US2875485A - Precision casting mold and method of making the same - Google Patents

Description

March 31, 1959 H. SCHNEIDER PRECISION: CASTING MOLD AND METHOD? OF" MAKING-.THE SAME Filed Dec. 14., 1954 lllndllllld lfllllallirrflllllllllflllllllllllll x ,frail JNVENToR. Hons Schneider Ffa@ @Ml/nw MTV@ ATTORNEYS The pattern assembly as shown in Fig. 2 is ready for coating by immersing or spraying with a mixture consisting of a solid filler material and a suitable liquid binder. The binder serves'to provide a close refractory film and the refractory filler material should have the property of forming a very smooth and refractory surface in contact with the wax pattern so that the cast article will be as smooth as possible.

The pattern assembly is then placed on a molding ask base plate 14 provided with a hole or perforation 1S as shown in Figs. 2 and 3. A cylindrical flask'16 is then fitted over the whole pattern assembly and the joint between base plate 14 and the'flask 16 sealed with wax or by any other suitable sealing composition. A quick setting refractory mixture is thenpo-ured into the flask thus formed and permitted to stand until the mixture has suiciently solidified to form a plate 19. Thereafter the dry, pourable, backing material is filled into the ask. The backing material consists of a refractory composition, a binder, or bonding agent, and a carrier for the bonding agent. The refractory material may consist of a granular material such as molding sand or quartz. The binding material is composed of a dry, finely ground powder at room temperature which becomes an effective bonding agent at the temperatures used to bake the mold. The carrier for the bonding agent is a pulverulent substance having a large active surface and which will readily absorb the inorganic bonding agent. Advantageously, the pulverulent carrier substance is impregnated with a solution of the bonding agent or with a fluid medium containing the bonding agent in a fairly finely divided solid form and subsequently dried. The dried pulverulent mass is then mixed with the refractory component of the investment composition. An advantage of using such a carrier for the bonding agent is that it brings a very large total surface area of the granular refractory molding compound into contact with the bonding agent adhering tothe surface or pores of the pulverulent carrier.

The pattern assembly provided with the thin non-selfsupporting coating 17 is thus completely embedded in the backing material 18. The whole assembly is then placed on a vibration table, and vibrated sufficiently to insure that all surfaces of the coated pattern assembly are backed bythe backing material. Thereafter a cover 20 is formed by pouring the same refractory mixture as used for the plate 19 on to the surface of the backing material.

In the next operation, the assembly shown in Fig. 3 is transferred into a furnace and fired by heating at a temperature of approximately 80G-900 C. for 6-8 hours. This lfiring treatment causes the pattern material and the sprue material to melt. The wax or synthetic resin will partly flow out through the hole or perforation 15 or be burned. On baking the mold the bonding agent imparts a uniform cohesive strength to the investment. The investment compound having thus been consolidated has the property of protecting the thin mold layer effectively against destruction by the inowing metal.

Fig. 4 shows the fired mold ready for casting. Molten metal is poured through the ingate 21 While the mold is still hot. The ring of the mold transforms the backing material into a refractory body 22 to which the coating 17 adheres.

Various granular, dry, refractory materials such as molding sands or quartz may be utilized as the refractory constituent of the backing composition Without departing from the scope of the present invention.

The utilization of the dry pourable backing composition of the present invention entails a particularly careful bonding of the granular, refractory constituents of the backing up composition which on baking the mold im- Vately high temperature.

4 a high adhesivity or a great capacity of sticking to the granular refractory compound of the backing up composition. Particularly suitable are gel-like inorganic compounds pulverulent at room temperature and which become fusible at temperatures above 600 C. and preferable between 800 and l000 C. The backing up composition, having been bonded, is adapted to protect the thin mold layer reliably against destruction by the intlowing molten metal. l

Some examples of suitable inorganic bonding compounds are liquid dispersions of gelatinous bonding agents such as complex aluminium pyrophosphates, alkali-metal Zirconates, alkali-metal titanates, titanic acids, zirconic acids, or oxychlorides such as magnesium oxychlorides. These compounds are distinguished in that at room temperature they exhibit a gel-like structure in a dry condition. Other examples are solutions of borates, such as borax dissolved in water, phosphoric acid and boric acid. Solutions of phosphates such as primary sodium phosphate may also be used.

lVarious carriers for the inorganic bonding agents may be used in accordance with the present invention. Some examples of carriers which have large active surfaces and exhibit a high adhesivity or capacity of absorbing the inorganic bonding agents due to their porous constitution and their highly particulate surface are infusorial earth, magnesium silicates such as meerschaum, pumice, and asbestos. Other substances have a large active surface and the above properties may also be used as a carrier in accordance with the present invention. The above named compositions, however have been found to be particularly advantageous in the present invention.

When larger size patterns are to be invested'or backed it is also advantageous to admix with the backing up composition comprising a dry, pulverulent, refractory material, an inorganic bonding agent effective at ternperatures above 600 C. and a carrier for the inorganic bonding agent, a second bonding agent dry at room. temperature and becoming an effective bonding agent at temperatures below the baking temperatures of the mold, or below 600 C. Examples of such bonding'agents are nely ground synthetic resins, such as silicone resins, which become an effective bonding agent at a moder- This second bonding agent may be used when the rigidity of the raw unbaked molds do not resist the vibrations occurring during their transportation.

Example A plurality of wax patterns of a small flat bottomed hollow cylindrical article were assembled together with a sprue in the form of a tree-like structure which serve as a positive pattern for therproduction of the casting mold. The sprue was similarly -made of wax. The composite Vtree-like pattern thus constructed was then dipped into a liquid mold forming composition of a mixture prepared from finely comminuted quartz and partly hydrolized yethyl silicate dissolved in alcohol as is well known in the yart. The thin non-self-supporting film deposited on the pattern was about 0.5-1 mm. in thickness. This coating was then dried and the pattern placed with the sprue downward upon a plate perforated for the subsequent discharge of the molten wax pattern. A cylindrical molders flask made of heat-resisting sheet metal was then placed over the entire coated pattern and the joint between the flask and the perforated plate was then sealed olf with wax. A small amount of a liquid mixture of quartz sand and water' glass to which an acid, such as hydrochloric acid, had been added for the purpose of accelerating the hydrolysis, was then added to the ask. This fluid mixture was permitted to stand until solidified to form a cover plate surrounding the sprue. The molders flask was then filled up witha dry, pourabley backing up composition which was prepared asy follows: 600 gramsof primary sodium phosphate was dissolved in v2 liters of Water and the solution added to 2.4 kg. of comminuted infusorial earth. Infusorial earth consists of the minute fragments of the shells of silicious algae (diatomae). It is characterized by a very large active surface. The mixture of primary sodium phosphate and infusorial earth was thoroughly kneaded and dried overnight at about 120 C. The dried composition was then ground and particles above 40 microns removed therefrom by means of a ne mesh screen. 6 parts by weight of the pulverulent substance thus obtained were mixed with 94 parts by weight of quartz sand having a particle size between 0.5 and 1.0 mm., and the whole thoroughly commingled in a rotary drum. This dry, pourable backing up composition thus obtained, containing an inorganic bonding agent, was packed into the molders flask and iirmly settled therein on a shaking table. The top face of the molders tiask was then covered with a slurrylike mixture of quartz sand and water glass, to which an acid had been added to accelerate the hydrolysis, to form a second cover plate the same as that formed above. After the cover had hardened the molders flask was transferred into a baking furnace and gradually raised to a baking temperature of 800 C. during a period of 6-8 hours. During the baking operation the sodium phosphate deposited upon the infusorial earth fuses and in that state exerts the desired bonding action on the investment composition. If the wax pattern is to be recovered, the mold prior to being baked, is transferred into a drying oven in which the wax is caused to run out at a slightly raised temperature. Immediately upon removal of the molders ask from the baking furnace, liquid metal was poured in the mold. After cooling the mold, the two 'covers were broken up and the waste mold forming cornposition knocked out of the molders ask. The castings may be readily taken out of the backing up composition and freed from any mold forming composition still adhering thereto in any conventional manner. The finished castings exhibited a perfectly smooth surface free from blemishes and form a particularly true replica of the pattern parts.

In case of larger size molds, it is advantageous to use armoring members for the purpose of strengthening the investment compound. Casting molds produced in accordance with the process of the invention are adapted for static castings as well as for making castings on a centrifugal casting machine.

Iclaim:

1. The method of making a unitary precision casting mold by means of a pattern destroyable in the mold which comprises coating the pattern with at least one mold forming layer having the form of a thin non-selfsupporting coating, placing the pattern in a molders llask, backing up the coated pattern with an investment composition comprising a dry pourable composition containing a granular, refractory, molding composition, an inorganic bonding agent, and a carrier which is a member selected from the group consisting of infusorial earth, magnesium silicate, meerschaum, pumice, and asbestos impregnated with the bonding agent comprising a dry,

pulverulent composition having a large active and porous surface and a high capacity for absorbing inorganic bonding agents, the particle size of said refractory composition being substantially larger than the particle size of said impregnated carrier, and heating the mold to the baking temperature of the mold to destroy the mold pattern and to permit adherence of the pattern coating to the backing up composition, said carrier impregnated with said inorganic bonding agent being dry and pulverulent at room temperature and said bonding agent becoming an effective bonding agent at the temperature used to bake the mold.

2. The method of claim 1 in which the carrier is impregnated with a composition selected from a member of the group consisting of phosphates, phosphoric acid, borates, boric acid, complex aluminium pyrophosphates, zirconic acid, alkali-metal zirconates, titanic acid, and

alkali-metal titanates.

3. An investment back up composition for unitary precision casting molds in which the mold pattern is defined by at least one mold forming layer in the form of a thin non-self-supporting coating and in which the molds are formed by baking the coated pattern and back up composition comprising a dry, pulverulent, pourable composition containing a granular, refractory, molding composition and a carrier which is a member selected from the group consisting of infusorial earth, magnesium silicate, meerschaum, pumice, and asbestos impregnated with an inorganic bonding agent, said carrier comprising a dry, pulverulent composition having a large active a'nd porous surface and a high capacity for absorbing inorganic bonding agents, and said bonding agent being dry and pulverulent at room temperatures and becoming an effective bonding agent at the temperature used to bake the mold, said back up composition. comprising between about to 96 parts by weight of refractory material, between about 3 to 10 parts by weight carrier, and between about 1 to 5 parts by weight of inorganic binder on a dry basis.

4. The investment, back up composition of claim 3 in which the carrier composition is impregnated with a composition selected from the group consisting of phosphates, borates, phosphoric acid, complex aluminium pyrophosphates, zirconic acid, alkali-metal zirconates, titanic acid, and alkali-metal titanates.

References Cited in the tle of this patent UNITED STATES PATENTS 2,027,932 Ray Ian. 14, 1936 2,388,299 'I'hielemann Nov. 6, 1945 2,441,695 Feagin et al May 18, 1948 2,521,614 Valyi Sept. 5, 1950 2,682,692 Kohl July 6, 1954 2,720,687 Shaw Oct. 18, 1955 2,736,077 Bartlett Feb. 28, 1956 FOREIGN PATENTS 585,665 Great Britain Feb. 18, 1947 703,607 Great Britain Feb. 3, 1954

Images