Wax patterns are the heart of the investment casting process. With a good quality pattern and precise control over the casting process, the metal part will probably be good too. Conversely, any defects in the wax will carry over to the final part.

If you’re searching for a foundry capable of investment casting complex, high value parts, you need to understand how wax patterns are made and used. Here’s a detailed look.

One Pattern, One Part

Investment casting is an expendable mold process: once the metal has solidified the mold is broken apart to release the cast part. Like other expendable mold processes, the mold is formed around a pattern, but in the case of investment casting, the pattern is destroyed to get it out of the mold.

To do that, the pattern must be:

• Inexpensive, as it will be produced in the same quantities as the finished part
• Easily removed from the mold
• Capable of reproducing the features needed in the part being cast

These requirements are met by wax. It’s inexpensive, can be injection-molded to the required shape, and melts at a low temperature so it can be melted-out of the mold.

Pattern Production

The process of injection molding wax patterns is essentially the same as injection molding plastic parts. Wax is melted and pumped into a mold cavity that forms the shape required. The temperatures and pressures involved are lower than for plastics, so the machines are smaller and the molds need not be so robust.

As with injection molding, the mold must be designed to let the pattern release easily. However, as waxes expand more than metals there is no need for draft angles on the sides.

Complex patterns are often assembled from multiple pieces This allows for the inclusion of cores that will remain in place after the wax is melted out to create voids in the metal part.

Wax sprue or runner systems are attached to each pattern to provide channels for metal to flow into the mold cavity. To keep costs down, when parts are small multiples will be attached to a central sprue to form a “tree”. The tree is then coated with a ceramic slurry that dries to form a hard shell. After melting out the wax the mold is ready to receive molten metal.

Process Control and Pattern Quality

Molding wax patterns involves tradeoffs between speed, temperature and quality. The cavity must fill completely and the wax must set before the mold opens. Trying to reduce cycle time by increasing temperature or injection pressure risks creating cavitation defects in the wax. This is where the wax sets with voids or bubbles, which unless repaired, will appear as defects in the metal part.

Wax formulation determines flow and molding characteristics like viscosity and melting point as well as surface finish. The pattern-maker selects one of the many possible types of investment casting wax to suit the part design while minimizing defects and cycle time.

To arrive at metal parts of the required dimensions, injection molding tools for patterns must incorporate appropriate shrinkage allowances. This has to cover both metal shrinkage in the ceramic mold and wax shrinkage. It’s a complex problem that can take some time, plus some development work, to solve. However, as the mold tools are often made from aluminum, (a result of the low injection pressures), modifications are easier than when working with tool steels.

Consider Pattern-Making Capabilities

Investment casting is a cost-effective production route for complex metal parts. High precision, good surface finishes, and the ability to reproduce fine detail mean it can eliminate many secondary machining operations.

Finished part quality depends largely on the quality of the pattern, (plus melting and pouring parameters). This is why pattern quality should be a primary consideration when evaluating investment casting foundries as potential suppliers. As a leader in investment casting, Impro has both the expertise and the equipment necessary to mold quality wax patterns. Contact us to discuss your needs.