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Metal Casting Technologies : September 2007
54 www.metals.rala.com.au TECHNICAL FEATURE M By Dr. Anchalee Manonukul & Dr. John Pearce etal injection moulding (MIM) and precision casting are potential manufacturing routes for the production of complex shaped metal parts. Following the widespread commercialisation of MIM in the 1980s  MIM has become the main competitive process to precision casting for small complex parts. The term 'precision casting' can refer to any casting process that can offer close dimensional control, e.g. investment (lost wax), high pressure die casting and precision sand core assembly, however, for comparison with MIM in this article, investment is chosen as the competitive casting process A basic outline of the MIM route is also included for readers in the metal casting industry who may be unfamiliar with the process. Although MIM is a powder metallurgy process, MIM has not inherited the lower mechanical properties associated with the conventional powder metallurgy route. MIM parts are nearly fully dense and hence, with respect to mechanical performance can be equivalent to or even better than cast or wrought parts. Factors other than mechanical properties then have controlling influences on the decision to choose investment casting or MIM. OUTLINE OF METAL INJECTION MOULDING Metal injection moulding (MIM) is a relatively new metal fabrication technique that merges plastic injection moulding, metal powder and sintering together . The shapes of parts produced by MIM can be as complicated as any of those formed by plastic injection moulding and multi-cavity tooling can be used. The strength of MIM parts is developed during sintering. The MIM route thus combines the benefits of the durability and strength of metal powder sintered to high density with the geometrical flexibility of injection moulding. Until quite recently the MIM process has been mainly used to reduce manufacturing costs by conversion of parts originally designed and produced by other processes such as precision casting or machining. A published example from the UK  of part conversion including the corresponding manufacturing costs is illustrated in Figure 1. This trend has changed since designers have become more aware of MIM. Increasing numbers of small parts are now designed to be manufactured by MIM and some designs cannot be easily produced by other processes. A fibre optic connector which is designed with MIM in mind is shown in Figure 2. Choosing between investment casting and metal injection moulding Figure 1. Price comparison of parts manufacturing by MIM and other processes  (a) (b) Figure 2. Fibre optic connector designed for production by MIM Casting MIM Machined MIM £12 £3 £60 £2