This invention relates generally to investment casting, and more specifically relates to water disruptible cores useful in investment casting procedures, and to the method of preparation of such cores.
In the well-known procedures applicable to investment casting, a precise wax pattern is initially prepared corresponding to the metal article intended for ultimate manufacture. The wax pattern is thereupon repeatedly coated or "invested" with particulate ceramic material, so as to build up a ceramic shell mold of a desired thickness. Since the article to be manufactured often includes complex openings, cavities and the like, one or more ceramic cores are commonly disposed with respect to the wax pattern, so that when the wax is later removed from the shell mold, the cores remain in place, to form the appropriate cavities and the like, when the final metal casting is prepared.
Several techniques are known for removing the wax pattern from the thus formed shell mold. Among the most common and acceptable is the use of high pressure and high temperature steam, which is accomplished by subjecting the shell-encased wax pattern to autoclaving. Once the wax is thus removed, the shell mold is fired in a furnace to remove wax residue and moisture and then the shell mold carrying the core or cores is ready for use for casting of the metal article.
Upon the metal casting being completed, the ceramic core or cores must be removed from the cast article. In many instances this can be readily assured by forming the core from a material which is soluble in caustic alkali. However, this procedure cannot be used where one desires to cast metals or alloys such as aluminum, because in these instances the metal or alloys would themselves be chemically attacked by the caustic alkali. Accordingly, where casting of aluminum and similar metals and alloys are involved, it has been known in the prior art to form the ceramic cores of a material which is soluble, or at least includes sufficient soluble portions as to be thoroughly disruptible by water, so that after the casting is prepared, the core or cores remaining with the casting may be removed by use of water. Prior commercially available cores, of this type however, have not been capable of satisfactorily withstanding the autoclaving conditions which are discussed above, and which constitute an intermediate step to which the core is subjected prior to its use in casting.
Prior art disclosures of water soluble or disruptible cores of the general type discussed above, include, for example, U.S. Pat. No. 4,629,708. The core described in this patent is a refractory body based upon a water soluble salt and a calcium silicate, the latter being present in amount of at least 45% by weight. The water soluble salt and the calcium silicate have a specified particle size distribution, and the calcium silicate is preferably a wollastonite. The patent discloses that this core may be removed from the cast or molded article by dissolution in water. The said patent also states that one can enhance the resistance of bodies made according to same to adventitious water, e.g., during storage or transport, by treating the cores after forming with a waterproofing material material. The cores may thus be coated and slightly impregnated by dipping them in an unsaturated polyester resin, which may then be cured to give a surface coating providing increased strength and resistance to water attack. This coating is burned off either before or during the casting process.
U.S. Pat. No. 4,361,181 discloses a casting core produced from a granular based substance, in a sugar derivative as binding agent.
U.S. Pat. No. 3,407,864 discloses use of a fugitive core comprising at least one soluble metal halide salt, and up to about 10% be weight of at least one second constituent selected from the group consisting of borax, magnesium oxide and talc. The patent teaches that it is preferred, for economic reasons, to use sodium or potassium chloride as the metal halide salt. One example of a core made according to the patent contains 95 weight percent sodium chloride, 3 weight percent borax, 1 weight percent magnesium oxide, and 1 weight percent talc.
U.S. Pat. No. 3,645,491 discloses water-soluble cores for use in the casting of metals, which consist of a water-soluble salt and a synthetic resin. The soluble core may consist of a mixture of the water-soluble salt and a liquid resin binder such as to retain the salt grains in the agglomerate; or alternatively, the soluble core may consist of grains of a water-soluble salt, preferably of a controlled grain size which are coated with a synthetic resin and subsequently dried to a powder. One example of the core composition consists of granular sodium chloride, bonded together in an agglomerate by a synthetic resin of a phenol formaldehyde-furfuryl alcohol type.
U.S. Pat. No. 3,548,914 discloses soluble cores comprising a mixture of sodium chloride and sodium silicate, or sodium sulfate and sodium silicate.
U.S. Pat. No. 4,438,804 discloses a water-soluble core prepared from a mixture of sand, potassium carbonate as a first binder, and at least one of barium carbonate and alkali silicate as a second binder.
As indicated, the principal problem to which the present invention is addressed is imposed by the use of the high pressure steam and temperatures which occur during the autoclaving step which has been previously discussed. Steam temperatures during autoclaving are typically of the order of 340.degree. to 350.degree. F., accompanied by pressures of about 100 psi. While the prior art water soluble core structures, for example of the types above discussed, have in many instances performed successfully with respect to their ability to be dissolved or disrupted by water, the prior art known cores have not been able to successfully withstand the said autoclaving conditions. Very typically, when a prior art core is thus subjected to autoclaving, the water-soluble salts present in same are found to undergo leaching. This weakens, and indeed may begin to disrupt the entire core structure; and at a minimum, will produce a leached residue of salt on the surface of the core. When such a core is thereupon utilized in the subsequent molding operation, the presence of such salts at the core surface, produces an unacceptable casting.
Another difficulty found with certain of the prior art cores, arises from too high thermal expansion characteristics. This can occur, for example, where the sodium chloride content is too high. Lowering the sodium chloride content will reduce the coefficient of thermal expansion, but depending upon the replacement components, leachability may be lost in the fired core--which defeats the purpose of using same.
In accordance with the foregoing, it may be regarded as an object of the present invention to provide a nonferrous ceramic core for use in investment casting procedures, which is readily disruptible by water following the casting operation, but which, further, possesses appropriate characteristics as to enable the core to fully withstand the autoclaving conditions which are commonly incident to the investment casting procedures.
A further object of the invention, is to provide ceramic cores as aforementioned, which have low thermal expansion characteristics, and good leachability as will facilitate removal of the cores following their use in casting.