I need to make a mold for a fibreglass box I want to make for my Grand Cherokee. Expanding faom sounds like the easiest way. Question is, since its a two part chemical that expandes on reaction with eachother, would it meld through a thick poly garbage bag?

Basically, I want to place a garbage bag into the area, tuck it into the areas that are crusial for shape, then pour in the foam and hope it expands into the areas well enough to make a mold for fibre glassing.

Anyone have experiance with this?

 

I've done that before for creating a seat mold that I used in a Formula SAE car and it worked just fine. Unfortunately the foam requires air for curing so the stuff wont fully set up like you want.

Another method you may want to try is to buy a couple sheets of two-inch styrofoam, then cut them and glue them together into a cube that is roughly the right size as your cavity. Then, using a cheap eversharp ginsu knife, carve the sides of the cube until it is the proper shape to fit into your cavity. Once you are done, spraypaint the cube (and I'm theorizing here), followed with a layer of bondo that is spread onto the cube to fill in the gouges. Sand it once the bondo begins to harden to smooth out the finish. Spraypaint the whole assembly with gloss paint, then use it as your mold. Dont forget to make your cube small enough to allow for the thickness of the fiberglass.

Now, what about subwoofer chamber volume? Isn't that important in terms of getting efficient response from the woofer element? Your cube will have an arbitrary volume.

 

I dunno, using plastic would have me worried. Why not use aluminum foil?