US2880486A - Method of making investment castings - Google Patents

Description

April 7, 1959 E. c. WALLACE 2,880,486

METHOD OF MAKING INVESTMENT CASTINGS Filed May 28, 1956 2 Sheets-Sheet 1 F v 2 j 15 INVENTOR a. M

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Apnl 7, 1959 E. c. WALLACE METHOD OF MAKING INVESTMENT CASTINGS 2 Sheets-Sheet 2 Filed May 28, 1956 INVENTOR ATTYS.

United States Patent METHOD OF MAKING INVESTMENT CASTINGS Edgar C. Wallace, Watertown, N.Y. Application May 28, 1956, Serial No. 587,808

6 Claims. (Cl. 22-196) This invention relates to the production of precision metal castings and more particularly to improvements in the lost-wax process.

An important object of this invention is to provide an improved process for making metal castings by which castings can be produced having close dimensional tolerances and smooth surfaces.

A more particular object of this invention is to provide an improved method of making metal castings by the lost-wax process in which shrinkage of the castings at the mold walls is minimized.

These, together with various ancillary objects and advantages of this invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description when taken in connection with the accompanying drawings wherein:

Figure 1 is a side elevational view of a master pattern of the part to be reproduced;

Fig. 2 is a sectional view through a master mold and illustrating the step of forming the master mold from the master pattern;

Fig. 3 is a sectional view through the master mold and illustrating the formation of a destructible pattern from the master mold;

Fig. 4 is a diagrammatic sectional view through a plating chamber for applying a thin metallic coating to the destructible pattern;

Fig. 5 is a sectional view through a mold flask and illustrating the destructible pattern encased in an investment to form a refractory mold;

Fig. 6 is a sectional view through the flask and refractory mold and illustrating the step of heating the refractory mold to bake and remove the destructible pattern therefrom;

Fig. 7 is a sectional view through the refractory mold after the melt has been poured into the mold; and

Fig. 8 is an enlarged fragmentary view through the casting.

The so-called lost-Wax process is utilized to produce castings of high precision and is particularly adapted for the production of small parts which, because of their intricate shape or of the hardness of the material used, would require expensive machining operations to produce. In the lost-wax process, a destructible pattern corresponding to the article to be cast is first formed, then coated with an investment to form a refractory mold, after which the pattern is removed by heating and baking and the melt then poured into the refractory mold to form the finished casting. Since the primary purpose of employing the lost-wax process is to eliminate or minimize the amount of machining'required in the completed casting, it is a desideratum to minimize the shrink age of the casting in the mold and to provide a smooth surface on the casting which requires little if any further machining.

In quantity production of a specific'article by the lostwax process, it is customary to prepare a master mold of the article from which the destruptible patterns are formed. In Fig. 1 there is illustrated a somewhat simple shaped master mold pattern of an article which is adapted to be reproduced by the lost-wax process. As illustrated, the master pattern includes a body 10 which has a cylindrically shaped upper end and a frusto-conical lower end. A re-entrant bore 11 extends into the lower end of the body and grooves 12 are formed in the frustoconical portion and intersect the bore.

The master mold is formed in a conventional manner of a suitable material such as metal or of wood which is adapted for repetitive use in the molding of the destructible patterns. As diagrammatically illustrated in Fig. 2, the master mold is formed of metal and comprises a plurality of mold sections, such as 13a, 13b and arranged to enable removal of the destructible pat terns 16 which are thereafter formed in the master mold. When necessary in the formation of complex castings, it is apparent that a plurality of different master molds may be provided to form various parts of the complete destructible pattern, and the separate pattern parts thereafter assembled. Since the method of the present invention minimizes shrinkage of the final metal casting, the master pattern is formed more nearly to the exact size of the article to be cast than the oversize master molds used in the conventional lost-wax process. A pour opening 15 is also formed in the master mold to enable injection of the material which forms the destructible pattern thereinto.

In the lost-wax process the destructible pattern may be formed of wax or any other suitable material such as fusible plastics which can be melted and completely burned out at relatively low temperatures to remove the destructible pattern from the investments surrounding it. The destructible pattern, when removed from the master mold, has the same shape as the master pattern 10 from which the mold was made and, as shown in Fig. 3, comprises a cylindrical body 16 having a frusto-conical lower end portion, a re-entrant bore 17 extending inwardly from the lower end thereof and grooves 18 formed in the frusto-conical lower end portion. The destructible pattern also includes a sprue portion 19 formed in the pour opening 15.

In the conventional lost-wax process, the destructible patterns are directly coated with an investment to form a refractory mold which is non-wettable to the metal to be poured therein. Since the melt does not wet the refractory mold, appreciable shrinkage of the casting occurs at the mold walls. In order to improve the dimensional accuracy of the final casting, the master molds and the destructible patterns produced therefrom are lic coating which is wettable and solderable to the metal to be cast. Such a metallic lining has been found to minimize the shrinkage of the casting at the mold walls so that the master mold and the destructible patterns produced therefrom may be made to conform more nearly to the exact size of the article being cast and in some applications may be made on size.

errors caused by variations in shrinkage are also reduced.

In the conventional lost-wax process, the smooth ness of the surface of the casting is determined by the type of investment used. In accordance with the present invention, the metallic lining for the walls of the refracwry mold is first applied to the destructible pattern in Since the shrink. age of the metal casting is minimized, the dimensional.

such :a manner as to forma uniform metal casting on the pattern having a smooth outer surface. The refractory mold is then formed around the coated pattern and the pattern thereafter removed. When the melt is poured into the .lined mold,-itdoes not fill all the small interstices for-med in the somewhat-grainy investment of the refractory mold. Instead, the melt solders to the metallic lining to produce a casting having a smooth outer surface determined primarily by the smoothness of the outer surface of the metallic coating formed on the destructible pattern.

As it is essential that the metallic coating be wettable and solderable to the melt, the metallic coating is preferably applied directly to the pattern 16 so that the inner surface of the metallic coating, after the destructible pattern 16 is removed by heating and baking, is clean and freeefrom all foreign material which would inhibit wettability and solderability of the coating .orwhich would otherwise adversely affect the surface layer of the completed acasting. Since the destructible pattern is customarily formed of a non-conductive material such as wax or plastic, the metallic coating may conveniently be applied to the pattern by chemical plating or by vapor deposition. As shown in Fig. 4, the pattern 16 is mounted in a vacuum chamber 22 having a removable cover 23 to permit insertion and removal of the pattern. A high vacuum pump, diagrammatically indicated at 24-, is connected with the vacuum chamber as by a conduit 25 to substantially completely evacuate air from the chamber after placing the destructible pattern therein. A plurality of vaporizable metallic elements 2527 are disposed at appropriate locations in the vacuum chamber and arranged to be heated by any suitable means. As shown, these elements 2527 are heated by suitable electrical heating units 29 which extend through insulating bushings 28 in the vacuum chamber for connection to a suitable source of electrical energy (not shown). In the vapor vacuum plating process, the elements 25-27 begin to vaporize appreciably at temperatures below the melting points of the elements, by reason of the high vacuum in the chamber, and the evaporated metal is deposited in a substantially uniform layer on the destruc: tible pattern 16'. The vapor vacuum plating process has very good throwing power and consequently uniformly 1 coats the destructible pattern on all surfaces thereof including the bore 17 and grooves 18, and does not tend to form bridges and trees" as occur in electro-deposition of metals. The deposited layer 31 of metal on the destructible pattern is not subjected to any mechanical stresses during the subsequent steps of forming the refractory mold and removing the destructible pattern and may therefore be made relatively thin and preferably of the order of .0002", it being understood that the thickness of the coating may be varied as desired.

The destructible pattern 16 having the coating 31 thereon is then removed from the vacuum plating chamher and coated with an investment to form a refractory mold. As is conventional, the destructible pattern is first coated with a primary investment indicated at 33 in .Fig. 5 and thereafter positioned in a mold flask 32 which is filled with a secondary investment 34 around the investment coated pattern 16. The investment is allowed to set and solidify to form the refractory mold, after which the refractory mold is removed from the flask 32 and dried and baked in a suitable oven to harden the refractory mold and to melt and destroy the destructible pattern 16 and efiect removal of the latter from the mold. As previously described, the destructible pattern 16 is formed of a material such as wax or plastic which is arranged to be destroyed at a relatively low temperature to effect complete removal of the destructible pattern from the refractory mold, without damaging the thin metallic coating 31 so that, after the baking of the refractory mold, thecoating 31 remains in the refractory mold as a lining.

..The-metal from which the article isto-be cast is then ..2,sso,ese

heated to its pouring temperature and preferably maintained at as low a temperature as permissible consistent with proper pouring of the melt. The melt is poured into the refractory mold 34 to thereby form a casting 38. As previously described, the metallic coating 31 is wettable and solderable to the metal of the melt and hence materially reduces shrinkage of the casting at the mold walls. As is apparent, the coating may be formed of any material which is wettable and solderable to the melt and which, when present in the outer surface of the completed casting will not produce an undesirable alloy therewith. Thus, metals of the type described, which are different from the metal or alloy of the casting may be used when it is desired to provide a surface coating or surface alloy on the casting which is different from the casting. Preferably, however, the coating is formed of substantially the same metal as the melt or a metal closely akin thereto so as to provide a uniform casting. Since the metallic coating 31 is relatively thin, it blends and alloys with the base metal of the casting, as, diagrammatically shown in Fig. 8.

From the foregoing it is apparent that the shrinkage of the castings at the mold walls is reduced by the provision of a metallic lining which is wettable and solderable to the casting. Therefore, the errors caused by variations in shrinkage are also reduced so that cast articles may be produced having higher dimensional accuracy than heretofore obtainable by the lost-Wax process. Further, this metallic lining also tends to inhibit mixing of those materials in the refractory mold which melt at high temperatures, with the base metal of the casting.

It is also deemed apparent that the surface smoothness of the completed casting is improved by depositing the metallic coating on the destructible pattern in the form of a smooth outer surface which in turn forms the outer surface of the completed casting.

I claim:

1. The method of making precision investment castings comprising covering the surface of a destructible pattern with a thin metal coating, the major metal constituent of which is the same as the metal to be cast and is wettable and solderable thereto, encasing the covered pattern in an investment to produce a mold, removing the destructible pattern from the mold while allowing the metal coating to remain in the mold as a lining, pouring the metal to be cast into the mold, maintaining the mold lining sufficiently thin that the entire metal mold lining blends and solders to the poured metal to reduce shrinkage at the mold walls, and removing the mold from the casting to thereby provide a completed casting having a substantially uniform composition throughout.

2. The method of making precision investment castings comprising vacuum plating a destructible pattern with a metal having a major metal constituent the same as the metal to be cast and is wettable and solderable thereto to provide a thin metal coating on the pattern, encasing the metal covered pattern in an investment to produce a mold, removing the destructible pattern from the mold while allowing the metal coating to remain in the mold as a lining, pouring the metal to be cast into the mold, maintaining the mold lining sufficiently thin that the entire metal mold lining blends and solders to the poured metal to reduce shrinkage at the mold walls, and removing the mold from the casting to thereby provide a completed casting having a substantially uniform composition throughout.

3. The method of claim 2 wherein the thickness of the metal mold lining is about .0002 inch.

4. The method of making precision metal castings;

the application of the investment a thin coating of a metal having a major metal constituent the same as the metal to be cast and which is wettable. and solderable thereto,

5 and blend with the poured metal to reduce the shrinkage of the casting at the mold walls and provide a completed casting having a substantially uniform composition throughout.

5. The method of making precision metal castings according to the lost wax process which comprises vacuum plating directly onto the destructible pattern and before the application of investment a thin coating of a metal having a major metal constituent the same as the metal to be cast and which is wettable and solderable thereto, maintaining the coating sutficiently thin to entirely blend and alloy with the poured metal to reduce shrinkage of the casting at the mold walls and provide a completed casting having a substantially uniform composition throughout.

6. The method of making precision metal castings according to the lost wax process which comprises plating the destructible pattern before the application of investment with a metal coating of about .0002 inch in thickness and having a major metal constituent the same as the metal to be cast whereby to provide a coating which is wettable and solderable with the poured metal to reduce shrinkage of the casting at the mold walls and to provide a completed casting having a substantially uniform composition throughout.

References Cited in the file of this patent UNITED STATES PATENTS 2,205,854 Kroll June 25, 1940 2,510,735 Bodger June 6, 1950 2,644,208 Auphan July 7, 1953 FOREIGN PATENTS 945,912 France Dec. 6, 1948

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