Hmmm. Well you try inflating a round balloon to the correct diameter,
coat it with plaster of paris and plaster-soaked rags (which will
harden quickly), that would then give you a spherical mould. If you
want the OUTside of your fibreglass dome to be smooth you then need to
make an inverted mold from the plaster one.
Go and talk to a blow moulder and see what they already have that they do for
other folks. Vacuum formers are also worth talking to.
The big bird warning globes that hang on power lines for example, and there are
probably decorative globes that can be picked up.
Just a couple of thoughts...
make a mould from a childs plastic ball after checking dimensions are
within tolerance I think balloons will not achieve stated accuracy
requirement. As stated previously depending on which side is required to be
fair an intermediate mould may be required. This can be GRP as well if it
is properly waxed and polished and release agent is used. For high density
moulding consider "vacuum bagging"
Half a sphere good enough? You could mould some sand by making a baseboard
with a perpendicular bar to carry a former the shape of the dome. By
turning the former 360 you can carve a pile of sand into shape. This method
has been used to make microwave dishes. The pile of sand as formed can be
fibreglassed. I can't remember which book I've seen this written up.
If you mix dry cement with damp sand, it will harden, giving a shape to
work with. You don't want it too hard to easily remove from the fibreglass
Alternatively, get a ball slightly larger ( wall thickness is going to
be about 3mm at a guess ), slice it in half and you have a ready-made
mould you can fill. The ball will probably be flexible enough to allow
you to peel it off the fibreglass matrix when set.
Might be an idea to use the ball to make a hemispherical impressing in
a bucket of damp sand first, to give you somewhere to pop the mould
while the fibreglass sets.
You'd need a fairly stiff mix of resin to prevent the matrix from
slacking though. I'd be inclined to build it in layers.
Stephen Howard - Woodwind repairs & period restorations
The top of a balloon covered in papier-mache? Perhaps not a perfect
dome, but it would be smooth and you can inflate the balloon to get
the diameter correct.
A standard football is 22cm wide, so perhaps a cheap plastic football
could be warmed and over-inflated.
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