The Benefits of Metal Injection Molding

May 25, 2004

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While many are shying away from metal injection molding because of its huge startup cost, one contract manufacturer is realizing substantial gains. Learn the pluses of this specialized process:

While some thousands of North American companies are well acquainted with plastic injection molding, a much smaller number has ventured into metal injection molding because of its sizeable startup cost. But the process is proving to be cost-effective for one Massachusetts contract manufacturer, Morgan Advanced Ceramics (MAC). The job shop uses metal injection molding to make surgical tools, fiber optic connectors and various components for the aerospace and military market, and it's deriving a lot of benefits from the specialized process.

For starters, the shop is finding that metal injection molding can yield small, complex parts at a far lower cost than other processes such as machining, investment casting, and stamping and attaching components together. For example, the process could produce a part that would have cost $100 to machine for roughly $20 per part in a mid-sized run, says Mike Hill, the injection molding manager at MAC. And the savings only increase with bigger runs. In batches of 130,000 to 140,000, parts can be molded for as little as $5 a part.

Additionally, metal injection molding can produce parts with complex features, such as cross-drilled holes, undercuts and fins, usually requiring no secondary machining. What's more, it delivers net shape parts in a single molding operation, followed by a sintering operation. The process can even hold tolerances that are half of the industry standard of +/- 0.003 inch per inch of dimension, though achieving such tight tolerances could necessitate a secondary operation.

Fortunately, the cost of tooling is low, compared to die casting and investment casting. For example, a good-quality one-cavity mold will run from $15,000 to $25,000, estimates Hill, which is cost-effective as long as the mold is utilized for 20,000 to 25,000 parts per year. Moreover, a metal injection molding machine can produce most parts in 10 seconds—much faster than the 15 to 20 minutes that machining usually requires. Even if the time needed for de-waxing and sintering is added up, the process is still cost-efficient.

Another major advantage of the method is that it allows shops to tailor materials to specific applications. Different powdered metals and alloys can be mixed together to make composite materials with customized or enhanced properties. For example, MAC uses tungsten, nickel and iron alloys to produce unique mixtures such as tungsten-copper, moly-copper and tungsten-iron. One of the shop's projects for an aerospace company, for instance, called for a metal density of 17 grams per cubic centimeter for a part in a missile guidance system. Since this density does not occur naturally, MAC created it especially for the customer by blending tungsten, nickel and iron in specific proportions.

The process does have some limitations, however. According to Hill, metal injection molding is not suitable for components with thin cross sections. "If you have a wall that is only four or five thousandths of an inch across, it's very difficult to get the material to flow through that thin section without plugging up," Hill tells Job Shop Technology. "So we reach a point where we can't get some walls any thinner." In addition, metal injection molding is not appropriate for larger parts because of the high cost of powdered metal.

Clearly, when it comes to small, complicated parts, metal injection molding can challenge conventional machining and investment casting. As one job shop has discovered, the process's speed, affordability and versatility make it the ideal choice for demanding applications requiring complex geometries or specialized material properties.

Source:

Metal Injection Molding Offers Complex Geometry for Small Parts
David Gaines
Job Shop Technology, March 2004
www.jobshoptechnology.com/features/0302/mim.shtml

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