Questions Concerning Shrinkage When Making Fiberglass Resin Molds

[mmanolis2001]

I am attempting to make a 2 piece mold out of Fiberglass (Bondo) resin for the first time and have a question for the experts here.

I have read that polyester resins like bondo tend to shrink when curing. I’m planning to make a two piece mold out of a hard master (Baked Sculpey). I will have a mold base filled half way with leveled clay and the master embedded half way into this clay base. The master is cylindrical with no overhangs to catch or lock it into place while casting. I also plan on embedding marbles half way into the clay at various locations for the mold keys.

My main concern is the shrinkage during curing and possible miss alignment of the mold keys? How do you handle keying these types of resin molds to ensure they mate properly after curing?

Should I cast both half’s without keys and drill holes through both pieces before separating them, then glue rods into one half to act as the keys? Can the cured resin even take a small drill bit without cracking?

Thanks for any tips.

[SHK]

only suggestion i have is mix the resin 60/40 with body filler, it will reduce the shrinking. i would be concerned with using a hard master with a hard mold as well.

[mmanolis2001]

only suggestion i have is mix the resin 60/40 with body filler, it will reduce the shrinking. i would be concerned with using a hard master with a hard mold as well.

I thought about this as well. Im not worried about the master locking into the mold due to its shape but you did get me thinking about the shrinking resin cracking due to the hard master....

Now im worried about that.

I guess all i can do is give it a shot and post the results. Its only $18 down the drain if it fails.

[mmanolis2001]

When mixing the body filler with the resin can you just use the resin hardner or do you need to mix the hardner 60/40 as well?

[Largehead Louie]

I still think your hard master is going to lock into the resin. Have you thought about RTV?

[SHK]

just use the resin hardener. im thinking the same as louie there

[Frank]

When mixing the body filler with the resin can you just use the resin hardner or do you need to mix the hardner 60/40 as well?

Not sure if the filler will shrink less it is the same resin, just has a filler added. But if you do add the the filler when you add the liquid hardener it is WAY stronger than the paste that comes with the filler. So work fast and beware it will harden fast. Also when it shrinks it will lock on to the master. Frank

[Romeo D]

Why not use RTV for a Hard master? I've been down your road with the Bondo resin. It was fun and I learned a lot. Now I only use Oomoo 30 or Plaster of Paris. Give it a try, You may loose your hard master in the process but you will gain a little experience. Be sure to use the bondo in a well ventilated area as well. I don't think those fumes are good for you.

[wchilton]

I tried using marbles for keys in a POP mold and they caused it to crack. Better to just impress the key into the first half and let the second half fill the impression. That way your keys shrink or expand with the mold material.