Metal Injection Molding Becomes a Go-To Manufacturing Process

Once only employed when other part-making methods failed, metal-injection molding (MIM) has matured to become the process of choice in many applications.

“MIM used to be the last resort for making parts and was used only when the part couldn’t be made any other way,” said Jim Adams, vice president, technical services for the Metal Powder Industries Federation (MPIF), parent of the Metal Injection Molding Association (MIMA), Princeton, N.J. “But the technology is at a level where many parts are now being designed for MIM, such as hearing aid components, surgical tools, automotive turbocharger components and brackets for braces. Today, more multiple-machined part assemblies are being replaced with a single MIM part, reducing energy, assembly time and, ultimately, cost.”

MIM can produce high-volume, high-precision components for a range of industries. Its use is growing in the manufacture of medical devices, firearms, automotive parts, dental devices, electronic packages, cutting tools and other industrial parts.

Courtesy of Megamet Solid Metals

An array of parts made via metal-injection molding.

In its annual members-only PM Industry Pulse Survey, MIMA asked its members in what North American markets they are seeing most of their products used. Last year, firearms and medical were the two primary areas, with roughly 25 percent of the market for each, followed by automotive at around 15 percent, dental at less than 5 percent, electronics at less than 5 percent and telecom at 1 percent, with the remaining percentage being general industrial applications, according to Adams.

The percentage for auto, however, may not trail the leaders for long. “The automotive industry is where the largest untapped opportunities remain for the North American MIM industry,” said Bruce Dionne, president of MIMA and general manager of Megamet Solid Metals Inc., Earth City, Mo., which produces MIM components.

Courtesy of Megamet Solid Metals

A typical mold Megamet uses to make MIM parts.

One area where Randall M. German sees growth is implantable medical devices. German is professor of mechanical engineering at San Diego State University and a noted MIM expert. “The innovators are doing implants, not just the hand tools,” he said. “MIM is going into bone reconstruction, heart valves and implantable hearing aids. Hot new topics are advanced surgical stapling devices and small robotic manipulators that use MIM parts for linkages.”

In North America, stainless steel is by far the most common MIM material, accounting for about 45 percent of the total, according to Adams, with steel in second at 30 percent. Soft magnetic materials are significant at 10 percent, but tungsten, nickel, titanium and aluminum are each used in only about 1 percent of applications.

“Titanium is very difficult to process,” Dionne said. “It has a strong affinity for oxygen and as a result it is hard to maintain a pure titanium grade throughout all phases of production. Some companies are doing it successfully, but in very low quantities in specialty applications.”

Courtesy of Metal Powder Industries Federation

A stainless steel shuttle used in a “smart” stapling device for arthroscopic surgery is made via MIM. The 5g shuttle has two components that previously were combined via laser welding. Secondary operations are reaming and tapping the small hole.

Courtesy of Ryer

Ryer’s 17-4 PH D90-8 powder metal is suitable for molding MIM parts.

Another material challenging to work with is aluminum. “It is difficult to sinter and so the process has to be done precisely,” said Ron Peterson, vice president of Ryer Inc., Temecula, Calif., a MIM feedstock supplier. “But it is good for components that are weight-sensitive and parts subjected to elevated temperatures where plastic can’t be used.”

The MIM process involves mixing metal powders with thermoplastic binders to form a feedstock that is injection-molded using standard plastic injection-molding machines.

After ejection from the mold, the binders are removed (see page 66). The parts are then sintered at a temperature high enough to create metallurgical bonds between the powder particles, but below the melting point of the metal.

During the sintering process, the parts typically shrink 15 to 22 percent, depending on the debinding system used, to achieve the final component density of 96 to 98 percent of the full-density solid metal (2 to 4 percent porosity).

Small World

MIM’s strengths lie in high-volume production of small, complex parts. Tooling and setup costs are difficult to justify for low-volume work, so MIM works best for production quantities exceeding 20,000 annually.

“MIM is good for long runs so you can get your pricing down, and it provides very accurate part-to-part consistency,” said James Liddell, sales and marketing for Mold Craft Inc., Willernie, Minn., a moldmaker whose customers make medical devices, dental products and consumer goods.

MIM is especially effective at producing components that require holes, slots, ribs, bosses, grooves, protrusions and multiple features. The process also can be used to combine two or more simple components into one more-complex component. “Small parts that have complex geometry, fairly consistent wall thicknesses and a flat surface are good candidates for MIM,” Dionne said. “Examples of parts that are not competitive include ones that can be easily stamped or turned on a CNC lathe or Swiss screw machine.”

Generally, MIM parts weigh less than 250g. Typical maximum dimensions are from 25mm to 35mm (0.98 ” to 1.38 “) and wall thicknesses are typically 1mm to 3mm (0.04 ” to 0.12 “). A MIM part can have as few as three or as many as 100 or more measurable dimensions.

According to German, the average MIM part produced has a mass of about 6g, based on reports from companies about how many parts they make and from powder makers about how many tons of powder they sell per year. “It used to be 10g about 10 years ago,” he said.

Heavier and larger parts can be made via MIM but it is typically not cost-effective. The parts take longer to process, severely impacting energy utilization. With large parts there are also issues with wall sections or lengths. “There are cross-sectional thickness limitations of around 3⁄8 “, typically,” Dionne said. “Any thicker and the parts take an extremely long time to debind and run the risk of cracking.”

The larger the part, the greater the part-to-part variation. Typically, the process can maintain dimensional tolerances of ±0.003 ” to ±0.005 ” (±0.076mm to ±0.127mm) per linear inch in each direction, depending upon geometry. “Using this as a basis, a 10 “-long part would only be able to maintain ±0.030 ” to ±0.050 “, which for most precision engineered components would not be acceptable,” Dionne said. “Additionally, the parts would consume the entire tolerance range, so extreme-sized parts on either side of the normal distribution curve would exist in production, making it possible to have parts vary at a potential extreme of 0.1 ” in this 10 ” example.”