US3506061A - Apparatus for vacuum-casting a plurality of metal parts in a single mold - Google Patents

Description

April 14, 1970 P. J. CLEMM 3,506,061

LITY

Filed June 10, 1965 /NVENTOR.'

APPA US FOR VACUUM-CASTING A PLURA v METAL PARTS IN A SINGLE MOLD 2 Sheets-Sheet 1 PE TER J'. CLEMM, BY 4/4661 vaudwm- ATTORNEY Aprll 14, 1970 P. J. CLEMM 3,506,061

APPARATUS FOR VAGUU AST A PLURALITY v OF METAL PARTS A S LE MOLD Filed June 10, 1965 2 Sheets-Sheet 2 INVENTOR.

PETER J CLEMM,

United States Patent 3,506 061 APPARATUS FOR VACUUM-CASTING A PLURALITY 0F METAL PARTS IN A SINGLE MOLD Peter J. Clemm, Malvem, Pa., assignor to General Electric Company, a corporation of New York Filed June 10, 1965, Ser. No. 462,835 Int. Cl. B22d 27/16,- B28b 7/26 U.S. Cl. 164-254 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to apparatus and a method for vacuum-casting a plurality of metal parts in a single mold and relates, more particularly, to a permanent mold from which the parts, after being cast, can easily be removed without damage either to themselves or the mold.

The molding apparatus that I am concerned with comprises a plurality of cores disposed in stacked relationship and containing recesses defining cavities of a shape conforming to that of the parts being cast. These cores are made from a high strength material, such as graphite, that allows the cores to be reused for many repeated casting operations. In view of this repeated reusability, the mold is referred to herein as a permanent mold. To permit repeated use of the cores, it is important that the casting operation not impose damaging stresses on the cores, and it is also important that the cast parts be removable from the mold without damaging the cores. It is, of course, just as important that the cast parts themselves not be damaged either by the casting operation or by their removal from the mold.

If, during cooling, the metal of the cast parts contracts to a greater extent than the core material, then there will be a tendency to stress both the cores and the cast parts. In most mold constructions, this differential contraction will load the cores in compression and the cast parts in tension.

An object of the present invention is to prevent the stresses produced by such differential contraction from damaging either the cast parts or the cores.

Another object is to construct the mold in such a manner that the cast parts can easily be removed without imposing damaging stresses on either the cast parts or the cores.

In carrying out my inventionin one form, I provide a mold assembly in which separable, reusable cores are disposed in stacked relationship with the above-described cavities located between adjacent cores. A reservoir, or entry region, for molten metal is disposed adjacent one end of the mold assembly, and molten metal is fed from this reservoir into the cavities. The cavities are interconnected by gates of small cross section that provide passageways between the cavities through which molten metal flows from the reservoir into the cavities to successively fill the cavities,

The interior of the mold is maintained under a vacuum while the cavities are being filled, and thus there is no air or other gas present to interfere with filling the cavities through the small gates. After the cavities have been filled and the molten metal has subsequently frozen to form the cast parts, the cast parts are removed from the mold. This removal is effected by fracturing the metal in the small cross-section gates between adjacent cast parts, each such fracture permitting removal of one of the cast parts and an adjacent core. This fracture is produced by a predetermined separating force applied either to the cast part being separated or to the core between the adjacent cast parts. The cross-sectional area of the metal in the gate is sufiiciently small that said predetermined separating force fractures the gate metal before it produces damaging stresses elsewhere, either in the cast part being removed or the immediately adjacent core.

For a better understanding of the invention, reference may be had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a cross-sectional view through a mold assembly embodying one form of the invention;

FIG. 2 is a sectional view taken along the line 22 of FIG. 1;

FIG. 3 is a diagrammatic view illustrating the general manner in which the mold and cast parts cool after the mold-filling operation; and

FIG. 4 is a sectional view of a portion of a slightly modified mold assembly.

Referring now to FIG. 1, there is shown a mold assembly 10 comprising a plurality of cores 12 disposed in vertically-stacked relationship. Each of these cores 12 has a recess 14 in one or both of its horizontally-disposed faces, and this recess 14 together with a recess 14 on an adjacent core 12 defines a cavity 16 of a shape conforming to that of the part to be cast.

The lowermost core 12 is imperforate, but each of the other cores includes a centrally-located passage, or gate, 20 extending vertically therethrough. These gates 20 are preferably disposed in aligned relationship along an axis 21 that is centrally located with respect to the cavities. Each of the illustrated cores 12, except the top one, is of a disk shape with a circular outer periphery. The top core 12 may be thought of as being cup-shaped with a circular outer periphery. The circular outer peripheries of all the cores are in alignment. The top core 12 forms a reservoir, or entry region, 22 for molten metal that is fed downwardly through the gates 20* to fill the cavities 16, as will soon be described in more detail.

Surrounding all the cores 12 is a jacket 24 of cup-form that has a cylindrical portion 25 with internal threads 26 that mate with external threads on the top core 12. When these threads are tightened by suitably rotating the jacket 24 relative to the upper core 12, the lower wall 27 of the jacket 24 engages the lower core 12 and clamps all the cores together. Each of the illustrated cores 12 has a planar surface 28 that engages a corresponding planar surface on the adjacent core or cores to provide a good seal between these parts when the cores are clamped together by rotation of the outer jacket 28. These planar surfaces are referred to as the parting surfaces of the cores. In the illustrated mold assembly, these parting surfaces 28 extend substantially perpendicular to the central axis 21. If desired, the parting surfaces of adjacent cores may be of a conical form or some other form capable of providing a good seal therebetween at the outer periphery of the cavity 16 to prevent the escape of molten metal from the cavity.

The heat used during the casting operation can be derived from any suitable source, but I prefer to use an induction coil 32 that surrounds the entire mold assembly. When this coil is appropriately energized, it heats the mold assembly to the desired temperature.

A first step in the casting operation is to place a charge of solid copper in the reservoir 22 at the top of the mold. This copper has been suitably pre-treated to free it of all but a tiny part of its sor-bed gases. The whole mold assembly together with the copper charge is heated by the coil 32 while under vacuum. As the copper melts, it is fed by gravity through the gates 20 and into the cavities 16. The lowermost cavity 16 is first filled, followed by successively higher cavities.

In a preferred application of the invention, it is desired to employ the illustrated mold assembly for making precision castings that have a high freedom from included gases. It will be assumed that the castings are to be made of copper, though it should be understood that many other metals are equally usable.

The cores 12 and the jacket 24 of the mold assembly are preferably made of a high purity graphite. Before the casting operation, the mold assembly is heated to a high temperature, e.g., between 1,000 and 2,000 degrees F., while in a high vacuum. This heating in a vacuum drives off most of the gases from the walls of the mold, thus assuring that these gases will not find their way into the castings during the casting operation. A high vacuum is also maintained immediately before and during the casting operation, and this allows the cavities 16 and gates 20 to be completely filled with molten metal without any interference from gases. While I prefer to provide a vacuum in which the pressure is 10 mm. of mercury or lower, many of the advantages mentioned hereinafter can be realized with lesser degrees of vacuum, e.g. below several millimeters of mercury. The term vacuum as used herein is intended to denote a pressure below several millimeters of mercury. In the preferred form of the invention, I employ a very high vacuum, primarily because I wish to maintain the cast parts substantially free of internal gases. The desired vacuum is obtained, preferably, by locating the mold assembly in an evacuated chamber (not shown) provided with a suitable vacuum pump that holds the pressure in the chamber at the desired low level.

As was mentioned hereinabove, the gates 20 have a very small cross section. The vacuum environment makes it feasible to rely upon the small gate cross-section since there is no gas present inside the mold to inhibit flow through the gates and no need to provide for gas escape therethrough, as would usually be the case with casting in air.

Each of the gates can be of a uniform cross section, but in the preferred form of the invention illustrated, each of the gates has a single restricted portion 40 intermediate its ends. By way of example, this restricted portion has a diameter of about 5 inch. On opposite sides of the restricted portion 40, the gate diameter gradually increases to about .275 inch. Gates of substantially this same size and shape can be used whether the casting metal is copper or some other metal. By confining the maximum restriction 40 in the gate to a small portion of its length, the flow resistance of the gate to molten metal being fed therehrough can be maintained low in comparison to the flow resistance of a gate of the same length with a uniform cross section equal to that of the maximum restriction. This lower flow resistance serves the desirable purpose of reducing the period of time required for filling cavities 16.

When the cavities 16 and the gates 20 have been filled with molten metal from the reservoir 22, the induction coil 32 is deenergized to permit cooling of the mold assembly and the metal therein. When suflicient cooling has occurred, the molten metal freezes, or solidifies, thus forming cast parts of the desired shape in the cavities .16. This cooling action proceeds concurrently from the bottom of the assembly upward and from the outer periphery of the assembly radially inward. FIG. 3 illustrates with the dotted lines A, B, C and D successive positions that the freezing front passes through as the temperature drops. It will be noted from this figure that the freezing front passes upwardly and radially inwardly, passing through successive positions A, B, C, and D as the temperature drops.

To effect cooling in the illustrated manner, the mold assembly is positioned with its bottom portion contacting a base (not shown) that is cooled at a suitable rate, preferably from outside the evacuated chamber in which the mold is located. Alternatively, suitable thermal insulation can be provided about the jacket to control the cooling pattern. By making this insulation progressively more effective, proceeding from the bottom to the top of the mold assembly, cooling can be caused to follow approximately the pattern illustrated in FIG. 3.

By maintaining the assembly at a high temperature during the entire filling operation, I maintain the metal in each of the gates in a molten state until all the cavities being fed through it have filled. This insures that there will be no premature freezing in the gates that could interfere with filling cavities therethrough. By cooling in the general manner illustrated, I maintain a centrallylocated head of molten metal in communication with the still-molten metal in each cavity until substantially all the metal in the cavity freezes. This permits metal to be fed through the gates from points above the still-molten metal to maintain the cavities filled despite the tendency of the cooling metal to shrink at a greater rate than the graphite of the mold.

When the metal is copper, its contraction during cooling exceeds that of the graphite by a factor of about 2.5, its actual shrinkage relative to the graphite being almost 2%. This relative shrinkage causes the core 12 between a given pair of castings in adjacent cavity 16 to be tightly compressed between the cavities, while loading the metal in the connecting gate 20 in tension. To prevent these stresses from damaging the cores 12 or the cast parts in cavity 16, I allow plastic deformation of the metal in the gates to occur before these stresses can reach damaging levels. The cross section of the gates 20 is made small enough to permit such plastic deformation to occur before damaging stresses develop. Where a restriction such as 40 is provided in the gate, this deformation will tend to concentrate in the region of the restriction.

After the cast parts have been removed from the molds, the metal that was in the gates 20 is suitably detached from the cast parts and discarded. Any plastic deformation or shrinkage defects which may have occurred in the gates do not affect the quality of the cast parts, in view of this detachment of the gate metal from the cast parts.

The small cross section of the gate also serves in an important manner to provide for removal of the cast parts from the mold without damage to the mold or the cast parts. Such removal is effected after the casting op eration by first unscrewing the jacket 24 and then separating it from the mold assembly. This frees the bottom, or first, core 12 for removal. After the first core 12 is so removed, the second core 12 is suitably gripped at its outer periphery and a downward separating force is applied thereto. This fractures the metal in the lowermost gate 20 and separates the second core 12 and the casting in the lowermost cavity 16 from the other castings. This procedure is repeated, successively freeing each newly exposed core and casting from the assembly until finally all of the castings have been removed from the assembly. If desired, each newly exposed casting can be freed from the assembly by applying a separating force directly to it instead of applying the force through the core 12 just above it. If the separating force is applied directly to the casting, it can be either a twisting force or a force that loads the gate metal in tension, or a combination of these forces.

Whether this separating force is applied directly to the cast part or indirectly through the immediately adjacent core 12 or in some other way, the metal in the gate, being relatively weak due to its small cross section, fractures in response to the separating force before the stresses elsewhere produced by the separating force are high enough to damage either the casting or the core. The cross-sectional area of the gate is purposely made small enough to impart this amount of weakness to the metal in the gate.

Despite the small gate cross section, adequate flow is obtained through the gates during the previously-described casting operation because the vacuum environment eliminates any gas interference problems, as was pointed out hereinabove. As explained above, after the cast parts are removed from the mold, the gate metal that remains attached is suitably detached and discarded.

When the gate 20 is shaped in accordance with the preferred design of FIG. 1, i.e., with a zone 40 of maximum restriction, fracture in response to the above-described separating force occurs at the restriction. Since there are no additional restrictions in gate 20 on either side of restriction 40, it should be apparent that the gate walls do not interfere with release of the metal therein.

By way of example and not limitation, each of the cast parts in the illustrated apparatus has a diameter of about 2.7 inches and a minimum thickness of about inch. Each of the cores has an outside diameter of about 3.5 inches and a minimum thickness of about inch.

Since the above-described casting operation and the above-described removal operation can take place without damaging either the castings or the mold assembly, it should be apparent that my apparatus lends itself to the production of high quality castings and a high degree of mold reusability. The gates have a very small volume in comparison to that of the cavities, e.g., a few percent or less, and this contributes to a high casting yield with little scrap.,That is, only a few percent or less of the metal is scrapped when the gate metal is detached from the cast part and discarded after the cast parts are removed from the mold.

A further benefit of using the constricted gates is that, due to intimate contact between the graphite and the molten metal as the metal passes through the constrictions, a substantial percentage of the minute amount of oxygen remaining in the molten metal is extracted from the metal.

In the above-described process, I have melted the solid metal while it was present in reservoir 12. If desired, however, the metal can be melted elsewhere and then suitably poured as a liquid into the reservoir. In either case, the amount of metal placed in the reservoir preferably should substantially exceed that required to fill all the cavities 16 and gates 20. The excess metal is reusable as melt stock on the next casting operation.

Although the castings are shown as being of circular disk form, it is to be understood that other more complex shapes can be produced, if desired, by suitably shaping the recesses that form the cavities 16. It is to be understood, of course, that by varying the number of cores the mold assembly can easily be adapted to accommodate fewer or more castings. By producing a large number of similar parts in a single mold assembly which is of a highly compact design, I can reduce the space, time, and equipment required for a given casting.

Although each of the cores illustrated in FIG. 1 is of a one-piece construction, it is to be understood that the cores can, if desired, be of two or more pieces of suitable form. For example, in FIG. 4, I show each core constructed of two pieces, 12a and 12b, with mating parting surfaces 29. Each of the pieces 12a and 12b can, if desired, be considered as a core. The cast parts can be removed from a mold of this construction in substantially the same manner as described hereinabove with respect to the embodiment of FIG. 1.

Although I prefer to use only a single centrally-located gate 20 between adjacent cavities 16, it is to be understood that my invention, in its broader aspects, comprehends the use of more than one gate between adjacent cavities.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications may be 'made without departing from my invention in its broader aspects; and I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Permanent mold apparatus for vacuum-casting a plurality of metal parts comprising:

(a) a plurality of separable reusable cores disposed in stacked relationship,

(b) adjacent cores having recesses formed therein defining cavities between the cores having configurations conforming to those of the parts to be cast and of such shape that the cast part is freely removable from the cavity when the adjacent cores are separated,

(c) means including an opening into one of said cavities defining an entry region for molten metal for feeding the cavities,

(d) gates of small cross-section extending through said cores and providing passageways between said cavities through which molten metal is adapted to flow from said entry region into said cavities to successively fill the cavities, all of the molten metal fed to said cavities passing through said entry region and through those gates located between a given cavity and said entry region,

(e) means maintaining the interior of said mold under a vacuum while said cavities are being filled,

(f) each of said cast parts being separable from its adjacent cast part after a predetermined separating force is applied either to the cast part being separated or to a core between adjacent cast parts,

(g) said gates being sufiiciently small in cross-section that the solid metal in a gate adjacent a cast part being separated has a breaking strength low enough to fracture in response to said predetermined separating force before the stresses elsewhere produced by said separating force are high enough to damage either said cast part or said adjacent core,

(h) at least one of said gates has a constriction intermediate its longitudinally opposed ends, thereby providing at said constriction a relatively weak zone in the metal of said gate that is more susceptible than the rest of said gate metal to fracture by said separating force.

2. The apparatus of claim 1 in which said gates are generally centrally located with respect to said cavities.

3. The apparatus of claim 1 in combination with means for maintaining the metal in the gates at a high enough temperature during the mold-filling operation to remain molten until all the cavities being fed through a given gate have been filled.

4. The apparatus of claim 1 in combination with means for cooling the mold from the end remote from the entry region toward the entry region after said cavities have been filled with molten metal and for cooling the molten metal in accordance with a pattern that maintains a head of molten metal outside each cavity in communication with the still molten metal in said cavity until substantially all of the metal in said cavity freezes.

5. The apparatus of claim 1 in combination with:

(a) means including a removable jacket surrounding said cores for holding the cores in assembled relationship,

(b) said cores being individually separable from the remainder of the mold when said jacket is removed.

6. The apparatus of claim 1 in which the surfaces of of graphite.

, 7 8 said cores that are exposed to molten metal are made 2,804,664 9/1957 Brennan 164-65 2,940,142 6/ 1960 Wells et al 249-52 X References Cited 3,233,294 2/ 1966 Carpousis et a1. 249-126 X UNITED STATES PATENTS 0 FOREIGN PATENTS 241899 Harmon 16441 1,258,736 3/1961 France. 4/1951 Kroll 22200 544,269 4/ 1 932 Germanv. 8/1913 Mackenzg 2%90'1 12 795 1 /1885 Great Britain 11/1915 ijoolidge 22-204 11/1915 Coolidge 22-204 8/1925 U dale 249 126 X 10 I. SPENCER OVERHOLSER, Prlmary Examlner 11/ 1920 Frank 164-129 X V. K. RISING, Assistant Examiner 8/1938 Stoody 164-337 9/1941 Hansen 249-126 9/1951 Brunner 18-30 164-65, 69, 129, 131, 264; 249-52, 126

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