Metal Injection Molding (MIM) technology has grown in sophistication to become a leading manufacturing method for many medical-device OEMs since the commercialization of Metal Injection Molding in the mid-1970s. What is the attraction? Basically, MIM can produce complex parts more quickly and at a lower cost than conventional techniques such as machining and investment casting. The technology also allows device engineers a lot of design freedom, and it produces high-density components. Common medical components produced by MIM include laparoscopic parts such as biopsy cups, dissectors, and instrument handles, as well as hearing-aid cans and orthodontic brackets.
Under the hood of MIM
Metal Injection Molding combines powder metallurgy and thermoplastic injection molding. The injection molding aspect of MIM takes place at relatively low temperatures and pressures in conventional injection-molding machines. Molds are similar, with slides and multicavity configurations. After injection molding, the binder in the MIM feedstock is removed and the part is sintered (changed to a coherent mass by heating without melting) to form the metal part. Sintering takes place under a reducing gas atmosphere at high temperatures (above 2,000° F), near the melting point of the metal.
As the furnace reaches this “solidus” temperature, pores in the part are eliminated and the part shrinks. Part shrinkage can vary from 15% to 25%, depending on the alloy. Tooling must be precisely oversized and compensated so the sintered part shrinks to the correct dimensions. Shrinkage is predictable and uniform along all axes and finished parts retain the original complex shape of the molded part since the powder and binder are mixed to a precise homogeneous blend. Scrap is eliminated or significantly reduced since machining the part after sintering is usually unnecessary.
MIM versus Machining
As mentioned, Metal Injection Molding can be more cost-effective than machining, but it’s important to take into account factors such as anticipated size, unit volume, component geometry and complexity, and material and tolerance restrictions. MIM tooling can cost more than $30,000, so it represents the largest investment in the manufacturing of a component. When the anticipated volume isn’t high enough, the investment may not justify the means.
Parts with a simple geometry can be produced more cheaply by stamping or, possibly, Swiss screw machining. However, Metal Injection Molding can cut costs substantially for components with features such as cross-drilled holes, threads, and fins, all of which require expensive secondary machining.
Miniature components are not a limitation for Metal Injection Molding. In fact many MIM parts are nearly invisible. On the other end of the size spectrum, a golf ball is often used as a measure of the process’s limit. Actually, large size has less to do with process capability and more to do with capacity. For large parts, molds produce fewer components and furnaces are limited by pounds per batch or hour. It goes without saying that it’s possible to sinter many more small parts than large parts in a session because of the difference in weight.
Materials, too, can govern whether a part is machined or produced by MIM. Suitable materials for MIM are stainless steel, nickel iron alloys, and F-75 alloy (a biocompatible cobalt chromium often used in implants). Some materials, such as the purer forms of titanium and aluminum, do not sinter well because they have a tendency to oxidize too easily. Stainless steel and alloys containing a high percentage of nickel are good candidates for MIM because the soft and gummy materials are difficult to machine. MIM also allows for custom alloys. Sometimes an alloy needs tweaking to meet specific design objectives or exhibit needed properties. Small batches of customized alloys can be processed to ensure application-specific objectives are met.
And when it comes to tolerances, the sintering process along with the effects of friction and gravity can introduce slight dimensional variations. Therefore, components with extremely close tolerances might not be the best candidates for MIM.
MIM and Medical
The medical industry began using Metal Injection Molding earnestly in the late 80s or early 90s. Laparoscopies were increasingly performed, causing medical OEMs to compete furiously for this business. After the “dust” settled, the market demanded disposable laparoscopic instruments. Now the trend is towards computer-controlled, robotic surgeries requiring miniaturized graspers, scissors, and sewing units. Traditionally, components used in minimally-invasive surgeries measured about 11 to 12 mm. Today, they measure 5 to 6 mm. Orthodontics and elective surgeries have been adversely affected by the economy, but mainstream medical work remains steady. Laparoscopic components, for instance, provide consistent work in the re-engineering of components to be ever smaller and smaller.
The hearing-aid market also has become hot recently. Previously, many manufacturers bought 80% nickel alloy in sheet form to fabricate RF-shielded hearing-aid cans. During the process, the sheet had to be deep drawn and annealed four to five times. In contrast, MIM molds the can net-shape with walls measuring 0.011-in thick. In comparison to the old method, MIM cuts costs by 30% to 40%.