The machined, natural EDM finish is the surface that the EDM process leaves on the metal at the end of the finishing cycle. The finishing cycle puts the most stress on the graphite electrode. Finish settings are high frequency cycles with more sparks, or discharges, per second due to the short on and off times required to remove a small volume of work metal in order to create a smooth surface finish in the cavity.

The cavity surface will reflect the quality of the electrode material used to finish the cavity. If the structure of the graphite is not capable of producing the specified surface finish, the machine will continue to run without ever achieving the desired result. When the finishing time becomes excessive, the grade of graphite is likely the problem.

The uniformity of the microstructure (particle size and pore size) of each manufactured graphite grade is different, much the same as the machine technology is different for each manufacturer. As an example, if you select the machine setting for a specific surface finish on two separate brands of equipment, the cavity inevitably will be slightly different. Even on the same machine, if two grades or brands of graphite are used for the electrode, the cavity will be different. Although the cavity will have the correct surface finish, the surface finish reading is an average of the high and low areas, and the textured surface may be very uneven with high spots that are shiny or low areas that will shadow. The uniformity of these areas is critical to the finished plastic and how light reflects off the surface. The shape of the high and low areas is determined by the graphite grade. Graphites with poor microstructure – such as large and irregular shaped particles with weak bonds between the particles – will wear unevenly, causing high and low areas in the cavity.

Another problem is inconsistent surface finish on multiple-cavity jobs. Because graphite grades from different manufacturers will wear differently, they will produce a different looking finish even though the machine was set to a specific surface finish setting. This is most often seen when molds are produced by different suppliers. Unless the graphite grade is specified, the EDM finish will not be the same on all of the molds.