: This board was a wealth of information for me when i was building my kayak,
: now I'm hoping the resident fiberglass/resin experts can help me with a
: slighty OT project. I want to make a fiberglass mold of an original
: sculpture I've carved out of foam.

Do a few samples on scrap pieces of the same typ of foam, with whatever coating you are going to use. a lot of foams will dissolve when they come in contact with polyester resin. For cheap protection you may want want to spray on a few layers of latex paint, or a couple layers of epoxy. Getting foam out of a mold might be tricky, so assume you will lose your original. If the original is destined to be sacrificed, then cover it with epoxy resin and when that has cured, you can dissolve out the foam with some acetone.

Sure you don't want to make a latex mold? You can back that up with fiberglass, and the latex just paints on easily.

: My plan is to seal the prototype, coat
: it with a mold release and then use polyester (cheaper than epoxy) resin
: impregnated fiberglass to build up the mold shell.

I'm not sure what kind of detail you wish to preserve in your model, but glass cloth is not going to flow around any kind of small curves, and complex curves of any size will give you problems. Glass mat is not much better. A vacuum setup can pull things in together and force them against your mold a bit, but that is a whole 'nother matter.

An arrangement that sprays on random chopped glass fibers mixed with a resin (called a "chopper gun") might supply the material and resin you need. Take a look at the materials and information which is online at www.fibreglast.com They have videos on a lot of fiberglass fabrication techniques, and they might have one on how to do this, too.

: My question is if I
: could use unfaced fiberglass insulation (maybe 2" thick), satured
: with resin, to build up the shell quickly and cheaply?

Yes and don't. The fiberglass we normally use is coated so that it works better with resins. I've heard the term "volane" used in talking about the coating applied to some of these amterials. i have no idea what technical properties this imparts to the fabric or mat, I just know that there is a difference. On the other hand, if you have a thick enough resin, you can cast a solid block with just about anything sealed inside. So, you could probably use straw, sawdust, wheat flour, glass fibers, glass insulation material and even pennies, (I made a paperwight with pennies cast in resin years ago) with varying degrees of strength.

But, the main reason you don't want to use a 2 inch thick layer of home insulation is that it is 2 inches thick. As these resins cure they produce heat. A mass of resin 2 inches thick is going to produce a lot of heat, and could seriously deform, if not entirely melt, your foam original. And did we mention how many gallons of resin it might take to saturate a single square foot of 2 inch fiberglass? Consider the math: There are about 8 gallons to a cubic foot. Or, to put it another way, a gallon is about 1/8th of a cubic foot. A square foot of 2 inch thick glass mat is 1/6th of a cubic foot. fortunately you are not going to get a solid 2 inch thick block. You'll somehow (vacuum?) manage to compress these fibers into something that weighs about 16 to 24 ounces per square yard, and use about a quart of resin per yard of material.

Definitely consider using lighter materials for your mold, or even making the mold in layers, with a first layer following the fine details, and another layer bonded to the back of it to support it. This can be made of sticks if you like, covered with layers of glass cloth and resin. Or, you cna shape the glass and reson in some shape to give you the stiffness you want with minimal materials.

: Or do you think i
: should stick with multiple layers of good old 6 oz. cloth?

If you have some around, try it. If i was experimenting with a small model, I'd cut it into thin strips ( 1/4 to 3/4 inch) and pull them apart, giving me long and short strands. I'd put a layer of resin on my original and sift handfuls of these chopped strands over it, letting them settle into the wet resin, reinforcing it. After the resin hardened a bit, I'd coat it with another layer of resin and sifted glass fibers. Then I'd cut my fabric into squares of about 2 inches to a side, and apply these like patches, saturating them in resin first, and putting on enough to cover the thing with two to 5 layers. in curved areas I'd orient these patches at an angle so the fabric was effectives placed on a a bias.

After that set up I'd look at adding some reinforcement, like sticks, to larger areas, and cover those with larger "patches" of glass cloth, maybe 6x6 up to 1 foot square. I would not try to keep these pieces flat! Instead I would want them to have curves and be bent, rising in ridges above the mold, so that when they hardened they gave me some thickness and added stiffness.

Mold making is an art in itself. Have fun experimenting.

PGJ