Evaporative Pattern Casting Process

This invention relates to the production of castings by the evaporative pattern casting process. In particular, it relates to the coating of the pattern so as to provide adequate strength during the pouring of the molten metal and also provide adequate permeability for the venting of the gases formed by the vaporization of the expendable pattern by the molten metal.

The evaporative pattern casting process is a procedure that has become of importance in the production of castings for several reasons including the freedom of design that it permits. In addition, it requires only a limited outlay of capital and allows the foundry to pour the same pattern in different metals.

The technology for the preparation of the pattern is readily available since the usual material for its construction is polystyrene although other materials such as polymethylmethacrylate have been used for certain applications. A vital step in the process is the coating of the pattern since the coating not only controls the rate at which gases are produced and removed, but also must be strong enough to support the molten metal during the vaporization of the pattern. In the event that the coating does not provide adequate permeability for gases formed by the evaporation of the pattern during the pouring of the molten metal then the surface of the casting can be marred by what is known as the lustrous carbon defect. Normally only one coating of the refractory material is applied to the pattern to furnish the required thickness of refractory coating.

It is, therefore, an object of the present invention to improve the coatings used on patterns in the evaporative pattern coating process.

Another object of the present invention is the coating of the patterns used in the evaporative pattern coating process in a manner that the gases formed during the process can be properly vented thus avoiding the marring of its surface of the casting by the lustrous carbon defect.

Still another object of the present invention is the production of a coating for the patterns used in the evaporative pattern coating process which retain its shape during the time period when the expendable pattern is being vaporized.

Other objects will be apparent from the ensuing description.

It has now been found that the use of refractory material containing a sufficient amount of a substantially spherical shape will give the required combination of gas permeability and strength to support the pattern during the pouring of the molten metal for the effective performance of the evaporative pattern casting process. This unique refractory composition produces castings that are of the desired shape all free of the lustrous carbon defect. The present invention contemplates a refractory composition comprising a sufficient amount of substantially spherical refractory material to support the pattern during the pouring of the molten metal and evaporation of the pattern and to permit the venting of the gases formed by the evaporation of the pattern. Normally the refractory composition must contain, at least, about 10 percent by volume of substantially spherical refractory material. The refractory composition of this invention can contain from about 10 percent by volume to about 100 percent of the substantially spherical shaped material. For most applications from about 10 to about 80 percent by volume of substantially spherical shape of the total refractory composition will suffice.

In essence, the improved evaporative pattern casting process of the present invention comprises forming a pattern of the desired article from vaporizable material such as polystyrene; coating the pattern with a refractory material comprising, at least, about 10 percent by volume of a substantially spherical shape, placing the pattern in a container, vibrating unbonded sand around the pattern and pouring molten metal into the pattern to form the casting and evaporate the pattern. The permeability of the refractory composition of this invention permits the gas formed by the evaporation of the pattern to be vented while containing the molten metal during the evaporation of the pattern. Failure to properly vent the gases causes the lustrous carbon defect which mars the casting. Failure of the coating to contain the metal when it is being poured allows the molten metal to penetrate the coating and flow into the sand. The present compositions and methods prevent either of these defects.

In order to determine the significance of the use of substantially spherically shaped refractory material, a series of experiments were performed using refractory material containing varying amounts of the substantially spherical refractory material.

In these experiments, a product sold by Zeelan Industries under its trademark ZEOSPHERES was used as the substantially spherical refractory material. ZEOSPHERES are thick walled hollow spheres made of aluminum silicate. The non-spherical refractory material presently used in these experiments was aluminum silicate sold under the trademark KYANITE. This is a standard refractory material used as a coating for expendable patterns. Three formulations were prepared. In each formulation the only variable was the refractory material. The components such as the binders, suspending agents, etc. remained constant as to identity and amount, i.e., the refractory component constituted 36.4 percent by volume of the formulation and the remaining components constituted 63.6 percent by volume of the formulation. The non-spherical refractory component of the three formulations were as follows:

Expendable polystyrene patterns coated with the same approximate thickness of each of the formulations obtained by multiple coatings of the three formulations were used to prepare a series of identical castings. These castings were inspected as to their shape and surface with the following results:

The castings prepared from the patterns coated with Formulation A had severe lustrous carbon defects on their surfaces.

The castings prepared from the patterns coated with Formulation B had lustrous carbon defects on their surfaces less severe than the lustrous carbon defects on the castings made from the patterns coated with Formulation A.

The castings prepared from the patterns coated with Formulation C did not have lustrous carbon defects on their surfaces.

A second set of experiments was performed using KYANITE 200 mesh size as the non-spherical refractory material and containing 15, 45 and 100 percent by volume of the substantially spherical refractory material. Sufficient coatings were put onto each pattern to have a similar thickness of refractory material on each polystyrene pattern having the same volume percentage of substantially spherical refractory material. Two levels of binder material were used for each level of substantially spherical refractory material. Ten castings were poured from each formulation. The surfaces of the castings were rated by two persons (D, E) skilled in the casting field on a scale of 1-5, 1 representing a surface with no visible defects, 5 representing a rough surface and 2-4 representing intermediate levels of visible defects; 2 representing very slight visible defects, 3 representing a casting having slightly more defects and 4 representing a casting demonstrating lustrous carbon on the upper 2/3's of its surface. The results were as follows:

The results of the tests are as follows:

The castings made from the refractory material containing the substantially spherical refractory material had surfaces improved in comparison to surfaces obtained by expendable patterns coated with refractory material not containing the spherical refractory material; however, the use of 100% of the spherical refractory material reduces the refractory characteristics of the coating and can cause the formation of a rough surface. Accordingly, it is preferred to use a maximum of about 80% by volume of the spherical refractory material and the remainder the normal refractory material which, due to its shape, will provide improved on refractory coating when applied to the pattern. The substantially spherical refractory material used in the present process can be hollow. The optimum amount of the substantially spherical refractory material in the refractory composition will vary depending on factors such as the shape of the casting, identity of the other components of the formulation, the metal being poured and the like.