LEGO building blocks are great for building small mold boxes
submitted by Jason C. Gares "jasoncg1971"
LEGO building blocks are great for building small mold boxes. You just snap them on top of one another, as if you were building up a small brick wall. Surround your parts with them, in any shape or heighth you may desire.
When your RTV rubber has fully cured, just unsnap all of the bricks. The RTV will peel right off of them. Molding clay peels right off of them, too, without much hassle.
I've found that even though the LEGO blocks have small gaps between them, that the RTV generally is thick enough not to leak out. It tries to, but does not make it very far. This means you don't have to necessarily have an air-tight box. You end up with a lot of weird flash-type things sticking out of the outsides of your boxes, but they tear right off.
On multi-part molds, using LEGO bricks means you can easily rebuild the same exact box again, after tearing it down between steps. This repeatability makes things much easier.
With a little extra creativity, it is easy to create molds that have built-in, perfectly aligned "keys". Just leave some exposed blocks sticking up inside the mold box itself when you pour your first batch of rubber. When you pour the second one, it will duplicate the pattern of the top of those LEGO blocks, automatically creating great mold keys.
Places like Toys-R-Us sell big bins of them at around $20 each, if you shop around a bit. (That is for 1100 individual pieces, plus a reclosable storage bin.)I would also recommend picking up a couple of LEGO's flat "floor" type sheets, to attach the other LEGO's to. They are around $5 each. It is sometimes handy to have more than one.