision is required, rigid patterns are necessary. Wax patterns are traditional, but polystyrene has come into use more recently by reason of its greater rigidity and less tendency to distort while being handled.

In either case, construction of patterns prior 25 to the present invention has been a costly procedure, especially whenever numerous undercuts are involved. The expense has been due to the necessity for constructing complicated molds for the patterns, complicated because the various parts of the mold must be capable of being taken apart without bending either the molds or the patterns molded in them. Mold costs of thousands of dollars are not at all uncommon.

The present invention eliminates for the first time the construction of complicated piece molds for each part to be precision cast. It is particularly advantageous in cases in which the objects to be precision cast are themselves individual parts of a, piece mold, and therefore must match exactly. The process of the present invention also is especially useful in the production of a mold for the casting of a realistic glove used to cover an artificial hand in which the undercuts are very numerous and the detail so ne that machining is impossible or uneconomical.

Briefly, the present invention comprises spraying a pattern onto the surface of a suitable model, the model itself being hollow and iiexlble, that is, collapsible or semi-collapsible, but filled, at the time of spraying, with a removable rigid core. The pattern, which usually is undercut with respect to the model, is removed after the rigid core amended April 30, 1928; 370 0. G. 757) 2 has been removed and the model collapsed. Th pattern then may be invested and cast by techniques familiar to those skilled in the art of precision casting.

The process oi' the present invention is adapted to cases in which accuracy is required of one surface and the edges," but it is usually inapplicable where close thickness tolerances exist or where the texture of the back surface must be other than that produced by spraying.

The hollow model onto which the pattern material is sprayed is lled with a soluble or fusible core capable of being dissolved, melted, or softened, to the extent that it can be removed at a temperature below the temperature of decomposition or flow point oi' the model, or the temperature at which appreciable distortion of the sprayed model would take place.

As one example, the method of the present invention has been used to prepare patterns for a mold of a glove to simulate the appearance of a human hand, although it will be understood that the invention is not so limited.

As a iiexible, hollow model, there is preferred a model or form produced by heating a dispersion of polyvinyl chloride or a copolymer of vinyl chloride with vinyl acetate, methyl or ethyl acrylate, methacrylate, fumarate, or maleate, vinylidene chloride or other monomer, the said polymer or copolymer containing at least thirtyilve per cent chlorine in a solution containing from 0.5 to 5.0 per cent by weight of a polymer or monomers listed above, the balance of the solution being made up of a plastlcizer for the chlorine-containing resin. While this is a preferred composition, use also may be made of hollow, rubber models, or those of compositions similar to the above in which the chlorine-containing resin is dispersed in a plasticizer, or mixture of plasticizers alone, rather than in a solution of a polymer in such a plasticlzer.

Examples of rprepared compositions are as follows:

um Mun-AMWWWWWW n man Example II Pipetlions Constituent ar Y Weight Geon 100 x 210 45 Di-Z-ethylhexyl phthalate... 26. 7 'Iricresyl phosphate... 26. 7 VYDR. 1.

In the foregoing compositions, VYNU-l and VYDR are vinyl chloride copolymer resins manufactured by the Carbide and Carbon Chemicals Company, New York, N. Y., while Geon 100 x 210 is a vinyl chloride resin produced by the B. F. Goodrich Chemical Company, Cleveland, Ohio.

The present process also may be carried out over a hollow, flexible model composed of an acrylate such as ethyl acrylate or a plasticized alkyl methacrylate, or acrylate-methacrylate mixture, but the vinyl chloride polymer compositions are preferred because of theirv ease of handling, durability and chemical resistance.

4Several materials may be sprayed onto the vrigid yet potentially collapsible form. Wax is one such material but it suffers from the disadvantage of being readily deformed by handling. Polystyrene is another, although the technique Y of spraying it has not been worked out satisfac-K.

torily. It possesses the advantage that it is neither ductile nor malleable and patterns break rather than dent if struck. When wax or poly styrene are sprayed, the patterns may be invested and burned out in the customary manner.

Preferably, however, patterns are made of a metallic material melting below 100 C. The well known Woods metal has proved in practice to be a suitable metalas has the alloy sold to the trade as Cerrosafe which, like Woods metal, is a non-eutectic alloy of bismuth, lead, tin, and cadmium, produced by the Cerro k1de Pasco Copper Company, New York, N'. Y.

The procedure of the present invention as applied to the production of a multi-piece mold ofan object such as a glove, is illustrated by the accompanying drawings, wherein:

Figure l represents diagrammatically one step in the process of a mold produced in accordance with the present invention;

Fig. 2 represents a further step in the production of the mold;

Fig. 3 is a diagrammatic representation of the iiexible' form or model being partially collapsed for removal from the pattern formed in accordance with Figs. 1 and 2;

Fig. 4 is a diagrammatic View showing the pattern after removal of the ilexible model but before separation of the parts;

Fig. 5 is a diagrammatic view showing one portion of the pattern of Fig. 4 in an investment provided with suitable sprues for removal of the pattern;

Fig. 6 is a view similar to Fig. 5, but with the pattern removed.

With reference to the drawings, the flexible model or form used as the starting shape, when a cosmetic glove is the finished article, is made first, preferably from a resinous composition of the type illustrated above in Examples I and II. This form maybe prepared by gelling the resin in a suitable impression of a model hand, and, after gelation, the resulting resinous positive model hand is removed from the impression. The resulting resinous positive is hollow, and open at the wrist or arm portions of the resulting posi- It will be understood in this connection'that the process of the present invention is not limited to this adaptation, but it is of general application for the reproduction of any shape from -a hollow. exible' starting form. In any event, the hollow, exible form is rendered rigid by introducing therein a fusible core, such as wax,

which has a melting point below deformation temperature ofthe flexible form. A low melting point waxmay be used or a mixture of such wax and sand 'may' be employed, vthis core material being tamped into the flexible form until the form is ililed completely therewith.

In the drawings, the flexible form is represented hy the numeral B, the rigid core material being indicated at lll.

In producing the mold oi the present invention, where a multi-piece mold is desired, a portion of the rigidied flexible form is masked with a suitable masking material, such as clay, as indicated at l2. The exposed portion of the form is sprayed by Va low melting alloy of the characteristics described above, thus forming a layer i4 of this metallic material covering the exposed, or unmasked, surface of the form. When the alloy layer I4 has been built up to requisite thickness, the mask is removed and thin separator strips It are applied to the edges of the sprayed coating, and the remainder of the form is sprayed, as indicated at I 8.

When the spraying is completed, the core material l0 is removed" by melting or other operations which will readily be suggested to an operator, depending on the character of the core material, the removal being accomplished through the opening in the hollow form 8. The form then may be collapsed as indicated by Fig. 3 and removed from the corresponding opening in the coating, leaving the coating as a hollow pattern which, in the illustrated embodiment, is composed of the parts 20 and 22.

The parts 2l and 22 then may be separated and invested as shown in Fig. 5.

The critical surfaces of the melting pattern, that ls the surfaces which are to be duplicated iinally, are the inner or front surface or negative of the piece intended for production in the iinished mold, and the bearing edges or parting surfaces of the pattern. In investing each part of the pattern, the investment is poured in two sections, after providing for suitable sprues, indicated at 24 and 26, for example, or registration marks, grips and the like. One section 28 of the investment is in contact with all of the critical surfaces of the pattern, as mentioned above, the other section 30 of the investment being arranged so that, when it is removed, traces of the low-melting sprayed metal alloy, remaining after melting the pattern and allowing the maior portion of the alloy to run out through the sprues, can be removed with a stream of hot water. thus leaving the mold cavity 32 when the second section of the investment is replaced in position.

The casting operation is carried out after putting the second section 2l back inplace and sealins the cracks 34 between the investment secf tions. 'I'he material to be cast is introduced into the cavity 32 through the sprues 24 and 26, the resulting protuberancesin the cast shape being trimmed oil' from the resulting casting.

The process of the invention is applicable equally to single-piece molds having open ends and to multi-piece molds. Where a single-piece tive. 75 mold is to be produced, the original mask l2 is omitted. Such single-piece, open-ended molds frequently are employed where elimination of any flash line, however fine, is necessary. In this case, no marking is employed, the pattern being sprayed directly onto the rigidifled form, which form being .flexible subsequently is pulled out of the inside of the pattern after removal o! the rigidifying core.

pattern. When a sprayed metal pattern is used,-

it frequently is desirable to invest it in two pieces, only one oi which is in contact with the surface the reproduction of which is desired.

In accordance with the present process, the

` procedure is to mask oil' the important surface,

suitably with clay, and to add clay cylinders to the back of the pattern wherever sprues are desired in the investment.

The investment composition, usually a mixture of a. calcium sulphate hemi-hydrate, water and asbestos is poured over the marked pattern, and allowed to harden.

When this has taken place, the clay mask is removed, and after painting the exposed portion of the investment with a separating medium, such as shellac or mineral oil, investment compound is poured upon the important surface as Well.

When this hall.' of the investment has hardened also, the whole sandwich of investment pattern and investment is heated to a temperature alcove the melting point of the patternmaterial. When the latter melts, the two halves of the investment easily can be separated so that any particles of the melted pattern material which cling to the surfaces of the investment can be rinsed off with hot water or blown of? with steam. Following cleaning of these surfaces, the two halves of the investment are reunited, and clamped or taped together. For very critical work, the outside of the investment frequently is shellacked or sealed with some other sealing medium except for an area on the bottom.

When the investment has been brought up te a suitable temperature, depending on the metal to Ibe cast, metal may he poured into the sprue holes and, on solidication, a casting results. Where extreme perfection ci detail is important, a vacuum is applied to the interior of the investment through a hole in the sealing medium lett at the bottom. Since a dried out investment is highly porous, the vacuum existing within the apparently solid body of the investment material holds the molten metal to itself and prevents faults due to the trapping of air between the investment and the metal.

Having thus described our invention, what we claim as new and wish to secure by Letters Patent is:

1. A process for producing metal molds for producing molded replicas oi an original object, which comprises forming an open, seamless, negative impression of the object duplicating on the inner surfaces of the impression, in high fidelity, all surface characteristics of the object, casting in the impression a flexible, hollow, open, seamless positive replica of the object duplicating on its exterior surfaces all surface characteristics oi' an original object, removing the positive from the impression, rigidli'ying the said positive with a low melting point wax composition, coating the resulting rigid positive by applying a masking material to predetermined areas of the positive while leaving other areas unmasked, spraying Ithe unmasked areas of the positive-with a low-melting point, fusible alloy, removing a portion of the masking material from the rigid positive thereby exposing further area of the positive, applying separator means to terminal portions of the resulting sprayed coating, spraying the resulting ex areas with the low-melting alloy, continuing the exposing and coating until the entire rigid positive is coated except for the opening in the positive, thereby producing a plurality of separable sections interiorly duplicating the surface details of the ilexlble positive, removing the rigidifying composition from the exible positive, collapsing the flexible positive and removing the same from the coating, separating the sections of the coating, separately investing each section in a high-melting pointl investment material, each section forming a pattern, forming sprue holes in the investmentl material, melting the pattern from the investment, draining the resulting molten' pattern metal through the sprue holes, thereby leaving a mold cavity in the investment material, filling the mold cavity with molten mold metal introduced into the cavity through the sprue holes, allowing the metal to solidify in the mold cavity, removing the investment material from the solidified metal, and assembling the resulting mold sections into an `assembled mold. 2. A process for producing metal molds for producing molded replicas of an original object, which comprises forming an open, seamless, negative impression of the oblect duplicating on the inner surface of the impression, in high fidelity, all surface characteristics of the object, casting in the impression a flexible, hollow, open, seamless, positive replica of the object duplicating cn its exterior surfaces all surface characteristics of -tne original object, removing the positive from the impression, rigidiiying the said positive by illling with a readily removable core material, applying a masking material to predetermined areas o the rigidiied positive While leaving other areas unmaskecl, spraying the unmasked areas ci? the positive with e. low-melting point fusible alloy, removing a portion of the masking material from the rigid positive, thereby exposing further areas of the positive, applying separator means 'to terminal portions of the resulting sprayed coating, spraying the resulting exposed areas with the low melting alloy, continuing the exposing and spraying until the entire rigid positive is coated except for the opening in the positive, thereby producing a plurality of separable sections interiorly duplicating the surface -details of the positive, eil'ecting separation between the resulting sections and the positive, separating each of the sections from one another, separately investing each section in a high-melting point investment material, each section forming a pattern, forming sprue holes in the investment material, melting the pattern from the investment, draining the resulting molten pattern metal through the sprue holes, thereby leaving a mold cavity in the investment material, illling the 75 mold cavity with molten mold metal introduced intothecavitythmuhthmmm UNI'I'EDBTATES PATENTBY`- l the investment material from the lolidied "937g www am 10' 1905 metal, and assembling the resultins mold muws 996 733 Mol-eau July 4, 1911 inw an assembled mld V 1,1r19hs2 schaap Apr. 1a, 191e GLARE I MIUTON' 5' 13:18:18:y Kralund ,.---epn 4,1911 CARL A' NELSON- 1,c92,4aa Barkscnat i Nov. 2o, 1924sl 1,856,394 Lettel'lnhn May 3, 1932l l REFERENCES CITED 2,280,014. Halsanv p1-.21,1942 The following references are o! record in `the 1 2,288,840 Raiche July 7, 1942 111e of this patent: 3,345,977 Howald et al Apr. 4, 1944

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