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?
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?
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.
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 .
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.
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.
