3D-forming Process: Plastics 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.

In the first presentation Colin Hindle, SPRA Education Officer and Lecturer in Polymer Technology at Edinburgh Napier University, traced the evolution of plastics injection moulding from its origins in the ‘Stuffing Machine’ patented by the Hyatt brothers in 1872.  To mould cellulose nitrate, they had adapted a metal pressure die casting machine by introducing a mandrel in the barrel, to improve heat conduction to melt the cellulose nitrate, and a water cooled mould.  Because the early plastic industry was dominated by thermosetting materials, compression moulding was the main process and development of injection moulding did not get going until the 1930s and the ram machine and the reciprocating screw machine in the 1950s.  After that process control became the focus, with first solid state control and later microprocessor control.  More recently electric drives have taken over from some hydraulic components and the process has diversified with gas injection, water injection and MuCell innovations.

The benefits of plastics injection moulding include:

     Near net or final shape production
     Part count reduction
     Fewer secondary operations
     Piece part costs
     Rapid scaling to production volume.

Colin took the audience through the basic steps of an injection moulding cycle, what a modern machine looks like and the wide range of industrial and consumer goods produced by injection moulding.  He explained each of the individual stages of the injection moulding cycle in detail:  plasticisation, mould filling, compression, compensation, cooling and ejection.

In conclusion Colin cited 6 reasons why the plastics injection moulding process has been so successful:

    1. shear thinning of thermoplastic melts:  viscosity decreases at higher shear strain rates (flow rates)   
    2. poor thermal conductivity of plastics: prevents early solidification
    3. shear heating: high flow rates generate heat and compensates for heat loss to the mould during filling

    4. Fountain flow: melt front lays down a skin and fills through the centre, which influences molecular orientation in the skin
    5. Compressibility: thermoplastic melts are compressible, which helps reduce shrinkage.
    6. Sympathetic design of parts and mould: the real key to success.

Andrew Russell then took over to talk about metal injection moulding, a process in which the principles of plastics injection moulding have been developed to produce 3D components in metal.

Report by Charlie Geddes, SPRA Hon Secretary          March 2011