"Help me understand this picture..." Topic

Howdy, I'm about a month out from casting my first figure and I have a question about the mold (or mould if you prefer) making process.

Specifically, take a look at this set of pictures:
link

My question is: what are the ball bearings for? I thought they were placed in the plasticine in order to create keys for joining the mold halves, but the pictures show the bearings being left in through both pours of the silicone. Wouldn't this leave a spherical empty space? If so, why is that necessary?

Thanks heaps

I use acorn nuts, but the idea is the same: They are left in as a sturdy key to match up the mould halves. Metal keys, acorn nuts or something else, won't warp or wear so the mould will always match up correctly…if you remove the metal pieces the key will wear and eventually fail.

Oh, OK so you leave them in forever, throughout the casting process. They're an integral part of the mold. That makes total sense now; thanks again.

I'm sorry. I just looked at those pictures more closely…In the pictures they have created hollows in the mould around the pieces and separate keys. This is occasionally done for a dual purpose:

- to aid in venting (you can cut your vents to the hollows if need be)
- to aid in demoulding (some people feel the thinner mould wall around the casting cavity makes it easier to get the pieces out)

I don't do this with my moulds so I missed it the first time I looked at the pictures.

This may be a bit confusing because they are making "Master Molds" These will have wax injected into them so that a lost wax mold and a brass master can be made.

In typical spin casting molds, you want something quite hard to put up this the ware and tare. You will note that they did not vulcanize the rubber so this is RTV (Very Soft).

Acorn Nuts:
link
are used in spin casting molds and with luck, stay in the entire life of the mold. The ball barring are most likely used to provide a void space for air that is forced out of the mold when the wax is injected in. If the photos were larger I suspect you could see fine vent gates cut from the mold parts to the voids.

DyeHard.