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So you have a piece to mould with an undercut
like that pictured below? The key to this
is selecting the right place to split the
mould. Every application is different and
so we can only look at the basics here, but
apply these basics to your own project.
To create a two piece mould you must decide
on the split point. A flange must be constructed
to allow the two halves to be separated and
rejoined to allow removal of the cast. These
must be bolted together before casting or
lamination begins but the halves can be prepared
before joining.
To produce the flange a line of clay or plasticine
is laid along the split point. Using fillet
balls or marbles or clay balls, make small
half circle pips along the clay to act as
keyways. The clay and keyways are then painted
with PVA. The same process as a single piece
mould is use on that half, except that you
continue up the side of the flange. Allow
to fully cure.
Now remove the clay strip and fillet balls.
You will notice that there are small round
dips in the flange to act as keyways to ensure
correct mating of the mould halves. Paint
with PVA and wax heavily and repeat the whole
process for the second half.
At this stage consider the points at which
you will bolt the halves together. Select
some large washers with small holes and encapsulate
these under a small piece of glass mat. Make
sure these match on opposite sides as these
will be drilled to accept bolts and wing nuts.
The reason for these washers is to prevent
wear to the mould from frequent bolting actions.
Clamp the two halves together and drill through
the centre of the washers using an HSS drill
bit. Once complete you can remove the clamps.
Allow the whole mould to cure fully before
un-bolting and gently separating one half
at a time in a similar way to single piece
moulds.
Clean up the surface, wax and polish, paint
with PVA and bolt back together. Along the
seam line use some slurry wax to help mask
the join. The mould is now ready for use!
We have reproduced this in pdf format for
you to download and refer to. do not reproduce
without prior permission from www.tomps.com:
GRP
Sectional Mould Guide
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