Designing with Metal Injection Molding
Designing with metal injection molding (MIM) allows for multiple technical advantages. MIM allows for the same part design freedom used for the production of plastic parts, but the final MIM component is wholly comprised of metal. Metal injection molding allows the design engineer to re-imagine components from fresh perspectives, which can allow a reduction of material content, the combining of multiple components into one, or the molding of functional and decorative features from the start. Further, the MIM process can eliminate frequently required secondary processes like knurling, assembly, and lettering. Ultimately, the elimination of secondary processes leads to lower cost of the part.
GENERAL DESIGN GUIDELINES
What makes a particular component an ideal candidate for metal injection molding fabrication?
The justification for using MIM often results from various intersections of quantitative design considerations that include material performance, shape complexity, production quantity, and component costs. The most effective MIM components will meet each of these requirements. The Venn diagram illustrates target metal injection molding applications.
- Shape Complexity
- Material Performance
- Production Quantity
- Component Costs
Metal injection molding offers the same design freedom as plastic injection molding. Moreover, the more geometrically complex a part is, the more solid the rationale for manufacturing it via the MIM process. Parts may include cross holes, angle holes, internal threads, irregular shapes, splines, undercuts, side holes or grooves, complex contours, or cantilevers.
Parts that would usually be made by assembling multiple components can be designed as a single MIM part. Some parts that could not be fabricated via any other process can be made through MIM. Complexity that would be cost prohibitive to do via multiple machining operations or by casting and then finishing can be achieved cost effectively through MIM processing.
Medium to high volumes of components ranging from thousands to millions of parts annually are typically needed in order to be able to amortize costs associated with tooling and start-up engineering. The best economic advantages are achieved at the highest quantities due to the benefits of larger material purchases, multi-cavity tooling, and dedicated production units. Cell phones, eyeglass hinges, and orthodontic brackets are examples of well-designed components for MIM.
MIM fabrication is ideal where near-fully density, high-impact toughness, fracture toughness, and corrosion resistance are required. Additionally, MIM is compatible with most ferrous and non-ferrous alloy systems. If non-standard material properties are required, these can be developed with new alloy systems.
MIM is also appropriate for materials that are complex to machine, have multi-phase microstructures, or involve high work-hardening materials . It can deliver a high-quality surface finish (32 rms or better) and cleaner feature detail than investment casting.
There is a place for each of the traditional metal-forming processes: each has its own strong suits as well as its limitations. Nevertheless, wherever a component fabrication choice exists between MIM and one or more of the other processes, it pays to see how they stack up in a head-to-head comparison.
The following table shows how four major processes fare in some of the more important parameters to consider.
|Parameter||MIM||Conventional PM||Machining||Investment Casting|
|Range of Materials||High||High||High||Medium-High|
MIM vs. Conventional PM
- MIM can produce geometries that eliminate secondary operations
- MIM offers superior density, corrosion performance, strength, ductility
- MIM can combine two or more PM components into one, reducing part count
- MIM parts offer superior magnetic performance
MIM vs. Machining
- MIM designs can save material and weight
- MIM is designed for high-production volume and can provide cost savings through better material utilization—sprues and runners can be reground and reused as feedstock with no compromise to final properties
- Molding from a single tool eliminates multiple set-up operations
- Difficult-to-machine materials can be molded into a net shape
MIM vs. Investment Casting
- MIM can produce thinner wall sections, sharper cutting points
- MIM produces better surface finish
- MIM is better for small-diameter blind and through holes
- MIM greatly reduces requirements for finish machining
- MIM produces high volumes of small components at a lower cost, faster lead times