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Metal Casting Technologies : September 2007
TECHNICAL FEATURE For most MIM parts, flat surfaces are normally required to provide support during debinding and sintering. In addition, the metal injection mould in MIM can limit geometrical features in a similar way to die casting. ■ Dimensional accuracy: MIM can provide tighter tolerance than investment. Although MIM involves large dimensional changes during debinding and sintering, the process is highly controllable and reproducible. Information  collected from several European manufacturers by the European Powder Metallurgy Association (EPMA) is shown in Table 2. ■ Surface finish: MIM can produce a better surface finish than investment casting. The as-sintered surface roughness of MIM parts is smooth but not mirror finished, the surface roughness is highly dependent on the powder size. MIM parts will contain blemishes, examples include gates, ejector pin marks and parting lines, which should be located in non- critical positions. ■ Order quantity: The economical order quantities for MIM and investment casting are significantly different. MIM is a mass-production process. Production quantities from 5,000 parts per year e.g. for a specialty firearm trigger guard to more than 100,000,000 parts per year e.g. for mobile phone vibrator weights are examples of the range . A typical order of exceeding 20,000 parts per year is normally needed to justify high tooling and set-up costs . Investment casting is more competitive for small to medium volumes but because of limitations in automating the process may be less suitable than MIM for higher volumes. ■ Porosity control: Typical MIM parts have high relative density exceeding 95% theoretical density which compares with good casting parts. However, the pores within MIM parts are micro-pores which are usually smaller than shrinkage/gas pores within cast parts. Hence, MIM parts normally have better mechanical properties. Control of the level of porosity obtained during the sintering process is also possible for specific applications, for example, in filtration, lubrication and flow control. Casting porous parts may also possible by adding a foaming agent, however the porosity cannot be controlled uniformly. In addition, cast porous parts are contaminated by the foaming agent and by the addition of small ceramic particles, for example silicon carbide, used to stabilize the foam structure before solidification . ■ Novel compositions: Novel compositions, e.g. metal matrix composite and two-material structures can be easily obtained by MIM, e.g. using mixed metal-ceramic mixed powders can produce metal matrix composite parts. In addition, the co-injection moulding technique similar to that used in polymer processing can be applied to conventional MIM to obtain a dual material structure. ■ Novel processes: For powder metallurgy, there are many novel processes that can use the advantages of raw material in a powder form. For example, green machining, green assembly and gas-assisted injection moulding. For situation where certain geometrical features cannot be obtained from a single injection moulding shot, it is possible to remove or add material. Parts after injection moulding have reasonable strength and can sometimes be machined to remove material and obtain required geometrical features. An injected part can also be used as an insert in the moulding step of the second part or several injected parts can be assembled before debinding and sintering. All component parts included in green assembly should be made of the same material intended for the same sintering process. In some cases the strength of www.metals.rala.com.au 58 Table 2. Comparison of typical dimensional tolerances for MIM and investment casting  Typical dimensional tolerances (± mm) Nominal size (mm) MIM Investment Casting <3 ± 0.05 3-6 ± 0.06 ± 0.12 6-15 ± 0.075 ± 0.15 15-30 ± 0.15 ± 0.20 30-60 ± 0.25 ± 0.30 >60 ± 0.5% from nominal size