Note: This is the first of a three-part series covering metal injection molding (1) trends, (2)material/process advances and (3) applications.

Metal injection molding (MIM) offers a manufacturing capability for producing complex shapes in large quantities. The process utilizes fine metal powders typically less than 20 micrometers in diameter, which are custom formulated with a binder made of various thermoplastics, waxes and other materials, converted into a feedstock that is granulated and then fed into multiple cavities of a conventional metal injection molding machine.

Feedstocks come in various grades of stainless steel, iron and nickel alloys. Soft magnetic, tungsten, and titanium are also available. Historically, metal parts fabricators developed MIM feedstocks in-house. They'd formulate mechanical blends of powdered metal and binder systems by trial and error.

As with any injection-molding material, feedstocks that are dependable and consistent can enormously improve processability and part performance. Many in-house 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, Ludwigshafen, Germany
  • Advamet from Advanced Metalworking Practices Inc., Carmel, Indiana, USA
  • PowderFlo an aqueous agar-based feedstock from Latitude Manufacturing Technologies Inc., Flanders, New Jersey, USA

European Powder Metallurgy Association
Metal injection molding manufacturing.

After the "green" component is removed, most of the binder is extracted by thermal or solvent processing, and the rest is removed as the component is sintered, or solid-state diffused in a controlled atmosphere furnace. Green parts are oversized and somewhat fragile. They are often 10 to 20 percent larger than the final part size. It's possible to grind off excess material when the part is still in its green state.

MIM parts contain up to 40 volume percent binder, which needs to be removed. The debinding method and its duration depend solely on the binder carrier material used. The binder has the greatest effect on cycle times.

Polyolefin and wax systems may be debound either thermally, with solvents, or both. Acetal binders must be catalytically debound with nitric acid and a nitrogen-gas atmosphere in a separate debinding oven. Partially hydrolyzed polyvinyl alcohol is soluble in water; remaining polypropylene and plasticizers are thermally debound. Agar-based binding systems undergo debinding by simple air-drying at ambient temperatures.

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 namely, 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.

Metal injection molded parts.

The MIM process is similar to plastic injection molding and metal isotactic pressing, and it can produce much the same shapes and configuration features. However, it is limited to relatively small, highly complex parts that otherwise would require extensive finish machining or assembly operations if made by any other metal forming process.

The advantages of metal injection molding lie in its capability to produce mechanical properties nearly equivalent to wrought metal materials, while being a net-shape process technology with good dimensional tolerance control. MIM parts offer a nearly unlimited shape and geometric feature capability, with high production rates through the use of multicavity mold tooling. Typical specialty markets using MIM parts include medical and dental devices, electronic parts, specialty industrial components and consumer products.

NetShape Technologies
Optimal MIM target application focus.

The primary attraction of MIM is economical production of complex parts from high-performance engineering materials. The circular figure that follows illustrates the concept.

The intersection of the three regions delineates the most attractive area for the application of MIM. Using MIM, complex metallic shapes can be inexpensively formed to nearly full density through a polymer powdered metal compound. As a result of the high final density achieved, MIM products are suitable for high performance levels.

MIM competes with other shaping technologies, including plastic injection molding, die/investment casting, slip castings and cold isostatic compaction. The success of MIM versus its rivals lies in the combined attributes.

MIM overcomes the property limitations inherent to plastics, the cost of machining, productivity limits of isostatic pressing and slip casting, defects and tolerance limits of casting and the shape limitations of traditional powder compaction. A further advantage of this process is that it is possible to mold materials that can only be formed from powders, such as tungsten heavy alloy, super alloys and so on, which are quite difficult to produce by conventional routes.

Because of the ability to make high-density parts with close tolerance limits and excellent surface quality, MIM technology has a wide range of applications and has been used to manufacture parts and components for aerospace, automotive, electronics, telecommunications, consumer, medical and dental industries.

To be globally competitive, U.S. metals molders will have to move into niche areas, which are more technology intensive. Such niches include micromolding, molding highly complex medical components, molding high-volume automotive components, and molding more exotic materials.

Furthermore, the MIM industry is not a "shoot-and-ship" type of environment. Suppliers can't just supply widgets but must also offer full-service support to their OEM customers. The survivors provide upfront design assistance, expertise in design/construction of intricate tooling, and in-house secondary operations or subassembly. Metal injection molders need to take a close look at maintaining/improving their core engineering competencies and continuing to invest in new technology to sustain their U.S. manufacturing operations.

In the final analysis, however, effective competition is all about price. While it is important to improve innovation and service in all production and business functions, it is also essential to be the low-cost provider. For the most part, survival will lie in automation, using the least expensive materials available, and providing excellent customer service. Reduce costs: automate, innovate, reduce manufacturing steps, and increase yield.

If a supplier can beat the overseas competition, which may be machining or casting parts, then the business can be won. One of the most cost-effective ways for the U.S. to maintain its global dominance in the MIM industry is to follow jobs overseas with joint ventures. Because building an overseas plant is a costly undertaking for most companies, current MIM molders should work closely with the OEM via joint ventures and alliances, so as production moves overseas, the related suppliers can follow.

Another option for U.S. MIM companies is to provide value-added services in order to maintain or secure international business. The tremendous domestic consumer markets, which countries like China and India offer, must also be kept in mind.