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Silicone vs. Metal Molds in the Medical Industry

Ilene Wolff
By Ilene Wolff Contributing Editor, SME Media

Some in the medical industry are using silicone rubber molds made with a 3D-printed master pattern for low-to-mid production runs of cast polyurethane device housings.

Among the reasons they’re doing so are the ability to be first to market with a new product; be able to enter new markets; make small runs to test new devices; make design changes more easily; and even achieve full production for items needed in small numbers.

3D printing speeds up the cast urethane process, according to Chuck Alexander, director of product management, Stratasys Direct (Valencia, CA), the manufacturing division of Stratasys Ltd. “Creating a master with 3D printing allows you to get to a completed mold and the casting process quicker,” he said.

In addition, 3D printing enables masters with complex geometries and internal features for housings for such items as testing, diagnostic and therapeutic devices; instrument panels; medical and dental capital equipment; and lifestyle items such as treadmills and exercise bicycles.

Once the mold is made, the polyurethane’s liquid components are poured in at a specific ratio. The polyurethane is cured with heat, which gives the thermoset a chance to harden and develop ultimate mechanical properties. External features can include snap fits, interlocking joints, threaded fasteners, and living hinges.

Generally, a silicone mold makes sense for production runs of 500 units per year or less, according to Alexander. Typically, when medical manufacturers introduce new devices, the volumes are generally low—tens or hundreds of units annually. Injection molding tooling can be cost-prohibitive in markets with that low of an annual sales volume. For some applications where production runs remain small, a soft silicone tool may be all that’s ever needed.

Stratasys Direct partnered with EMD Millipore Corp. to make cast polyurethane housings for this cell analyzer.

“We’ve had customers sell hundreds of units a year and they never go to a hard tool,” Alexander said. “They stay with cast urethanes.”

It’s also cheaper and faster to make a master silicone tool that takes days and costs thousands of dollars vs. creating a metal mold that takes weeks to make and costs tens of thousands of dollars for injection molding.

“You can use that soft material since it doesn’t have to hold up to the pressures of injection,” Alexander said. “If you’re adding pressure, even if it’s low pressure, you need a hard tool to do that. And that’s where the cost goes up.”

For markets where innovation is the norm and product life is short, silicone tooling to cast polyurethane also allows manufacturers to improve their designs at a faster pace without having to amortize the cost of hard tooling. “Any design change becomes easier to incorporate as you go along,” Alexander said.

Devices can also incorporate fillers such as fiberglass, carbon fiber, and Kevlar to add rigidity, strength or, in the case of Kevlar, impact resistance. The fillers are laid into the mold by hand, then polyurethane is cast into it and subsequently permeates the weave of the filler fiber.

Other available material features important in the medical industry are compatibility with medical imaging, electrostatic dissipation and electromagnetic shielding to block electronic signals either from or to a device. Copper or silver coatings can be applied in a layer 0.005” (0.127mm) thick using standard spray paint equipment after the casting process is done.

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