smart51
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| posted on 19/7/10 at 03:18 PM |
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which way does fibreglass shrink?
I'm planning to make a mould for pulling cycle wings for my new build. My experience of GRP is that is shrinks or stretches so that the shape
of the moulding is not the same as the buck the mould was taken from. To counter this, I want to make the buck either over size or under size so that
the shrunk part is the shape I want. which way does GRP go? If you imagine a cycle wing, does it go more "pinched" or does it splay?
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twybrow
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| posted on 19/7/10 at 03:20 PM |
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It is the resin that shrinks, and the fibre resists the shrinkage. If it is a CSM laminate you have made, then it will shrink proportianately in both
directions. However, the factors that affect shrinkage will include the geometry, the cure schedule, the resin system etc. To be honest though, we are
talking of 1-2% shrinkage, so is it really needed to make your buck 1-2% larger/smaller?
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smart51
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| posted on 19/7/10 at 03:29 PM |
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the "flat" tops of my MNR cycle wings were always a bit concave, suggesting that the laminate wanted to flatten out. The body panels for
my cabin scooter did the same thing, both along and across the panel. As my new cycle wings will be a close fit to the tyre, I don't want them
to pinch and rub against the side walls or to flare out more than I intended. 1 or 2 percent won't be noticeable.
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clairetoo
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| posted on 19/7/10 at 03:37 PM |
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The only real shrinkage problem with GRP I find is in the gelcoat - when heated (caused by a thick layup and plenty of hardener) it expands a little ,
but then shrinks back to slightly smaller than before - this is what causes molds to wear out (they will gradually curl up over time.....)
The best tip I can give to making perfect moldings is make the molds well thick (at least 5mm) , then leave the moldings in for a good 36 hours before
removal - that way they will be hardened enough not to move without the support of the mold .
Its cuz I is blond , innit
Claire xx
Will weld for food......
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40inches
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| posted on 19/7/10 at 07:37 PM |
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quote:
Originally posted by smart51
the "flat" tops of my MNR cycle wings were always a bit concave, suggesting that the laminate wanted to flatten out. The body panels for
my cabin scooter did the same thing, both along and across the panel. As my new cycle wings will be a close fit to the tyre, I don't want them
to pinch and rub against the side walls or to flare out more than I intended. 1 or 2 percent won't be noticeable.
Can GRP panels be removed from the mould too early? i:e when they are still "green"and slightly soft, and then sag and set.
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clairetoo
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| posted on 20/7/10 at 10:07 PM |
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quote:
Originally posted by 40inches
Can GRP panels be removed from the mould too early? i:e when they are still "green"and slightly soft, and then sag and set.
They sure can - when I got the bodywork for my Fury from Fisher's , the molders were taking them out of the molds as soon as they could , then
standing them on end - which resulted in them sagging and distorting 
Mine was 2" too short to fit the chassis , and took several weeks of stretching to get it to fit properly .
Its cuz I is blond , innit
Claire xx
Will weld for food......
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