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MIM parts: Myths vs. realities

May 7, 2000

3 Min Read
MIM parts: Myths vs. realities

Editor’s note: Selling MIM services often means overcoming misconceptions about what the process can and cannot do. For MIM processors facing this hurdle, or for molders looking to join the MIM industry, Greg Brasel, ceo of MIM molder Megamet LLC (St. Louis, MO), offers to clarify some common myths.

MYTH: MIM cannot compete with press and sinter powder metallurgy.

P/M has been known as a source of inexpensive parts, yet MIM can be just as economical given the right volume, alloy, part size, and requirements. The total cost must be taken into account for a P/M part that requires secondaries such as machining, tapping, and plating. Whereas, a MIM part made from stainless steel does not require plating and can be cheaper.

MYTH: MIM requires proprietary feedstocks.

More and more suppliers of over-the-counter MIM feedstock have entered the market, offering consistent batch-to-batch quality and short lead times. By providing processing assistance and data on shrinkage, suppliers have helped to standardize materials and equipment for many MIM vendors, reducing the need for specially developed feedstocks.

MYTH: MIM orders must be greater than 100,000 pieces to be economical.

This can be traced to the cost of a MIM mold and the amortization of engineering or preproduction costs over the order. However, a MIM tool does not have to be expensive. Multiple part numbers can be made from a family tool or from inserts placed in a master mold frame. Also, as the MIM process becomes more common and the industry matures, molders are drawing on the experience of others, gradually making these preproduction costs a nonissue.

MYTH: MIM is suited only for small parts.

Commonly, molders accept that the size of MIM parts must fit inside a tennis ball, or have a maximum length of 7 inches and top weight of 100g. But as the feedstock, molding abilities, and knowledge base of MIM companies have improved, so have their capabilities, as in the case of a pump housing from Phillips Metal Injection Molding (see December 1999 IMMC, p. 44). In addition, the entry of more feedstock suppliers to the market will increase competition, forcing down prices.

MYTH: MIM is good only for complex geometries.

Although MIM is often considered for a product when the part is too complicated to be produced economically by other processes, MIM has replaced machining, powder metallurgy, and zinc diecasting in simple parts as well. In these cases, the tool steel was too hard on the inserts, setup costs were prohibitive, impact loading conditions were too severe, or service temperatures were too high.

MYTH: MIM can hold tolerances only to ±.3 percent of a dimension.

This should be considered a rule of thumb and not a limitation. The amount of tolerance depends on the dimension and geometry. It is possible to hold ±.0005 inch on an ID hole with proper mold design and a stable process. Linear dimensions less than .020 inch should have dimensional tolerances that are practically as good as a machined part. A 3-inch-long part given the requirement of ±.3 percent would have a tolerance zone of .018 inch, but most MIM vendors can produce parts to a tolerance of ±.005 inch, and as a result can compete with machining.

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