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Part-Making Hurdles Spur Changes in Metalworking

  Originally Published MDDI June 2006 METALS   New materials, tools, and processes help ease the production of demanding metal components.   William Leventon  

METALS

Medical device designers are specifying metal parts that are smaller and more complex than ever. A tall order? You bet. But that's not all—designers also want higher part quality, better performance, tighter tolerances, lower costs, and shorter cycle times.

It's a daunting list of demands. But they're being met, thanks to a variety of innovations. New material options are appearing, and new products are improving lab work. New machines are offering greater speed and precision. And new processes are tackling the toughest metalworking challenges. Here's an overview of developments in five key areas.

Materials

For metal-testing operations, Buehler encapsulates specimens in epoxy and places them into these specimen holders to facilitate mounting in grinding and polishing machines.

Medical components are getting ever smaller, but their functional use stays the same. Today, “a device has to perform like a crowbar but look like a toothpick,” says Chuck Edwards, executive vice president of Micro Medical Technologies (Somerset, NJ), which manufactures metal parts for medical OEMs.

In many cases, commonly used metals don't meet the needs of these demanding new products. So Micro Medical often asks suppliers to change the composition of generic metals. “We'll ask for a certain nickel content or a certain chemical alteration that adds strength or reduces ductility,” says Frank Jankoski, the company's director of technical services. For example, he notes, oxygen content is critical to the strength of titanium. The company works with a materials supplier to alter the oxygen content of the material to get the performance characteristics required for a given small medical part.

Memry Corp. (Bethel, CT), a company that manufactures nitinol alloys, also has a titanium project of note. The company is studying a titanium material that has some of the properties of nitinol but contains no nickel. This superelastic titanium material offers good strain recovery and biocompatibility. It also offers flexibility and a low modulus that's close to that of bone, making it a candidate for use in orthopedic implants, says Mark Polinsky, Memry's director of engineering.

But a number of challenges must be overcome before the material is ready for the implant market. For one thing, the material would have to be supplied on a scale that meets the needs of casting and machining operations. Today, nitinol is normally provided in amounts suitable for products such as wire, strip, and tubing, Polinsky notes.

Top inset: Micro Medical's metal-injection molding (MIM) processes create surgical parts that need no secondary sharpening. Main photo: Micro Medical created this surgical instrument with a rolled tube in a progressive die. The tool's other components were also made with MIM processes (bottom inset).

As for nitinol itself, Polinsky adds, some researchers are evaluating methods of creating microscopically thin layers of the material. Among other things, these researchers want to discover the temperatures that trigger phase changes in the material. Phase changes are significant because they alter the size and strength of a nitinol component.

In addition, Polinsky says, researchers want to determine the amount of movement or force produced by the phase changes. The movement of superthin nitinol layers could make them suitable for implantable microswitches. A tiny amount of current would be sufficient to trigger a phase change that flips such a switch, according to Polinsky.

Superthin nitinol layers could also produce the small amounts of motion required by tiny dispensing and actuation devices. For example, Polinsky notes, nitinol could be a key component of a micropump used for drug delivery. Normally, such devices require multiple parts to perform pumping functions. But a micropump could also be actuated by a thin piece of nitinol that moves in response to temperature changes. Such a device would be smaller and require less current than conventional micropumps, Polinsky says.

Testing Specimens

In some testing operations, metal specimens are encapsulated in an epoxy that facilitates mounting in grinding and polishing machines. By filling the voids in the specimen, the epoxy also clears out liquids that can emerge to cause problems when the specimen is being studied.

Although they're helpful in the laboratory, some epoxies used for test specimens pose a danger to humans, who must wear respirators when working with the materials. But Buehler Ltd. (Lake Bluff, IL) has developed a new epoxy that allows testing personnel to remove their respirators. Six years in the making, EpoHeat helps produce excellent samples and is safer to use than competitive products, says George Vander Voort, Buehler's director of research and technology.

The epoxy is a low-viscosity material that flows easily into the crevices of metal samples. It's also less gummy than some epoxies, which eases grinding and polishing operations. “If I were holding a sample [that included some other epoxies] in my hand, I would be able to feel the grabbiness of those epoxies as I grind and polish,” Vander Voort says. “That grabbiness isn't as bad with EpoHeat.”

Agie's Micro-Nano EDM system has three axes that are based on a parallel kinematic structure. It features rigid flexible bearings that are friction- and backlash-free. Here, a technician adjusts one of the parallelograms.

Buehler also aims to improve the metal-testing process with a new line of grinder-polisher machines. These machines feature platen cooling that limits the heat introduced into the epoxy during grinding and polishing of specimens. Excessive heat can melt thermoset epoxies, Vander Voort explains, causing mounting and polishing problems.

In addition, he notes, such machines offer a powerful head motor. Dual head speeds allow both slow operation for materials prone to smearing and high speed when time is critical. Some of the machines also measure the removal rate during grinding and automatically stop when the right amount of material has been removed from a specimen.

Manufacturing Equipment

Medical device manufacturers may benefit from new machine technology for metal-part manufacturing such as electrical-discharge machining (EDM). Agie Ltd. (Lincolnshire, IL) has developed two new EDM options. The Agietron Micro-Nano is a high-end die-sinking EDM system for high-precision medical components. Equipped with a technology called parallel kinematics, the Micro-Nano improves resolution and repeatability by reducing the friction between moving parts. The machine offers a rough positioning accuracy of ±1 µm and a fine positioning accuracy of 0.2 µm, according to Ken Baeszler, sales support manager for Agie. The Micro-Nano cuts tiny three-dimensional features into metal parts. Surface finishes can be very fine, with a roughness (Ra) of 0.1 µm or better, Baeszler says.

Less expensive than the Micro-Nano is the company's Agiecut Attak, a wire EDM system designed for high-volume applications. It features a low-maintenance design with no belts and a wire-threading system with few moving parts. That means a low operating cost, says Baeszler. The system can use any type of wire to cut surgical tools and implants that require EDM. It can also use indexing equipment for cutting products such as bone saws, Baeszler notes.

Currently, Norman Noble uses Swiss machining for six-axis EDM work. However, in the future, the firm hopes to use wire EDM systems to cut intricate features into small implantable parts.

Unlike other EDM options, six-axis wire EDM work has long been a very slow and expensive process, according to Kevin Noble of Norman Noble Inc., a contract machining firm based in Highland Heights, OH. But the needs of the medical market have pushed machine manufacturers to improve wire EDM technology for six-axis work. “They've done everything they can to make this [process] work a lot quicker in medical machining applications,” says Noble, the firm's chief technology officer.

Currently, Norman Noble uses Swiss machining for six-axis work. But EDM technology has made such strides that Noble believes his company may soon be using six-axis wire EDM systems to cut small features into bone screws and other implantable parts.

Another machine-related advance helps manufacturers of very small medical parts. Normally, bar stock measuring 1/8 in. in diameter is the smallest that can be fed into a machine's bar feeder. “If it's smaller than that, the bar stock wants to flex and bow and do a jump-rope type of motion,” Noble says. All of this movement in the machine translates into parts made inaccurately.

Recently, though, a Swiss machine manufacturer introduced a unit that can handle bar stock with a diameter of less than 1/32 in. According to Noble, this supersmall bar stock doesn't jump around inside the new Swiss machine, so it can make parts accurately. Smaller bar stock also means significantly lower material costs—particularly if the manufacturer is using an expensive material like nitinol or platinum. “Instead of buying 1/8-in.-diameter platinum, you can buy 1/32 in.,” Noble says. “That makes an enormous difference in the materials cost.”

Metalworking

Novel metalworking processes have found favor in the device industry. For example, Noble reports, many medical OEMs specify nitinol parts that are made using a technique called shape setting. In the shape-setting process, a piece of nitinol is bent into a certain shape in a machine that also exposes the metal to heat and chemicals for a set period of time. When the nitinol is removed from the machine and cools down, it returns to its original shape. But when it's heated up again, Noble explains, it remembers its shape in the machine and once again takes that shape.

During the shape-setting process, “you're teaching the metal to take a certain shape at a certain temperature,” Noble says. This property can be useful when nitinol components are inserted into the body during surgery. Body heat causes these components to take the shape they were given during shape setting. The resulting movement may help surgeons maneuver instruments inside patients or anchor components securely in the body, according to Noble.

Buehler's variable-speed, programmable grinder-polisher features material removal and can prepare specimens up to 2 in. in length.

In some cases, processes change to meet special needs. American Micro Products Inc. (Batavia, OH) needed to change its gauging process to handle the submicron tolerances of a machining job for a medical customer. Part of a so-called capable manufacturing process is the ability to measure amounts one-tenth as large as the process tolerance, notes American Micro's J.W. Childs. For example, manufacturers must be able to measure 0.1 µm to prove that they are capably running a process with a tolerance of ±1 µm.

American Micro was forced to deal with even smaller tolerances when it took the job of grinding a valve for a medical metering device. One of the product's dimensions has a submicron tolerance, so the capable manufacturing rule requires that the process take measurements at a scale of hundredths of a micron.
To achieve the required measurement accuracy, the company worked with another firm to develop a proprietary process that involves measuring the flow of a certain material through a valve. “You measure tolerances by measuring the flow, because you know what the flow has to be for the tolerancing to work,” Childs explains. “Not many people in the world use that approach,” which cost about $750,000 in this case, he adds.

Manufacturers are modifying metalworking processes in many ways to improve results and lower costs. American Micro is working on a process that could significantly reduce manufacturing time and costs by pressing features into metal parts instead of machining them. According to Childs, a $400,000 machine might take 20 minutes to cut the required features out of a piece of metal. By contrast, he says, a $100,000 press might be able to create those same features in a single 10-second cycle.

On the downside, pressing metal can produce stress risers that eventually result in broken parts. Fearing broken implants in the bodies of patients, medical device manufacturers usually prefer machining to forming, Childs notes. But he maintains that testing of individual components can minimize the dangers from stress risers. “If we can prove to [our customers] that this process works, it will be a real money saver for them,” he says.

In tube manufacturing, tolerances typically increase with tubing size. So relatively large tubes like those used for stents are usually made with larger tolerances than smaller tubes, Memry Corp.'s Polinsky says.

To get better control of the wall thicknesses of larger tubes, Memry employs a two-step manufacturing process. The company's standard drawing process is performed on a hollow bar with a deformable core of soft material. In the new process, the soft core is used for only part of the drawing process. When the tube is near its final size, the soft core is replaced by a nondeformable hard core that remains in place for the finishing drawing steps. According to Polinsky, this so-called hard-mandrel tubing process gives the manufacturer better control over wall tolerances, thereby reducing wall thickness variations around the circumference of larger tubes.

Normally, metal components are stamped and then moved to another location for secondary welding operations. But Micro Medical is working on a system consisting of a laser welding head mounted inside a progressive stamping die. To control the welding process, a computer or programmable logic controller is integrated with the press controls. With each stroke of the press, the laser is fired to perform one welding operation.

By reducing part handling, in-die laser welding reduces the time and cost of manufacturing operations. Reduced human handling should also improve part quality, Jankoski notes. The cost of a press with laser heads mounted in dies could be more than $1 million. But, according to Edwards, such a large capital investment could be justified by process-related savings in high-volume production of a single product or a number of products similar enough to allow rapid changeovers from one product to another.

For a detailed examination of resistance and laser welding processes, see the article “Resistance and Laser Welding for Medical Devices” on page 98.

Metal-Injection Molding

Manufacturers form a variety of metal parts using the multistage metal-injection molding (MIM) process. Phillips Plastics Corp. (Menomonie, WI) has had success using MIM to make metal jaws for cutting tissue. However, MIM can make it difficult to maintain the tolerances required for such parts because shrinkage occurs during the final sintering step. Therefore, the jaws are normally machined, according to Tony Pelke, engineering manager for the firm's MIM operation.

To solve the tolerance problem, Phillips came up with a two-step approach. First, the company employs special fixtures that allow the jaws to be sintered very close to the required tolerances. Then the jaws are stamped using a coining die, which reshapes the parts to bring them within the tolerance limits.

Phillips Plastics uses extensive automation when machining metal parts to maintain constant cycle times and minimize part handling.

At Micro Medical Technologies Singapore, manufacturing personnel have identified material homogeneity as a key to MIM success. The material fed into the molding machine is a mixture of powdered metal, binders, fillers, and other ingredients. The consistency of this mixture—or the lack of consistency—has a major effect on the results of the molding process.

For example, says Edwards, “if there's more binder in one part than the next part, the shrinkage that takes place during the sintering operation will vary. That variation changes dimensions from part to part.”

According to Jankoski, most MIM operations in the United States buy a commercial mixture for their molding processes. But Micro Medical's Asian facility prepares its own molding compound using a proprietary vacuum mixing technique to ensure material homogeneity and part consistency.

The vacuum mixing technique also helps the company eliminate cracking in parts when they're in the fragile so-called green state before sintering. If a part is cracked before sintering, “there's an inherent failure mechanism in that part,” Edwards says. “If the part is an end effector for a medical device, for example, the crack could cause the end effector to break during surgery.”

Another key to the elimination of green-state cracking is automated handling of the fragile parts. At many MIM facilities worldwide, Edwards says, green-state parts are hand loaded into sintering containers or simply drop off the end of a conveyor belt into boxes. Fragile parts that are handled by humans or fall haphazardly into boxes are more likely to crack than parts that are handled by robots, he maintains. In medical product manufacturing, using robots to place green-state parts into sintering trays helps ensure part quality.

Conclusion

It seems that medical device OEMs consistently ask for parts that are less expensive and more complex. But metals suppliers are rising to the challenge by developing new processing techniques or materials options. Some are changing the composition of the metals, while others are developing metalworking processes that yield more-precise results. In addition, metal machining equipment is getting faster and is producing parts with tighter tolerances. The bottom line is quality, and metals suppliers are answering the call.

William Leventon is a freelance writer based in Somers Point, NJ, who frequently contributes to MD&DI.

Copyright ©2006 Medical Device & Diagnostic Industry

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