Investment casting by the lost wax process can be traced to ancient Egypt and China. The process as practiced today, however, is a relatively new technology dating to the 1930's and represents a rapidly growing business and science. Investment casting technology simplifies production of complex metal shapes by casting molten metal into expendable ceramic shell molds formed around disposable wax patterns which duplicate the desired metal shape. "Precision Investment Casting", i.e., PIC, is the term in the art that refers to this technology.
The conventional PIC process employs six major steps:
(1) Pattern Preparation
A disposable positive pattern of the desired metal casting is made from a thermoplastic material such as wax that will melt, vaporize or burn completely so as not to leave contaminating residues in the de-waxed ceramic shell mold. The positive pattern is prepared by injecting the thermoplastic material into a negative, segmented, metal die or "tool" designed to produce patterns of the shape, dimension and surface finish required for the metal casting. Single or multiple patterns can be assembled by fusing them to a disposable wax "sprue system" that feeds molten metal to fill the shell mold;
(2) Shell Mold Construction by
(a) dipping the pattern assembly into a refractory slurry having fine particulate refractory grain in an aqueous solution of alkali stabilized colloidal silica binder to define a coating of refractory material on the pattern; PA1 (b) contacting the refractory coating with coarse dry particulate refractory grain or "stucco" to define a stucco coating, and PA1 (c) air drying to define a "green" air dried insoluble bonded coating. These process steps can be repeated to build by successive coats a "green", air dried shell mold of the desired thickness. PA1 1) nitric acid acidified sodium silicate slurry to gel set an alkaline sodium silicate slurry, PA1 2) a phosphoric acid acidified potassium silicate slurry system to gel set any of: PA1 3) an acidic ethyl silicate slurry to gel set any of: PA1 1) a sodium stabilized negative sol colloidal silica binder slurry, and PA1 2) an alkaline ionic silicate slurry binder system.
(3) Dewaxing
The disposable wax pattern is removed from the "green" air dried shell mold by steam autoclaving, plunging the green shell mold into a flash de-waxing furnace heated to 1000.degree. F.-1900.degree. F., or by any other method which rapidly heats and liquefies the wax so that excessive pressure build-up does not crack the shell mold.
(4) Furnacing
The de-waxed shell mold is heated at about 1600.degree. F.-2000.degree. F. to remove volatile residues and form stable ceramic bonds in the shell mold.
(5) Pouring
The heated shell mold is recovered from the furnace and positioned to receive molten metal. The metal may be melted by gas, indirect arc, or induction heating. The molten metal may be cast in air or in a vacuum chamber. The molten metal may be poured statically or centrifugally, and from a ladle or a direct melting crucible. The molten metal is cooled to produce a solidified metal casting in the mold.
(6) Casting Recovery
The shell molds having solidified metal castings therein are broken apart and the metal castings are separated from the ceramic shell material. The castings can be separated from the sprue system by sawing or cutting with abrasive disks. The castings can be cleaned by tumbling, shot or grit blasting.
Binders used in the refractory slurries affect the shell building process and ultimate shell mold quality. Binders should be chemically stable to ensure long service from a refractory slurry used for repetitive dip coats. Binders also should form insoluble bonds with the refractory grains during air drying to permit redipping of the pattern as well as to permit removal of the pattern during furnacing. The stabilized ceramic bonds produced in the shell during furnacing mold must also have adequate fired strength and refractoriness so as to withstand casting of molten metal.
Standard refractory slurry binders which have been employed in manufacture of ceramic shell molds include hydrolyzed ethyl silicates and small particle size sodium stabilized colloidal silicas having an average particle size of about 8-14 nanometer. The latter includes alkaline aqueous dispersions of colloidal silica stabilized with sodium hydroxide which are non-flammable and have low toxicity. The former is acid stabilized with sulfuric or hydrochloric acid added during hydrolysis to form colloidal silica in situ. The former, however, employs flammable, toxic alcohol solutions to maintain solubility. The ethyl silicate binders, however, permit faster drying and use lower levels of flux promoting sodium oxide.
In the conventional process for making ceramic shell molds, the interval required for drying between coats may vary from 30 minutes for refractory prime coats to 8 hours or more for back-up coats depending on mold complexity and shell wall thickness. Completed shell molds are usually air dried an additional 24 hours or more to assure adequate green strength for pattern removal. This dependence on air drying for shell mold quality accounts for a major portion of production time, contributes to high production costs and is a serious shortcoming.
Because of this shortcoming, numerous efforts have been made to shorten or eliminate the time interval required for drying between coats by using chemical methods to rapidly set the refractory slurry binder. These chemical methods have broadened the choice of refractory slurry binder candidates beyond hydrolyzed ethyl silicate and sodium stabilized colloidal silica to include ionic alkali metal silicates, and acid stable alumina modified colloidal silica. These prior art chemical methods include:
(1) Use of a Gaseous Gelling Agent to Gel Set a Slurry Binder System
U.S. Pat. No. 2,829,060 teaches the use of carbon dioxide to gel set an ammonia modified sodium silicate slurry binder system.
W. Jones, in a technical paper presented to the Investment Casting Institute in October of 1979, disclosed the use of carbon dioxide or acidic alumina solutions to set alkaline silicate binder slurries. Alkaline silicate binder slurries, however, can cause undesirable fluxing at high temperatures.
U.S. Pat. No. 3,455,368 teaches the use of ammonia gas to gel set a hydrolyzed ethyl silicate or acidified colloidal silica binder system. Ammonia gas, however, is toxic.
U.S. Pat. No. 3,396,775 teaches the use of volatile organic gases to gel set a hydrolyzed ethyl silicate slurry binder system. Volatile organic gases, however, present a ventilation problem that contributes to poor acceptance in the foundry.
(2) Use of Two Interacting Slurry Binder Systems to Gel Set One Another When Applied as Alternating Coats
U.S. Pat. No. 2,806,270 teaches the use of:
(a) an alkaline potassium silicate slurry, PA2 (b) an alkaline piperidine modified ethyl silicate slurry, and PA2 (c) an alkaline mono-ethanolamine modified ethyl silicate slurry system; PA2 (a) an alkaline potassium silicate slurry, PA2 (b) an alkaline piperidine modified ethyl silicate slurry, and PA2 (c) an alkaline mono-ethanolamine modified ethyl silicate binder system.
U.S. Pat. No. 3,751,276 and U.S. Pat. No. 3,878,034 teach the use of an acid stable alumina modified colloidal silica slurry binder system to gel set either an alkali stable ionic silicate binder slurry system or an alkali stabilized colloidal silica binder slurry system. The use of two interacting slurry binder systems, however, requires a change in conventional shell making procedure.
(3) Use of a Chemically Treated Stucco Grain to Gel Set a Binder Slurry System
Dootz, Craig and Payton in Journal Prosthetic Dentistry Vol. 17, No. 5, pages 464-471, May 1967 describe the use of monoammonium phosphate and magnesium oxide treated stucco to gel a sodium silicate binder slurry system. This approach, however, suffers the disadvantage that its effectiveness degrades over time and can contaminate the refractory binder slurry.
(4) Use of a Gelling Agent Solution to Gel Set a Binder Slurry System
U.S. Pat. No. 3,748,157 teaches the use of a basic aluminum salt setting agent solution to gel set
Although these methods of the art have varying degrees of usefulness in preparing ceramic shell molds for use in PIC, they nevertheless require multiple catalyzation steps or substantial time intervals between successive coatings of refractory slurry materials. A need therefore exists for materials and methods which rapidly form ceramic shell molds.