So I’m trying to speed up my mold making process. I was thinking today, could you ditch the glass reinforcement behind the gelcoat, and instead use pour foam to reinforce the gelcoat layer? I was thinking 16lb density foam, but would it adhere well to polyester gelcoat? Or even epoxy gelcoat?
You could try it, depending on the force you will be applying to your mold, and how many times you plan to cycle the mold. I would gel coat followed by a coating using veil and polyester to give a little reinforcement for the gel coat, then use the pour foam. I use 16lb pour foam to reinforce molds, but that is after a significant amount of reinforcement is laid up, it does offer a fair amount of support to the mold. Epoxy is considerably stronger than polyester gel coat and standard tooling gel coats.
I’ve found that if you plan to use your mold for infusion, significant amounts of foam make it harder to evenly warm your molds, if needed This is obviously more noticeable in larger molds.
Urethane pour in place foams are not terribly stable. As your temp changes they tend to expand which would distort your mold.
The mold would be me room temperature infusion. So no oven curing. The only heat would be from the resin curing. Does the foam warp under low temps?
Yes. When they produce foam blocks they store the expanded foam in a warehouse for at least a month while they monitor the internal temp. This will let them know when it may be stable enough to slice. You might be able to get away with using it if you aged you molds the same way?
Sounds like this isn’t as viable an idea as I’d hoped…
Here is a technique that I developed for my self. This is how I build all my production moulds now. Some mould are almost 2 years old and show no signs of problems and pulled over 100+ planes from them.
To make it even quicker you can just use the corian mix and no tooling coat. And the surface will be as good as you plug 
Tim
Hrmm…I am goign to look into your method Timbuck, thanks!
Another idea was to just back the molds using epoxy tooling putty, but then I’m forced to use the more expensive epoxy gelcoat as well.
My whole idea is to try and eliminate as much fiberglass as possible. I’m pretty horrendous with any type of hand layup. I can never seem to get all of the air bubbles out, and I end up with weak spots behind my gelcoat.
You should look at urethane casting resin. Stable, rigid, no shrinkage during cure, no movement in use. I’ve used Adtech brand with good results in a semi-production setting.
Adtech reps will tell you you can cast and then pull directly off the urethane surface (after release agents, of course) but I would recommend gelcoat and a vinylester skin before the urethane. Their product is a little soft and the surface can be damaged pretty easily.
Since it is a pourable product, you’ll need to build a containment dam to make the mold. I incorporated the dam into the pattern and laid it up with the skin. I added a 1708 biax to the walls of the dam to provide some structure to the mold. After casting, I added a 1708 to the back side of the mold. The urethane is a little brittle so having some structure will help with longevity. These molds were hatch covers for a boat and we were building one per week for over three years.
For most of the molds I make, it would be more time consuming to create the dam sections. With foam it kicks so fast that I’d be OK, but pouring a casting material isn’t really feasible for these molds.
What if I were to just use a Polyester gelcoat, backed by a thickened VE or PE resin? Using Cabosil or even chopped fibers? Then I’d at least be able to fill all of the low spots, making the glass reinforcement easier to lay down.
That can be done but it is very easy to cause exotherm and shrinkage due to the mass of resin. Use small amounts with a fiber filler.
You can do that but just be warned, polyester and vinylester are relatively brittle. Details can break off in use. I would recommend milled fibers as a filler if you go this route.
Also, I agree with AVT, shrinkage might be a problem.
I hear you…I think even resin/fibers would be better than current method. I have yet to get a mold done without resin voids behind the gelcoat. I’m not talking a thick fiberous resin mixture, just a skin layer. Essentially just a tad bit thicker than what a surfacing veil would be. Just enough to eliminate voids