Part Making on a Very Small Scale: Micromolding for Medical Devices

Originally Published MDDI May 2002

PLASTICS MOLDING

Some medical microparts measure less than a cubic millimeter, but micromolders and their customers want to go even smaller.

William Leventon

As medical devices continue their shrinking act, manufacturers are coming up with part designs that are tinier—and tougher to mold—than ever. "Customers are asking for smaller, lighter, thinner components," observes Stuart Kaplan, president of Makuta Technics Inc. (Columbus, IN), one of a small number of companies that mold so-called "microparts."

In the medical industry, microparts include sensors, catheter tips, tubes, and implants. Demand for these supersmall parts is growing rapidly, notes Donna Tully Bibber, vice president of sales for Miniature Tool & Die Inc. (Charlton, MA), a maker of steel micromolds. "We're being inundated with really funky, small applications that we would have giggled at not long ago. But now that we can make them, we're not giggling anymore," says Tully Bibber, whose company has made micromolds for parts less than 2 mm long.

Although there's much talk about micromolding these days, there's no set definition of the term. "Plenty of manufacturers say they do it," says Tully Bibber, "but they really just mean that they make small parts."

Miniature Tool & Die defines a micropart as anything smaller than a single plastic pellet or weighing less than a tenth of a gram. But current micromolding technology can do even better than that, turning out parts just a few ten thousandths of an inch thick and weighing less than a thousandth of a gram.

How small is small enough? We're not there yet. Micromolders predict that demand and technology will continue to drive down part sizes—perhaps to the point where molds can no longer be used for molding.

MICROPART MACHINES

To manufacture microparts, the molding industry depends on a new crop of injection machines, molds, and part-handling systems. Some micromolders use screw-and-barrel molding machines similar to conventional machines except that the components are much smaller. Makuta Technics, for example, uses screw-and-barrel injection systems with diameters that range from 14 to 20 mm and nozzle orifices of 1.5 mm. These systems can produce shots weighing less than a gram, according to Kaplan.

Another micromolder using screw-and-barrel machines is Empire Precision Plastics Inc. (Rochester, NY), which has made parts weighing less than 0.0008 g. The key to molding on this scale is getting the right amount of plastic into the mold—also known as "dosing the shot," according to Neil Elli, president of Empire Precision. When molders don't accurately dose their shots, Elli explains, they risk overpacking their parts, which can cause the parts to stick in the mold or even to break it.

According to Elli, molders who want precise shot dosing can buy electric injection machines with servomotors capable of very accurate screw positioning. Or, if they plan to stick with hydraulic machines, molders can install a valve gate that shuts when the right amount of plastic has been injected into the mold.

A new option for micromolders is an injection machine that replaces the traditional screw with a motor-driven plunger. Named for the part sizes it produces, the Sesame is a small electropneumatic micromolding machine made by Hull Corp. (Warminster, PA). The machine can mold 20 thermoplastic "sesame seeds" from a single pellet, according to Bob Boland, Hull's sales manager.

Instead of a screwing action, the Sesame uses a tiny plunger to push material into the mold. This plunger, which can be as small as 1.5 mm in diameter, is driven by an electric servomotor. To ensure accurate shot sizes, the servomotor can control plunger position to within 5 µm. Total injection time can be as brief as 0.020 second, Boland says.

To keep plastic flowing through its small passages, the Sesame relies on a combination of high injection pressures (up to 50,000 psi) and high melt temperatures. High temperatures degrade materials during the time they spend in a conventional machine, but the Sesame is designed to minimize so-called residence time. Material is in the Sesame for only "a couple of minutes, compared to a couple of hours in a regular machine," notes Andy Leopold, vice president of Medical Murray Inc. (Buffalo Grove, IL), which developed the Sesame technology and has licensed it to Hull.

What accounts for the difference in residence times? During a molding cycle, the Sesame holds an extremely small amount of material, far less than the smallest screw-and-barrel machine. "Even a short screw would hold 20 times more material" than the Sesame, Leopold says. The less material there is to heat, the shorter the required residence time.

A conventional method for removing material from a machine before it degrades is by using a large runner and sprue. Sometimes, Leopold says, the runner and sprue are so big that the part material makes up less than 1% of the shot. That means that more than 99% of the shot material is wasted. Worse, the manufacturer loses control of the part-molding process. "What you're actually doing is molding the sprue and runner, and the part becomes a kind of by-product," Leopold says.

The Sesame is designed so that molders can use a smaller runner and sprue, which gives them more control over the amount of plastic and pressure used to form the part itself. A smaller runner and sprue also means less material waste. While screw-and-barrel systems waste as much as 99.7% of the shot material, the Sesame wastes less than 80%, according to Leopold. This is particularly important when molding expensive materials like biodegradable plastics, which cost as much as $10 per gram.

The Sesame can handle any type of moldable plastic, as well as silicone rubber. Supersmall medical parts that have been molded by the machine include:

THE MICROMOLD CHALLENGE

Of all the challenges in a micromolding operation, Leopold puts mold making at the top of the list. Dennis Tully would probably agree. "In the macro world, even tight-tolerance tools are much more forgiving than they are in the micro world, where a 0.0005-in. variation could kill a project," says Tully, vice president of engineering at Miniature Tool & Die, which makes molds for parts weighing as little as 0.0006 g. "With the small sizes and tight tolerances, there's really no room for error in the micro world."

Microscale mold making has gotten a boost from recent developments in electronic signal sensing, part measurement, and process control. According to Makuta Technics's Kaplan, these improvements allow mold makers to hold tolerances of ±10 nm while cutting mold steel.

To make tiny features, mold makers sometimes turn to unconventional processes, such as reactive ion etching (RIE). In RIE, mold makers use reactive ions to knock metal atoms out of a mold surface, explains Jonathan Colton, head of the micromolding program at the Georgia Institute of Technology (Atlanta).

Other mold makers use lasers to create extremely small features. For example, some micromolds require holes so small that they can't be made with a conventional electronic discharge machine (EDM). In cases like this, Medical Murray uses lasers to burn mold holes as small as 0.001 in. in diameter.

Some conventional high-volume molding operations use 64-cavity molds. But such molds won't do for micromolding, Leopold contends. "You lose control of the individual cavities, so the parts don't come out the same." For micromolding, Leopold prefers two- or four-cavity molds, which yield more-repeatable parts.

Microscale parts can be ruined by a rough mold surface, so mold makers must pay close attention to surface finish. An EDM surface that looks smooth from a distance will probably appear much rougher up close, Leopold notes. To smooth these surfaces, mold makers often polish them with brass or copper tools. Another option is plating the mold surface with nickel or tin. Elli recommends plating for molding thermoplastics containing an abrasive material.

Once microparts have been formed in the mold, they must be removed and placed in containers. To manage this process efficiently, some micromolders have developed clever micropart-handling systems. At Makuta Technics, for example, production lines can run unmanned thanks to automated venturi systems that suck microparts out of the molds and place them in tubes.

Regardless of the microparts on the drawing board, mold makers should enter into the design process as early as possible. "Often, an engineer will design something that we can't tool," Elli says. "So we have to be part of the design team. That way, we can help customers design parts that are manufacturable."

MATERIAL CONSIDERATIONS

One of the most important decisions facing micropart designers is what material to use. Colton recommends plastics with a high melt-flow index and low viscosity at processing temperatures. He also prefers low-temperature polymers that protect molding machinery from exposure to excessive heat.

Elli tries to avoid plastics with glass-fiber reinforcement that doesn't flow easily through long, thin-wall mold features. And he sticks with premium suppliers that can deliver material with consistent melt flow from one lot to the next. "When you're making microparts, the cost of the plastic isn't significant," he says. "So you shouldn't be trying to save a few cents on materials."

Though material costs are usually negligible, the cost of specialized machines, molds, and secondary equipment makes micromolding "a pretty expensive process," Elli notes. OEMs should be prepared to pay as much for microparts as they do for larger components. "I make parts that weigh a third of a gram and are just as expensive as parts that weigh 4 g," Elli says. "Buyers tell me that they expect [microparts] to cost less because they require less plastic. But there's no correlation between size and cost."

GETTING SMALLER

Although today's medical microparts seem incredibly small, demand from device manufacturers will drive part sizes down even farther. To get smaller from here, Kaplan believes micromolders will rely primarily on new mold-making technologies. "That's really where the magic is," he says.

One such technology could be LIGA, a lithography, electroplating, and molding technique developed in Germany. Already, Tully notes, companies are producing LIGA structures that could be converted into molding cavities. "The technology allows [mold] sizes to be minuscule—probably much smaller than anything plastic will flow through," he explains.

To accommodate supersmall part-forming processes, materials could be inserted into a mold in a new way. For example, different monomers could be separately injected into a mold, where they would combine to form polymers. This way, Colton says, "you're not trying to shove in big molecules. You're using smaller molecules and then reacting them once they are in the mold."

At the submicron level, Colton believes melt-and-squeeze methods of shaping plastic may reach their limit. Then, he says, manufacturers may turn to reactive part-forming techniques. For example, a manufacturer could fire several lasers into a box of gas. At the point where the laser beams intersect, the energy would produce a reaction that causes a part to form. According to Colton, this is one way part manufacturers could realize a long-discussed dream: molding without a mold.

CONCLUSION

New machine and process technologies have combined to make molded microscale plastic parts a reality. In the medical industry, micromolding techniques are used to manufacture parts such as sensors, catheter tips, and microsurgical components. Tiny as they are, however, today's microparts will be too large for some of tomorrow's applications. As a result, micromolders are considering unconventional techniques that could help them make components even smaller than the current crop of flyspeck-sized offerings.

William Leventon contributes frequently to MD&DI.

Photo courtesy of Empire Precision Plastics Inc.
Photo courtesy of Phillips Plastics Corp.

Copyright ©2002 Medical Device & Diagnostic Industry