Within rapid prototyping, various processes exist to translate an image on a screen into a physical model. Our experience with casting resins and advanced polymers allows us to create working prototypes, which can mimic almost any industrial method of production. We use various polyurethane resin systems for vacuum casting, enabling us to produce fully functional components quickly and affordably in small quantities.
MASTERS FOR VACUUM CASTING
We typically use various 3D printing methods to create master models. While this allows complex models to be created straight from CAD, it is usually not a complete solution in itself. To produce the top quality prototypes that we are renowned for, 3D prints still need a great deal of specialised hand-finishing by our skilled model makers. Only after this process is the casting master is ready to be used for tooling.
Mould-making, or tooling, is a process where the casting master is suspended in a box, with risers and runner branching off it, to create channels through which residual and reaction gases can escape when the part is cast. Liquid silicon is carefully introduced under vacuum, capturing the surface texture perfectly. The tool is then placed in a vacuum chamber, removing any trapped air or bubbles, which would otherwise create voids in the tool that could potentially deform or spoil the final cast. The tool is then placed in a temperature controlled oven for curing. Once cured, the silicon block is split open and the master removed, leaving the negative space formed by the master model within it.
Liquid resin is then poured, once again under vacuum, into the tool, which is placed in a temperature controlled oven for curing. Once cured, the tool is opened, revealing the final cast part. Finally, the casting is then meticulously cleaned up to become a finished component. A micron-thin seam of excess material, known as ‘flash,’ occurs where the two halves of the tool meet; this must be gently cleaned away. The risers and runners are snapped off and sanded away.
“Rubberised” elastomeric or soft-feel parts of a design can be ‘overmoulded’ onto the finished hard parts. We create two versions of the same tool, the first excluding the overmoulded areas, the second with them included. The cast from the first tool is placed into the second tool, and a different, softer grade of material introduced, filling the overmould cavity. This method allows us to achieve a range of effects, for example, soft rubber grips perfectly moulded over hard handles. Alternatively, overmoulded components can be made separately, and attached afterwards, to save on production costs and time. The suitability of these each of these processes depends on the nature of the part and the level of finish required.
Amalgam has made many 3D printed and vacuum cast parts for us, from flow straighteners to custom seals. Each time they offer sound and relevant advice on how best to achieve the result I’m after. The realised parts we then receive are always impressive, excellent value for money and the turnaround time for the parts is such that prototype testing is commonly accelerated compared to the traditional machined part route. A pleasure to work with them.
Jon Powell – Malvern Instruments Ltd