Vacuum casting is ideal for producing small quantities of parts,
especially if aesthetics are important. The master part, typically
produced by Stereolithography or CNC machining is dressed and
textured as required before establishing parting planes, feeds etc.
The master is then mounted within a frame and liquid silicone cast
around it. After curing the master is cut out to leave a cavity
into which a variety of polyurethanes can be cast under vacuum.
The silicone has sufficient flexibility that undercuts can be
released by flexing the tool, although deep draws may require
additional tooling splits.
Tool life varies with resin cast, but is typically around 20 parts
by which time the tool surface has started to harden and the tool
flexibility deteriorates. Using multiple masters and producing
multi cavity tools can produce greater quantities, although cavity
life will remain around 20 parts.
The process is relatively labour intensive and typically requires
between 2 and 5 days to produce first off castings. This is on top
of the time to manufacture and dress the master part. These times
are dependent upon part size as this affects material curing times.
In evaluating vacuum casting, especially if quantities require
multiple cavities, it may be worth considering RIM or injection
moulding. Alternatively if quantities are low it may prove faster
and more cost effective to make multiple laser sintered parts.
These alternatives are dependent upon part size, complexity and
Tolerances vary depending on material and supplier, however as a
general rule using ± 0.25mm to 100mm and ± 0.25% thereafter will
provide a 'safe' estimate. Note these are process tolerances and
should be applied on top of any master part tolerance.
Minimum Feature Size: In a similar way to injection moulding, this
is determined by the ability to get the polyurethane to 'flow' into
small features. As a wall section, 1mm is about the minimum
achievable, however if the area is small enough this can be
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