A Primer on the Benefits of Metal Injection Molding

Today, medical device manufacturers are able to produce high-performance, complex geometric parts and components for surgical instruments at high volumes cost effectively using a metal injection molding (MIM) process. This flexible, hybrid technology should be part of any full-service contract manufacturer organization’s (CMO) offerings for its ability to deliver high-quality medical device parts consistently using a single manufacturing process.

Demand for lightweight disposable surgical devices has been steadily increasing as hospitals see a distinct cost advantage for off-the-shelf, single-use products that do not require sterilization. With COVID-19, disposable sterile solutions that can help minimize risk of infection transmission in the hospital offer a safer alternative versus reusable instruments that undergo repeated use and sterilization. CMOs and their original equipment manufacturer partners (OEMs) can work together to address changing market needs with innovative technology, and MIM can help manufacturers meet the increased demand.

Both MIM and traditional machining processes can be used to produce medical device components effectively and efficiently. MIM does not replace machining but it offers distinct advantages for certain high-volume, high precision projects where parts may be smaller and require more maneuverability along with strong mechanical properties.

The MIM Process
The MIM process is similar to plastic injection molding, but with metal feedstock. It integrates the shaping capability of plastic injection molding and materials flexibility of conventional powder metallurgy to efficiently produce small complex parts at high volumes using high temperature and pressure. The process uses combinations of metal powder and plastic binders that are blended and compounded so an injection moldable feedstock can be produced to fabricate the part.

Using an injection molding machine, the parts produced are then subjected to a binder removal process. Depending upon the type of binder used, different methods of debinding are applied, and a thermal process to remove polymer is applied. The parts, after debinding, then go through a sintering process to ensure the right material composition, physical properties, and correct geometry. Newly molded parts are the shape of the final part but larger. The sintering process allows controlled shrinkage by 15 to 25 percent of full density. The end result is a net shape part. Further post-processing may be needed to arrive at the final part.

MIM Versus Machining: Factors to Consider
CMOs should consider several factors when determining whether to use MIM versus traditional machining, as both have advantages and disadvantages depending on each project scope and requirements. With MIM, components can be produced in a single manufacturing process. Machining and other technologies typically use multiple processes and require secondary operations, which require more labor. MIM may have several distinct advantages when considering the following factors:

• Volume: MIM is cost-effective when large volumes are needed and lends itself well to automation where high volumes, tight tolerances, and consistent quality are required. In contrast, machining is more cost-effective for lower production volume runs. With MIM, the biggest cost is setup to create the mold, but after this initial investment, the cost can quickly be recovered when amortized over high-volume runs.
• Material: A wide variety of materials can be used with MIM and the process allows a great degree of flexibility to customize material compositions to meet specific attributes required by an OEM customer. Selecting the right materials and feedstock composition are critical factors for success. Some of the compositions are stainless steels, low alloy steels, carbon steels, Ni-alloys, tool steels, and tungsten alloys. Unlike machining, there is less waste and scrap produced with a MIM process and fewer raw material inventories are needed, which can help reduce costs.
• Properties: MIM is best suited for very small, high performing precision products and parts with tight tolerances and consistent dimensions over high-volume runs. Components can achieve 95 to 98 percent of wrought material density at a much lower cost.
• Design Complexity: MIM is ideal for producing complex-shaped components with complicated design geometry, eliminating the need for multiple steps and assembly required with a machining process. In addition, MIM allows significant design freedom as parts can be designed and manufactured with minimal design restrictions. Nearly any design change can be made within a short development cycle and turnaround time. Design features such as thin wall sections, exterior and interior threads, holes, grooves, words, and numbers can all be achieved with MIM.

Early Collaboration Is Key
Today’s competitive medical device marketplace requires manufacturers to continually seek ways to reduce development cycles, production time, and costs in order to maximize value and return on investment. An experienced CMO partner can help OEMs select materials and optimize designs to ensure the success of any project. Collaboration early on in the development cycle is paramount for ensuring a component produced through metal injection molding meets the desired mechanical properties. A CMO partner with experienced MIM engineers on staff can help ensure MIM-produced parts are uniform and have consistent qualities without requiring assembly.

Engineers with expertise in both MIM and machining processes can suggest design modifications and provide input to get the best performance out of a product no matter what technique is ultimately used. An experienced CMO partner can work with their OEM customers to factor in design considerations upfront that can save time and costs.

Conclusion
A MIM process is effective for production of small, intricate parts and products with complex geometries at high volumes, including handheld tools used for cutting and articulating during surgery. Devices produced through MIM have excellent strength and properties, offer great design flexibility, and can often be produced at a lower cost than traditional metal machining and die casting. A MIM process also reduces the need for secondary operations and raw materials, which add costs. For the right project, MIM can help CMOs and their OEM customers achieve a competitive market edge.

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Steve Santoro is EVP of medical device contract manufacturer MICRO, responsible for directing corporate technical and commercial teams. He previously held high-level operations, sales, engineering, and general management positions, and was recently appointed a charter board member of the School of Applied and Engineering Technology at the New Jersey Institute of Technology.