Metal injection molding (MIM) is a metal processing method that has become a beneficial manufacturing technology for the medical device industry, as it can produce complex-shaped, high-density, and high-performance metal parts at a low cost. Designers and engineers in the medical devices field are able to compare MIM to traditional machining techniques in the small parts arena.
Among the various technologies for metal parts production, metal injection molding (MIM) offers a number of advantages and cost benefits for certain applications. In general, the process is particularly suited to small components, typically less than 100 grams, and can produce complex-shaped, high-density, and high-performance metal parts at a low cost. As a result, MIM is a metal processing method that has become an essential manufacturing technology for the medical device industry, and one that is garnering increased attention from designers and engineers in the medical devices field.
By combining the processes of thermoplastic injection molding and powder metallurgy to produce net-shape metal parts, MIM has, over the past decade, established itself as a competitive manufacturing process for small precision components. The process is comprised of feedstock preparation, injection molding, debinding, and sintering. Given that the process features high and uniform density, surface finish, strength, and shape complexity, MIM is a favorable comparison to traditional machining techniques in the small parts arena.
In the medical industry to date, MIM has been used to produce components for hearing aids, implants, and orthodontic brackets, with additional applications being developed as the technology continues to advance. For example, tungsten heavy alloys are being produced to form radiation shields for syringes used in the delivery of radioisotopes during chemotherapy. Another example is an articulation gear made of stainless steel 17-4PH used in a surgical stapling unit. There has also been much progress toward the development of implantable MIM products for applications such as drug delivery devices and joint replacement.
One of the companies at the forefront of MIM production is SolidMicron Technologies Pte Ltd. (SMT), a Singapore-based manufacturer that specializes in the mass production of precision metal parts through MIM technology. Among the research SMT is actively pursuing is the development of an MIM process for titanium material—in particular the Commercially Pure Titanium (CP-Ti) and Ti-6 A1-4V—which is of certain interest in the medical industry due to its combination of high strength and light weight corrosion resistance.
Compared to other shaping technologies that produce small, complex components in large volumes, MIM offers plastic injection molding, die casting, investment casting, machining, and conventional powder metallurgy. According to SMT, MIM can perform more tasks because the starting powder used for MIM is of a very small particle size that allows the material to fill out small details of the mold. Consequently, the smaller the component, the lower the cost will be.
MIM also enables the production of components with a density of up to 95-98%, a level of densification that is impossible with wrought materials from conventional powder metallurgy. The density improves the mechanical property of the components, which in turn results in higher mechanical strength and ductility. For instance, SMT’s MIM process improves percentage elongation (the force a component can withstand before breakage), yield, and tensile strength of the components.
Furthermore, MIM allows for a high volume of production that drives costs down. Compared to other metal processing methods, MIM is a high-repeatability production method that provides a consistent component result. Additionally, MIM produces near-net forms resulting in very minimal waste material, while contributing to a lower production cost. MIM can generate thousands of parts in a day, a volume that is normally unachievable by other methods, such as machining and casting.
Seeing as MIM requires upfront investment for the tooling, it is not an optimum choice for projects involving low volume and simple geometry components. For small, complex components required in large volume, however, MIM offers high production rates made possible through the use of multiple-cavity tooling. MIM also provides a nearly unlimited shape and geometric feature capability, including undercuts, threads, and thickness variations.
Specific comparisons to other production methods clearly demonstrate the advantages of MIM. Investment casting, for instance, is slow, labor-intensive, and expensive, requiring many secondary operations, and hard to control tolerances. MIM has a lower cost, short production cycles, and high repeatability, with minimal secondary operations and an excellent surface finish.
Compared to die casting, MIM offers high-quality mechanical properties and no rough finishes, while enabling the use of a wider range of materials. In contrast to machining, MIM has the ability to manufacture intricate designs with almost no material waste; while the densities capable with MIM are higher than with conventional powder metallurgy.
SMT, founded in 2006, has been continually expanding its production capacity to meet the increasing demand for MIM products, and operates a 20,000-plus square foot manufacturing facility under industry standards ISO2009:2008 and TS16949:2009. Among its key capabilities with MIM technology is the ability to incorporate several small components into a direct injection molded element. The incorporation eliminates the machining process to produce the individual components, as well as subsequent processes needed to join the components together. Accordingly, significant cost savings are imparted to customers.
Moreover, SMT incorporates complex inner features into products with the use of removable plastic inserts. The undercut feature that is unable to be formed by normal tooling, is incorporated into the plastic insert and formed during the injection molding cycle. The plastic insert is then removed during subsequent processes, leaving the desired shape on the product itself.
In its research and development of the MIM method utilizing titanium, SMT is addressing the high reactivity of titanium, and has sourced an MIM-viable powder to use in the process, with particular attention being given to the binder composition and the sintering atmosphere of the MIM processing environment. The goal is to reduce reactivity toward interstitial elements, and to minimize the reaction between the Ti powder and other contaminants that reduce the mechanical properties of the consolidated components. The company plans to begin mass production of titanium-based MIM components in early 2012, and will make the process available to medical device customers worldwide.
Tai Chee Keong is a Metallurgy Engineer at SolidMicron Technologies Pte Ltd.