A mold with a good EDM finish is ready to use right out of the machine (tank-to-part technology), while a mold that needs to be etched or polished to get the desired finish requires extra steps that increase cost and extend production time.

An EDM finish on a mold can provide the same appearance on the molded part as a more expensive etched mold. Due to the geometry of the cavity, there may be areas that need polishing for quick release, better fit or as part of the product design. Since the mold etching and polishing are often outsourced, extra scheduling, coordination and cost are required. If this operation is performed at multiple locations, there is an increased risk that the textures will be mismatched.

In the etch process, all of the EDM finish must be removed before the cavity can be etched. Even though this finish will be removed, it is important that the cavity has a quality surface and that enough stock remains to allow the polisher to obtain a fine finish. Knowing this process, some production people believe that the grade of graphite is not important, so they try to cut costs by selecting graphite based on price rather than selecting the appropriate grade of graphite based on application. But choosing a grade of graphite that produces a poor finish increases both the polishing time and the cost of the job .

For example, graphite with poor grain structure allows large grains to fall out of the electrode. Because the working gap is narrow during the finishing operation, these large grains get trapped in the gap, causing a secondary discharge that results in pinholes in the cavity. The working gap can be opened up with higher voltage settings to allow small grains to escape the gap, but this leads to slower performance.

Since imperfections in the cavity – including pits or pinholes created during the EDM process – must be removed, the additional polishing necessary to remove blemishes that extend below the surface may remove too much stock, causing rework (welding and reburning) or remake of the cavity. Etching adds time and cost to the job when a good EDM finish on the cavity can provide the same appearance on the molded part.

Mold polishing is a very skilled operation because too much polishing can create a cavity that is out of spec. Whether the polishing is to be done by hand or by mechanical means, the same care that enough metal remains for the polishing operation to produce a blemish-free EDM surface must be taken, thus the quality of the cavity as it comes out of the EDM machine is very important to the polisher. Ideally, a fine finish would be reached on the machine, requiring only minimal hand polishing. Under some conditions, the finishing cycle may be lengthy, but due to critical dimensions, it is the best approach. Sharp corners and thin, deep ribs may be impossible to polish; therefore it is important that a good surface finish is obtained in the tank .

Some EDMs will produce a mirror finish using diffused discharge machining technology (powder additive) that eliminates the need for hand polishing. This technology requires a quality graphite electrode .

In theory, technical advances allow EDM cavities to have a specified surface finish, including a mirror finish right out of the tank. In practice, the electrode material is the key to achieving this performance. For example, ultrafine graphites with their small grain size, high strength and uniform microstructure will resist wear and particle loss to produce quality surface finishes in less time than graphites with low strength and poor microstructure.