Composite mold making questions Options

[TonyAKAVW]

I want to make a diffuser/engine pan for my car, and last night I started gluing foam sheets together to make a mold. My plan is to form the foam into the shape I want and then seal it with body filler, and then primer and polish. I have been reading up on making molds for composite work and most seem to talk about making a copmosite piece for the mold rather than what I am planning. If I make my mold as described will my composite part rip the mold apart when I seperate the two? (assuming I use a PVA mold release film).

Also, if I make the part with carbon fiber, how do I finish the edges of the piece when I'm done?

I would like to vacuum bag this part because the part will be visible from the top and the bottom... Can I get away not bagging it and get a decent finish? (I'm not super concerned that it has a perfect surface really).

-Tony

Finished part will look something like this...

With the long straight edge meeting up against the firewall and the two tunnels exiting right under the bumper.

[TonyAKAVW]

Okay, so it sounds like there are a bunch of different ways to do this. First off, yes carbon fiber is expensive (~$60-$80 a yard depending on patern) and I won't start with that. If however my mold is only good for one shot then I might as well go with it, but thats a big risk....

Eventually I would like to use carbon because it looks cool and can be made thinner (lighter) than a glass counterpart. I am not going for the smooth glass finish like you see on vehicle interiors etc., but rather for the minimum epoxy lightweight construction.

So my initial plan was to make my mold as in figure D below. Basically shape the foam as I like, fill in the voids and make a smooth surface with body filler, and then sand some more. On top of that spray a primer, sand, polish and wax. Then coat with PVA and then start making a part. The vacuum bagging would probably not be done on the first article. Instead I would just lay up a few layers of glass, squeegy out extra resin and then let it cure. Then after that is done, pry the part from the mold and repeat, this time with vacuum bagging. And after I get that right, go for carbon.

So Paul's option as I see it is to cover the foam portion of the mold with a layer of fiberglass itself, with poly resin, and then use that part as my mold. I think if I ever want to vacuum bag it, that wouldn't be the preferred way to go, as my mold would now be a rather flexible large piece of fiberglass.

I think the method closest to D is what andy reccomends.

The whole deal with mylar is just that it has a very smooth surface? Last time I remember playing with mylar though, it has a tendency to wrinkle... Am I thinking of a different kind of mylar? Also as I said earlier, surface finish isn't my primary conern, and I really don't actually want a glassy layer of resin, rather I would like minimal weight, which means peel or perf-ply and an abosrbing mat layer during vacuum bagging.. right?

-Tony