The following article was reprinted with permission from Job Shop Technology magazine :

Metal Injection Molding Offers Complex Geometry for Small Parts

Besides producing small, complex parts more cost-effectively, a Massachusetts job shop is able to blend together powdered metal and alloys to form composite materials with unique qualities. 

By David Gaines

With metal injection molding (MIM), a contract manufacturer can produce small, complex parts much less expensively than with machining, investment casting, or by stamping two or three parts and attaching them together. The process often yields parts that are net shape in one molding operation, followed by a sintering procedure. Parts can be produced with cross-drilled holes, undercuts, fins, and other complex features, usually with no secondary machining.

While probably thousands of companies in North America do plastic injection molding, far fewer achieve custom injection-molded parts with metal. Most subcontractors are hesitant to initiate the process due to the costly startup involved, according to the production manager at Morgan Advanced Ceramics (MAC) in New Bedford, Massachusetts. The dynamic job shop applies the specialized process to surgical tools, fiber optic connectors, and a variety of aerospace and military applications.

"The high cost of purchasing molding machines and sintering furnaces, and the learning curve to learn this specialty keeps a lot of companies from initiating the process," Mike Hill, the injection molding manager at MAC-New Bedford, points out. "We were lucky to get funded by Morgan, our parent company, when we started this process in the early '90s. We already had some of the equipment because there are commonalities between our core business, ceramic-to-metal assemblies, and MIM."

What sets MAC apart from most of its competitors is its ability to perform medium, as well as large production runs. Many metal injection molders, Hill maintains, will only do large volume runs with a minimum of 100,000 pieces. "Our average production runs go from about 10,000 to 15,000 parts per year upward to about 800,000 to one million parts," says Hill. "It wouldn't be cost effective with this process to make tooling and then only do short runs of a few hundred or a few thousand parts per year."

Metal injection molding is able to successfully compete with traditional machining and investment casting in the small parts arena. For example, Hill says, a part that might cost $100 to machine can be molded for about $20 per part in a medium run. In a larger run, in the 130,000- to-140,000 range, the cost can be reduced to $5 per part.

Metal Molding Routinely Achieves Close Tolerances

Standard tolerances in the industry are fairly conservative at +/- 0.003 inch per inch of dimension. But Hill says that MAC often holds half of that tolerance. Nevertheless, it may be necessary to perform a secondary operation in order to come up with an extremely tight machining tolerance.

Another benefit of the MIM process is the versatility with which powdered metals and alloys can be blended together to yield composite materials that offer special or added qualities. Morgan Advanced Ceramics routinely works with tungsten, nickel, and iron alloys to formulate mixtures like tungsten-copper, moly-copper, or tungsten-iron.

In one molding project for an aerospace firm, MAC worked on a part that goes into a missile guidance system, in which centrifugal weights were used as a switch. In addition to certain size and weight requirements, the weights required a metal density of 17grams per cubic centimeter. Mother Nature doesn't make an alloy with that density, Hill explained, so MAC formulated it for the customer by mixing the right proportions of tungsten, nickel, and iron to get the correct density.

Morgan Advanced Ceramics-New Bedford occupies a 55,000-sq-ft plant, with 170 employees. Ceramic-to-metal assemblies constitute the bulk of its business; injection molding, about 20%. Morgan Advanced Ceramics, a global manufacturing company located in Windsor, England, bought out the original company, Alberox, in 1989. It now holds 12 similar firms in the United States that are active in materials technologies.

With MIM, the company's primary markets are medical equipment, which is about 60% of its work; aerospace, fiber optics, electronics, and firearms. For the medical sector, MAC often molds laparoscopic parts like graspers, dissectors, and instrument handles. For the aerospace market, the company produces parts for missile guidance systems. It also manufactures stainless steel fiber optic connectors, and many small parts—such as gun sights and trigger mechanisms—for firearms.

Customer satisfaction rates highly with MAC. Its operation is ISO 9000-certified, and the company is now applying for the newer, more stringent ISO 2000 standard. In 1999, Textron Systems Corp. awarded MAC "Approved Supplier" status, a prestigious honor considering its relationships with thousands of suppliers. One of only 30 recipients, MAC received the award for 100% on-time delivery and 100% quality for one year.

Cost Effective for Small Parts

Metal injection molding is a very cost-effective process for small, complicated parts. A MIM machine can mold most parts in ten seconds—parts that could take 15 to 20 minutes to machine. And even with added time for de-waxing and sintering, the process still maintains cost effectiveness. Moreover, compared to die casting or investment casting, the cost for tooling is minimal. A decent one-cavity mold, Hill says, costs anywhere from $15,000 to $25,000, a satisfactory cost if a customer is able to do 20,000 to 25,000 parts per year from a mold. Even a more intricate four-cavity mold tops out at $50,000 to $60,000.

The process, however, is not conducive to parts with thin cross sections, Hill confides. "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," says Hill. "So we reach a point where we can't get some walls any thinner." Another disadvantage, one that makes it too expensive for larger parts, is the high cost of powdered metal.

What Engineers Need to Know to Design for MIM

While many engineers and designers are comfortable in a machining environment, they are often in the dark about metal injection molding, where 90-degree corner angles don't exist. And when they are confronted with flash or ejector pin marks and draft or parting lines, their eyes can easily glaze over. Consequently, MAC urges engineers to brainstorm with them before designing a part.

"Metal injection-molded parts will have about a two-degree draft, so the part will come out of the mold easily," Hill reveals. "Often designers don't allow for draft, excess flash, or even location of the gate; these areas must be considered."

Besides helping customers redesign parts, MAC is willing to innovate. One of its customers, a manufacturer of paintball guns, wanted to increase the velocity of the CO2 gas charger on its gun barrel. Initially, the guns only used 80% of the gas to fire the paint ball and 20% to recock the trigger. More energy was needed in the barrel to fire the ball farther.

"We came up with a very effective solution for them," Hill beams. "We decided that a counter balance on the trigger would force out more CO2. We first engineered a metal mixture for them and then molded it into the net shape they wanted. To do this, we devised a counter weight on the trigger that had about 12 grams of material per cc. It increased the barrel charge to 90%."

Quick-Change Production Tooling Eliminates Costly Revisions

Although MAC vends out most of its tooling and secondary operations, it handles some operations in-house. Quick-change production tooling is one technology that MAC has developed so that a customer doesn't have to buy completely new tooling to revise or add to a part. With MAC, all tooling is hard steel, since soft aluminum tooling would not withstand the high heat and pressures required. This constant also holds true for prototype tooling.

"We have a low-cost process to make prototype tools that easily transitions into production," Hill explains. "In this way, we can make prototype tooling for a lot less money than a full production tool. If the prototypes are OK, we can complete the tooling quickly for production runs."

One of the firm's projects was a fiber optics connector that the OEM had outsourced to an EDM contractor. Part holes were machined with an electrode that plunges in and out of the workpiece. Since the part was very small, about 1/2-inch deep, it was very hard to sink holes that were 0.005 inch to 0.010 inch in diameter.

After EDM drilling, the holes were left with tiny ridges from the electrode. The holes looked smooth to the normal eye, but inserted fiber optic strands would catch on the ridges and knot up. Threading fibers through the holes became almost impossible.

Morgan Advanced Ceramics solved the problem with a core pin insert that was 0.005 or 0.006 inch in diameter. As planned, the injected material filled in around the insert pin to form the specified hole. Next, MAC took the workpiece out of the mold and very gently slid the pin out of the green part. What was left was a tiny, smooth hole the size of the pin.

"Because the pin was highly polished and was pulled out in the green state, we were able to wipe the ID of the hole very smooth," says Hill. "Extremely small holes, like this one, are very unique for this type of technology. Without our process he wouldn't have been able to make the part at all."

In another situation, MAC was working on a metal housing for a microscope slide used for DNA sampling. Originally, three stampings had to be brazed together. The part is the size of a thumbnail, with a wall thickness of 0.020 inch. The part was constantly twisting and bending during stamping, causing it to easily go out of spec. Repeatability was hardly ever achievable.

The company was able to devise a way to injection mold the housing with all of the features the part required. Moreover, when the OEM's engineers saw what MAC could do, they designed five more features into the part.

"When the housing was finished, it was even more complex than when they tried to make it with the stamping press," Hill recalled. "The features could have been machined in initially, but it would have been cost-prohibitive. We were able to make the parts for $2 each, and when we added on the new features, we didn't have to add any additional cost for the production run, only for changes to the tooling."

For more information on Morgan Advanced Ceramics, visit the company's web site at www.morganadvancedceramics.com.