At Matra BAe Dynamics, the SLA process is being applied to building wax injected tools for investment casting and delivering a saving in time of around 12 weeks on the production programme. The rapid tooling came as a direct result of 3D Systems’ QuickCast process which is used to build highly accurate open matrix resin patterns for investment castings on the SLA machine. The honeycomb shell, which is normally up to 1mm thick, is dipped in the same way as a traditional wax pattern but, instead of autoclaving, the epoxy resin is burnt out afterwards.
As Mike Ince, production engineer at Matra BAe Dynamics, explained, `although QuickCast is designed to provide fully functional metal prototypes in a very short time, as every casting needs its own pattern, economics demand that an alternative process is used for the production of more than five or six castings. My idea for using the SLA to build wax injected tools came when I realised that I would be asked to produce 60 or 70 components as part of the long range Trigat Missile design programme.’
The cores and cavities were built in accurate clear epoxy solid (ACES) and, instead of using a normal solid build style as would normally be employed for a straightforward prototype model, the cavities were built as a 5mm thick shell. This was subsequently backfilled with an aluminium filled epoxy, by the foundry. The technique not only speeded up build time but also, through the use aluminium, helped to dissipate heat when the wax was injected.
At the foundry, the toolmaker fitted them into aluminium bolsters and the resulting tool was used to produce traditional wax patterns for investment casting. Subsequently they were cast in 1169 aluminium alloy. The whole process from CAD to castings took just five weeks as opposed to 17 weeks by traditional methods which would also have involved at least £3000 of hard tooling per component.
Figure 1: At Matra BAe Dynamics, the SLA process was used to build wax injected tools for investment casting, saving 12 weeks on the production programme. The photograph shows the ACES cavities fitted to aluminium bolsters