Designing Metal Injection Molded Parts

Metal Injection Molding technology is most suitable for small complex high quality ferrous parts that would otherwise need to be individually machined or cast and then finished. The Metal Injection Molding process fills a special niche for the manufacturing of parts which, because of their complexity, either cannot be made by other methods or cannot be made at competitive costs.

Designing for Metal Injection Molding (MIM) requires close cooperation between the responsible engineer, the buyer, and the technical staff at FloMet, especially in the initial design stages. The Metal Injection Molding process, similar to other part fabrication methods, has its own set of guidelines for producing soundly engineered and economical products. Design advantages can be gained, but limitations will have to be taken into account as well.

In the case of machined parts, for example, Metal Injection Molding may allow a single complex composite part where presently a set of individual parts are machined and assembled. But unlike machined parts, injection molded parts inherently exhibit features such as parting lines, gates, and ejector pin marks. While these features are kept to a minimum, it will be necessary to discuss alternatives and identify critical surfaces if any.

New or revised prints specifically for the molding technique are recommended.
The optimally designed part is complete as processed, with no secondary operations required. Secondary operations to achieve tight tolerances, such as ±.0005 inch (0.013mm), are possible. However, they can soon negate the original cost advantages of molding and batch processing. 

TOOLING, VOLUMES, AND COST 
CONSIDERATIONS OF METAL INJECTION MOLDING

Tooling designs and costs are comparable to high quality Class I molds for the plastic injection industry.

Since these tooling costs and start-up efforts such as engineering and pre-production first articles need to be amortizable, orders generally need to be high volume to be economical. However, for very complex or expensive parts, this process may offer economies in small quantities.

The following diagram compares the relative costs of fabrication as a function of manufacturing method and complexity.

PART SIZES AND WEIGHTS

Theoretically almost any size part can be manufactured via Metal Injection Molding. Realistically the process is only economical for relatively small parts. The important cost factors are as follows:

• Weight - Above 100 grams the expensive powders prohibit cost effective production unless the part is extremely complex.
• Length - Material flow limitations restrict part size by limiting the distance from the gate to the furthest point on the part to about four inches.
• Envelope - The number of parts that will fit in a furnace determine the per part cost of this operation.

GEOMETRIES

To a large degree Metal Injection Molding (MIM) is not shape sensitive. The more complex the part, the more advantage MIM offers over other fabrication methods. Unlike conventional powder metal pressing, for example, Metal Injection Molding feedstock readily flows - like plastics - during the molding process. As a result, undercuts, complex contours, and cantilevers are all permissible.

• ACCEPTABLE QUALITY LEVEL (AQL)
  The AQL level required for the component must be specified.  It should be noted that the AQL required can - as with other fabrication methods - materially affect the cost of the Metal Inection Molded part.