Metal founding – Process – Shaping a forming surface
Reexamination Certificate
2003-03-10
2004-08-24
Lin, Kuang Y. (Department: 1725)
Metal founding
Process
Shaping a forming surface
C164S519000
Reexamination Certificate
active
06779590
ABSTRACT:
TECHNICAL FIELD
The instant invention is directed to methods for making molding casts by a process typically referred to as “investment casting” or “lost wax casting.” More specifically, the instant invention is directed to methods of using gas permeable phosphate bonded investment compositions that can be burned out rapidly, without fracturing, to produce strong defect-free molds.
BACKGROUND ART
“Investment casting” or “lost wax casting” is a process employed in a number of industries to make small parts out of metal, glass, porcelain and other ceramic materials. For example, the process is heavily employed in the dental industry to make dental prostheses and is utilized in the jewelry and gun industries to make small metal parts.
The first step in investment casting is to create a wax model of an object to be reproduced. This model is affixed to a wax pin called a “sprue pin” which, in turn, is positioned on a base generally made of rubber or plastic. A cylindrical ring of metal, plastic, or paper is then placed on the base so that a cylindrical cavity is created with the wax model positioned in the center of the cavity.
In the next step, investment powder, which consists of refractory fillers and a self-hardening refractory binder, is mixed with a liquid, commonly water or a silica sol, to produce a thick slurry. The investment slurry is poured into the cavity formed by the cylinder and base and over the wax model and sprue pin. The cylindrical cavity is completely filled with investment slurry so the model and sprue pin are completely covered. The investment slurry is then given time to “set” (harden) and dry. Drying can be accelerated by forced air and other techniques.
After the investment has set and dried, the base is removed and the mold (at this point consisting of the wax model, sprue pin, and overlying investment) is placed in a “burn out” furnace. The burn out furnace fires the mold at very high temperatures. The intense heat causes the wax model and sprue pin to melt and burn—hence the name “burn out.” In addition the heat removes water and other volatile components. Finally, the heat chemically alters the investment material. Obviously, the type of chemical alteration that takes place will depend on the type of investment material utilized.
The mold is then removed from the furnace. The end result is a mold with a cavity that can be used to reproduce a desired object from metal, glass, porcelain, and other ceramics. To reproduce an object, a molding material (metal, glass, porcelain, ceramic) is heated until fluid and packed into the cavity. Then the mold is allowed to cool. When this has been accomplished, the mold is then broken and the casting is removed and trimmed. In the dental industry, when making prostheses, the mold material is removed by sandblasting the mold with glass beads. However, other industries use other methods—such as dropping the mold in water, creating a heat differential between the outside and inside surfaces of the mold that cracks the mold open.
There are many types of investments used in the art. Phosphate bonded investments are one type of investment material. Phosphate bonded investments are discussed extensively in Ralph W. Phillips,
Skinner's Science of Dental Materials
, 8th ed., pp. 406-410, W. B Sander's Co., Philadelphia, 1982.
One of the principle drawbacks to “investment” casting is the amount of time it takes to burn out the mold. This time is due to limitations on the rate at which the temperature can be raised during burn out.
If heat is applied too quickly, a rapid ejection of steam and other volatiles cause the walls of the mold to flake. In addition, if heating is too rapid, radial cracks will develop in the mold due to the resulting heat differential. In fact, if heating is rapid enough, the mold may actually explode.
In order to avoid the aforementioned problems, the temperature of the burn out oven is raised very slowly until the final firing temperature is reached. For molds made from conventional investment materials, the heating cycle is at least one hour. However, burn out can last as long as four hours. Moreover, the burn out furnace must be cooled between runs, further decreasing throughput.
A few attempts have been made to create investments that can be heated faster without the formation of flakes or cracks. These attempts always involve the use of additives to increase the porosity in the material. Porosity increases gas permeability, enabling steam and other volatiles to more easily escape from the material during the heating step without causing damage. However, porosity also decreases the smoothness of the mold cavity walls which may cause inclusions to form in materials molded therefrom. In addition, the inclusion of additives increases the complexity of the composition and, thereby, increases the possibility of other deleterious effects.
For example, U.S. Pat. No. 4,604,142 describes the use of starch as an additive to a gypsum-bonded investment to create, among other things “good air permeability.” Similarly, U.S. Pat. No. 4,814,011 describes the use of starch as an additive to phosphate-bonded and gypsum-bonded investments that contain a wide variety of refractory fillers to provide “good air permeability.” In both patents, the permeability of the investment is controlled by the use of additives and not by a judicious control of the composition of the basic investment formulation.
In addition, U.S. Pat. No. 5,373,891 describes the use of gas permeability improving additives to provide gypsum-bonded investments that can be burned out in short periods of time, thereby, producing shorter production cycle times. Once again, the permeability of the investment is controlled by the use of additives and not by control of the composition of the basic investment formulation.
SUMMARY OF THE INVENTION
The instant invention is directed to new phosphate bonded investment casting materials. The most outstanding feature of the new investments is the short mold fabrication and processing time that they allow.
For instance, some investments must be thoroughly dried before they can be burned out. In contrast, molds made from the instant investments can be placed in a furnace as soon as the investment has set. Although the molds are quite wet at this point, there are no adverse effects. This eliminates much of the processing time required by other products.
More importantly, most investments must be heated very slowly to prevent damage from volatile formation and temperature differentials. This means that the burn out oven must be cooled between each run. The slow heating ramps, and intervening cool down ramps, are very time consuming. In contrast, the new investments are sufficiently gas permeable to enable rapid heating. In fact, heating is virtually instantaneous after setting since the new investments can be placed directly into a preheated burn out furnace that is maintained at a constant elevated temperature. This completely eliminates the need for slow heating ramps and intervening cool down cycles. As a result, the burn out process is greatly accelerated. Conventional investment casting takes as long as four hours whereas casting the new investments takes less than an hour. The new investments decrease processing time at least three fold.
The new investments are well suited to the production of rush or emergency castings as well as routine production. Although the investment materials eliminate much of the time required in investment casting, they still produce smooth, defect-fee casts that are free of hairline fractures and other common deformities. The new investments produce molds that are robust enough to be used in the fabrication of dental porcelain restorations and soft enough that pressed porcelain objects may be removed from the molds without undue difficulty by sandblasting with glass beads.
The inventive phosphate bonded investments do not require the addition of modifying agents that might add undesirable properties. Instead, the invention utilizes conventional ingredients, namely, silica
Chadwick Thomas C.
Pineda Rachel R.
Den-Mat Corporation
Dorsey & Whitney LLP
Lin Kuang Y.
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