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Scottish Plastics and Rubber Association Scottish Plastics and Rubber Association

3D-forming Process: Metal Injection Moulding

The joint meeting between the Scottish Association for Metals and SPRA, on Wednesday 16 March at the University of Strathclyde, Glasgow, explored both plastics injection moulding and metal injection moulding. It attracted a broad spectrum of interest from industry and academia, including teachers and student teachers.

Andrew Russell, SPRA Council member and Laboratory technologist at Rolls Royce plc, Inchinnan, developed the concept of injection moulding, as explained by the first speaker, Colin Hindle, to produce 3D components in metal. 

Existing 7-step process
As a case study Andrew selected a stator vane from the compressor of a Rolls Royce aero engine.  Up to now the component has been manufactured in Inco 718 alloy, a Nickel-Iron-Chromium precipitation hardenable superalloy which has good creep properties at high temperatures, by a forging process which involves 7 distinct steps.   Metal injection moulding offers a simpler and less expensive process. 
Metal Injection Moulding stages
The alloy powder, with a particle size of 5 – 40 µm, is mixed with a binder based on polymethylmethacrylate (PMMA) and a water soluble polymer and fed to a conventional plastic injection moulding machine to form moulded preforms. 

The preforms are then subjected to a debinding process using solvent extraction with water, followed by a thermal process to depolymerise the PMMA, the decomposition products being vacuum extracted.  This is a critical stage because of the associated shrinkage and the temperature profile and component support both have to be carefully controlled.  Tolerances of +/- 0.5 % can be achieved. The final step is a single forging stage followed by finishing processes.

Before and after the sintering process, showing the extent of shrinkage

The advantages of metal injection moulding over the conventional forging sequence are:
     Reduced component times
     Reduced raw material waste
     Reduced energy consumption

Report by C Geddes, SPRA Hon Secretary     March 2011

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