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Vacuum Casting

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, because we are able to produce realistic components affordably in small quantities.

MASTERS FOR VACUUM CASTING

We generally use a 3D printing process to create master models. This allows us to produce complex and high-tolerance shapes straight from CAD.  3D printing is not however a one-stop solution for really top quality prototypes. The objects created need a great deal of skill and hand-finishing to bring them up to the required standard. Once cleaned up and hand finished, the casting master is ready 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 silicone is carefully introduced under vacuum, capturing the features and surface texture of the master model 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 blemish or deform the surface of a tolerance part. The tool is then placed in a temperature controlled oven for curing. Once cured, the silicone block is split open and the master removed, leaving the negative space of the master within it.

Liquid resin is poured into the tool, again under vacuum.  The filled tool is then placed in a temperature controlled oven for curing. Once cured, the tool is split, revealing the cast part. Finally the cast is  meticulously trimmed and fettled 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 carefully cleaned away and the risers and runners are trimmed off.

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, because we are able to produce realistic components affordably in small quantities.

OVERMOULDED PARTS

Elastomeric parts of a design can be ‘overmoulded’ onto the finished hard parts. We can create two versions of the same tool, one without the overmoulded component (tool A) and one with (tool B). The cast from tool A is placed inside tool B, and a different material introduced, filling any spaces. 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.

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, Senior Systems Engineer, Malvern Instruments Ltd