maybe i should have used a more viscous resin…
Cool, very informative write up. Reminding myself that 770NC should not be used with gel coat.
Can 770NC be used with a clear epoxy surface coat and wet lay up following that surface coat?
Are you sure the reason the 770NC recomends not using gel coat is because it will repell the gel coat? I’m thinking that is probably why it recomends not using gel coat with that product.
About the sealer… is sealer applied before the NC770 is applied?
I know on the epoxy resin parts i make there really isn’t a need to use any gel coat or surface coat because the part comes out of the mold with a “surface coat” to it… due to being infused, via VARTM. Sometimes there are small resinless areas inbetween the weave but those easily fill in with 2K automotive clear coat. Spray on 2 heavy coats of it after the part is released,trimmed and preped for clear.
I think I understand what Hojo means about flanging up the ends of the bumper. And yep i can say that should be a one piece mold, otherwise you are going to have to “egg crate” the backside of the mold with a support structure that keeps the two piece mold from wanting to bend in the middle where it joins. It is that, or build some damn heavy duty flanges where the mold joins together and use some heavy duty bolts and nuts to hold it in place. Plus now you have the unwanted problem of having to seal the split mold in the center each time you use the mold. For some parts it is necessary to have a multi-piece mold, but only when there is a negative draft geometry to the plug that would cause mold lock.
epoxy surface coat is a thick paste, so it doesn’t run ,like the polyester ones which are liquid. also, the epoxy mold resin (epoxy tooling gelcoat lets say) doesnt attack the plug’s paint.
the sealer is used before frekote. (though i didnt have any sealer)
on the case of the join flanges, i went the way of making tough flanges with heavy duty bolts. the mistake i did on making this mold is that i let the end of each edge roll back, thus creating a problem for me in demoulding. parts of the item that will not be shown in the final use (for example, this is a rear bumper spoiler. we don’t care how perfect the underside’s edge is.) could be molded in the way that they dont cause problems in either the layup or demolding.
I would have made my joining flanges here:
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Judging by the thickness of your molds, my guess is that thats whats causing the voids on the surface of your vacuumed part. Might be pulling air through the gelcoat.
Fastrr: You can spray gelcoat onto semi perm. You HAVE TO mist the part first with a single pass with your gelcoat gun. Let it sit long enough to flash the solvents off, and let any trapped air raise to the surface. Not doing this step will also give you an extremely porous surface. Once the solvents flash, spraying is a breeze. If your using a brushed gel with semi perm, cant help you there.
i used an epoxy surface coat which is thick and very strong, and after it had dried completely i vacuum bagged the carbon, so all the mold would be airtight. judging by the way the mold was holdig the vacuum, there were no (significant) leaks
but what you say might be true as well…
Only reason I mentioned it is that I ran accross the same problem 2 weeks ago. Infused a flat panel on a piece of melamine, a single pinhole in the surface of board let the pump pull air through 3/4" thick wood. Either that or the air was coming from the edges, but then that would mean it pulled air through 2 feet of wood.