Dome

Radius I mean.

Use a ball as your mould.

How accurate (sphericity/dimension) does it need to be?

About 5mm either side of 26 diameter.

Could you find a toy ball of the right dimension perhaps?

Bob

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 dome...

Steve

fibreglass,

Hi

Thanks for all the replies. Yup, it is a half sphere. Ill try that sand method, its sounds promising.

Michael

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.

ChrisH

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.

Tim

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...

Peter

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.

Regards,

RSGB VHF/UHF Manual (Page 8.56 in the 3rd edition).

Steve R

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"

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