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.

[highways]

Oh boy, don't burn that foam. No hot knife allowed. It will produce cyanide gas. We generally work with blue foam... which can be a little hard to come by in Southern California because it's used more up north. But I think Home Depot at least has like the 1" thick sheets of it.

Your MDF vacuum table sounds good. As far as pumps- an 'adequate' setup is more complicated then you would think. You do want to pull 15-25lbs per square inch... for pretty much a 24 hour period. Most pumps if hooked up directly will have a hard time maintaining a steady pressure and they may burn up before the end. The best home setup I've seen/worked with had a good pump, a propane tank vacuum reservoir- to ease the burden on the pump, and vacuum pressure regulator, and a pressure trigged relay to turn the pump on when vacuum was needed. A good system wont be cheap.

Don't get an old medical pump. You have no idea what it was used for... it probably hasn't been cleaned well enough, and it will spit out diseased exhaust all over your workshop. Definitely not cool.

Mylar will give you a great finish on the top. it comes in different thicknesses. We would use fairly thick stuff (1mm or 2mm?) doing wings... but we didn't go around any tight radiuses. I think a thinner piece that completely pulls down to the shape of the mold would yield good results.

The key to good composites (and the reason for using vacuum) is so that you can squeeze the epoxy out of the part... leaving the glass with the minimal amount of excess epoxy, ie un-wanted weight. Without vacuum bagging you just can't make really lite weight stuff.

As far as your mold plans- it looks pretty good. The mold, which sorta resembles what you would normally call the plug.. anyway... your bottom part that you're gonna lay the glass up on... the finish is everything. You're gonna want to get that surface so polished and glassy- so that you can actually seperate your big flat stuck part off of there. I don't put much faith in PVA. The mold release wax is where the money's at. Put some on. Then put some more on. Then put some more. Get the idea? At the end of the day you'll probably still have to use some plastic spatulas to pry the part off.

As far as fabricating your mold... I would probably fabricate it the same as the top of your part- with mylar. Shape the foam, lay up a few layers of 8 or 10 ounce glass, finish with a layer of 2 ounce, then vacuum bag it with mylar so you get a glassy surface.

Shoot... with how flat your part is.... why don't you just mylar both sides? Build your foam mold to shape... then skip the one week of work polishing and waxing... instead just lay the first piece of mylar over your shaped foam... wet out your clothe on top of that... slap the second piece of mylar on top... then into the vacuum bag. Make the mold... mylar... and glass extend about 2 inches past where you want the part to end.. then just jigsaw your final part to the correct dimensions.

Prepare for a 2-5 times learning curve. In other words don't waste your money on carbon fiber the first go around. Infact, where do you plan on getting carbon fiber in the first place? When the war started carbon fiber was basically rationed- and if you could even get it for a non military application it would be really really expensive. Maybe it's available again, I don't know. But your part doesn't really call for using carbon fiber anyway. You would be better off using S glass cut on the diagonal. Personally I would do one layer of 2 ounce, one layer of 4 ounce on the 45 degree bias, another layer of 4 ounce straight, a third layer of 4 ounce on the bias, and finish with 2 ounce again. You can even paint the mylar.

There are ways to make the part super lite weight... but then the finish on one side of the part would come out textured instead of glassy smooth. It would involve peel-ply and bleeder clothe. Definitely the most hi tech lite weight way to go.

I wouldn't worry too much about baking the part. Heat cured epoxies are another ball game entirely.. and they operate at like 250 degrees or something I think. Instead I would just throw a space heater in your garage over night. Epoxy curing in an 85 degree garage does really well since it was designed to work at 70 degrees room temp.

Anyway- we used to build model sailplanes as well. And some top secret stuff for the F-22. Now I have to kill you.