PECM Eats Up Tough Metals

Less stress, greater reliability in medical manufacturing

When Koninklijke Philips N.V. needed a fine cutting edge on its Norelco electric shavers sold in the United States, engineers at the Dutch company tweaked electrochemical machining (ECM) to get the quality and reliability they needed. The modified process devised by Philips’ engineers is now used in many industries outside of personal care, including medical manufacturing.

"They were going through a lot of punches for dies to produce that cutting edge," said Scott Kowalski, who works with two of the engineers on Philips’ patent in his role as president of RM Group Holdings LLC in Ridgefield, N.J. "They took ECM, which is just a standard DC (direct-current) technology, and they introduced a variable pulse and an oscillation that would not only remove the edge, but would create the profile."

Both ECM and its modified form—precision electrochemical machining (PECM)—machine via an electrochemical process. The technologies are used for different purposes and produce significantly different results.

"The ECM is fixing a problem," Kowalski said. "There's a burr, there's an edge, there's something there that the customer didn't want or expect. PECM is more value added. We’re introducing shapes, profiles, geometries," he added, noting that ECM was introduced about 50 years ago while industrialized PECM has only been around for about 10 years.

ECM Technologies, Leeuwarden, Netherlands, described PECM this way: "During the PECM process, metal is dissolved from a workpiece with direct current at a controlled rate in an electrolytic cell. The workpiece serves as the anode and is separated by a gap (which can be as small as 10µ) from the tool, which serves as the cathode. The electrolyte is pumped under pressure through the inter-electrode gap, thus flushing away metal dissolved from the workpiece. As the electrode tool moves toward the workpiece to maintain a constant gap, the workpiece is machined into the complementary shape of the tool."

PECM is ideal for handling the tough-to-machine materials and complex geometries used in modern medical parts, and it produces the fine surface finishes the industry requires for hygienic use, according to Philips.

The tough alloys and super-finished surfaces are great for medical implants because of sterility, as well as for their biocompatibility. Nickel titanium, also known as Nitinol, is a great alloy for some bone fixture implants, but it would be hazardous if the nickel inside should enter the bloodstream. Implant manufacturers want super-finished surfaces to mitigate this risk; rougher finishes corrode and chip off easier, which could be extremely dangerous for a patient.

"There's a number of applications that are out there for various implants and surgical tool components," said Don Risko, owner, DGR Consulting LLC, Jamestown, Pa. "A big one now seems to be staplers."

PECM's capability isn't widely known, due in part to closely guarded manufacturing secrets in the medical and other industries, according to Risko, who's worked with ECM for nearly 40 years. This has "stifled" PECM technology somewhat, he said, citing several instances of companies developing capabilities for clients who forbade disclosure, therefore precluding references to them in future sales pitches.

However, the use of PECM has grown in medical manufacturing.

"Because they’re small parts with stresses on them (during use), they tend to be made out of harder materials, higher-strength materials," explained Bruce Dworak, president, Hobson & Motzer Inc., Durham, Conn. As a precision metal component manufacturer that has long supplied the medical device market, the company has worked through the evolution of stapling technology and the demands placed on device components.

"And those higher-strength materials, the processing of them, can lend itself to PECM," Kowalski said. "We typically get the materials that other (machine tool) manufacturers don't like."

As in electrical-discharge machining (EDM), materials must be electrically conductive to be suitable for PECM. While this has created some confusion about the two processes, their similarities end with conductivity. In EDM, metal is melted or eroded by an electric spark, effectively burning the material; in PECM an electric current and an electrolytic solution work together to dissolve the metal on a workpiece. The EDM electrode wears away, while in PECM it remains intact.

"Another aspect of PECM is that it can remove material fairly quickly, and a lot of times (can machine) multiple configurations and components simultaneously," Risko added. "So PECM is considered an area-machining process as opposed to a point-machining process like milling or turning. Machining an area simultaneously can have significant advantages compared to multiple setups or multiple machining operations using conventional machining."

One of the main advantages of the PECM process is that hardened materials are machined almost as easily as those in a softened state because the hardness does not affect the material removal process. With conventional machining, the harder the material is the more difficult it is to machine. As a result, the PECM process excels in machining materials such as stainless steel—and those that have tight grain structures —even in a heat-treated state.

"If you have something that needs to be processed after it's been heat treated, like say a 465, something that's really hard, a 440 or 420," Dworak explained, "those would be some of the better applications because it doesn't care that they’re heat treated or not, it doesn't care how hard they are."

In addition to its knack for machining hard-to-process materials, PECM is notable because it doesn't add stress that can lead to cracks or part failure—a critical concern for the medical industry. The process is also known to produce good finishes.

"Your finish is superior to any other manufacturing method, whether it's milling, grinding, or lapping," Kowalski said. "I mean, the finish that comes off the PECM is truly a mirror finish."

According to Kowalski, PECM can achieve a surface finish of 4 Ra, which measures the average roughness across a surface. "But the technology could go tighter than that," he added. "I want to say we can get down to a 2 Ra (the lower the Ra, the smoother the surface), depending on the application and part geometry, if you really do a super fine finishing."

PECM's reproducibility is helpful to create anvils for staplers used in minimally invasive surgery. "We have customers that produce 14,000 to 15,000 of these per week, with a 6-8µ tolerance," Kowalski said, noting that "repeatability is paramount" for such applications.

Another benefit of PECM is that multiple parts can be machined simultaneously with the proper tool.

"It’ll take the exact same amount of time to make two or three or five parts as it would one, once you get going," Dworak said. "So it's a matter of that initial investment (in the tool) and having enough energy and electrolyte in the machine to accommodate it."

Daniel Herrington, CEO of Voxel Innovations Inc., Raleigh, N.C., compares a multi-electrode tool to a multi-cavity mold for injection molding.

"If you make your tool twice as big, or you put two parts in a single tool, you need twice the amount of amperage from our machine to make that, but your sinking rate is unchanged and so, effectively, your removal rate is doubled," he said. "And so, if you follow that logic, it makes sense that whenever we can, we machine lots of the surface simultaneously. That could be one bigger part, machining multiple features or complexity in one shot, or multiple smaller parts—two at a time or five at a time or 10 at a time—all in parallel."

The high productivity Herrington described has led one of Kowalski's customers to assign three operators to one machine to increase throughput.

The beauty of the technology, he said, is that it doesn't split a spark. "So if it takes me five minutes to make one part, it takes five minutes to make 50 parts."

The electrolytic solution plays three roles in PECM. Along with conducting the electric charge to machine the workpiece, it also acts as a flushing and cooling agent.

"It's primarily there to conduct current and make sure the reaction happens," Herrington explained. "But if that was completely static, then your process will be very limited in its speed, because now you’d fill all that electrolyte with the waste products, metal, and hydrogen gas, and it would stop functioning. You have to flow the electrolyte through that gap so you’re continually flushing those waste products out."

Managing the solution is key, Kowalski asserted, pointing to three limiting factors in any PECM machine:

Of the three, Kowalski characterizes flow rate as "the big constraint." Although PECM is a different process, Kowalski believes its best operators come from an EDM background because they understand fluid dynamics and flow.

The uniformity of the electrolyte solution moving between the tool and the workpiece is critical. "If you have air bubbles or voids in the flow, then that affects how the current travels between the tool and the workpiece," Risko explained. "And hence, it affects the metal removed. Many times a tooling configuration called ‘closed tooling’ or a flow box that encompasses the tool and the workpiece is used. The electrolyte flows through with some back pressure on it to make sure that there's sufficient uniform flow between all areas of the gap between cathode and workpiece."

Controlling flow isn't the only variable in managing the solution. The process uses the same electrolyte, sodium nitrate, for different metals, but the pH, conductivity, and concentration changes for different materials.

"For the most part, we run about an 8.2, almost a balanced pH," Kowalski said. "But certain materials, you’re going to want to increase your pH."

The system can automatically add sodium hydroxide to increase the pH and nitric acid to decrease it. In addition to an overall system filter, the PECM machine has a fine inline filter to remove precipitated metal in the solution that helps prevent it from re-entering the gap between the anode and the cathode, which would diminish machining quality.

"The machine has a filtration process that captures all of what we call the sludge, all the oxides and hydroxides that we’re dissolving," Kowalski said. "It goes through our product filter, and comes out in cake form."

Some PECM byproducts are toxic and require special handling and disposal. Because the majority of medical components are chrome-containing stainless steels, the toxic metal hexavalent chromium-6 is likely present in the filtered byproduct, Risko warned.

"A number of these more advanced systems now can reduce that hexavalent chromium-6 to chromium-3, which is easier to dispose of, because it's not a toxic liquid," he said.

No less important than the electrolytic solution is the tool itself."One of the exciting benefits of this process is the tool is not consumed over time," Herrington said. "Think about CNC machining, where the end mills get worn down over time, or even electrical discharge machining where that wire or the EDM electrode gets destroyed. That's not the case for us."

Voxel has used CNC, micro-machining, wire EDM, sinker EDM, 3D printing, and photolithography to make tools out of stainless steel, titanium, and other conductive metals.

"It basically affords us the ability to manufacture tools in some really interesting and exotic ways," Herrington said. "Sometimes, even if the tool costs us $10,000, that's not a big problem for PECM. We’re not going to throw it out after a few parts."

RM Group's tools "are very similar to that of a precision die set that you would see in a stamping press," according to Kowalski. "Most of our tools, if not all of our tools, utilize five-place decimals. So we are building our tools to within microns, not within tenths."

The sodium-nitrate electrolytic solution is relatively inexpensive and, because the machines are high amperage and low voltage, they don't cost a lot to operate compared with other machining methods. They are also virtually self sustaining and require little maintenance, according to Kowalski.

"It's almost like a chef's frying pan," he said. "It gets better with age."

The costlier items are the machine, process development, and tools.

"With the cost of the machine and the tool and everything else, you’re looking at seven figures when it's all said and done," Kowalski noted.

Front-end costs are associated with developing the electrode and figuring out pulse widths, vibration profiles, and how the electrolytic solution will flow, Herrington explained.

"You might be spending another additional tens of thousands of dollars in engineering time to develop the technique and the process, how you are going to do it in production, and those types of things," he said. "It's not uncommon for an application, from the time an engineer or a customer sends us a model, (for a customer to) spend anywhere from 50 grand to 150 grand or more to develop the process and the tooling."

He compared PECM's costs to those of good injection molding. Although they both entail high upfront costs, they provide affordable, high-quality production parts in return.

"In many, many cases, PECM is less expensive because you are doing area machining, you are not creating any burrs, and you have excellent surface finish," Risko said. "With a conventional machining process, you would have a secondary process of removing burrs and improving the surface finish."

Due to equipment and development costs, PECM is not suited for low-volume production, Risko said. However, medium- to high-volume applications can take advantage of the extremely long tool life and area machining capability.

"So when you take all of these attributes of PECM into account, it means that in applications where the process excels it is competitive and less expensive," Risko said.

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