The drive in the healthcare industry toward less-invasive, less-damaging medical procedures is causing medical devices to shrink. But smaller devices require smaller components, thereby placing increasing pressure on manufacturers to keep pace with miniaturization. Not immune to these pressures, some providers of molding services and manufacturers of molding equipment are downsizing--they're adopting micromolding techniques that ultimately enable medical professionals to perform procedures that result in smaller incisions, shorter recovery times, and less scar tissue.

Ruling the Micro-Realm

Providing components for a range of medical specialties, Saint-Gobain Performance Plastics Corp. (Portage, WI; www.medical.saint-gobain.com) is at the cutting edge of micromolding. "When you're talking about really small parts, your production speeds and processing capabilities have to be on the dime," remarks Jason House, applications engineer at Saint-Gobain. "The silicone products that we make using our liquid injection micromolding (LIMM) process are replacing some existing products that were once made from rigid thermoplastic materials." Such products could conceivably be fabricated using a compression molding process, House adds, but that process would not be able to compete with the rapidity of the LIMM process.

To meet the needs of shrinking devices, the company can manufacture silicone components as small as ±0.001 in. for drug-delivery applications, vascular devices, catheter components, IV therapy, and minimally invasive surgical procedures such as laparoscopy, thoracoscopy, and ophthalmology. In the area of tolerances for holes or similar features, the company's molding operations can insert throughholes on miniature parts down to 0.010 in. in diameter. It also has expertise in creating blind shutoffs--holes in the thin sidewalls of hollow cylinders that do not protrude completely through the wall. "To make such a feature in the micro-realm, I have to come in and shut off on a quarter pin," House says, "and I can form that feature on parts that are 0.012 in. in diameter."

The challenges of creating minuscule components with even more-minuscule features are daunting. "We've mastered the process quite well, but we're consistently learning," House notes. "We perform data studies and field-test studies to better define our microcapabilities."

For example, one of the company's biggest difficulties is refashioning a product that was once larger while maintaining appropriate flash- and gate-defect tolerances. Flash defects can occur where a parting line is present in the tooling. In such areas, the steel can separate when the mold shuts, resulting in the flashing of silicone through the seam. Gate defects can occur in two- or three-plate molding applications that require an edge gate or a round drop gate. While a land area on the gate in the silicone is supposed to be flush with the surface of the part, occasionally the gate is not formed properly, causing it to break up in the land area and leaving a residue of silicone that corresponds to the size of the gate. "In microparts manufacturing, flash- and gate-defect tolerances become much stricter," House explains. "The customer wants to see less flash and less gate vestige."

As for what's next, House insists that multimaterial overmolded microcomponents are the wave of the future. In the micromolding world, the trend is shifting from the assembly of two separate components to the fabrication of a single smaller component in which silicone is overmolded on a rigid substrate, House says. "We see this as driving not only toward stand-alone silicone parts but also toward combination products with very high silicone functionality."

Variations on a Micromolding Theme

While the goal of micromolding is the manufacture of tiny components, some companies vary their offerings by providing hybrid products. A case in point is microPep (East Providence RI; www.micropep.com). Specializing in microsized parts, the company can also form microfeatures on larger molded parts. "There are myriad challenges involved with micromolding," says Scot MacGillivray, microPep's business development manager. "In micromolding applications, everything is tiny, and the specialty toolmaking required demands a high level of skill."

From minuscule wall thicknesses and gate sizes to demanding aspect ratios between hole diameters and lengths, micromolding is more complicated than standard molding, MacGillivray stresses. And handling microparts is a trickier proposition. "It is imperative to handle tiny parts properly," he says. "They need to be removed from the mold and packaged before they end up in places they shouldn't be."

Offering parts as small as 0.007 in. for such medical components as catheter tips, microPep can fabricate objects that weigh as little as 0.00012 g, have wall thicknesses down to 0.003 in., and boast features as small as 3 µm--not to mention geometries such as microwells measuring 3 µm and holes for fluidic transfer measuring 0.0025 in. Despite all the rigors, micromolding offers cost advantages over machining, according to MacGillivray, making it an attractive option for medical device manufacturers.
 

Nanomolding Challenges

"The general drive toward micromolding is that everyone wants to have their components--particularly ones that go into the body--smaller and smaller, more detailed, and more feature packed," remarks Brent Roland, vice president of marketing and sales at Medical Murray Inc. (North Barrington, IL; www.medicalmurray.com), a supplier of nanomolding equipment and a provider of in-house nanomolding services. "The point is for doctors to make as few trips as possible into the body to diagnose, sense, intervene, or deliver something--whether it be medicine or a device."

Designed to meet the challenges of shrinking device sizes, the company's Sesame nanomolding machine can hold low volumes of melted material and offers controlled high-speed and high-pressure injection capability. These attributes enable the equipment to make submicromolded-size parts with complex geometric features from all types of thermoplastic and silicone rubber materials, as well as from bioabsorbable polymers that would degrade in standard equipment. The machine can also be used to make insert-molded parts requiring tiny, integrated features for such applications as overmolded polymers, electronics, and radiopaque markers.

In nanomolding, small means really small. "When you start to play with the language surrounding sizes such as fractions of a cubic meter, a nanoportion of a cubic meter is 10^-9," Roland says. "And when you take one sliver of a cubic meter, that's a 1/10^-9 sliver, or a cubic millimeter. That's the ballpark upper-end size that we play in." Most molders categorize their parts either in terms of weight or volume, Roland adds. The top weight of a typical Medical Murray shot size is approximately 0.08 g, but it can be 100 times lighter than that. In volumetric terms, a nanomolded component can be 1 mm³ at the top end and at least 100 times smaller than that at the bottom end.

The main challenge facing Medical Murray has been training its tooling designers to be able to work in the nanomolding size range. "It's an art," Roland says. "We've been fortunate enough to cultivate three or four tooling people that no longer laugh at us when we bring them our ideas on part sizes. They've dug down and developed processes to build tools for us that can accomplish complicated, complex geometries in very small real-estate packages."

It has also had to acclimate its tooling suppliers to the tiny molds it requires to fabricate miniature components. "It's been an educational process for them," Roland comments. "They were quite good at making larger things--what we would call micromolding--but they did not have a lot of experience until we helped them to get access to other processes that would allow them to design tools to make the complex features we need."