|Composites Design and Manufacture (Plymouth University teaching support materials)
Extrusion, Injection Moulding and Rotational Moulding.
Extrusion (see also Ăström pp 293)
Extrusion normally uses a screw in a barrel (similar to an injection moulding machine) to push a continuous stream of material through a die to produce constant cross-section material. It is confined to use with short fibre reinforcement (or unreinforced grades) as the action of the screw breaks the fibres into shorter lengths. This is not a significant technique in the production of composite components, but does find use in compounding (mixing) materials to provide pellets for subsequent injection moulding.
Note that pultrusion is the equivalent process for continuous fibre composites of constant cross section, albeit that the fibres are normally impregnated with resins without the use of the screw-in-a-barrel process.
Injection moulding (see also Ăström [1 pp 219-222])
If a closed cavity mould tool is placed at the output end of an extruder, and discrete charges of material are delivered, then it is possible to "injection mould" components. This is a significant industry for the production of unreinforced polymeric components and it can produce short fibre reinforced thermoplastic (SFRTP) parts. Machines also exist which can process liquid resins to produce thermoset components by reaction injection moulding (RIM). The inclusion of short fibre reinforcements leads to reinforced reaction injection moulding (R-RIM). A further development is structural reaction injection moulding (S-RIM) where continuous fibre reinforcements may be incorporated. In this latter process, the resin system (normally polyurethane) is usually of very low viscosity and very rapid reaction time. There are superficial similarities between S-RIM and resin transfer moulding (RTM): initial viscosities may be similar but the timescale for mould fill is typically <1 minute for S-RIM and >>1 minute for RTM with significantly shorter cycle times due to the faster cure in S-RIM. Pressures of 10000-40000 kPa may be generated during R-RIM but are normally limited to 100-1000 kPa in RTM [Ăström]. In consequence, the RIM processes are more normally undertaken with steel mould tools to react the higher pressures.
Shear Controlled Technologies
The Wolfson Centre for Materials Processing (Brunel University) developed a range of multiple live-feed moulding (MLFM) Shear Controlled ORientation TEChnologies (SCORTEC) which can impose improved orientation distributions on extruded or short fibre composites, including shear controlled orientation in extrusion (SCOREX) and shear controlled orientation in injection moulding (SCORIM) [2-4].
During rotational moulding, a polymer powder is slowly tumbled in a heated, enclosed mould. The polymer should uniformly coat the inside surface of the mould tool to produce a hollow object. It is possible to incorporate short fibres into the product.
The Tecni-Form website has a good description of the process with and animated schematic.