Moldmaking Resource Plastic Mold Makers, Mold Designers, Mold Repair Guide When CNC WEDM, or wire EDM, first became available as a viable machine tool, somewhere around 1976, the buzz was that it would never be applicable for injection moldmaking. The thinking was that mold cavities are blind, meaning that the shape is cut into the steel, not through it.
Wire cut EDM and band saws
Because the WEDM operates somewhat like a very accurate band saw, the wire must pass through the workpiece in order to do any cutting. Since mold cavities are, by nature, not able to be machined using a wire cut EDM, the machines were largely used for tool-and-die making, where it excelled.
Cavity pockets
The first thing I recall was that I no longer had to use a drill press to drill 4 corner holes in the mold plates and then use a band saw to rough out the cavity pockets in the mold base. The WEDM did a great job, with straight side walls, even corner radii and accurate pocket sizes. I didn’t mind this at all!
Apparently it works better to cut these cavity pockets with an end mill on a CNC machining center, CNC horizontal mill or CNC boring mill, because you very rarely see wire cut pockets anymore.
Mold components
Nevertheless, within a few years, electrodes, core inserts, insert pockets, ejector pin holes, dowel pin holes, slides, gibs and many other components were manufactured on a WEDM. The ability to wire ejector and core pin holes has proven to be especially useful. This helps eliminate most lapping and honing of the holes to achieve a proper fit. Another benefit is that the holes can be finished after heat-treat, unlike the old method of drilling and reaming before heat-treat.
Ganging up in wire-blanks
Mold designers have developed interesting strategies to manufacture mold components on the wire EDM. By using CAD design programs, such as SolidWorks or Unigraphics, they can design wire blanks that are pre-drilled, pre-tapped and pre-ground for subsequent use in the wire machine. One blank might provide the steel for 12 molding inserts, for example. These blanks are heat-treated prior to wiring, and often the only thing left to machine afterward is the wire tab. This can easily be removed on a manual surface grinder.
Lights out operation
The wire EDM is one of the best CNC machines for operation 24/7-light’s out. The workpieces are pre-set outside of the machine on a surface plate, and can usually be quickly set-up in the machine by using quality tooling, such as System 3R, Erowa or Hermann Schmidt.
Because the machines can re-thread themselves in the event of wire-breakage or for other reasons, it is quite common for one toolmaker to operate several machines simultaneously. This saves time and money, especially on overnight and weekend runs. It is amazing that a tiny wire of .008 in diameter can be threaded through the wire guide automatically, but it happens all day long by using a water jet to guide the wire through the guide.
Little waste
Essentially the only significant waste by-product of the WEDM is the spent brass wire used to do the machining. Fortunately, this brass can be cut-up and recycled rather easily. There is no oil used as the dielectric fluid, as in sinker EDM. Rather, the wire cut EDM using de-inoized water to flush away debris and contain the spark.
Conclusion
The wire cut EDM, or WEDM, has transformed mold making in many ways. It is a dependable workhorse that has replaced a great deal of precision surface grinding and milling. The uses are limited only by the creativity of the designer or toolmaker.