I’ve now made 3 roof blanking panels and the problem is they don’t stay the correct shape! Meant to be flat. last one i kept in the mould for a month then left it in my car for a few days and this is how it looks today. Getting frustrated now as it’s for a friend and he is waiting for it. Any ideas? it’s polyester 3 layers. 1 layer csm 2 layers cloth and gel coat. Do you think it would help with more layers?
pic of it now
I don’t know for sure but I’m thinking uneven shrinking. If your layup isn’t semetrical, both sides the same as it cures the side with the gelcoat will shrink a little bit more than the other side with just the resin. Are you limited to a specific thickness? Maybe a coremat might help.
Can you incorporate some stiffeners? I’ve used those small fiberglass rods from McMaster Carr, they come in carbon fiber tubes as well, you split them down the middle to form a channel as a stiffener. Put them on the underside if it’s not cosmetic.
Post curing it in your car might have been uneven. Just build a cardboard box big enough to accommodate the part, line it with tin foil and stick a hair dryer in it to get the temp up. Follow post cure limits for your resin. You probably won’t have to get it much over 130 degs or so. Too much heat is as worse than not enough.
Have the other parts curved the same? You might have to tool it with a bit of a bend to get it to come out flat.
Other than that I’m just spitting dustballs here.
hi Winty. What is the cloth ( weave and weight ) , weight of CSM, and what was the laminate schedule ( order of lay-up )?
Use an epoxy and cure it an oven of some controlled heat box. Polyester sucks for this kind of applications, at least the cheap resins.
Incorporating some stiffeners is an option I was thinking about as it will probably be covered on the underside. I’ve made two so far and they have both warped.
I used 300g csm for the first layer and two layers of 400g woven cloth.
If I make an oven how long will it need to be heated for?
Your laminate schedule is exacerbating the effect of the polyester shrinkage. You would be better off putting a layer of cloth either side of the CSM.
You should check the technical data sheet for your resin. It should have guidance on post cure times & temps. Most formulators have them available for download from their web sites.