Moldmakers benefit the most, by far, from fully digital sinker EDM generators. The digital technology has advanced the functionality of generators to the point where mold shops can now precisely control and manipulate the EDM spark. Such control allows them to produce components that reduce overall mold maintenance costs for their customers. Molds that require less maintenance provide better part consis-tency, produce a greater number of quality parts and have increased life. A good, friction-free mold surface finish can help, and advancements in generator technology can create such surfaces.
Fully digital generator technology that controls the spark and gap voltage during the EDM process provides options in burn parameters to achieve different levels of surface finish. Sinker EDM functionality in the generator settings and control software help reduce friction on the surfaces of molds, including inside any small details and ribs. Molds fill quicker, shaving seconds off the molding process. For manufacturers that make millions of injected-molded parts, this saves hours of cycle time and helps them produce hundreds of thousands of additional parts.
The resulting surface finishes from this EDM generator advancement also reduce the chance of residue sticking to the mold after the plastic is injected. This is a problem that grows worse after repeated injections, leading to uneven surface finish on parts.
While a polished surface finish may seem ideal for every mold, this is not always the case. A highly polished finish requires that a great amount of pressure be used to inject the liquefied plastic into the mold. If the finish is too flat, or too smooth, the plastic suctions to the surface—the way two pieces of glass with water in between them stick together. This exceptionally strong suction effect slows the flow of plastic into the mold.
Additionally, greater ejection force is required to push the molded part out of the cavity of a highly polished mold, because the smooth surface causes it to stick in the same way. And if the newly formed part is still warm and a bit soft, the ejector pins will push into it and deform its surface.
This generator functionality smooths out the distance between the peaks and valleys on mold tooling surfaces, although not to a point where the peaks are eliminated. In a sense, it stretches the surface’s root mean square (RMS) value for roughness, while the roughness average (Ra) value remains the same. Because the peaks are more spread out, there is reduced surface sticking. This optimized surface finish also ensures that less force will be needed to eject a molded part from a mold. While tiny plastic particles may still be present on the part, the amount is not enough to consider the surface contaminated.
As a result of all of this reduced friction, molds can fill faster, and parts can be ejected quicker and with less effort. This means moldmakers need fewer ejector pins, which helps reduce machining and overall moldmaking lead times.
In operation, after rough-burning a mold cavity surface, a sinker EDM equipped with this friction-reducing functionality follows the standard finishing operation, but then activates the new function for the last two or three burn settings to impart the enhanced surface finish. The machine uses the same standard finishing electrode, and machining cycle time remains unchanged. This feature can also be used on existing mold tooling surfaces.
In addition to molding, other applications that would benefit from this type of surface finish are those in which the final part produced through EDM will come in contact with fluid. In such instances, the frictionless surface finish facilitates smooth and fast flow of those fluids.