Various methods of the Lost Wax Casting (LWC) process are employed in the casting of platinum with a varying degree of success. We shall concern ourselves with: "pouring" the Gravity Casting Method; "pulling" the Vacuum Assist Casting Method, "spinning" the Centrifugal Casting Method; and "pushing" the Pressure Casting Method.

"Pouring" or Gravity Casting

This method, employed for thousands of years, makes use of the basic principle of gravity, all objects fall towards the earth. Molten metal is either poured from a ladle or bottom poured from a special crucible with a stopper and a hole. This method makes use of the principle that objects with heavier specific gravity displace materials of lower specific gravity, hence heavier liquid (molten metal) displaces lighter gas.

However, only larger objects can be cast this way, as in industrial settings. Other disadvantages include the need for a large sprue reservoir and the contamination that exposure to oxygen can bring to the cooling metal.

"Pulling" or Vacuum Assisted Casting

This method relies on the use of a vacuum pump drawn on the refractory mold while molten metal is poured into it. The main advantage to the vacuum method is the removal of back pressure pockets of gas formations by the vacuum.

However, for the casting of platinum we are required to use phosphoric acid bonded refractories because of their ability to withstand greater temperatures than gypsum bonded products and for the strength of the refractory mold they provide. These investments are very poor at allowing a vacuum to be drawn through them and therefore impractical for use in the casting of platinum. Also, a large "feeder head" or button is required to preserve metal temperature and therefore extra metal is being wasted as it is also with centrifugal and gravity methods.

"Spinning" or Centrifugal Casting

Centrifugal casting uses the action of spinning to generate a force for the molten mass of metal entering the refractory mold. There are three methods of using centrifugal force: horizontal rotation, vertical rotation, and slanted rotation, each with its own characteristics.

Studies show that this creation of gas formations in centrifugal casting methods may be the most serious detriment to this kind of casting. Minimizing metal thrust to decrease gas formations causes miss fills and no compromise to the centrifugal speed can eliminate this problem.

"Pushing" or Pressure Casting

The methods of pressure casting are the least known of the various methods of LWC. Steam pressure is used inside this closed system to cast small quantities of metal. Pressure techniques are also used in the casting of steel, aluminum, and ceramics.  Simply put, in pressure casting a pressurized force (be it steam, compressed air or inert gas) pushes against the molten metal in order to fill the mold refractory.

Pressure is applied to the sprue reservoir and continues as the metal is cooling and hardening. Even with the surface apparently hardened, the inner mass of the metal is still molten. As the metal solidifies it is contracting and therefore with pressure continuously applied the density of the hardening metal increases. As the metal is solidifying at different rates, according to the mass of the design and its internal alloy components, the pressure applied to the metal organizes the solidification process and insures an even crystalline pattern.

It has been largely accepted in the western hemisphere that centrifugal force is the only way to successfully deal with the difficult nature of platinum as a casting metal. A variety of solutions have been taken to improve the problems of casting centrifugal. The addition of vacuum into the centrifugal method has been considered advantageous for the ridding of back pocket air pressure but appears only to accomplish the protection of the metal from oxide formations during its melt. Various configurations have been used to maximize the initial thrust of the centrifugal force including redesigning the traditional swing arm with one or even two breaks in the arm. These solutions appear to minimize the problems of metal spillage and crucible particle embedding.

The drawbacks of the centrifugal method, however, specifically, the gas formations caused by the excessive turbulence in the filling of the mold, as well as alloy separation giving rise to the mixing of gas and metal have been well known in the casting of gold and silver. In fact the centrifugal casting of gold and silver is done less and less. The use of vacuum casting, while proving successful in the casting of gold and silver, is impractical for casting of platinum because of the density of the refractory used, and additionally, it would appear that the force of the vacuum is too limited for the characteristics of platinum.

Given the difficulties in casting platinum centrifugal or with vacuum, it is no wonder that the impression has been created that working with platinum is so difficult and that the accepting of high failure rates is unavoidable.  However, if we consider the advantages of working with a pressure system for the casting of platinum, the problems inherent in centrifugal and vacuum methods can be avoided.
The introduction of pressure in the casting of platinum began in Japan in 1966. Its integration with more modern methods of induction melting began in 1985. This process was introduced in North America at the MJSA Expo NY in 1998.
The means by which the metal enters the mold refractory during the pressure casting process is smoother than with centrifugal. First, the metal enters the refractory mold by the force of gravity, and then pressure is applied to the solidifying sprue as well as throughout the mold chamber. The metal is pushed into the various intricacies of the mold while pressure is maintained. The force is adequate to accomplish the tilling of the mold without the excessive conflict caused by centrifugal filling.

Additionally, the various factors in terms of delivery of the force for filling the mold is much more easily controlled in pressure casting. Variations on the amount of force, when the force is applied in relationship to the timing of the metal cooling, and the duration of the force are all much more controllable with the method of pressure.
While the use of pressure in the casting of platinum is relatively recent, at least in this country, the potential for success with this method is much greater than with the other methods we have discussed.

 

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Pouring, Pulling, Spinning, and Pushing: Various Methods of the Lost Wax Casting of Platinum Both Old and New
Roger Greene
Eisinger Enterprises, Inc

This is an abbreviated version of the original work. For full technical details, please consult the original paper.