Moldmaking Using Wire EDM
This article explores the applications in which wire EDM has found its way. It will examine the reasons for this new interest in a supposedly unrelated machining method, and its impact on moldmaking, both physically and fiscally.
Since the introduction and acceptance of EDM in manufacturing, it has been assigned to more or less "typical" roles and disciplines - "typical" meaning that vertical (or ram) EDM was delegated to moldmaking and mold-type operations, while wire EDM was most commonly associated with the tool and die or stamping industries.
In the last 10 years, wire EDM has found its way into production and other applications. Perhaps the single fastest growing market for this type of product is in moldmaking. We will examine the reasons for this new interest in a supposedly unrelated machining method and its impact on moldmaking - both physically and fiscally.
Moldmaking is an age-old craft that, in theory, has not changed since the Bronze Age - a tool or part is made by pouring, forcing or compressing material into a pre-shaped impression or cavity, creating a replica of the cavity when the material cools and solidifies. Obviously, many refinements to this process have been made throughout the centuries, but the basic premise of moldmaking still remains the same - creating an impression or cavity from which a part is replicated.
There are many different types of molds and molding processes. Some molded materials are poured or cast, while other materials are injected, inflated, transferred or expanded by chemical reaction. Other types of molds are not poured or injected but are struck, such as forging dies or coining dies that make everything from the change in your pocket to automobile crankshafts, from the cutlery on your dinner table to steel railroad wheels.
As a rule-except when leaving stock for machining on the molded or cast part-the closer the shape of a cavity is to that of the molded or cast object, the better. Plastic parts are usually molded in their final shape with very few secondary operations required, if any. Except for some small, precision die cast parts, very few metal parts that will be part of assemblies can be used as-cast or as-struck. Most will need some type of secondary machining operations.
Many parts will require several subsequent machining operations before they can be put into service. Finishes and accuracy can be as different as a mold for a cast automotive cylinder head or a precision injection-molded medical part used to meter the flow of blood. Regardless of the required part quality, better accuracy and finish of any die or mold will result in better replication of the part dimensions and will provide longer service life of the mold or die.
Vertical EDM in the Mold Shop
Vertical EDM has been a mainstay in moldmaking operations for decades. It also is commonly called a "sinker" or "die sinker" derived from "hobbing." Hobbing is the process of forcing a pre-shaped, hardened die into an unhardened mold insert, cold-forming the resulting cavity. This was called "die sinking" and the term "sinker" for EDM evolved naturally because now an electrode was sunk into the workpiece instead of a hardened die.
Moldmakers quickly learned the valuable time and material-saving advantages of using EDM for moldmaking. They now could EDM complex shapes into prehardened inserts while eliminating the costly procedures of inserting and laminating sections to get square corners or special shapes that conventional cutting tools could not produce. The accuracy and finishing capabilities provided by orbiter-equipped manual machines and CNC EDM further refined this craft. It is very rare today to find a toolroom engaged in full-time moldmaking operations without at least one EDM machine.
Wire EDM in the Mold Shop
In the last few years, more and more wire EDMs are being installed in mold shops than ever before. This has become one of the fastest growing market segments for wire EDM sales. Let's examine some of the reasons for the intense interest in this product.
Many years ago, this was one of the first uses for wire EDM in moldmaking. Early mold shops had vertical EDM machines with technology developed around copper electrodes. Since the early wire machines cut copper quite readily, this was a natural alliance. Complex copper electrodes could be easily and accurately wire-cut with no operator intervention. If multiple electrodes were required, accuracy from part to part was consistent and, unlike other methods, they never required deburring.
Early attempts at wire-cutting graphite electrodes proved to be very slow, but with the improvements in generator technology coupled with high-performance EDM wire and the increased refinement quality of modern graphites, cutting speeds of up to 18 square inches per hour in premium graphite are now possible.
Rule: When choosing a graphite for wire-cutting, the best cutting speeds and finish will be obtained when cutting a premium, isotropic graphite with the finest grain and highest density possible. Often, the extra expense of selecting a better grade graphite than necessary for EDMing is offset by the savings in the fabrication costs by using wire EDM.
Mold Base Work
Perhaps the second most common use of a wire EDM in a mold shop is the quick and accurate cutting of insert pockets in the holder plates of the mold base.
Question: Why reduce that large slug, where your insert is to go into a bunch of oily chips at the expense of numerous end mills and machining time?
Question: Why make a set-up to drill four corners of a square or rectangular opening, then spend hours welding blades and band sawing the slug free of the mold base, only to then go back to the mill (again) for another set-up?
Question: Why mill the pockets and deal with tapered openings, cutter whip and large radii in the corners?
Answer: Exactly my question...why? Why not wire-cut them instead?
Wire-cut insert openings are done precisely without the cutter whip and tapered openings typical of milling operations. Square or radiused corners are easily produced using wire EDM, often enhancing the basic design and simplicity of manufacture while reducing total operations on the insert itself. Even if the original design didn't warrant through-hole pocketing, often the cost savings realized by cutting pockets in this manner can justify the expense of the extra back-up plate.
For those who make their own mold bases-if we can quickly and accurately wire-cut the pockets for inserts, then why couldn't we also cut all of the other through-hole details in the mold base in the same set-up?
Other details, like holes for leader pins, return pins, taper-locks, etc., can be dimensioned from the same datum and all holes and openings are effectively "line bored" to tenth accuracy for perfect registration.
Granted, secondary operations such as counterboring would have to be made for pin shoulders and guide bushings, but no subsequent operation would require elaborate set-ups or jeopardize the accuracy of previous operations, relying primarily on the use of piloted counterbores.
Despite the fact that the wire must pass completely through the workpiece, many times cavities can be wire-cut. Flexibility and vision in the design stage will allow the use of wire EDM with substantial cost savings in machining and polishing time.
For example, a plastic tumbler or container mold, conventionally machined, might be designed in a simple two-plate mold configuration. This requires that, while soft, the insert be turned on a lathe or milled using tapered end mills (for draft), often requiring considerable polishing after heat treat.
This same job could be wire-cut "in the hard" and could be done almost totally unattended to better-than-required dimensions that will need little or no polishing.
The outside diameter of the cavity (including draft) can be cut in the B-plate insert by allowing stock for the bottom radius of the tumbler in the A-plate insert.
In the assembly previously mentioned, the radiused bottom of the tumbler can easily be turned, milled or EDM'ed in a tool steel insert. Since this type of mold would most likely have a hot runner system, the hardened, removable insert makes designing, manufacturing and maintenance much easier.
Core Pins and Inserts
Many types of core pin shapes lend themselves to wire cutting. Connector molds, for example, typically require cores with multiple, close-tolerance laminations that are quite labor-intensive and therefore expensive to grind and assemble. Many hours are required to dress numerous grinding wheels with the proper geometry to be used in sometimes elaborate set-ups to provide necessary draft angles. Better results can be achieved faster, in fewer set-ups, and with much less operator presence using wire EDM.
Core Pin and Ejector Pin Holes
Many moldmakers use wire EDM to finish all through-hole work in cavity inserts. One method becoming popular is: after drilling all the necessary start holes, mounting holes and any rough cavity machining, the part is hardened. After the cavity itself has been finished, it can be completely polished without undue concern over minor bell-mouthing or slight rounding the edges of the ejector or core pin holes because they are not finished.
The final operation on the nearly finished insert will be the wire cutting of all the ejector pin and core pin holes. These openings will be straight, to size, and will have dead-sharp edges at the cavity for minimal witness marks and no possibility of flash.
Tip: Wire cutting and sizing ejector and core pin openings offers a special advantage-venting. An operator can program a 0.0002" "bump" in the insert wall in strategic places around the pin that can provide badly needed venting where any other method would be difficult or impossible. These vents can be "designed-in" and executed at the same time the openings are being cut or added as corrective action after learning their necessity and location from short-shots, gassing or burning of the resin.
Shutoffs and Stepped or Contoured Parting Lines Besides the obvious applications of wire cutting stepped or contoured parting lines on cavity inserts or mold bases, many times difficult shutoff details can be wire-cut on core pins, inserts and slide components. Even long or delicate parts can be successfully machined because, unlike conventional milling or grinding, wire EDM imparts neither cutting pressure nor heat from friction to deflect the part or to adversely affect the part's temper or surface integrity. Besides providing flash-free shutoffs, these details can be executed time and time again with the same consistent results, oftentimes running almost totally unattended by an operator.
Another little-used potential when using wire EDM to produce molds is the ability to design and use tapered inserts and pockets. Both can be wire-cut with ease and accuracy. Inserts tapered in this manner need no shoulder, heel or mounting screws to secure them, but when assembled they are securely locked into place. Besides making mold assembly and maintenance faster and easier, it can greatly simplify design problems often encountered when considering optimum water placement or hot runner configurations often complicated by mounting screw placement. This is another example of time and money saving potential that can be realized by using wire EDM in a mold shop.
Related Mold Components
Remember the slugs that were left after wire cutting the insert pockets? Many times, especially if the material is of cavity steel, these can be used as the core, saving money in both raw materials and heat treating. If the slug material is from the mold base and can be hardened or heat treated, these can be used as structural mold components such as slide bodies, locks and wedges, etc. Any of these practices can cut costs by reducing or eliminating the purchase of additional raw material because the slugs can become the core or other components instead of oily chips and wasted time.
Steel #1 SAE 1030
Steel #2 AISI 4130
Steel #7 AISI 420-F
Steel #3 AISI 4130 (P-20)
Steel #5 AISI H-13
Steel #6 T-420 SS
This chart shows the most common mold steels and their uses. Any time you have slugs or blanks of types 1, 2 or 7, they can be hardened and used for different mold components. Steel types 3, 5 and 6 are used for cores and cavities.
Reduce or Eliminate Polishing
Here's where it can all come together (or apart) - on the polishing bench. Unlike all the other operations it took to get the mold inserts this far, mold polishing is comprised almost entirely of labor-intensive handwork. True, the modern mold polisher has many mechanical devices to aid in this difficult and time-consuming craft, but from the most sophisticated ultrasonic polisher to the coarsest EDM stone, all of them have one thing in common - they all must be manipulated by hand-a hand that belongs to a highly skilled craftsmen. The mold polisher must be highly skilled because many an insert or detail has been scrapped on the polishing bench by an unskilled or inexperienced hand or eye. This skill is acquired in one way - through experience, and experience costs money and can only be charged against direct labor. To reduce direct labor costs and eliminate the possibilities of incorrectly polished or damaged details, wire EDM can save enormous amounts of time and money while relieving the pressure on this most common bottleneck in mold production.
Sinker technology has continued to evolve and can provide press-ready finishes as a matter of routine. The newest technology for sinkers involves the use of special powdered additives in the dielectric to speed finishing operations even in large cavities that would usually take too much time. Augmenting this finishing capability with a wire EDM machine is the next obvious strategy.
Dielectric additives for wire machines have been introduced, although they are still considered to be in the development stages. The typical wire EDM finish is already often satisfactory and can be run "as is" after only two or three skim cuts on a standard power supply. Some machines provide fine-finish circuitry options (<0.5 fmRmax) that provide near-mirror finishes. Should any further processing be necessary, such as draw polishing or processing to optical-quality finishes, the recast or "white-layer" left by wire-cut EDM is softer and thinner than the recast surface left by sinking operations and in comparison, can be stone-polished quite easily with a minimum of time and equipment.
Hopefully, through this article, a nonuser comes away with a greater understanding of the potential savings derived by exploiting the capabilities of a wire EDM in a mold shop. Keep in mind that due to this article's brevity some salient points have been missed, but to review, here are 10 of the important advantages of having a wire-cut EDM in your mold shop:
- Electrode making
- Mold base work
- Cavity work
- Core pins and inserts
- Core pin, ejector pin and ejector sleeve holes
- "Designed in" or emergency venting
- Related components
- Shutoffs and stepped or contoured parting lines
- Reduce/eliminate polishing
- Significantly reduce direct labor costs
These examples alone should be enough for you to consider examining how a wire-cut EDM can benefit your moldmaking operation.
Just as is done with vertical EDM, all aspects of mold manufacture should be examined to determine if part of the solution could be wire EDM - or wire EDM in conjunction with vertical EDM. This has already proven to be a very good marriage. While vertical EDM in moldmaking is engaged primarily in cavity work, wire EDM is applicable in almost all aspects of moldmaking: cores, cavities, electrodes, mold base work and components. The combination of the wire EDM's flexibility and ability to run unattended make it an ideal addition to any moldmaking facility. Of this, there can be no question as purchases of wire EDMs by mold shops make it the fastest growing segment of manufacturing today.
Looking back through the list of some of the different moldmaking disciplines at the beginning of this article reminds us that moldmaking of any type is both an art and science and therefore, will never be easy-but it can be made easier.
Learn the advantages that a wire EDM can bring to your shop. Discover how you can wire-cut difficult and labor-intensive parts and details unattended to split-tenth accuracies while often bypassing the polishing bench entirely.
Sound easy? Well, at least it's easier.
While EDM speeds today are impressive, there are a number of machining variables that influence the machining speed of the wire EDM.
Both copper and graphite provide approximately the same end result, so it is important for a shop to consider the advantages and disadvantages of each material in order to discover what would work best in their shopfloor environment.
A look at some of the factors influencing the success of your machining center investment.