Guys,

A couple of tips on making moulds, nothing earth shaking, but they help.

1) Use Lego blocks (or cheaper equivalents) to make mould boxes. Reuseable and really easy to get pretty much any size you want. Careful spraying mould release as many releases use solvents to get them to evaporate quickly and the solvent can attack the blocks. Use the release sparingly and you'll have no problems.

2) Reuse the rubber from your molds in new moulds. Silicone RTV doesn't stick to virtually anything. Except itself! I chop up old moulds and add them into new moulds to lessen the amount of new rubber I have to pour. The old sticks to the new as if they were one pour and you save a bunch of cash on expensive rubber. This really only applies to home casters as this weakens the mould somewhat and lessens the number of parts you'll get from the mould. However, I have gotten 50+ parts from some of these moulds, so it's not too bad, just not "production" quality.

I make sure the master is covered in new rubber and then insert pieces of old rubber down the sides and into the hollow areas to get more mould volume and stability.

Make sure you cut away any old mould surfaces that have been exposed to mould release as they won't stick and the new mould will likely tear on the seam. Also cut away any old mould material that may be discoloured by resin as these sections have degraded properties and, again, the new mould may split there.

Try it first on a small, unimportant mould to make sure your materials are compatable.

HTH

Paul

Great Tip - reuse the rubber. I haven't been able to bring myself to throw away old molds. Now I know why.

I had never thought of that one.

Thanks.

it does work with some types of rubber sometimes it just won't stick (I've read this on a couple Spec sheets from the Manufact.) it seems to apply to the more Tear resistant mold materials......

I have only tried this with RTV silicone and not with other rubbers, be they thixotropic (smells like vinegar, like caulking) or vulcanised (baked until hard like a tire) or other mould materials. I have used it with several diferent rubbers, but certainly not with all, so make a trial piece first and see how it goes.

Again, you have to make sure you have good, clean rubber an not a piece contaminated with mould release, resin or with plain finger oils or dirt from excess hadling.

Paul

Great advice here. For most of my molds I use paper cups, the waxed ones. The round cup molds hold their shape better over long runs. As cups come in a variety of sizes there is usually one that will fit the parts. There can be a trade off in rubber, but by reusing old molds you can off set this "extra". To contain the resin during processing (vacuum only) I wrap the top of the mold with masking tape. This is a mold saver as you don't need a big pour block in the mold. My average mold life for a detailed part is 20 with out mold release, up to 35 with.

Here's a quick tip..

No matter how careful I am I always make a little more mold material than I need... better too much than too little...

so I have a couple of smaller molds of items I might be able to use and use the excess mold material.

Same with extra resin.. I have a few small molds standing by to pour any extra into...