By Sean Fenske, Editor-in-Chief 

Molding is almost synonymous with medical device manufacturing as many parts within a product are fabricated via the process. What isn’t as common, however, is when metal is used in place of plastic. Metal injection molding, however, brings with it numerous benefits for device makers of a wide array of healthcare technologies.
 
With today’s trend toward miniaturization of devices and minimally invasive procedures, the demand for micro-sized components has never been greater. Metal injection molding offers specific attributes in the components crafted using this method that meet this need. Strength, repeatability, and favorable cost are just a few reasons device makers may want to further explore this option.
 
Fortunately, Nicholas Eidem, Director of Business Development at Advanced Powder Products Inc., took time to address a number of questions around this manufacturing method. In the following Q&A, he provides insights on metal injection molding for components at a “typical” size as well as at the micro scale.
 
Sean Fenske: While many are familiar with injection molding of plastic, can you please explain the process of metal injection molding?
 
Nicholas Eidem: Metal injection molding, commonly referred to as MIM, is a manufacturing process that combines the most useful characteristics of powdered metal and plastic injection molding. The MIM process starts by formulating a “feedstock” from fine metal powders and polymer binder used to carry the powder. The feedstock is then palletized and injected into a mold using plastic injection molding machines.
 
This is where the synergies between plastic injection molding and MIM really shine. MIM and plastic injection molding tooling use very similar design principles; however, MIM parts are less forgiving than plastic parts. Once the part is ejected from the mold, it is called a “green” part. A green part is 20% larger than the final geometry due to the addition of the binders. The green part is then sent through a first stage debind, where most of the binder is removed but the part still retains the 20% increase in volume.
 
The part is now ready for sintering. The MIM part is staged for sintering using ceramic supports to help retain the final geometric shape. During the sintering process, the binder is removed by heating the MIM part close to its melting temperature, which may vary based on alloy. When the binder is removed, the MIM part shrinks to its final geometry and densifies to 98% of theoretical density. Any features not supported during sintering may distort; therefore, custom ceramic fixtures are developed to keep distortion to a minimum. Once the part is sintered, it may be finished with a number of secondary processes just like a machined counterpart. These processes include heat treat, coatings, coining, machining, passivation, etc.
 
Fenske: For what types of applications is metal injection molding ideally suited?
 
Eidem: MIM is used where high volume, complex machining is not cost-effective for the OEM. MIM parts can be found in your iPhone and Apple Watch. Globally, MIM is used in automotive, electronics, outdoor recreation, hand and powder tools, military and defense, and medical device applications. Metal injection molding has a broad range of applications within medical devices, especially single-use products such as surgical instruments, dental brackets, orthodontic procedures, robotic surgery components, orthopedic implants, cardiac implants, and drug delivery devices.
 
Fenske: Are there material limitations with metal injection molding? What metals can be used?
 
Eidem: MIM can produce components in a wide variety of ferrous-based materials. Typically, if the powder is available in a small enough particle size (10-25 micron) and it can sinter to a high enough density, it can be used in metal injection molding. The most common alloys used in medical device applications are:
 

• Stainless Steel (ASTM B883) – 17-4PH SS, 316L, 420
• Bioimplantable Cobalt Chrome (ASTM F2886) – F75 ASTM, MP35N
• Titanium (ASTM F2989) – Ti-6Al-4V Grade 5

 
Fenske: How much complexity does metal injection molding at the micro level add? What are the primary challenges?
 
Eidem: There are numerous challenges and considerations that need to be identified and resolved throughout the MIM process to manufacture MicroMIM components. The feedstock needs to be specifically designed to process components with small features. The mold used to form the geometry will have very delicate components. These components need to be accounted for during the MIM design for manufacturing (DFM) and tool design process. The injection molding process is also tailored to process micro components. One additional aspect that may be overlooked is material handling and inspection. MicroMIM components are very small and difficult to see with the naked eye and often difficult to measure.
 
Fenske: Why is micro metal injection molding necessary? For what types of applications is it being used?
 
Eidem: Today’s product designs for medical devices require smaller and smaller components that are extremely durable and meet rigorous tolerance specs. Minimally invasive components deliver huge patient-care advantages by shrinking the size of devices ranging from minimally invasive biopsy cutters to catheters. Standard manufacturing processes have limitations at a micro scale. Even if the part can be manufactured, scalability and throughput can be challenging, resulting in high-cost components.
 
MicroMIM technology was developed to meet requirements for metal components with extremely small-size specifications. While the average mass of a conventional MIM component is 15-20 grams, MicroMIM is defined as the molding of components less than 1 gram or have critical features less than 100 microns. Some current MicroMIM parts are as small as a few tenths of a gram.
 
Fenske: What considerations should manufacturers keep in mind when considering or specifying metal injection molding—traditional and/or micro?
 
Eidem: The biggest issue we see when evaluating a MicroMIM part is wall thickness. Typically, the minimum wall thickness that can be molded is 10 thousandths of an inch (0.01”), however MicroMIM components can have walls as small as 4-6 thousandths of an inch (0.004-0.006”).
 
Another major consideration is flash management. Just like in plastics, flash is always a design consideration. At the micro scale, 1-2 thousand of flash that would otherwise be irrelevant now becomes a concern requiring mitigation built into the process.
 
Fenske: Do you have any additional comments you’d like to share based on any of the topics we discussed or something you’d like to tell medical device manufacturers?
 
Eidem: In addition to expanding design engineering freedom, MicroMIM delivers major business advantages in terms of sustainability. Because it is a highly repeatable process, with consistent results, production can ramp up quickly to support critical new product introductions. This is in stark contrast to machine tool production, where each individual component has a statistically significant likelihood of unacceptable dimensional variations. The batch production of micro-components also facilitates assembly flexibility for the final devices where they are used.
 
Cost savings with MicroMIM give additional support to profitability. Traditional manufacturing requires high up-front investments in tooling and fixturing to produce parts with tight tolerances. MicroMIM is a cost-effective alternative, with lower set-up costs as well as a lower marginal cost-per-piece during production. MicroMIM’s reliable product quality during production runs also drastically reduces manufacturing scrap, driving further overall cost reductions.
 
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