Metal injection moulding (MIM) is a relatively new metal-forming process. As the technique has matured and the number of manufacturers grown, medical OEMs have begun to use it to expand their design capabilities. Increasingly complex applications mean that sometimes MIM is the only appropriate process for the task at hand. With the inclusion of biocompatible and implantable materials, components that were routinely machined can be ‘mimed’ for a tenth of the cost. Some OEMS have brought this process in-house with mixed results. However, as it requires specialised skills and equipment, contract manufacturing can be an attractive alternative.

Orthoscopics was one of the first medical markets to adopt MIM. Graspers, blades, staplers and cauterising cutters are small, geometrically complex and must be manufactured in a biocompatible material (typically 17-4 or 316 stainless steel). Early designs were machined, which made each component expensive. As MIM stabilised and became established, suppliers became more consistent, and more medical companies began using the process.

Today, MIM applications include bone drills, robotic arms for surgery, bone rasps, cutting jaws, biopsy jaws, needle guides, saw guides and hundreds of endoscopic instruments.

With the ability to manufacture cobalt chrome (F-75), titanium and 316 stainless steel, MIM can also create implants, from very small, spinal staples to joint replacement including knees (femoral and tibial).

Complex designs have made tooling, mould and processing components increasingly difficult, and moulding has become much more sophisticated, with pressure sensors in each cavity, as well as in runner systems.

Parts of the process

MIM unites the shape-making capabilities of plastic injection moulding with the strength of metal. It is essentially a four-step process: compounding, moulding, debind and sintering. During compounding, a fine metal powder (5μ-20μ) is mixed with a plastic binder (approximately 60% metal and 40% binder). This compounded material is then put in a modified plastic injection moulding machine and injected into a mould. At this point the component is around 20% larger than the final size. This component is called a ‘green part’. This is put into a chemical bath and most of the plastic binder is removed. Some binder remains to keep the metal powder in the moulded form. This process can take up to five hours, depending on thickness and size. The green part is then put into an oven where the plastic binder is removed through evaporation. This process can last between several hours and several days, depending on part size. Most MIM companies are moving away from this process.

After debind, the green part becomes ‘brown’, and goes into a furnace (vacuum or continuous) where any remaining binder is removed and the temperature increased to near the metal’s melting point. The component shrinks by around 20% to its final size.

"Metal injection moulding unites the shape-making capabilities of plastic injection moulding with the strength of metal." 

The biggest issue with MIM is the lead time. Even a smooth development could take up to 24 weeks (from PO for tooling to production): a problem with a mould could add up to four weeks, depending on the magnitude of the change.

The main advantage of MIM is the choice of materials it offers OEMs. From copper, nickel-iron, soft magnetic materials to hard materials, HK30 and hastelloy, medical materials include 17-4SS and implantables, titanium, 316SS and F-75 (cobalt chrome).

MIM competes with other metal forming techniques including die-casting, machining, stamping and investment casting. It is best suited to shaping complex components in high volumes, (10,000 or more parts a year). Material selection is also a major factor. The die-cast element for an 18g component would be around $0.15, whereas the material and processing costs would make using MIM for the same piece about six times as expensive. MIM is only advantageous if higher material properties, or implantable materials, are needed, and should not be considered wherever magnesium, aluminium or zinc can be used instead.

Many OEMs have considered bringing MIM manufacturing in-house, but few have succeeded. MIM is a capital intensive process; startup costs can exceed $5 million, just for the equipment (including compounding, moulding, debind, sintering and secondary apparatus), and without accounting for the experienced personnel needed to run the facility. Recently, a multinational die-casting company started their own MIM capability. After three years of development with little success, they acquired an existing MIM company, mostly for its expertise.

MIM is 85% science: the rest is art. Understanding the dynamics of sintering, and how to minimise the effect of gravity and friction, is essential, and developing a consistent process takes years of experience. Of course, a lot of this can be minimised if experienced personnel can be hired, although they are hard to find and in high demand. Another issue is how to design for the MIM process. Most specialists will help with redesigning components, but this can take time, as most are also programme managers and have limited time for actual development. Some companies are now helping OEMs design components to optimise the advantages of MIM, however. This can save time and money, as they can go directly into production without any costly redesigns during the production process. These organisations can also manage the entire process from design, through production, while teaching the OEM more about the process.

Choose wisely

With more than 200 qualified contractors worldwide, finding the right supplier can be tricky. Many newer suppliers may be too optimistic of their capabilities and underestimate the amount of work it takes to get a medical component into production. An established facility that is well-versed in medical component manufacturing may meet the timelines requested by a customer, as it will understand the procedures required to meet medical specifications. The extra work created by increased quality requirements, along with the required documentation, means that established MIM facilities will almost always quote higher part prices. Companies not familiar with these requirements may initially be cheaper, but will soon discover they need additional resources and/or training to satisfy the needs of medical customers. It is important, therefore, for OEMs to do their homework and prequalify any potential supplier well in advance of choosing one.

"With increasing demand for smaller, more complex and multifunctional components, MIM is set to be a major source of top-quality, competitively priced products." 

A visit to a potential supplier’s manufacturing facility in order to meet the engineering team and discuss their programme management and learn how their processes are implemented ought to be an important part of the decision-making process.

Many suppliers make commitments they can’t meet, or do not understand what it will take to meet OEM requirements. It is imperative you do your research and do not choose a contractor on price alone. A supplier that has the lowest price usually does not have the supporting equipment/personnel to supply a medical component to 13485 standards. Of course, some medical companies do not require this certification, but most are moving in this direction and it proves that the supplier is serious in investing in procedures to supply a quality component.

The future of MIM is exciting. Each year, more materials are developed and advances in 3D printing and prototooling in developing speciality trays for sintering have shortened cycle times significantly. Sintering and environmentally friendly debind equipment are also adding to process improvements. With increasing demand for smaller, more complex and multifunctional medical components, MIM is set to be a major source of top-quality, competitively priced products.

A MIM contract manufacturing checklist

  • Does your contractor track each programme, and does the customer receive weekly progress updates?
  • Are the engineering, quality and manufacturing departments 13485-certified, and do they work together?
  • Are procedures followed stringently, and is there complete traceability from incoming material through compounding, moulding, debind, sintering and secondary operations, such as coining or machining?
  • Do they check chemistries and dimensions on each lot out of sintering before the product is moved to the next step?
  • Do they work with qualified OSPs?
  • Do their work instructions include monitoring and certifying OSPs?
  • Who is responsible for a non-conforming product from an OSP?