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Metal Casting Technologies : September 2007
TECHNICAL FEATURE www.metals.rala.com.au 56 PROCESS COMPARISONS MIM and investment casting can be considered as the main competitive processes for small sized parts with complex shapes. For parts that are less complex diecasting, PM die pressing, precision forging and machining may also be considered. The following factors must be taken into account in choosing between MIM and investment casting. ■ Materials: The choice of alloy material for investment casting is extensive whilst MIM can theoretically produce parts in any metal that can be prepared in suitable powder form and can be sintered. The majority of MIM parts are currently Stainless Steels, Low Alloy Steels, Low Expansion Alloys (e.g. Kovar) and Magnetic Alloys. Aluminium was originally though to be non-sinterable. However, MIM production of Aluminium Alloy is now possible with recent development in sintering technology. Because materials for MIM have to be in powder form with controlled shape, size and size distribution, the availability of alloy materials is clearly limited when compared to investment casting. Metal powder is normally more expensive than metal ingot so it is more cost effective to use readily available powders. MIM is highly suitable for difficult-to-cast materials such as Refractory and Hard Metals, e.g. Titanium and its alloys and Cemented Carbides. In addition, the MIM process can be applied to ceramic powders, e.g. Alumina, Zirconia and Silicon Nitride. MIM is therefore the optimum route to produce a large number of small parts in alloys with limited castability if they can be obtained in suitable powder form at economic cost. ■ Weight and Size: Investment can cast parts up to 30kg in Aluminium Alloy and up to 300kg in Steel, while MIM is suitable for producing small parts with a usual weight limit of less than 100 g when using conventional thermoplastic binders. The majority of MIM parts are between 8 - 16 g in weight  where MIM becomes highly cost effective. The typical limiting linear dimension for investment is 500 mm, while for MIM it is 100 mm . The weight and size limitations for MIM are based on technical and economic factors. Larger sizes need more expensive tooling and larger processing equipment. In addition, heating rates on debinding are required to be slower since as parts become larger and heavier, it becomes increasingly more difficult to debind without causing any shape distortion. Thus production of a larger part, if at all possible, involves increased processing time on more expensive equipment. With the recent development of an agar based binder which has water as the main ingredient, larger MIM parts can be produced albeit with lower accuracy. Figure 5 shows a 1.7 kg Nickel-based super alloy (IN718) aerospace part which has high strength, durability and oxidation resistance at elevated temperature . ■ Geometrical complexity: Geometrical features that can be obtained for both MIM and investment are undercuts, complex contours, both blind and through custom shaped holes, internal and external treads, and very thin sections. In some aspects, MIM has better design flexibility than investment and vice-versa as shown in Table 1 [3, 5, 6]. Figure 5. Metal injection moulded 1.7 kg Nickel-based super alloy (IN718) aerospace flow body part  MIM ,  Lost wax  Minimum wall thickness 0.3 mm 1.5 mm Maximum wall thickness 10 mm Over 10mm depending on strength of shell Minimum bore diameter 0.4 mm 2mm Maximum depth of a hole 10 times diameter From 1 to 4 times diameter depending on diameter Maximum depth of a blind hole 5 times diameter From 0.5 to 2 times diameter depending on diameter Table 1. Comparison of geometrical complexity for MIM and lost wax