Demoulding Fibreglass

PlankWalker

PlankWalker

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I am about to build a mould for a fibreglass deck box (about 750mm cubed). The mould is mainly MDF with plasticine in the corners (to be destroyed on de moulding),
I wont be using gellcoat as the exterior will be sanded and 2 pack painted.

My question is regarding the release from the mould, Is a wash or two of watered down PVA glue adequate, should I varnish and wax polish? All of these materials I have and I'm loath to wast beer tokens on special release agent.
Also what softens up very old plasticine which is too hard to work properly?


Eagerly awaiting your advice as allways


Plank
 
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PlankWalker said:
I am about to build a mould for a fibreglass deck box (about 750mm cubed). The mould is mainly MDF withplasticine in the corners (to be destroyed on de moulding),
I wont beusing gellcoat as the exterior will be sanded and 2 pack painted.

Myquestion is regarding the release from the mould, Is a wash or two ofwatered down PVA glue adequate, should I varnish and wax polish? All ofthese materials I have and I'm loath to wast beer tokens on specialrelease agent.
Also what softens up very oldplasticine which is too hard to work properly?


Eagerly awaiting your advice as allways


Plank
Click to expand...

If its epoxy then Brown Packing Tape every time, haven't tried it on poly resin though

roger
 
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If it's a straightforward cuboid, MDF should be fine. It has to have a high gloss finish however, otherwise it'll be a swine to release. We used to spray mdf with polyurethane varnish and polish the surface before painting with release agent. Even then we often took the top surface off the finished article when releasing. We also made the box so we could remove the panels from the mould individually rather than try to pop the box out of the mould. Haven't done this for 25 years so techniques might have changed
 
A lot depends on what sort of draft angles you have on the object, and how deep it goes, plus how flexible it (and the mould) are. I'd be wary of not having any gelcoat at all. I know the 2-pack ought to cover all the ends of the fibres, but you might still see some print-through of the fibre pattern on the finished one. Lastly, sometimes (depending on the mould material) pouring hot water down the gap between the mould and the item can help. If you have access to compressed air, some moulds have a hole in the "worst" place that you screw an airline fitting into and put a bit of plasticine or tape over when laying up, then just connect an air line to it when ready to eject it from the mould.
 
No gelcoat. Lots of primer and top coat.

Bonnetgloss05.jpg
 
If you haven't already made the mould, you might consider making up GRP sheet and then 'flat-packing' the box. You could even base the sheet on proprietory GRP sheet (bought quite cheaply from roofing suppliers), although this is usually quite thin and would need additional laminations inside.
 
macd said:
If you haven't already made the mould, you might consider making up GRP sheet and then 'flat-packing' the box. You could even base the sheet on proprietory GRP sheet (bought quite cheaply from roofing suppliers), although this is usually quite thin and would need additional laminations inside.
Click to expand...

Had thought of this but decided the reduction in size at the top, for the lid to fit over, was too fiddly.
Also forward corners need to be rounded to take Calor bottles and to deflect water, it has also to be bomb proof about 8 or 9mm thick.

Plank
 
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