US2229946A - Composition for molds for metal casting - Google Patents

Description

aiented Jan. 28, 1941 D UNITED STATES PATENT OFFICE 2,229,946 comrosrrron Fon Moons non METAL oas'rmo George Clyde Van Allen, Maplewood, N. 1., assignor to Baker & Compa corporation of New Je ny, Inc'., Newark, N. J., a my No Drawing. Application February 8, 1938,-

1 Serial No. .189,319

1 Claim.

This invention relates to investment compositions for the making of refractory molds for use in casting metals, alloys and other suitable castillg materials. The composition is particularly intended for use in molds for casting dental inlays, dentures and the like, although the comnet, for example calcined gypsum or plaster of Paris. Other ingredients or agents for the purpose of accelerating or retarding hardening, setting or expansion of the composition, or to modify the plasticityof the composition to facilitate mixing and shaping thereof, are also frequently included in such investment compositions.

The customary procedure in making molds of such compositions and casting articles therein,

consists in shaping a preliminary wax model or pattern of the object to be cast, for examplea *dental inlay, mixing the investment composition with a suitable liquid, for example water, constructing the mold around the pattern, allowing the mold to set, heating the set mold slowly to drive out the moisture and melt and burn out the wax pattern, and continuing heating until the mold is suillciently hot to compensate for the shrinkage of the cast material as well as to eliminate all wax and residue from same so as to leave a cavity corresponding in shape and size to the model, then pouring the molten casting material, for example gold, into the heated mold cavity and allowing the casting to cool.

Generally the molds are heated to a temperatureof from 900 F. to 1500 F. before the casting is formed, which causes the mold to expand, and

. the casting material after having been put in the mold cavity usually shrinks during cooling. Therefore, in order to obtain a casting which shall conform exactly in size and shape to the model, the expansion of the mold must correspond as exactly as possible'to the shrinkage of the casting material. Generally this expansion is slightly above 1%, for example in the case of casting gold, about 1.25%.

While such investment compositions comprising only aground siliceous refractory material, such as silica, and a binder such as plaster of Paris, have the property oi expansion under heat, the degree of expansion is not sufilcient to compensate entirely for the shrinkage of the casting 5 material. As an example, we find that a compo-- sition comprising seventy percent of silica and thirty percent (30%) of plaster of Paris has a maximum expansion of about eighty-six hundredths of one percent (0.860%) at a temperm ature of fifteen hundred (1500) degrees F. which is insuflicient to compensate for the average shrinkage of the casting material of from one (1) to one and one-quarter (1%) percent. A decrease of the silica content and a corresponding increase in the plaster of Paris content will result in a considerably smaller expansion of the composition during heating.

To increase the expansion of the investment composition, it is customary practice to include therein so-called expansion agents or expansion 20 promoters, such as oxalicacid or citric acid or sodium chloride.

Prime object of my invention is to provide an investment composition which shall include a new and improved combination of constituents or 25 ingredients one or more of which shall make it possible to obtain exactly the desired degree of expansion of the composition during heating thereof. 30

I have discovered that the desired expansion characteristics of investment compositions can be obtained, and the degree of expansion of such compositions can be increased substantially, by incorporating in such compositions small quan-- titles of one or more substances of the group consisting of ammonium chloride, aluminum chloride, ammonium bromide, and sodium nitrate.

As examples, sodium nitrate and ammonium bromide together may be added as expansion agents to the other constituents of the composition, or ammonium chloride and aluminum chloride together may be used as expansion agents;

and if desired all four of my enumerated expansion agents may be used together.

The addition of one or more of my expansion agents to an investment composition remarkably improves the expansion characteristics of composition and strikingly increases the degree of expansion. Whereas, as above pointed out a so Expansion agent 55 53 Sodium nitrate .7 1.035 Ammonium chloridm. 1. 040 Aluminum chloride. 1. 180 Ammonium bromide i 1. 205

The proportions of my expansion agents in the investment composition may be varied from one-quarter of one percent (0.25%) to about three percent (3%) by weight. The preferred range lies between three-quarters of one percent (0.75%) to two percent (2%), although the most satisfactory proportions with the proportions of the other ingredients now used, are from one percent (1%) to one and three-tenths percent (1.30%).

The proportion of the refractory base or filler, for example silica, may be varied widely, for example from forty percent to eighty-five percent (85%), while the proportion of the binder, for example calcined gypsum or Portland cement, may also vary within a wide range, for example from fifteen percent (15%) to sixty percent Preferably, in all cases, the materials comprise particles of different sizes.

Each of my expansion agents may be used alone, or in combination with. others of my expansion agents, or in combination with other expansion agents or promoters.

Also, my expansion agents may be utilized in investment compositions with ingredients or agents for accelerating or retarding Setting or hardening of the compositions, or for modifying plasticity or other qualities or characteristics of the compositions.

Having thus described my invention, what I claim is:

An investment composition comprising from forty percent (40%) to eighty percent of silica, from fifteen percent (15%) to sixty percent (60%) of plaster of Paris, and from one-quarter percent (0.25%) to three percent (3%) of ammonium bromide.

G. CLYDE VAN ALLEN.