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Investment casting mold base
An investment casting mold base has an enlarged, ellipsoidal-shaped body overlying a frusto conical-shaped body on the base. The enlarged body will form a molten metal staging cavity between a funnel cavity at the entrance of the mold formed by the frusto conical body and sprue channels leading to the article cavity in the mold. The staging cavity eliminates backup of molten metal at the mold entrance and possible spillover of the metal from the crucible mated to the mold during the centrifugal casting operation.
Metallic articles requiring detailed precision have been manufactured for many years by what was historically called the "lost wax" process. In more recent years, the process has become known as investment casting. The steps of the process may be divided into the two successive phases of: (I) investing, and (II) casting.
The basic steps occurring in the initial investing phase are: (a) creating a wax or plastic model of the article to be cast, (b) mounting the model and sprue forming rods made from the same or similar material on a rubber base, (c) placing a steel flask on the base around the mounted model, and (d) pouring suitable investment plaster into the container formed by the base and flask and allowing the investment to harden for five to ten minutes.
The final casting phase covers the steps of: (e) removing the rubber base from the flask after the investment has hardened sufficiently, (f) heating the flask containing the investment to between 900 and 1200 degrees F. to melt out the wax or plastic model and sprue rods, leaving an investment mold having negative voids or hollow channels and cavities in their place, (g) casting the desired article by (1) pouring, (2) pouring with vacuum assistance, or (3) centrifugally forcing, molten metal from a crucible into the investment mold cavities, wherein the metal cools and hardens to form the desired article, (h) destroying the investment mold in order to retrieve the formed article therefrom, and (i) finishing the article by machining off the connection with the metal of the sprue channels.
In step (j), casting the article by either pouring, with or without vacuum, molten metal from the crucible into the investment mold cavities, the speed at which the metal moves into the mold can be controlled. Consequently, if one finds that the metal being poured is overflowing the capacity of the sprue channels to feed the mold cavities, the rate of pouring can be decreased.
However, in casting molten metal by centrifugally forcing it into the mold cavities, one cannot control the speed at which it moves into the mold. Due to this lack of control, spillover of the molten metal out through the open top end of the crucible adjacent the mold frequently occurs. This is hazardous in view of the high temperatures involved and also wasteful and expensive in view of the high cost of metals, such as titanium, gold, or silver, commonly being cast. Also, some metal cools at the entrance to the mold, blocking the passage of sufficient molten metal into the mold to complete the casting of the article.
Common practice to overcome this spillover problem has been to increase the feeding capacity to the mold cavities such as by increasing the number of sprue channels or, alternatively, making them wider. However, this practice has several drawbacks. First, the machining time required to finish the final article is increased due to the greater surface area of contact between the metal residue of the sprue channels and the article. Second, a greater expenditure of material and energy results since it takes more wax material to form the sprue channels now greater in number or size than before, it takes more heat energy to melt out this increased quantity of material, and it requires a greater quantity of metal during the casting step to ensure completion of the casting product. Therefore, it is readily apparent that the above-described common practice has failed to provide a satisfactory solution to the metal spillover problem arising during the centrifugal investment casting process.
