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MIM – “Injection Molding” of Precision Steel Parts

By Parmaco

For the Production of small, sophisticated metal parts there are a great deal of different processes like various injection methods, powder-metallurgic production with the conventional press & sinter process or machining. Each of these methods has certain advantages and disadvantages, which determine the boundaries of use. An interesting alternative is the Metal Injection Molding (MIM) process, which specializes in the mass production of small, highly sophisticated steel parts with a relatively complex geometry.

MIMMIMGeorg N. Breitenmoser, CEO of Parmaco Metal Injection Molding AG in Fischingen (Switzerland), states: "Our manufacturing process is pretty similar to the conventional injection molding of plastic parts, which is why it allows us the same freedom of design." The SME is specialized in the production of sophisticated components of customized steels and special alloys, using the MIM process, a fusion between powder-metallurgy and injection molding. The starting material is a free-flowing mixture of metal powder and different plastics and additives. The equipment and molds used are similar to the ones used for the injection molding of plastics. The overall process chain, however, is more complicated, given that the plastic injection molding process results in finished parts, whereas for the MIM-process, injection molding is only the initial shaping operation. The resulting parts are "greenlings" which don't possess any mechanical strength. The greenlings are then put through a multistage process which consists of debinding and sintering. At the end of this process the parts will have shrunk by 20%. The outcome are geometrical, functionally sophisticated, high-quality MIMcomponents with lengths of up to 70mm and a weight range of 1 to 100g. Microcomponents from 0,01g up can be likewise manufactured. The range of available materials is really big, due to the fact that the alloys are mixed from powders. It reaches from the conventional carbon steels, over stainless and tool steels, up to highly alloyed steels like nickel-base alloys, titanium alloys or hard metals. The range of specific characteristics one strives for is very big, too. It reaches from tough carbon steels with medium strength to special alloys with high strength and hardness.

Convincing advantages...
MIMG. Breitenmoser continues: "On the market we're put up against a series of different manufacturing processes, such as die casting, recision casting, powder-metallurgic press and sinter processing or machining from scratch." Just like the MIM process, every process has specific advantages and disadvantages, so the best solution basically depends on the planned use and/or important boundary conditions such as the yearly amount of parts. The MIM process enables the production of very complex, filigree geometries with undercuts, always maintaining high precision and surface quality. Since very elaborate tools must be produced for each project, it normally pays off if a reasonably big amount of parts are required, meaning at least a few thousand per year. MIMThe dimensional tolerance is ± 0.3% of the specified dimension, whereas the surface roughness is at least 0.4 to 1.6 µm. MIM parts reach a low porosity and therefore a density of 96
to 100% of the theoretical material density. Their mechanical properties are similar to the properties of parts which have been milled from scratch. E.g., they are gas and liquid tight, even when submitted to high pressure. MIM parts can be mechanically finished, heat treated, hardened, and refined with all the common surface coating processes.

parmacoCompared to the powder-metallurgic press and sinter process, the MIM process is characterized by significantly greater freedom respecting the representable geometry, whereas the resilience of the parts is significantly higher than that of die casting parts made from aluminum or zinc. Also, compared to precision casting, the MIM process leads to higher exactitude and better surfaces. Milling from scratch only pays off when used for single pieces or small batches. Furthermore, as of today, the MIM process lives up even to
the highest quality standards demanded by the industry, e.g. from fields like the car industry, medical technology, aviation and astronautics.

Concerning costs...

MIMG. Breitenmoser is delighted: "In spite of the high price for the materials, the overall advantage offered by the MIM process is considerable. Savings of 50% upon switching to MIM are definitely no rareness." The fine, high-purity powders used as raw material require a priori a much higher material price than e.g. the casting process. However, if one tried, using another process, to produce a somewhat sophisticated component with suitable properties, dimensions and volumes, one would quickly find out that other process would struggle to keep up. This would apply particularly then, when the component is constructed in such a way that it's geometry and it's material properties make perfect use of the possibilities offered by the MIM process.

Cooperative development is essential
MIMG. Breitenmoser knows: "Design and manufacturing process should harmonize." When designing a new component the designer normally thinks purely in geometric function-related categories, whereas special MIMcharacteristics and restrictions of the different possible manufacturing processes don't play a major role to begin with. Often enough, the result were geometries which didn't harmonize with the process, e.g. material accumulation, wall thickness transitions, flow paths in the mold filling process, etc. Therefore it's useful when the experience of the supplier can, already in an early phase of the product development, have an influence on the design. For example, there would be a possibility, using the MIM process, to form threads directly inside the greenling with help from special cores, so that they don't need to be cut later on. At Parmaco, the personnel consists of numerous experienced professionals with loads of know-how. Furthermore, they are equipped with the latest CAD-CAM-development tools, such as the form-fillingsimulation software. These possibilities are happily presented to the customers within the scope of cooperative development.

Automation to the furthest extent
MIMG. Breitenmoser reveals: "Our philosophy regarding the manufacturing is to automate every process as much as possible." This has several advantages. On one hand one narrows the tolerances of the relevant production parameters as far as possible, which has a good effect on the constancy of the sequences and on the quality of the product. What is more, the automation is also noticeable in the costs, given that the wages paid to produce the product are less. This is especially important in a high-wage country like Switzerland, in order to keep up with the prices on the international market, as competitive as it is. For this type of automation tasks they dispose of a proper, highly qualified team, which has plenty of experience figuring out complicated sequences and solving very tricky tasks. As careful as possible with the material recipes G. Breitenmoser states: "Since we also produce parts for aviation and astronautics, MIMour processes must fulfill high requirements regarding documentation and traceability." All the used input materials - Metal powders, plastics, additives, etc. - are obtained exclusively from certified suppliers. The numbers of the charges are all documented and traceable. The recipes of the powder mixtures depend on know-how. Therefore Parmaco just buys the raw materials, taking care of the combination and preparation themselves. The mixing takes place with the help of the recipes, which are saved on the computer as order forms. This traceability also counts for regrind material like runners and sprues, as well as other rests of the injection molding process. These are traceable and get reused for production, causing the costs to sink significantly. Thanks to a quality documentation, the process history of any part can always be looked at, even after many years have passed.