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Metal Molding – It’s Not Just a Theory

A version of this article was first published in Medical Device Developments 2021 Vol. 1

Increasingly, the medical device industry is turning to metal injection molding to manufacture small complex components. The past 18 months have revealed just how important the technology has become. Donald F. Heaney, editor of the Handbook of Metal Injection Molding, as well as the Founder and CEO of APP, and Nick Eidem, APP's Director of Business Development, explain why:

Covid-19 has impacted many industries, but very few companies have felt it quite as sharply as medical device manufacturers. And, as if the pandemic wasn't enough on its own, over the past 12 months we've seen a cargo ship splayed sideways in a canal, a hacked pipeline, and global raw material shortages. It's no surprise that supply chain risk is the major topic of conversation. While pushing the demand for single-use devices into overdrive, the pandemic has made them consistently harder to source.

Enter metal injection molding (MIM), which combines the most useful characteristics of powdered metallurgy and plastic injection molding to facilitate the production of small, complex-shaped metal components with outstanding mechanical properties. MIM is a green technology built around specialists with the ability and metallurgical expertise to formulate feedstocks in-house which means it doesn't rely on fragile supply chains, greatly lowering risk for OEMs. Moreover, MIM is perfect for producing single-use devices because of its ability to meet the higher volume requirements of complex geometries at a cost-effective price point. That's not all. MIM can support a wide variety of medical device applications. These include dental/orthodontic brackets and devices, surgical devices, orthopedic components, delivery systems and robotic surgery applications. Stainless steels are among the most common alloys used In medical device components, especially single-use devices where long-term patient contact is limited. Advancements have also been made to utilize bio-implantable Cobalt-Chrome alloys as a cost-effective alternative to Titanium.

The MIM Process

Feedstock Compounding

The metal injection molding process can manufacture a wide variety of materials and alloys. Check out our full list of available metal injection molding materials. Once a MIM material is selected, the MIM process starts in house with mixing APP's proprietary MIM feedstock, a combination of fine metal powders and binders, such as waxes and different polymers. Once cooled, the feedstock is granulated into pellets to prepare for injection molding. APP’s proprietary feedstocks are designed for a specific shrink rate and allow for enhanced material flow and greater processing parameters. To ensure the reliability and repeatability during processing, the feedstock compounding process is tightly controlled.

Injection Molding

Processed with a fully automated injection molding machine, the MIM feedstock is heated and injected at high speeds into a tool or mold with multiple complex cavities. Once molded, the result is called a "green part". Due to the additional binding agents, the green part is roughly 20% larger than its final size allowing shrinkage during the debinding and sintering process.

Debinding

The parts then move through the first stage debinding. The binder systems used in the formulation of MIM feedstock requires a two-step debinding process. Although there are several methods to remove binder, metal injection molding uses two primary methods to remove the binders during this stage, catalytic and solvent, where most but not all of the binders are removed to prepare for sintering. Once this process is completed the part is considered a "brown part".

MIM Sintering

The MIM sintering stage is where the brown parts are placed in a high temperature furnace. The part is heated near its melting point, all the remaining binder is completely removed, and the metal particles are bonded together. The part then shrinks and densifies, and the final strength and geometry of the metal part is formed.

Secondary Operations

After sintering, metal parts can then be sent to additional operations to improve dimensional control, achieve tighter tolerances, increase mechanical properties, and visual appearance, such as heat treating and coating. These are called secondary operations and can be in house or contracted out. Finished MIM parts are manufactured to around 98% theoretical density of wrought metals resulting in similar mechanical properties.

The Art of MIM

Whether they're needed to minimize supply chain risk or provide the optimal medical device component, strong metallurgical expertise is a baseline for the top MIM manufacturers. This expertise is important for source appropriate raw materials and developing strong molding and sintering processes required to produce high-quality components at volume.

Metal injection molding also allows for design freedom that may not be possible through other manufacturing methods. MIM tooling can form complex geometries that would otherwise be very expensive to machine. Even if the geometry is able to be machined, scaling may be difficult with a large investment. By contrast, one MIM tool can produce anywhere between one and one million components with relative ease. The mechanical properties of MIM are also very good. MIM can achieve greater than 97% density on average and can stand up to the rigorous requirements of medical device manufacturers.

Typical Attributes Produced by the MIM Process

Attribute Minimum Typical Maximum
Component Mass (g) 0.030 10-15 300
Dimension (mm) 2.0 (0.08 in) 25 (1 in) 150 (6 in)
Wall Thickness (mm) 0.025 (0.001 in)* 5 (0.2 in) 15 (0.6 in)
Tolerance (%) 0.2% 0.5% 1%
Density 93% 98% 100%
Production Quantity 1000 100,000 100,000,000
*Features this small could have distortion

By Design

Speed is the essence when the goal is saving lives. Rapid prototyping has been shown to be perhaps just as important to the medical device industry as single-use devices. It’s also an ideal fit with MIM, and APP is utilizing metal 3D printing technology for early design testing and MIM experiments. Metal 3D printing can be use to get rapid prototypes in the hands of engineers without the up-front cost of tooling. APP’s propriety 3D printing technology, PrintAlloy®, uses MIM powders and sintering furnaces to create prototype components that meet MPIF standard mechanical properties of MIM components. The overlap in equipment and technology allows for testing during component development.

Many medical device manufacturers rely on the expertise of MIM suppliers to aid In the design of components specific for the MIM process. Design for manufacturing (DFM) performed by MIM experts can help alleviate component geometry risk in production. During the design phase, expert review tooling design, component modality, and sintering capability.

The metal injection molding market has grown around 6% annually is projected to grow at a CAGR of 11.9% through 2027. MIM is becoming more widely accepted among engineering and supply chain professionals. It will continue to grow as more and more companies push for single-use devices.

MIM Medical Applications

MIM Medical Applications