US3402754A - Method for casting in a shell mold - Google Patents

Description

P 1968 R. E. CHRISTIAN 3,402,754

METHOD FOR CASTING IN A SHELL MOLD Filed Jan. 13, 1966 2 Sheets-Sheet 1 INVENTOR.

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METHOD FOR CASTING IN A SHELL MOLD Filed Jan. 13, 1966 2 Shets-Sheet 2 INVENTOR.

0 1216 y 6272-3222171 BY M, p i u United States Patent 3,402,754 METHOD FOR CASTING IN A SHELL MOLD Rodney E. Christian, Birmingham, Mich., assignor t0 Traub Manufacturing Company, a corporation of Michigan Filed Jan. 13, 1966, Ser. No. 520,461 13 Claims. (Cl. 16435) ABSTRACT OF THE DISCLOSURE A method and apparatus of forming a precision casting.

A thin hollow mold is formed from the material that is to form the exterior of the finished casting by depositing the material on a destructible core or pattern. The pattern is then removed by destruction leaving a hollow thin shell mold into which molten material is poured. The molten material when poured fuses to the interior of the mold to form an intimate bond but does not substantially change the characteristics of the exterior of the mold so that a high degree of surface finish of the finished article reults.

The casting of finger rings is a good example of an instance where the article is cast, but it is desirable to minimize the subsequent finishing operations. Such rings have commonly been cast by a process known as the lost wax process, With the lost wax process, a wax pattern is formed in the shape of the ring. The wax is then surrounded by an investment of Plaster of Paris or the like, and the wax is subsequently removed from the Plaster of Paris by melting it. The gold or other precious metal which forms the ring is then poured into the Plaster of Paris mold filling the cavity left by the lost wax. Although the wax pattern has an extremely smooth surface finish which conforms to the master from which it is made, the Plaster of Paris surrounding the wax 'has a rough texture which the cast, precious metal assumes. Therefore, it has been necessary to perform considerable polishing operations upon the cast ring so that it will be salable. These subsequent finishing operations add greatly to the cost of producing the ring and requires expert personnel who are not always available.

It is, therefore, the principal object of this invention to provide an improved casting process in which the surface finish of the cast article is of high quality.

It is a further object of the invention to provide an improved low cost precision casting method.

It is still a further object to provide an improved precision casting process using the st wax method.

The method of making a precision casting embodying this invention comprises the steps of forming a thinshelled mold having a hollow interior and pouring a molten material into the interior of the mold. The material is poured at a temperature sufficient to fuse the interior portion of the material of the mold to the poured material and form an intimate bond therebetween. The mold is formed with sufficient thickness to preclude fusion of its outer periphery during the molding step.

Other objects and advantages of this invention will 3,402,754 Patented Sept. 24, 1968 become more apparent as this description proceeds, particularly when considered in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a perspective view of a ring master having the shape to be reproduced.

FIGURE 2 is a cross-sectional view of the master shown in FIGURE 1 embedded in a material for forming the wax pattern;

FIGURE 3 is a cross-sectional view, in part similar to FIGURE 2, showing the cutting of the embedding material to remove the master and to form the wax pattern;

FIGURE 4 is a cross-sectional view of the wax pattern;

FIGURE 5 is a cross-sectional, schematic view showing the apparatus for forming a metallic coating upon a series of patterns positioned upon a tree;

FIGURE 6 is an enlarged cross-sectional view taken along the line 66 in FIGURE 5 subsequent to the depositing of the metal coating upon the tree.

FIGURE 7 is a cross-sectional view illustrating the emtal covered tree embedded in an investment;

FIGURE 8 is an enlarged cross-sectional view, in part similar to FIGURE 6, showing the mold after the wax pattern has been removed;

FIGURE 9 is a cross-sectional view, in part similar to FIGURE 8, showing the mold filled with the poured metal; and

FIGURE 10 is an enlarged view of the encircled area in FIGURE 7.

Referring now in detail to the drawings, the reference numeral 21 in FIGURE 1 identifies a master ring having the desired configuration which will be reproduced. The master ring 21 may be formed by any known handforming operation or the like, as is well known in the art. The master ring 21 is embedded in a body of latex or the like 22 (FIG. 2) which flows around and completely encompasses the periphery of the master 21. The latex body 22 is then slit along a parting line, indicated generally by the reference numeral 23 (FIG. 3), so that the master 21 may be removed. The body 22 is then adhered together along theparting line 23 and a sprue (not shown) is formed so that a body ofwax 24 may be poured into acavity 25 which has been left by removal of the master ring 21. Thus, a wax ring 24 (FIG. 4) will be formed that will conform almost identically to the shape and surface texture of the master ring 21.

A plurality of thewax rings 24 formed in the described manner are supported upon r0d-like wax members 25 which may be separate pieces or may be formed by the wax which solidified in the sprue formed in the body 22. The rod-like members 25 are adhered, as by melting, to agate 26, that is also formed of wax. The adherence of a plurality of therings 24, androds 25 to thegate 26 forms what is known in the art as a tree, as indicated generally by the reference numeral 27 (FIGURE 5).

A metallic coating is deposited upon thetree assembly 27 by a process known in the art as sputtering using the apparatus shown schematically in FIGURE 5. The sputtering apparatus comprises an inverted vacuum jar 28 sealed around its lower periphery to a base 29 by means of agasket 31. Posilioned within the vacuum jar 28 is a target 32 that is connected by means of alead 33 to ahigh voltage terminal 34 positioned within the base 29. An anode '35 is supported above anopening 36 formed in the base 29 and is connected by means of a lead-inwire 37 to another high voltage terminal 38 formed in thebase 29. Theanode 35 and target 32 are similarly charged for a reason which will become more apparent.

Thetree assembly 27 is supported within the vacuum jar 28 upon the base 29 and an electro-magnetic coil 39 encircles the vacuum jar 28 adjacent the upper portion oftree assembly 27. The electro-magnet 39 receives electrical 3 2 power from any suitable source (not shown). A filament 41 is supported in the base 29 internally of a channel 42 that intersects the channel which forms the opening 39 at its lower end. A pair of terminals 43 and 44 are connected to the opposite ends of the filament 41.

The target '32 is formed from a solid piece of the metal to be deposited upon thetree 27. In a preferred embodiment of this invention the metal from which the target 32 is formed is the same metal as that from Which the finished ring is to be made, for example gold, white gold, platinum or some other precious metal. The advantage in the use of the sputtering process is that the target may be an alloy or mixture of metals, such as white gold, and the alloy will be deposited upon thetree assembly 27.

The metal is sputtered onto thetree assembly 27 by positively charging the target 32 andanode 35, energizing the electro-magnet 39 and heating the filament 41. The vacuum jar 28 is partially evacuated in any known manner. Heating of the filament 32 causes electrons to bombard the oppositely charged target 32. When the target 32 is bombarded by the electrons, minute particles of the alloy will be dislodged and repelled from the target 32 due to their like charge. These positively charged particles also will be deflected away from theanode 33 because of its like charge and will be focused upon thetree assembly 27 due to the opposite charge of the target 32,anode 35 and the field created by the electro-magnet 39. This leaves a thin coating 51 of the precious metal upon thewax rings 24, rods 25 and remaining portion of thetree assembly 27. The process is continued until the layer 51 has the desired thickness (FIGURE 6.)

After the metal coating 51 is deposited upon thetree assembly 27 to the desired thickness, the metallic coated tree assembly is removed from the coating apparatus and is embedded in an investment, for example a block of Plaster of Paris, indicated generally by the reference numeral 52 (FIGURE 7). Thegate 26 is positioned so that it opens through the top of the Plaster of Paris block 52. After the metallic coated tree assembly is embedded in the investment 52, it is heated sufficiently for thewax ring 24rods 25 andgate 26 to melt and flow through the open end of the investment 52 leaving a hollow metallic mold which is formed by the metal 51 that was deposited upon the tree assembly 27 (FIGURE 7).

After all of the wax has been removed leaving the metallic mold, the opening of thetree gate 26 through the top of the investment 52 provides a pouring funnel into which molten metal such as gold or some other precious metal may be poured. The molten metal flows down the gate left by the removal of the wax through the risers provided by the removal of therods 25 intocavities 53 formed from the removal of the wax rings 24 (FIGURE 9).

The poured metal, which is preferably of the same metal as the shell 51, is poured at sufficient temperature to cause fusion of the interior portion of the shell 51 with the poured metal. The resultant intimate bond between the shell 51 and the poured metal forms a solid body. as indicated in FIGURE 9. The metal may be poured under a vacuum pouring process in which thehollow interior 53 is evacuated and in which the investment 52 is rotated about a vertical axis that is intersected by the axis of thegate 26 so that the centrifugal force will insure complee filling of thecavities 53. The pouring temperature of the molten metal and the thickness of the shell '51 is selected so that the outer periphery of the shell 51 will be unaffected. That is, although its inner periphery is fusing, the outer surface will remain solid throughout the casting process. Thus, the smooth surface and the shape of the outer shell 51 is unaffected during the pouring process.

The disclosed casting method results in the formation of an extremely good surface finish that requires only a minimum of subsequent polishing. It will be noted from the enlarged view" (FIGURE that the investment 52 i 4 which forms around the metallic shell 51 leaves a plurality of small pockets or voids, indicated generally by the reference numeral 55 due to its texture. In a normal casting by the lost wax process, these voids 55 remain after the wax has been removed. When the percious metal is poured into the void left by the wax, the precious metal will fill the voids 55 resulting in a rough surface finish on the completed article. This rough surface finish does not result in the disclosed method, however.

In this casting process the outer periphery of the shell 51 actually forms the finished surface of the ring. As is well known, the sputtering process which has been described results in the formation of a substantially uniform metallic coating upon thewax tree assembly 27 so that the outer surface of the shell 51 will conform substantially to the outer surface finish of the master ring 21 but will be somewhat larger in size. Due to the thickness of the shell 51, the master ring 21 is made sufficiently undersized to compensate for the buildup of the metal shell 51 and its increased thickness in the finished product. Preferably, a shell thickness of about 3 to 10 mils will result in the desired surface finish.

Although the disclosed method has been described in conjunction with the making of an ornametal finger ring, it is to be understood that the method may be practiced in the use of casting any object with a precision finish and dimension. Various other changes and modifications may be made without departing from the scope of the invention, as defined by the appended claims.

What is claimed is:

1. The method of making a precision casting comprising the steps of forming a thin shelled mold by making a destructible pattern, depositing the mold material on the pattern to the desired thickness and removing the pattern from the mold by destruction of the pattern, the mold having a hollow interior and an exterior conforming in size, shape and surface finish to the finished casting, and pouring a molten material into the interior of the mold, the material being poured at a temperature sufficient to fuse the interior portion of the material of the mold to the poured material and form an intimate bond therebetween, the mold being formed with sufficient thickness to preclude fusion of its outer periphery during the molding step so as to preclude any substantial change in the characteristics of the exterior whereby the mold exterior forms the exterior of the finished casting.

2. The method of making a precision casting as set forth in claim 1 wherein the mold material and the poured material are the same metal.

3. The method of making a precision casting as set forth in claim 1 wherein the mold is surrounded by an investment prior to the pouring and the investment is removed after the pouring.

4. A method of making a precision casting as set forth in claim 1 wherein the pattern is formed from a material having a lower melting point than the mold material and is removed from the mold by raising its temperature sufficient to cause it to melt and run from the mold.

5. A method of making a precision casting as set forth in claim 4 wherein the pattern material is wax and the mold material is a metal.

6. A method of making a precision casting as set forth inclaim 5 wherein the poured material is the same metal as the mold material.

7. The method of making a precision casting as set forth in claim 4 wherein the mold is surrounded by an investment prior to the pouring and prior to the removal of the pattern, the pattern is removed subsequent to the surrounding of the investment and the investment is removed after the pouring.

8. The method of making a precision casting as set forth in claim 7 wherein the mold material is a metal and the poured material is the same metal. 7

9. The method of making a precision casting asset forth in claim 1, wherein the mold is formed by forming a non-metallic pattern and depositing a metal upon the pattern by the process of sputtering.

10. The method of making a precision casting as set forth in claim 9 wherein the metal is deposited upon the pattern by inserting the pattern and a bar of the metallic material into a partial vacuum and surrounding the pattern and body of the material with a magnetic field, positively charging the pattern and body of material and inserting a heated filament into the vacuum.

11. The method of making a precision casting as set forth in claim 10 wherein the pattern is formed from a material having a lower melting point than the mold metal and is removed from the mold by raising said temperature sufficiently to cause it to melt and run from the mold.

12. The method of making a precision casting as set forth in claim 11 wherein the mold and contained pattern are surrounded by an investment and the pattern is removed from the mold by melting subsequent to the inser- 6 tion of the investment, the investment being removed subsequent to the pouring.

13. The method of making a precision casting as set forth in claim 12 wherein the mold and poured material are the same metal.

References Cited UNITED STATES PATENTS 2,479,598 8/1949 Barber et al. 164-34 X 2,788,555 4/1957 Sukacev 164-34 3,064,112 11/1962 Hanzel 164-98 X 3,139,658 7/1964 Brenner et al. 164--46 FOREIGN PATENTS 378,355 8/1932 Great Britain.

1. SPENCER OVERHOLSER, Primary Examiner.

E. MAR, Assistant Examiner.

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