Sinker EDMing drives micro moldmaking

When it comes to micro moldmaking, there’s only one good way to create the smallest cavities and other features, according to Dennis Tully, president of Miniature Tool and Die Inc., Charlton, Mass. “Using sinker EDMs, we can make feature sizes smaller than anything we’ve managed to successfully produce using other removal methods,” he said. “With conventional milling, there are limitations on how low you can go on cutter size. I’ve read about people having some success with very small-diameter mills, and we have a Roku-Roku milling machine that is supposedly capable of using very small endmills to make very small details, but every time we try to do something like that, we realize that microEDMing is so much easier.”

Reasons to EDM

Kamlakar Rajurkar, a professor in the Department of Industrial and Management Systems Engineering at the University of Nebraska, agreed that sinker EDMing is the preferred method for creating micromolds. “Sinker EDMing, because of its ability to generate sharp corners and machine hard materials, is a predominant micromold machining technology. It is widely used either as a stand-alone technology or in combination with other technologies.”

Rajurkar and his research associate, Dr. Murali Sundaram, assistant professor of mechanical engineering at the College of Engineering, University of Cincinnati, cite four key reasons for using sinker EDMing.

1. Machining high-hardness materials that offer wear resistance, which produce the most desirable mold qualities, is very challenging using traditional machining processes, such as milling. EDMing is able to vaporize any conductive material irrespective of its mechanical properties, such as hardness.

2. Because EDMing produces negligible mechanical machining forces, it is preferred over other techniques, such as milling and grinding, for machining fragile molds.

3. Traditional micromachining requires tools that themselves require micromachining. This irony is avoided in microEDMing when simple-shaped electrodes, such as plain cylinders, are used to machine complex micromolds with the help of CAD/CAM programs.

4. EDMing is economically viable for small and medium-sized moldmakers.

At Miniature Tool and Die (MTD), Tully uses sinker EDMs to produce entire mold cavities, including finishing. The shop’s moldmakers manufacture intricate, 3-D electrodes with overcuts as small as 0.0002" to 0.0005".

Both of these parts (tension breakers) were made from cavity molds, but mold quality and surface finish are vastly different. Miniature Tool and Die made the mold for the part on the left using techniques it has developed for microEDMing, while the other part’s mold was made by another shop using conventional EDM methods and equipment. The top diameter of the MTD part is 0.075" while the radius of the top section of the feature is 0.004". Photo courtesy Miniature Tool and Die.

A micromold being made on a GFAgieCharmilles FO 350 MS sinker EDM. Photo courtesy GF AgieCharmilles.

A micropart for an insulin pump made from Acetal in a micromold EDMed by Phillips Plastics. The inside diameter is threaded and the part pivots along the journal axis. The pivot journals on either side of the part are 0.050" in diameter. The part is 0.288" long and 0.230" wide. The top edge of the thread is 0.0025". Photo courtesy Phllips Plastics.

The shop operates three sinker EDMs: a Herbert Walter unit made in Germany and not typically used in the U.S., an Agie Exact and a new Sarix. MTD has spent hundreds of hours tuning and adapting the power supply for the Herbert Walter Exeron sinker to enable it to produce precision micromold components. “The Exact is the workhorse of our sinker EDM arsenal; it consistently delivers reliable and repeatable results with no work- piece error,” said Tully. “The Sarix is a very unique EDM process. It utilizes a rotating electrode made from round, solid-carbide rod diameters ranging from 0.4mm to 0.005mm. The electrode is driven along a toolpath created from a CAD/CAM system. Using small submicron level stepovers and extremely low power settings, this machine can create details and components with near-polished surfaces and sharp inside corners.”

Tully said MTD modified its machines to work more effectively in microscale operations. “It’s really been a matter of fine-tuning the machines’ abilities to machine workpieces and create electrodes so that we can get results beyond what we’re supposed to be able to achieve,” Tully said. “Most shops operating EDMs are interested in increased speed and reduced electrode wear, but we don’t really care about those factors. We are more interested in dimensional control, good surface finish, fine replication of the electrodes and producing sharp corners. For the most part, EDM technology hasn’t evolved in ways that directly benefit us, so we’ve had to find ways to adapt it to make it work for us.”

While Tully declined to say how MTD modifies its EDMs, he said the company has improved the way it manufactures its own electrodes. Sometimes MTD does this on a sinker EDM, when fine detail is required, but most often the shop uses its Roku-Roku high-speed milling machine. MTD’s electrodes are made from ultrafine graphite blanks.

Tully uses his three sinker EDMs in different combinations for mold work. About 30 to 40 percent of the time the shop can create a micromold on one EDM, but because each machine has its own unique capabilities, the rest of the jobs require a second or third EDM. When that happens, the shop uses its ability to dial out error using electrode corrections within the machines’ software systems. “This gives us the ability to squeak out every last tenth that can creep in during the entire EDM operation,” he said.

Tully said its Agie Exact helps MTD produce accurate setups when multiple machines are involved. “When we first started using it in 2003, we were astounded at how accurate it was. We would set up a workpiece, machine it, take it out and it would have no detectable error using our measurement systems.” Using the electrode corrections allows MTD to sink multiple electrodes in the exact locations to produce crisp and accurate details.

Process variety

Unlike Miniature Tool and Die, Phillips Plastics Corp., Menomonie, Wis., uses a variety of machining processes to create micromolds, including sinker and wire EDMing, hard milling and grinding for finishing.

“Part design dictates the type of machining process we use,” said Dave Munkwitz, micromolding manager for Phillips Plastics. “Square corners require sinker EDMing if the mold detail cannot be created in some other way. It is also easier to use small-scale electrodes in an EDM than to machine 60-HRC steel with micro-endmills.”

According to Gisbert Ledvon, business development manager for EDM and machine tool builder GF AgieCharmilles, Lincolnshire, Ill., a key consideration for microEDMing molds is stabilizing temperature in the machining environment. Even small temperature changes make machine components expand and contract, affecting accuracy. “The machine tool itself must be in an environment that is temperature-stable as much as it can be; not only the dielectric, but the casting of the machine itself.”

Many of the micromolds created by Miniature Tool and Die are 3"× 5". Details are typically very small. “We try to use standardized mold cavity inserts that allow us to make them ahead of time and put them on the shelf in a hardened condition, and typically those inserts are ½"×3⁄4". About 90 percent of the things that we make fit into cavity blocks that size. But some of the inserts have to fit inside a cubic millimeter or even smaller.”

About 90 percent of MTD’s plastic injection micromolds are for medical applications, typically minimally invasive surgical devices.

Fine surface finish is a key attribute for medical micromolds, and MTD’s new Sarix EDM allows it to produce finer finishes than ever before. “Our Sarix machine, which we’ve had for about 4 months, produces negative VDI numbers, which we didn’t even know was possible,” Tully said. (VDI stands for Verein Deutscher Ingenieure, the Society of German Engineers, and is a widely used standard for measuring EDM surface finishes. Lower numbers indicate finer finishes.)

Advancing technology

MTD’s new Sarix EDM shows how new technology is improving microEDMing. Rajurkar and Sundaram identified several new developments that have contributed to improved EDMing:

1. Controller technology: Advances in solid-state electronics have resulted in transistor-based EDM controllers that have enabled more precise control of spark on/off timing than their relaxation circuit-based counterparts. Relaxation circuits, used in traditional EDMs, use capacitors for charging and discharging; transistor-based pulse generators provide better spark control.

2. Modern electrode materials: Materials such as conductive CVD diamond have enabled almost zero-wear microEDMing. This is a significant improvement over traditional electrode materials, such as tungsten, copper and graphite, which have experienced relative wear as high as 20 percent in experimental microEDM drilling operations lasting 20 to 30 minutes.

3. Machining strategies: Tool-wear compensation technologies—such as the uniform-tool-wear method and the uniform-tool-shape method—can successfully compensate for tool wear due to their strategic toolpath planning and repositioning. (In these two methods, simple, uniform electrodes with round or rectangular sections are used. Uniform wear at the end of the electrode is realized by layer-by-layer machining.)

4. Flushing techniques: Traditional flushing techniques such as jet flushing are not suitable for micromachining due to dimensional constraints. Another traditional technique, immersion flushing, is not particularly effective in microEDMing, especially in high-aspect-ratio work. Recently developed ultrasonic flushing, however, is very effective in microEDMing applications, particularly in micro deep-hole drilling with aspect ratios higher than 20:1.

According to Munkwitz of Phillips Plastics, advances in sinker EDM technology over the past 5 years have allowed greater use of the technology in micromoldmaking. “Improved generator settings for extremely small detail have allowed better control of detail and surface finishes along with reduced electrode wear,” he said. “These micromachining capabilities are available in many full-feature machines. Also, improvements in several microEDM-specific machines allow the creation of details in steel that would have been extremely difficult 5 years ago. These machines are very narrow in scope, but they do have their place in the industry.” µ

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About the author: Alan Rooks is editorial director of MICROmanufacturing and Cutting Tool Engineering magazines. Telephone: (847) 714-1074. E-mail: arooks@jwr.com.