Metal injection molding (MIM) can be a cost-effective alternative to EDM'ing, casting, or machining for relatively small medical parts. The process combines powder metallurgy technology with the complex shape-making capabilities of plastic injection molding. "MIM part sizes are usually described by weight," explains Director of Operations Mike Stucky of NetShape Technologies Inc (netshapetech.com) in Solon, OH. "The typical weight range is about 5 to 25 grams, which translates to about 1/4-in. long × 0.020 in. diameter up to about 4 × 3.5 × 3 in. But parts can go down to tenths of grams and up to around 100 grams."

MIM materials

Among the advantages of MIM is that it handles a wide variety of materials in low, or high-volume runs. "Medical parts such as debridement tips and dental bracket hooks are often made from 17-4 stainless steel," says Stucky. "But with the right equipment, shops can MIM tool steels, tungsten heavy alloys, magnetic materials, and even titanium."

MIM also lets designer pick material properties, such as strength, hardness and corrosion-resistance. "In addition, the process eliminates worrying about issues such as machinability and weldability," says Stucky. "MIM can build complex, high-tolerance parts containing small, precise features. Designers often look at die casting for complex shapes and high-production rates. However, die casting is typically restricted to low-temperature alloys such as magnesium and aluminum alloys. MIM provides better material properties without the expense of machining or casting and then machining."

According to Stucky, MIM tooling is similar to that used in plastic injection molding. "In our case, the shops that make our tools are primarily plastic-injection mold makers," he says. "There are minor differences in vent depths and gating schemes, but otherwise, the basic construction is identical."

In addition to metal powder, an important component of MIM is the polymer binder system, which mixes with the powder to create a feedstock. The type of binder system used dictates whether molds incorporate hot or cold runners. NetShape Technologies uses hot runners systems, whereas other shops might use cold runners.

"The kind of equipment a shop already has often dictates what kind of binder it selects, so the choice is mostly a matter of convenience, "says Stucky. "The binder a shop chooses has a lot to do with when the company came into the MIM game, whether the shop is going to compound its own feedstock, and where the shop learned the technology — did it license the technology or pick up the information from an academic paper? However, binders don't really play a role in part design."

Again, designers need to be concerned with selecting the correct alloy with the needed strength, hardness, and corrosion resistance. However, MIM shops can blend metal powders to create materials with very specific properties. "For example, should a designer want a 17-4-like stainless-steel alloy with more corrosion-resistance than the base alloy, it is possible to adjust the individual alloy element ratios," says Stucky. "Plus, there are a lot of commercially available elemental powders that let shops, say, buy iron powder and then blend in amounts of nickel, chromium, and molybdenum. There are even alloys available intended to be tailored. When working with magnetic alloys, designers are often looking for a specific magnetic response. Shops can adjust the alloy elements to get that response."

17-4 stainless steel is a very good alloy for medical parts because it has good corrosion-resistance and it is very strong and can be heat treated, says Stucky. "316 stainless steel has even better corrosion resistance but not quite the strength of 17-4. However, for a lot of applications it's an excellent choice."

"We don't handle implantable alloys, but there are a lot of companies that do," says Stucky. "Those alloys are specified by medical standards to ensure they are biocompatible. Examples are titanium and cobalt-chrome alloys for hip implants. But usually, alloys are not tailored for a specific application. Instead, they are designed to meet an acceptable standard of either the medical industry or some other governing bodies."

Relative to highly machined parts — those undergoing either multiple CNC operations or EDM'ing — MIM is considered high volume. "For medical, the range is around hundreds of thousands a year," explains Stucky. "However, MIM can handle a million parts a year. What really defines the quantities is the application and the willingness of the customer to pay the price that's needed to make the part viable for the producer. For some parts, we produce a thousand a year, while for others we might produce a hundred thousand a month."

As far as casting, there are a lot of cast 17-4 parts that could do well with MIM for the right size part, says Stucky. "MIM supplies better material properties and equivalent or better shape capabilities, and it does finer detail than casting. We do see a lot of parts that were cast and machined, and we can MIM the part and eliminate the machining operation."

When it comes to tolerances, most MIM companies quote ± ½%. "For example, when the part's longest dimension is 1-in. long, we quote ± ½%, or 0.005 in. As the dimensions get smaller, MIM holds even tighter dimensions. Large dimensions get worse because of the effects of gravity during sintering, distortions due to nonhomogenous molding or feedstock properties. ± ½% is a common number to quote to."