Injecting more "mettle" into P/M designs

Powder-metal parts as strong as solid steel. That's the allure of metal injection-molding (MIM) technologies which produce parts with near-theoretical densities (>95%). This results in strength and modulus values comparable to those of wrought metal and gives MIM parts mechanical properties that often equal or surpass those of other metalforming processes including investment casting, forging, and machining.

MIM is not a new technology. But, most MIM applications materialize to solve a problem in existing parts made some other way. This is changing as materials continue to improve and molders and designers gain experience with the process. More and more parts are specifically designed to be MIM. The ability to make larger parts has also bolstered use of the technology in structural applications ranging from precision medical and aerospace parts to sporting and recreational gear.

Injection molding is usually associated with plastics where it is known for producing complex, net-shape parts in quantity. Efficient cycle time is another point in the techniques favor. Additionally, it's often possible to consolidate multiple parts into one injection-molded version that needs no secondary finishing because the process accurately reproduces mold surfaces.

MIM works with an expanding array of metal alloys. Traditional MIM parts compete well in niche markets where parts weigh less than 7 oz (200 gm) and wall thicknesses range from 0.01 to 0.25 in. (0.25 to 6.25 mm). Tolerances are typically held to +/-0.3% and surface finishes of 32 rms (0.8 mm) are common. MIM parts can also be brazed, soldered, and welded as well as plated or ground to size if necessary.

MIM, however, may not be a shoe-in for every small, complex design. Poor candidates may include applications currently using screw machining, zinc die casting, or stamping. Designers must evaluate each part individually taking into account its complexity and whether there will be secondary machining and finishing operations. If a machined part, for example, needs three or more tool setups and will be made in volumes exceeding 10,000, MIM may provide better economics. The overall design requirements also play an important part in the decision-making process.

Material basics

MIM is a union of thermoplastic injection molding and conventional powder metallurgy. Binders consisting of wax and polymeric or aqueous systems, mix with fine (

Historically, metal-parts fabricators developed MIM feedstock in-house. They'd formulate mechanical blends of powdered metal and binder systems by trial and error resulting in "home-brew" feedstocks. As with any injection-molding material, feedstocks that are dependable and consistent can enormously improve processibility and part performance. So it's not surprising that many home-brew feedstocks are being replaced by commercially precompounded versions with a high degree of consistency and quality assurance.

Popular commercial feedstocks include Catamold from BASF AG, in Germany (www.basf.com), Advamet from Advanced Metalworking Practices Inc., Carmel, Ind. (www.advancedmetalworking.com), and Aquamim from Planet Polymer Technologies Inc., San Diego (www.planetpolymer.com). The newest offering PowderFlo is an aqueous agar-based feedstock compounded by RTP Co. under license from Latitude Manufacturing Technologies Inc., Hackettstown, N.J. (www.latitudemanufacturing.com).

Molding process

The materials selected for MIM binder systems are varied, but they all function as flowable carriers for the metal particulates. The feedstocks inject into molds similar to those used for plastic injection-molding via standard injection-molding equipment. Some MIM feedstock may require modified injection-molding equipment and tooling to accommodate the material's mildly abrasive nature and high viscosity.

Feedstocks are fed into the injection-molding press through a hopper at the back of the machine. The system heats the material, plasticizes it, compresses it via the screw inside the barrel, and then injects it into the mold cavity through the injection screw's shut-off screw tip. The material cools and solidifies in the mold prior to being ejected in what's called a "green" part. Molds incorporating slides and cores produce more complex features than is possible with machining or casting.

Green parts are oversized and somewhat fragile. They are often 10 to 20% larger than the final part size. It's possible to grind off excess material when the part is still in its green state.

Debind and sinter

The green part undergoes a debinding step to remove most of the carrier. In contrast to conventional powder metallurgy, MIM parts contain up to 40 volume percent binder which needs to be removed. Feedstock materials need to be consistent and offer a predictable and reproducible rate of shrink during further postmold processing.

First stage debinding typically removes a large portion of the total binder. Density of the part at this point remains below that of the base metal, reflecting both the presence of voids between the metal particulates and the volume of the binder still present. Following first stage debinding the part is even more fragile and is said to be in a "brown" state.

The debinding method and its duration depend solely on the carrier used. The binder has the greatest effect on cycle times. Polyolefin and wax systems may be debound either thermally, with solvents, or both. Polyacetal binders must be catalytically debound with nitric acid and a nitrogen-gas atmosphere in a separate debinding oven. Partially hydrolyzed PVOH is soluble in water; remaining polypropylene and plasticizers are thermally debound. Agar-based binding systems undergo debinding by simple air-drying at ambient temperatures (short oven drying may be required with larger or thicker walled parts).

Debinding is followed by sintering, a high-temperature firing process in a controlled atmosphere to consolidate powdered-metal particles by diffusion. This happens in a sintering furnace at temperatures elevated to just below the melting point of the specific metal. Sintering and densification occurs through multiple processes: including volume diffusion, grain boundary diffusion, and surface diffusion. In some cases a liquid phase is used to accelerate sintering.

Sintering begins with the molded part undergoing a preheating stage that removes the remaining binder. This stage is necessary to remove any residuals or potential contaminates that could compromise the metal's mechanical properties. A defined sintering schedule typically outlines temperatures, ramp rates, soak or hold times, and cooling rates required for both specific MIM materials and furnaces used. It also defines the atmosphere -- hydrogen, nitrogen, vacuum, or combinations thereof.

Successful sintering depends on both the metal being processed and the qualities of the particular furnace. Both should be considered in relationship to each other. After sintering, the metal particulates have been consolidated and densified into a solid mass with the nearly theoretical density of similar wrought metal.

MIM versus other processes

MIM has a number of advantages over traditional processes such as investment casting, press and sinter, forging, and machining.

Investment casting requires the construction of individual molds for each part produced. Poured rather than injected under pressure, recovered parts yield shapes that -- by injection-molding standards -- are relatively crude and need secondary finishing operations. Surfaces are rough and dimensional tolerances inexact. Sinks and voids are common in wall thicknesses over 0.75 in. and thin walls are difficult to fill. Cast parts need large gates to promote material flow which may not be easy to hide. The resulting features could detract from part aesthetics. Individual production steps can include pattern making, tree assembly, investing, stuccoing, and dewaxing. Firing, pouring, and mold knockout as well as finishing steps including deburring, machining, and polishing add more labor to the process.

Press-and-sinter processing, like MIM, uses powdered metal as a raw material. Physically akin to compression molding of plastics, powdered metals are placed in a mold and then ram compacted to form a fixed shape. Furnace sintering fuses the particulates and slightly increases finished part density. The process produces green parts in quick cycles and the mold is reusable. But only simple geometries are practical and mechanical strength is lower than both MIM and wrought metals.

Forging is also best suited for simple shapes. Heated metal is physically hammered into shape under high temperatures and pressures. First, metal ingots are cast and reduced to billets. The billets are heated and placed on formed die halves. Hammering metal against the dies begins to form parts. Heat loss dictates reheating and more hammering. The process is repeated until the final shape emerges. Multiple compaction adds strength. Cross holes are impossible and its tough to maintain tolerances and straight edges. Postforging operations include welding, heat treating, and final machining. Tooling is costly and has a short working life. Cycle times are extended and overall production costs are relatively high.

Machining of individual parts delivers both exacting tolerances and complex shapes, but there are some limits on part complexity compared to the intricacies of injection molding. Nearly any metal can be machined and machining normally doesn't alter the properties of the raw material. Machining centers are well established and accessible. This remains the preferred process for prototyping and short run production. Machining is, however, labor and capital intensive, design limited, and production times lag and costs do not decrease with volume.