MIM (Metal Injection Molding) is an innovative shaping process for the production of high strength, complex metal parts with small and micro dimensions.
Tools with MIM compatible part designs
An essential requirement for the production of MIM parts which are in accordance with their the drawings is a precise injection tool, which possesses a high resistance against abrasion, due to the abrasive effect of the MIM powder mixture (feedstock).
The MIM process combines two well-known production technologies:
- Plastic Injection Molding This technology, which has been known and well-tried for many decades, enables the production of complex parts out of plastic. Thanks to the use of tools with sliders, one can produce parts with undercuts. In the injection process, synthetic granules are melted into a special plasticising unit, and then in liquid state injected into steel molds under high pressure. After solidification the finished part can be removed. Injection molding revolutionized shaping in the synthetic field and is indispensable in industrial manufacturing.
- Sintering of metal powder compacts by ejection (P/M process, Press Sintering)This technology has also been in use for decades, in order to produce, metal parts out of metal powders with different compositions. In the first step, metal powder is pressed into a mold under high pressure. After this, a “green” compact is obtained. This compact then undergoes a high temperature heat treatment, where the metal particles merge by means of diffusion processes. Thus, a stable metal part is formed, which matches the shape of the pressing tool. P/M is a commercial process for producing simple metal parts.
Metal Injection Molding combined both technologies
First off, an injectable, pasty mass is created using very fine metal powder and synthetic material. The metal powder takes up over 90% of the weight. This material is then processed to formed parts using an injection machine, just like in the plastic injection molding process. Subsequently, the synthetic percentage in the part, which enabled the shaping in the first place, is removed, without the part losing its shape. In a following step, the parts are sintered and obtain the desired properties, particularly a high density and strength.
Advantages of the MIM process
Excellent shaping possibilities
With the MIM process one can produce very complex parts which need little to no retouching work. Undercuts, which are not possible with the conventional P/M sintering method, can be done using MIM without difficulty. A particular advantage of Parmacos perfected version of the process is the excellent reproduction of even the tools finest contours in the final product, due to the fact that the extremely easy flowing material enables the exact molding of sharp corners and edges and very fine details.
First-class material properties
In contrast to the conventional press and sinter, where usually densities of about 90% of the theoretical raw material densities are achieved, MIM components densities between 96% and 100%. Thus, material properties are attained, which extensively match those of parts that have been manufactured by machining from bar stock. An outstanding characteristic of the MIM process is the possibility to create “impossible” alloys using adequate powder mixtures; something that cannot be done at all or only with very high costs using the metallurgical melting process. A good example for this are alloys with an exactly defined thermal coefficient of expansion.
Outstanding surface quality
The surfaces of MIM parts are superior to those of precision casting parts. Even without the polishing treatment, a surface roughness < 1.2 is achieved. Thus, handling and polishing expenses can be eliminated or reduced considerably.
The MIM process allows us to achieve exactitude better than ± 0.3% of the desired size, without post-processing. This means that with extremely small parts, the tolerance is in the area of a hundredth of a mm. When working with parts that have very narrow tolerances or bigger dimensions, calibration or post-processing might be necessary.
Range of materials
Since MIM parts normally do not require mechanical post-processing, the harder a material is to machine, the greater the advantage offered by the MIM process. This applies especially to stainless steel, soft magnetic alloys, and iron/nickel materials, but also to tool steels and special alloys.
All hardening processes can be applied to MIM parts offhand, just like with comparable solid materials. All the surface treating processes like for example nickel-plating, hard chrome plating, etc. can be applied as well.
The commercialization of the MIM process depends a lot on how well the achievable material properties and the creative possibilities suit the intended use. Although the costs of the process seem relatively high, it enables considerable savings compared to alternative process chains. Therefore, each case is reviewed in order to determine if the application of the process pays off for the intended use. For that matter, it is of utter importance that the design engineer be completely aware of the possibilities of the MIM process. Principally, the same Guidelines apply to the design of MIM parts as that of plastic moulded parts. This means that thick cross-sections may have a negative effect on the dimensional accuracy, even when using the MIM process. The parts have to be designed in such a way that they can be de-moulded with slides or cores etc.
A stamped part which is one to one manufactured by MIM will most probably always be more expensive in MIM. However if the design freedom of MIM is considered in the design Phase of the final assembly it is often possible to integrate Features into the design which lead to better functionality and tower cost of the whole assembly. Often it is possible to reduce the number of components when using the design freedom of MIM. Redcution of assembling cost is the result.
Parmaco will gladly advise you in the Initial Phase of new development with regard to the MIM process.