Contrary to some views, it is possible to create small features in aluminum molds. Quickturn injection molding, using a combination of CNC machining and Electrical Discharge Machining, is one method.
Service bureaus continue to develop new manufacturing methods and processes that speed the prototyping of your parts. A recent addition is Quickturn injection molding. You can now receive injection-molded parts in days instead of weeks. The parts can be fully textured, have undercuts, and be made from engineering grade materials.
EDM can cut intricate contours or delicate cavities that would be difficult for other machining equipment, such as grinders and end mills.
The improvement in delivery time is due to CNC machining advances over the past several years. Using aluminum as the mold material, a high-speed milling machine (30,000-42,000 rpms) can remove material rapidly and accurately. In addition, the machines can cut smaller features into the mold, a capability that is not possible when using harder mold materials. Unfortunately though, high speed machining is not able to machine all the features into a mold. Some feature sizes and depths still require other manufacturing methods to create part features in the mold.
Thus, it is necessary to return to traditional methods of mold building. The typical process for creating small features in molds is to use Electrical Discharge Machining (EDM).
EDM is a machining method primarily used for hard metals or on features that would be impossible to machine with traditional techniques. It is especially well suited for cutting intricate contours or delicate cavities that would be difficult to produce with a grinder, end mill, or other cutting tools.
EDM removes very small amounts of metal through the production of a rapid series of electrical discharges. These discharges are passed between an electrode and the metal being machined and create a set of successively deeper craters until it produces the final shape. The surface finish of the metal after an EDM process can serve as a light texture on the molded part or it can be polished out for a smooth finish.
With a combination of EDM and CNC machining, you can obtain injection molds made of aluminum with sharp, clear features.
A common type of EDM is typically known as RAM EDM. It consists of an electrode plunged into a metal work piece to create the cavity or pass-through feature. Since this is a highly controllable machining process, small features can be created in a mold, features that are too difficult for typical CNC machining processes.
When is the best time to use EDM during the mold building process? The following chart is a guide.
The EDM process does add cost to the mold build process. It is also a very slow and meticulous process. Therefore, it adds time to the mold construction process as compared to high speed CNC machining. Now, though, as rapid tooling companies become more advanced, they can use a combination of CNC machining and EDM to help speed the process.