Medical Device Link.

MICROMOLDING

Micro-Mold brand thermoplastic parts from Accumold are used in a variety of medical and microelectronic applications. (ALL PHOTOS COURTESY OF DONATELLE.)

Contrary to its name, micromolding has become very big in the manufacturing of medical devices over the last decade or so.

The push for smaller and smaller devices is driven by the enormous growth in the number of noninvasive procedures being performed in outpatient suites and operating rooms. That means there is more need for tools and implantable devices that can transverse orifices and tiny blood vessels.

“They’re doing a lot of minimally invasive procedures, such as single port surgery and NOTES—natural orifice transluminal endoscopic surgery—where the surgeon enters the body through a natural orifice such as the mouth, navel, or urethra to avoid having to make incisions,” says Jim Jock, marketing coordinator of Micro Medical Technologies (Somerset, NJ). As a result, the endoscopes, catheters, needles, and other instruments the surgeons use for the procedures have to be that much smaller and more flexible, he says.

While it may seem impossible, device manufacturers have responded by molding parts with features that can’t be seen without magnification and that often are no bigger than a poppy seed. Aaron Johnson, marketing manager for Accumold (Ankeny, IA), says one of the micromolded parts it manufactures—though not for a medical device—is so small it takes 1000 of them to make any weight on the scale. Raghu Vadlamudi, director of process development for Donatelle, (New Brighton, MN) says his company has molded parts so small that it can make 1 million of them from 2.5 lb of material.

Defining Micromolding

There is some debate in the industry about exactly what micromolding is. Part of the issue is that “there is no industry standard that anyone can say, ‘That is micromolding’,” Vadlamudi says. However, the industry perception is that anything less than 1 mg in weight is considered micromolding, he says.

Technically, micromolding can refer to the microsized parts themselves or to larger parts that have miniscule features. “You can have a part that is bigger than 2 or 3 g in weight that might have a small feature that is less than 0.0002 g and that, too, is considered micromolding,” says Vadlamudi. “Usually, though, when people in injection molding refer to micromolding, it’s tiny parts.”

Donna Bibber, technical partner of microPEP (East Providence, RI), which specializes in micromanufacturing, says the definition for micromolding that she uses is fairly common in the industry. It means parts made from a fraction of a plastic pellet, weighing only fractions of a gram. “You need magnification to see their features or details,” adds Bibber.

Use of New Materials

As micromolding has taken hold, device manufacturers have noticed some trends, including the use of new materials and new methods for achieving smaller features, tighter tolerances, and better surface finishes. Device companies are demanding faster turnaround times, although that trend is true in the industry whether the part is microsized or conventionally sized, according to Johnson.

The medical field is moving more toward implants and devices made from bioresorbable polymers. These materials can be absorbed by the body weeks or months after they have served their purpose, and the tissue has healed. Physicians are moving in this direction, because screws and other devices made of metal can break or can cause adverse reactions. The price of metals is also increasing. Although plastics are increasing in price because they are oil-based, manufacturers have a wider choice of biocompatible materials, says Vadlamudi.

Micromolding and bioresorbable materials work well together, because microsized parts do not consume a great deal of material, and bioresorbable polymers are extremely expensive. Some specialized resins can cost between $3000 and $22,000 per pound. Resorbable polymers have become particularly popular in micromolding because the materials are so costly and using less is critical to keeping finished costs down, says Bibber.

A sensor housing implantable made from polyoxymethylene for hearing aids. Part volume is 3.7 mm3, and shot volume is 159 mm3.

One challenge however, is that micromolding can change the properties of a polymer as it is being squeezed into such a small area. The material has to be forgiving, and the manufacturing process must be designed so that there is little, if any, waste from the process, she says.

Bibber notes that resorbable materials are not that new. Although they’ve been available for at least 10 years, she says, the early ones were like mud, “and you just weren’t able to create thin-walled parts.” However, newer materials that are coming onto the market are much easier to work with. As a result, the number of applications is growing, and micromolders are having to step up to meet the demand for sensors, catheter tips, tubes, and implants that their designers want downsized considerably.

A micro filter made from polyoxymethylene for hearing aids. Part volume is 3.9 mm3, and shot volume is 288 mm3.

Johnson says that while he has certainly seen a lot more interest in bioresorbable materials lately, some issues remain. One is that they can start to dissolve, depending on the temperature. As a result, he says, manufacturers have to be more innovative than ever when it comes to designing the parts in which they are used.

A Slow Conversion

A catch wheel made from polyamide and glass for micro switch applications. Part volume is 5.3 mm3, and shot volume is 112 mm3.

John Whynott, technical product manager for Mikrotech, a division of Asyst Technologies LLC, (Kenosha, WI), expects the demand for micromolded devices from resorbable polymers to continue to increase. However, he says it will be a slow conversion. The medical field has been using metals, including stainless steel, for its devices for the last century, and as a profession, it is rather conservative. Even though “there may be better, newer, cheaper technology out there,” it will take a while to catch on, Whynott says. “Buyers have no experience with it and to them, it’s a huge element of risk going forward.”

However, he says, that lack of confidence in the newer materials and micromachining methods is going to slowly erode as they continue to prove themselves.

An 80-µm micro filter made from polyoxymethylene for applications in medical and acoustics. Part volume is 0.63 mm3 and shot volume is 112 mm3.

Johnson also expects that the demand for micromolding with bioresorbable polymers will increase as the technology improves and the price comes down. “We’ve been asked about it many, many times recently,” he says. “But often the customer goes down another route, because it is cost prohibitive. Eventually the price will come down or processes will be developed that help reduce the cost, and the interest will be there and companies will go with it.”

A locking lever made from polyoxymethylene for micromechanical and microswitch applications. Part volume is 0.70 mm3, and shot volume is 93 mm3.

Johnson says a lot is being investigated that holds much promise. For example, he says, one biomaterials solutions firm, Invibio (West Conshohocken, PA), recently announced that it embarked on a major project with Smith & Nephew plc (London), a global orthopedics maker. “[The goal is] to develop advanced structural bioresorbable materials with the performance specifications needed for more rigid, load bearing applications typically not attainable by today’s resorbable biomaterial technologies,” says Johnson.

New Techniques in Micromolding

Another trend in micromolding is the development of new techniques to provide tighter tolerances and better quality control and surface finishes.

Tolerance is a big issue nowadays. It’s not just about how small the part can be made, Bibber says.

Regardless of size, micromolded parts must have tight tolerances, ranging from ±0.001 to ±0.0001 in. and down. That presents designers and engineers with multiple challenges, says Mike Wilkinson, principal tooling engineer for GW Plastics (Bethel, VT).

Plastic micromolded and small parts made by Micro Medical Technologies used for RealHand High-Dexterity Surgical Instruments. The RealHand won a Medical Design Excellence Award in 2008.

Product and mold designs for microparts have been going through a learning curve over the last several years, Wilkinson says. “Mold manufacturing techniques have evolved. Specialized CNC [computer numerical control] machines, modified electrical discharge machining, and silicon water technology are now being used to produce precise cavity geometry for very small parts,” he says.

Specialized micromolding machines also have improved significantly over the last several years, according to Wilkinson. Today, some of the specialized micromolding machines offer integrated part removal, packaging, and vision inspection.

Johnson says that Accumold, too, “is continually being asked to push the envelope with what you can do with plastic, and we’re answering the demand from a tolerance and part-size standpoint.”

Bibber says one of the many challenges of micromolding is that with small parts, “what you have in steel is what you have in plastic.” Typically, a macromolded part will shrink away from the cavity and onto the core, making the transfer from the cavity side to the ejector side of the mold easier. However, because smaller parts have less shrinkage, the transfer forces are not as great, which means the release from the cavity side can be affected significantly by the surface finish. With conventionally sized parts, the machine cavity finish can be stoned and polished, but for a part with features as small as 0.005 in., it would be impossible to improve the finish by hand.

Like most micromolders, Bibber says, microPEP is developing ways to improve the surface finishes of micromolded parts. It has become an art form just to get the micro parts off the gates cleanly, and when parts go into the human body, they can’t have any jagged edges.

Vadlamudi says micromolders of medical devices are studying and adapting techniques used by watchmakers so that they don’t have to reinvent the wheel.

One technique to finish micromolded parts that has been on the rise is stereolithography.

Bibber says it also is becoming more common for micromolders to remove the miniscule parts from the gates with ultrasonic energy.

Developments with hot runner and cold runner systems also have helped to improve the precision of micromolded parts and the speed at which they can be manufactured.

D-M-E Co. (Madison Heights, MI) has adapted its hot runner system to work specifically for small parts, says Trevor Pruden, a mechanical engineer at the company. “We focus on going smaller and smaller with our channels and making our nozzles more precise,” he says.

A variety of micromolded and small plastic components made at the Largo, FL facility of Micro Star Innovations, a sister company of Micro Medical Technologies.

Validating the micropart is yet another challenge, Bibber says. Using certain vision systems and microscopes that magnify to 600X aids in measuring surfaces and tolerances, but this is an area where companies are always looking for better ways, she says.

Due to the high cost of some materials, companies get upset when they are throwing out more material than they are molding, says Whynott. Thus, the trend in the field has been to utilize processes that are precise and that waste little of the material as it is pushed into the cavities, he says.

As the parts get smaller and smaller, many manufacturers agree that the techniques used to build molds and to develop the molding process has to be changed.

Vadlamudi points out that the analytical technology aids such as pressure sensors and temperature sensors used to understand the process behavior must be redesigned to suit microsized parts. One promising technology uses ultrasonic waves to characterize the micromolding process, he says. “Atomic force microscopy and nanoindentation are the other techniques that are being developed to derive relationships between the process variables and product characteristics,” he adds.

Turnaround Time Improving

Yet another trend in micromolding is the increased speed at which the parts can be turned around.

“As manufacturers get more experience with these kinds of tolerances, it’s getting much easier for them to turn around projects,” Vadlamudi says. “It used to take a lot of time to make a micromolded part. Not anymore.”

Bigger parts afforded manufacturers more freedom, because if a part changed by 0.001 in., it didn’t matter to the finished product, but that isn’t the case with a microsized part. “Being off that much on a dust-speck-sized part and you could lose a quarter of the part,” Vadlamudi says.

Pruden says that thanks to its hot runner and other technologies, the turnaround time for equipment needed in micromolding is now four to six weeks. That’s down from the eight to ten weeks it would take when the process wasn’t as computerized, and the micromolded part geometries had to be input into the equipment by hand.

How Small Can It Go?

Micromolders see the demand for microsized medical device parts only continuing to grow. The question then becomes how small parts can go and what’s the limit.

Many micromolders don’t see an end to the downsizing of parts. “With the development of new technologies, new materials and new processes, there is no telling what tomorrow will bring in the world of micromolding,” Wilkinson says.

He and his colleagues agree—the trend in the world of micromolding is going to keep the sentiment that “if it can be conceived, it likely can be achieved,” says Wilkinson.

Beth W. Orenstein is a freelance writer based in Northampton, PA.