The basis of metal injection molding (MIM) technology involves fine metal powders, which are combined with thermoplastic binders and surfactants, allowing injection in a plastic injection molding machine. The MIM process is similar to plastic injection molding, but with metal feedstock to efficiently produce small, complex parts at high volumes, using high temperature and pressure. It’s an advanced metal forming technique that uses injection molding equipment for manufacturing both simple and complex metal parts to tight tolerances. The MIM process combines the design flexibility of plastic injection molding with the strength and integrity of wrought metals.
MIM is a net-shaping process in which metal powder is mixed with a thermoplastic binder and surfactants that are then fed into a cavity. The metal powder/organic mixture is commonly called feedstock. Feedstock starts as metal powders with an approximate average particle size of 10 μm. There are two strategies: Alloy powders can be used in which each particle has all the elements making up the alloy. Or, a master alloy powder can be mixed with a carbonyl powder (100 percent iron) to make up the alloy. Usually, a ribbon blender is used under vacuum with the surfactant added first then the binders. After molding, the parts are in the green state. The green-state parts are then subjected to a binder removal process.
Binder removal is usually a two-step process. The first step is organic debinding, followed by thermal debinding. After organic debinding, the parts are then loaded on the furnace furniture as the brown-state parts. At this stage, the parts are too fragile to touch. These brown-state parts from the furnace furniture are then loaded into the sintering furnace. Depending upon the overall organic content, the parts shrink between 12 and 15 percent. The result is a net shape part; however, further postprocessing may be needed to arrive at the final part.
MIM versus Machining
MIM does not replace machining, but the process offers distinct advantages for certain high-volume, high-precision projects where parts may be small and in a device requiring articulations, as well as needing strong mechanical properties. MIM is advantageous when considering large volumes, a wide variety of materials, small high-performing precision products with tight tolerances and consistent dimensions, and complex designs. MIM is versatile, low risk, highly scalable, and cost-effective.
Confidence in the MIM process has grown in the surgical instrument and implant manufacturing space. It is ideal for instruments that require high strength, wear resistance, and good corrosion resistance. MIM parts are being used in many applications, such as minimally invasive surgery, general surgical instruments, orthopedic surgery tools, dental products, and hearing aids. It is particularly well-suited for surgical instruments and implants.
MIM is the most suitable manufacturing process for complex parts that are used in minimally invasive surgical instruments being designed for higher degrees of articulation, which increases the numbers of complex metal parts used in the assembly. And metal-injection-molded components can significantly reduce corrosion since producing the desired component as a single part eliminates the need for brazing or welding, reducing resistance at the weld. Keep in mind that the corrosion requirements for metallic implants are much more stringent than for medical instruments, making MIM even more advantageous.
MIM parts are strong and can be bent, welded, hardened, and heat-treated like other wrought materials. A wide variety of metals can be processed with MIM, including stainless steel, steel alloys, iron-nickel alloys, cobalt alloys, and ceramics. Titanium MIM, however, is becoming the preferred material and manufacturing method for products with long life cycles and large annual production volumes. Many medical engineers think titanium MIM is prohibitively expensive, especially for upfront tooling. However, for high-volume MIM applications, tooling costs are generally not a barrier — they are typically recovered within the first year of production due to part-price savings.
ADVANTAGES OF MIM
Greater design freedom
With MIM, parts can be designed and manufactured with minimal design restrictions. In addition, almost all design changes are possible within the shortest development cycle and turnaround time.
Complex and intricate shaped parts
MIM is ideal for producing complex-shaped components as well as parts that require assembly or multiple steps to put together.
High production requirements
MIM is most beneficial in high-volume production of small precision parts with complicated design geometry. The process lends itself to automation where high volumes and consistent quality are required.
MIM technology is the most viable process for producing miniature parts economically.
Metal injection molding (MIM) is a hybrid technology that integrates the shaping capability of plastic injection molding and materials flexibility of conventional powder metallurgy. MIM is preferred for mass manufacturing of small, intricate geometric components of a variety of materials as it can achieve 95–98 percent of its wrought materials properties at a much lower cost.
MIM technology has found increased applications in the commercial world — from home appliances to watches, automobiles to aerospace, and medical devices to orthodontics.
Due to the flexibility of MIM technology, it is possible to customize material compositions according to the specific attributes required by the customers. Compositions include stainless steels, low alloy steels, carbon steels, nickel alloys, tool steels, and tungsten alloys.
The highly repeatable nature of the MIM process produces parts consistent in size, shape, and strength. However, MIM typically comes with a longer qualification timeline when compared to machining due to the need to build a complex mold. However, the long-term cost benefit makes it favorable.
Manufacturers need detailed knowledge of the costs and processing associated with technologies to choose the best process for each component. Detailed cost models and process capability data are needed to compare technologies and then identify the most suitable process before manufacturing begins.
Lower Costs than Traditional Metal Machining
Contours and shapes in medical devices can be especially complex. Demand for disposable minimally invasive surgical instruments has been steadily increasing as hospitals see a distinct cost advantage for off-the-shelf, single-use products that do not require sterilization. MIM can help manufacturers meet that increased demand. Injection molding can be automated when high volumes and consistent quality are needed.
Using the MIM process, many components can be produced in a single manufacturing process that offers great design flexibility. The ability to combine several operations into one process ensures that MIM is successful in reducing lead times as well as saving costs. MIM technology improves cost efficiency through highvolume production to net shape, negating costly, additional operations such as machining. MIM is scalable from low prototype volumes to millions of units per year.
With MIM, it’s less expensive to mass produce highly complex parts once a tool is constructed. Increased costs for special features — such as internal/external threads, complex profiles, chamfers, or identity marking — typically do not increase cost in an MIM operation because the features can be built into the mold. MIM can reduce the need for secondary operations and reduce overall raw material costs.
This flexible, high-quality process arms medical device makers with the ability to design unique products without the cost restrictions associated with conventional metal-forming techniques. High-volume, high-performance, complex surgical instrument parts with strong mechanical properties are achievable at an affordable cost with the MIM fabrication process.
This article was written by Steve Santoro, EVP, MICRO, Somerset, NJ. For more information, visit here .