Surface finish and integrity are two different facets of the cavity quality, but both play an important part in the characteristics of the mold. Much as the machine parameters affect the integrity of the sub-layers of the cavity,

they also affect the surface finish.
The type of electrode material being used is another factor. In terms of the sub-surface integrity of the cavity, the type of electrode has little effect. Where the electrode material does play an important role is the finish on the cavity’s surface itself. The surface finish of the cavity is an area where there is opportunity to reduce manufacturing costs and delivery time while still being able to provide a quality mold.

A mold with a fine EDM finish can be used right out of the tank while one that needs polishing or etching requires added costs as these processes are carried out. A fine EDM finish is achieved by using a high frequency EDM program with low amperages and short on-times. The quality of the electrode material goes hand-in-hand with the EDM program to achieve the desired results. Taking the EDM program out of the equation, the cavity surface will reflect the quality of the electrode material used to finish the cavity.

The material particle size and corresponding pore size play a significant role in the material’s ability to produce a fine finish. If the structure of the electrode material is not capable of producing the specified surface finish, the EDM machine will continue to run without ever reaching the desired surface finish.The material with the finer material structure provides the smoother surface finish and will require less polishing to achieve the desired finish results.

An electrode material with a poor microstructure—such as large or irregular particle shapes—will wear unevenly, causing the cavity to also have uneven surface areas. This situation is especially critical on multi-cavity molds where the surface finish is required to be consistent on all cavities.

When using a graphite electrode material, strict caution should be taken to ensure consistent quality for all electrodes. Because graphite grades from different manufacturers will wear differently, it is possible that the EDM machine may not provide the specific programmed surface finish. This is most often seen when molds are produced using different types of electrode materials or makes of EDM machines.