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Metal Casting Technologies : September 2007
WHO'S WHO OF METALS -- ANNUAL 2007/8 55 There are four main manufacturing steps in MIM as shown schematically in Figure 3. The first step is mixing and palletising (also called granulation). In this step, the metal powders are mixed with thermoplastic polymer binders and plasticizers to provide homogeneous material which is then appropriately sized as pellets suitable for feeding into an injection moulding machine operating at up to 250oC. The metal powders used typically have mean particle sizes below 20μm, normally 5-15μm, for rapid sintering. The mixing ratio between metal powder and binder is typically 60:40 by volume. For steel, the mixing ratio between metal powder and binder is 94:6 by weight. The typical binder used for MIM is a wax-polymer material. The second step is injection. The machine for injection is substantially similar to those used in the plastic industry. The metal powder + binder mixture is injected while the binder is in the liquid stage and the solid metal powders flow within the molten binder. Injected parts are then subjected to the debinding step to remove the binder material. There are two main methods of binder removal - thermal or solvent. In thermal debinding the injected parts are heated to cause the polymer binder to melt, decompose and evaporate. This should be done with care so as not to cause part distortion. Alternatively, the binder can be dissolved out with suitable solvents e.g. acetone and hexane, or just water for a water soluble binder. This is known as solvent debinding. Debinding is the most time consuming process of the four manufacturing steps. After debinding, a weak skeleton of powder is retained. Parts gain increased strength during the final step known as sintering. The objective of sintering is to densify the powders to remove most of the voids developed during debinding. This is nominally similar to sintering in conventional (press and sinter0 powder metallurgy. Sintering involves thermally activated diffusion mechanisms. Sintering must be carried out either in vacuum or in a controlled atmosphere to prevent oxidation and to reduce the oxide on the surface of the powders. Green MIM parts can be handled robotically and debinding and sintering can be done in batch or continuous furnaces. High density exceeding 95% theoretical density is normally associated with MIM . The key factor in the MIM process is the need to control shape and size during the large amount of shrinkage on debinding and sintering. Figure 4 shows a comparison of shape and size of an injected part and as-sintered part. Figure 3. Processing steps in metal injection moulding Figure 4. Comparison of shape and size of injected part (left) and as-sintered part (right)