What is metal injection molding (MIM)?
Metal injection molding (MIM) uses a paste made from metal powder and a binding agent. This paste can be squeezed through a nozzle to fill a mold. The binder is removed, leaving behind a metal piece that is ready for use, with very little post-processing required.
Mastering the proper technique for MIM can be challenging. The choice and ratio of binding agent to metal powder is just one consideration in forming quality pieces. A good binder allows the paste to flow through the nozzle and into the mold, filling it entirely. During the sintering process, the piece is heated to remove the binder. Between the filling of the mold and the heating process, the fragile “green” piece must maintain its structural integrity, which is aided by the proper choices in binder material and ratio.
During sintering, the binder must burn or evaporate cleanly, leaving behind no contamination. Furthermore, it must not allow any gas bubbles to form, which may damage the remaining structure. It also must not react with the metal, potentially contaminating the material. This is an important consideration during the heating process, when reactions can be accelerated.
Sometimes, the sintering process is accompanied by a hot isostatic pressing (HIP) step. In HIP, the part is pressed equally from all directions while heated, forcing the particles in close contact. The close contact removes pores and increases densification of the final parts.
Mold design is another important consideration for successful MIM. The fluid properties of the metal paste and the geometry of the mold will determine how well the mold will completely fill. The mold design will meet the design needs of the piece, but it can also include structural reinforcements, vents, baffles and other components.
The end result of MIM is a piece that is formed to a near-net shape, with over 99.5% density. It generates little waste and requires no assembly, such as welding or fastening. In fact, little post-processing is required, but it can be performed on the MIM piece if desired.
Why is MIM used over other methods?
MIM parts are used in a number of industries because of the flexibility in piece geometry. Not only can complex geometries be created, but pieces made from MIM also have strong dimensional stability. This means the mold’s dimensions, including its parallelism, will be maintained through the process. Furthermore, its surface finish is maintained. The dimensional stability means little post-processing is required.
In contrast, traditional metal casting requires high temperatures that can lead to warpage in thin parts. It also requires post-processing to remove sprues and gates where molten metal flows, parting lines between mold pieces, and risers where extra metal is stored to account for shrinkage during cooling. All of this post-processing is expensive and wastes material. Also, casting cannot produce the resolution in detail required for the small, intricate parts that are common in the medical and aerospace industries.
With machining, pieces can be machined from bar stock to precise dimensions. The machining process, however, wastes material and can be time consuming for complex shapes. Depending on the individual design and the machines available, the scrap material, programming time for computer numerical control (CNC) machining routines, and machine tooling costs can make this method more expensive than MIM.
Even with the rise of additive manufacturing techniques, such as 3D printing and selective laser sintering (SLS), MIM offers significant savings in comparison. MIM is repeatable and reliable, and has a much faster processing time than current additive manufacturing techniques, increasing the throughput and decreasing rework and scrap.
Titanium Metal Injection Molding (TiMIM)
One of the neat advantages to MIM is the flexibility in materials available for molding. Titanium alloys are notoriously difficult to cast, but adapt much more easily to the MIM process. Titanium metal injection molding (TiMIM) has become a preferred method for producing small implantable medical devices to tight tolerances.
Why is it important to find the best method to manufacture titanium parts? Because they’re critical for medical devices and implants. Titanium alloys are biocompatible, meaning the human body is less likely to reject or react to their placement. They are resistant to corrosion, including the harsh environment of the human body. They also have a good strength-to-weight ratio, meaning they will last a long time without adding undue loading on existing bones and tissues.
What types of parts can TiMIM make?
TiMIM parts can be small and complex, including pieces with tiny grooves, threads, scoring, and plenty of other features that modern devices may require. Besides the advantages of the MIM process, titanium alloys offer increased corrosion resistance over steel and biocompatibility that is necessary for implants and other medical devices.
One of the most common TiMIM alloys is Ti-6Al-4V, which consists of 90% titanium, 6% aluminum and 4% vanadium. This is a binary alloy with two phases, alpha (α) and beta (β), which have different crystal structures. The alpha phase is stabilized by the aluminum and the beta phase is stabilized by the vanadium. This two-phase alloy is capable of undergoing several different heat treatments to refine the microstructure. For TiMIM parts, the most common heat treatment is solution aging. Solution aging causes small, beta phase particles to form and strain the crystal lattice slightly. When aging is performed properly, the additional lattice strain makes the material tougher and less likely to crack.
The most important feature for biomedical applications is the biocompatibility of Ti-6Al-4V. The alloy is extremely resistant to corrosion, meaning pieces can be in service for a long time and do not generate toxic byproducts from the corrosion.
TiMIM medical parts include pacemaker components, bone repair and replacement components, cataract surgical equipment, ports for catheters, orthodontic brackets, and many others. In these applications, the lifetime of the component often is as long as the lifetime of the patient, and Ti-6Al-4V, with its strong resistance to corrosion and its favorable acceptance by the human body, make it an ideal candidate.
Outside of the medical devices industry, TiMIM components have been used to create brackets, fasteners, fittings, and other small components used in other industries. The tight tolerances required and the smooth surface finish make MIM a good choice for processing. Furthermore, the high strength-to-weight ratio of the titanium alloys make them a perfect fit for aerospace applications, where engineers search for ways to reduce mass in the final product.
Final Thoughts
Designs that require intricate, complex geometries are challenging for even the most modern manufacturing techniques. MIM and TiMIM offer much more manageable and affordable solutions for producing pieces with the fine details frequently encountered in the medical and aerospace industries.
For more information on MIM and TiMIM, please feel free to contact Praxis and one of our experts will be happy to answer your questions and discuss your unique needs.