US3311956A - Casting process employing soluble cores - Google Patents

INVENTORS l MERLYN E. TOWNSEND FIG. 2

C/ A TTORNE;

April 4, 1967 TOWNSEND ET AL 3,311,956

' CASTING PROCESS EMPLOYING SOLUBLE CORES Filed May 24, 1965 2 Sheets-Sheet 2 FIG.5

INVENTORS hgERLYN amwygamo ue E a. GUE BY Do /$1.0 6. LA RuE ATTORNEY United States Patent 3,311,956 CASTING PROCESS EMPLOYING SOLUBLE CORES Merlyn E. Townsend, Concord, Eugene J. Guelda, Oakland, and Donald G. La Rue, Orinda, Califi, assignors to Kaiser Aluminum & Chemical Corporation, Oakland, Calif., a corporation of Delaware Filed May 24, 1965, Ser. No. 458,066 3 Claims. (Cl. 22-194) This invention relates to improvements in the casting of materials, such as metals. More particularly, it is concerned with providing improved disposable or readily removable cores for use in casting from such materials articles, portions of which can have hollow configurations, hereinafter called hollow articles, and with methods of making or preparing these cores for such use. This invention also relates to novel methods of casting while utilizing these improved cores.

Cores for use in casting or molding of hollow metal articles and the like such as cores of sand, plaster or other materials which have the characteristic of being easily disintegrated in order to facilitate their removal from final solidified castings have been and are currently being used, Although cores of this type can be utilized in the practice of sand casting and semi-permanent mold casting with some measure of success, these cores ordinarily lack the requisite strength to withstand the destructive forces to which molding materials are subjected in present day die casting practices. Metallic cores are also available and, While these metallic cores may possess the requiste strength to Withstand the present day high molding pressures of die casting, problems are involved in readily removing these cores from the finished article. As a consequence, metallic cores are ordinarily limited in their usage to cores of simple configurations that have relatively few or no undercuts or backdrafts and which can be readily removed as complete units from the finished castmg.

Disposable or removable cores made of water-soluble salts, such as sodium chloride, have been previously proposed for use in casting practices and these cores can take relatively complex shapes or configurations. Although in instances where a particular casting application requires low molding pressures, these particular salt cores can be used; in the case of high pressure molding processes, however, such as die casting of metals, wherein a given core which may also be of a rather complex shape must be capable of withstanding high compressive forces throughout, these particular salt cores cannot be used because, among other considerations, they lack the strength to withstand the unbalanced forces to which they are subjected. The term core as used herein is meant to encompass either an insert in a mold that shapes the interior of an article being cast therein or an insert in a mold that shapes the exterior of the article being cast therein or various combinations of both. In other words, the term core simply means a mold insert used in the formation of either external or internal surface portions of an article.

Accordingly, the instant invention is concerned with providing a unique disposable core which has the requisite strength properties to withstand the high compressive forces which are experienced in present day die casting practices together with a unique method for making these disposable cores whereby these cores, even when provided with an intricate or complex configuration, will still be able to withstand the high casting or molding pressures to which they are subjected. The instant invention is also concerned with providing a novel method of casting while employing the unique cores of the instant invention.

The material of the cores of the invention possesses certain advantageous physical properties hereinafter discussed in detail which enhance its ability to be fused and cast into cores of complex and irregular configurations and shapes.

These and other purposes and advantages of the instant invention will become more apparent from a review of the ensuing detailed description taken in conjunction with the appended drawings wherein:

FIGURE 1 is a schematic diagram showing the interrelation of the several processing steps employed during the manufacturing of a hollow die cast article and illustrates a preferred series of process steps for preforming the disposable core element in conjunction with a series of process steps for forming an improved die cast article or the like by use of the aforesaid preformed core element;

FIGURE 2 is a perspective view of a representative disposable core element that can be produced by the series of manipulative steps set forth in FIG. 1;

FIGURE 3 is a perspective view of an improved hollow article manufactured by use of certain of the manipulative steps illustrated in FIG. 1

FIGURE 4 is a sectional view generally taken along line 4-4 of FIG. 3 and discloses the disposition of the core element in the hollow interior of the article being produced prior to dissolution and removal of the core element therefrom; and

FIGURE 5 is a perspective view of a further design for a disposable core element which can be formed by certain of the process steps illustrated in FIG. 1.

In the casting art, there has been a real need for a disposable or readily removable core which does not possess the serious shortcomings and handicaps of prior art cores; i.e., a disposable core which among other things has minimal shrinkage characteristics, can be easily removed from the finished casting and yet have sufficient strength to withstand the conditions to which it would be subjected in present day molding operations. The essential properties required in such an ideal disposable core may be summarized as follows:

(1) The core material should have minimal shrinkage as it passes from a liquid to a solid state or condition.

(2) The core should possess sufiicient strength to withstand all normal conditions to which it is subjected in the casting operation.

(3) The core should be capable of being formed so as to be an accurate reproduction of the core mold cavity and have good surface definition and dimensional accuracy so that even very complex castings can be made While using the same.

(4) The core should be capable of being initially solidified from its molten condition in molds made of any one of a number of conventional mold materials, such as sand, metal, plaster, etc.

(5) The core material after use should be readily recoverable by simple economical procedures and be reusable indefinitely, in other words, the core material should be readily dissolvable in and separable from and unreactive with water or an aqueous solution.

(6) The core should have a melting point which is higher than the melting point of the material which is to be finally cast or molded around the core so as not to melt, abrade, or distort during the casting or molding operation.

(7) The core should have a higher coefiicient of thermal expansion than the material being cast around it thereby eliminating the inducement of deleterious stresses in the cast material as it and the heated core contract during the cooling of the casting.

(8) The core, when solidified, should present a substantially smooth non-porous surface or surfaces to the material being cast therearound whereby the final cast product will likewise have a correspondingly smooth surface or surfaces which require little if any, machining or grinding.

(9) The core should have relatively low thermal conductivity so as not to extract heat too rapidly from the molten material being cast around it.

(10) The core should be unreactive with the material being cast around it.

(11) The core should not undergo reaction or decomposition under normal conditions of use.

(12) The core material should be essentially nontoxic so as to be capable of use with a minimum amount of danger to the user.

(13) The core material should be relatively inexpensive and easily obtainable.

(14) Finally, the molten core material should have good pouring properties including fluidity so as to completely fill the core mold cavity.

It has been found that anhydrous calcium chloride meets the above severe requirements and exhibits all or substantially all of the properties hereinabove set forth. Of particular significance is the fact that anhydrous calcium chloride is characterized by having a cubical shrinkage of less than 1% as it solidifies from the molten to solid state. Within limits the anhydrous calcium chloride can be used in admixture with other materials without adversely affecting its minimal shrinkage characteristics. For example, it has been found that up to 10% by weight of potassium chloride can be mixed with the anhydrous calcium chloride and the resultant molten mixture will shrink less than as it solidifies.

In this connection, it is to be understood of course, that, although the amount of shrinkage that can be tolerated for any given article to be produced by use of the cores of the instant invention can vary over a substantial range, the use of anhydrous calcium chloride is still advantageous because of its above indicated significant minimal shrinkage characteristic. It is to be understood that the invention is not limited to C.P. (chemically pure) or U.S.P. (United States Pharmacopoeia) grades of calcium chloride. Small quantities of additives, impurities, and fillers can be combined with the anhydrous calcium chloride, either in solution or as a dispersion, so long as their presence does not adversely affect the ability of the anhydrous calicum chloride to meet the above outlined criteria. Examples of such impurities, additives, and fillers are sand or alumina which are of sufiiciently fine particle size and/or in a sufficiently small quantity so as to form a uniform dispersion in the core materials and at the same time not adversely affect the ability of the anhydrous calcium chloride to meet the above-outlined criteria. In this connection, it has been found that the amount and type of impurities normally present in commercially available calcium chloride (e.g., about 94-97% pure) do not adversely affect the ability of the anhydrous calcium chloride to meet the above-outlined criteria. Commercially available calcium chloridenormally contains up to 5% other alkali chlorides, such as sodium chloride. Thus, it is to be understood that, as used herein, the terms consisting essentially of means the calicum chloride may include other ingredients which do not materially affect the basic and novel characteristics of the core elements of the invention.

7 The anhydrous calcium chloride core elements of the instant invention are normally self-supporting but may have metal pins or the like incorporated therein to support and position the core elements in the die casting apparatus. The core elements can be cast around the pins as the cores are being formed of the anhydrous calcium chloride. Removal of the pins from the core element after the casting operation may advantageously expose more surface area of the core element to water than would otherwise be the case thereby resulting in a more rapid dissolution of the core. If desired, the pins may be left in place within the die casting and the core element dissolved from around them. Metal inserts of various configurations may be positioned in the die casting in the same manner.

Ordinarily, calcium chloride is a deliquescent material and readily absorbs water from its surroundings to form hydrous calcium chloride. When fully hydrated it has the chemical formula CaCl -GH O. Hydrated salts are inherently unstable and when heated liberate their water of hydration. Under the high temperature-high pressure conditions present in the die casting of metal, the liberated water from a core made of a hydrated salt will ordinarily be converted to steam under pressure which will force its way through the metal leaving blow holes therein as well as causing a serious safety problem during casting operations.

When die casting a metal, such as aluminum or aluminum alloys around a core of hydrous calcium chloride, an additional problem is normally presented. The water liberated as the salt is heated reacts with the aluminum or aluminum alloy to form aluminum oxide and to liberate hydrogen gas. Inasmuch as molten aluminum has a substantial affinity for hydrogen, the metal under some circumstances could tend to become gassy and castings of high porosity could result.

As mentioned above, hydrated salts are also inherently thermally unstable. On the other hand, because of the criteria listed above, the core material must be a thermally stable compound. Even though calcium chloride is deliquescent in nature, it has been found that under cer tain critical operating conditions, both in the core formation and in the casting operation, anhydrous calcium chloride is an excellent core material meeting substantially all of the above listed criteria.

In accordance with the present invention, a disposable core element is provided for casting consisting essentially of a fused cast body of anhydrous calcium chloride. As used herein, the term fuse means: blend, integrate, or to :blend by melting together. The present invention is also concerned with providing a process for making the disposable core element and involves the preferred steps of heating a preselected amount of calcium chloride to a temperature sufficient to convert it and to maintain it in the molten state and to ensure the removal of all water of hydration whereby a fused molten body of anhydrous calcium chloride is obtained. The anhydrous molten calcium chloride i then cast into the cavity of a suitable mold. This mold may be a heated mold which is at a second temperature selectively lower than the first mentioned temperature, for example, about 500 F., in the case of a metal mold. The cavity of the mold has an overall configuration corresponding to the desired final shape of the core element being fabricated. The core element is removed from the core mold as soon as solidification of the same has been substantially completed.

In order to retain the anhydrous condition of the core element, the instant invention contemplates that after removal of the core element from the core forming mold, it will be transferred to a suitable depository within which it can be maintained in a true anhydrous condition until needed in a casting operation. Such a depository can consist of a warming furnace wherein the core can be kept at a minimum temperature of 200 C. or 392 F. (the temperature at which hydrou calcium chloride loses all of its water of hydration) or the depository can comprise a sealed container purged of all water vapor. With configurations where the core element is restricted in its contraction within the mold, it is important that the fused cast core element be removed from the core mold after solidification temperature has been reached and as soon as solidification of the core is substantially complete. If the fused cast core element were permitted to remain in the core mold and to further cool therein after solidification, in the case of certain configurations, undesirable: stresses Would be induced in the core element as it contracted during cooling below the solidification tempera-- ture because of its high coefficient of thermal expansion relative to the core mold material. This would result in failure of the core element.

The more complex the core configuration desired, the higher the temperature to which the molten anhydrous calcium chloride should be heated so that it will complete ly fill all of the intricate interstices or recesses of the core mold cavity before solidifying whereby the resultant fused cast core element will have good dimensional accuracy and surface definition and amount to a truly accurate reproduction of the core mold cavity. On the other hand, the molten anhydrous calcium chloride should not be heated to such an extent that undesirable cubical shrinkage will result as the molten anhydrous calcium chloride cools down to the solidification temperature and in practice operating techniques can be readily worked out so as to simultaneously meet these two conflicting requirements.

The die casting apparatus, represented at 30 can be of conventional design. Thus it can, for example, be comprised of the usual frame which supports opposed die sections having contacting faces adapted to he forced together and into pressure engagement with each other and with an appropriate core element interposed therebetween. Each one of the die sections has a recess corresponding to part of the overall shape of the product to be cast. One or more communicative openings is provided in at least one of the die sections for injecting the molten metal under pressure into the die cavity defined by the interior faces of the die sections. In die casting aluminum and its alloys, the die sections are normally preheated to approximately 450 F.

As an illustrative example of the preparation of a disposable core element according to this invention, a commercially pure (9497%) grade of granulated calcium chloride containing up to other alkali chlorides such as sodium chloride was employed in the following manner: The melting furnace was heated to approximately 1600" F. while the core mold was heated to approximately 500 F. The calcium chloride used had an approximate melting point of 1422" F. The calcium chloride was heated to .a temperature sufficient to convert and maintain it in the molten state and to ensure the removal of all water of hydration. The resultant molten anhydrous calcium chloride was then poured into the preheated mold and allowed to set until substantially solidified. The fused cast body of anhydrous calcium chloride was then removed from the mold and immediately transferred to a holding furnace or oven where the core temperature was maintained at a minimum temperature of 200 C. or 392 F. The core element so produced was found to have good dimensional accuracy, surface definition, adequate strength and be an accurate reproduction of the core mold cavity.

The core was used in the sequence of processing steps shown in FIGURE 1 to produce the die cast article shown in FIGURE 3. The core element was preheated to approximately 500 F. and positioned in the die mold. Molten aluminum alloy at a temperature of approximately 1100 F. was then cast into the die mold around the fused cast core element so as to surround it. The aluminum alloy used consisted of 3-4% copper, 7.59% silicon, 0.7% iron, 0.5% manganese, 0.07% magnesium, 0.5% zinc, 0.3% nickel, others 0.3%, with the balance aluminum. The initial injection pressure for the molten metal was on the order of 4,000 pounds per square inch and the final injection pressure was on the order of 10,000 pound per square inch. The molten metal solidified in the die mold and, after solidification, the solidified cast metal body surrounding the fused cast core element was removed from the die,mold. The fused cast core element was then removed from within the solidified cast metal body by dissolution in warm water. The die cast article had the desired dimensional accuracy and smooth surfaces requiring virtually no machining or grinding.

Advantageous embodiments of the invention have been disclosed and described. Although in the foregoing description of the novel core elements of the invention, the metal employed in the casting examples has been aluminum and its alloys, it is to be understood that the invention is also applicable to the casting of other metals, such as magnesium, zinc, lead, tin, and their respective alloys. Additionally, it will be obvious that various modifications and alterations may be made in the invention without departing from the spirit and scope thereof, and it is not to be taken as limited except by the appended claims.

What is claimed is:

1. The process of casting metal comprising the steps of (a) heating calcium chloride to a tempenature suflicient to convert it to and maintain it in the molten state and to ensure the removal of all water of hydration whereby a fused molten body of anhydrous calcium chloride is obtained,

(b) casting the molten anhydrous calcium chloride into a cavity of a mold, the cavity having an overall configuration substantially corresponding to the desired shape of the core element to be made,

(c) solidifying the molten anhydrous calcium chloride in the cavity,

(d) removing the fused cast body consisting essentially of anhydrous calcium chloride from the mold as soon as solidification is substantially complete,

(e) transferring the fused cast body to a second mold so as to be positioned therein,

(f) casting molten metal into the second mold around the fused cast body so as to surround it,

(g) solidifying the molten metal in the second mold,

(h) removing the solidified cast metal body surrounding the fused cast body from the second mold, and

(i) dissolving the fused cast body from within the solidified cast metal body.

2. The process of claim 1 wherein the molten metal is injected under pressure into the second mold.

3. In the process of die casting metal around a disposable core element positioned in a mold the improvement comprising employing a disposable core element consisting essentially of a fused cast body of anhydrous calcium chloride.

References Cited by the Examiner UNITED STATES PATENTS 2,420,851 5/1947 Zahn et al. 22-195 2,671,936 3/1954 Sundwick 2.270 3,218,684 11/1965 Spink 2 2-196 X References Cited by the Applicant UNITED STATES PATENTS 1,523,519 1/1925 Gibons. 1,554,697 9/1925 Alden. 1,603,262 10/ 1926 Alden. 3,094,422 6/1963 Reinhold. 3,131,999 5/ 1964 Suzuki et al.

J. SPENCER OVER'HO'LSER, Primary Examiner. E. MAR, Assistant Examiner.