The Concept of Mold Making

Sometimes, all it takes is the right frame of mind. Having the right concept, is half the battle.

What is a mold? A mold is the negative of your positive. If your fist is the final image and form, then your other hand wrapped around your fist would be the negative or mold.

Add to that concept, which materials are suited for which finished products. A rubber mold can turn out just about any finished material except ceramics. With ceramics, you are working with a clay mixture called ‘slip’ and slip requires a material that absorbs moisture, in order for it to form. So in the case of ceramic slip, your mold will be plaster.

Of all the materials you can make a mold from, only plasters will absorb and evenly, so that it’s possible to form a shell with clay.

Yes, it is possible to pour plaster into a plaster mold. In such a case, you have to watch for 2 things. One, a good separator inside your mold, to prevent added plaster from adhering. Two, undercuts.

Undercuts can be explained simply by imagining a ball within a ‘C’. If the ball fits snuggly against the inner walls of your ‘c’ then it can’t be removed through the opening. Or if you make a fist with one hand, and encircle it with the other, you can’t remove the fist without flexibility of the other hand. If your encircling hand is rigid, you can’t remove the fist. That explains an undercut.

Two hard, rigid forms must have clearance, in order to pull the inner form out of the outer form. So before you try plaster poured into a plaster mold (or any rigid mold), you need to carefully check that your poured form can be removed straight out of your mold. If not, other adjustments to your mold will have to be made. Such as cutting your mold and forming yet another parting line.

The most suitable mold materials for a variety of poured materials:

For any clay, including clay slip – plasters

For polymer clay – plastic, rubber or plaster

For paper mache’- plaster

For resins – rubber

You can find rubber compounds in both silicone and polyurethane. Polyurethane is your all around, multipurpose rubber.

Polyurethane also comes in what is called ‘wet’ or ‘dry’. Wet rubber constantly exudes a layer of oily substance, helping in later casting. The setback is that you can’t make repairs to your rubber mold by adding more. Once set, additional rubber will not adhere due to the oily surface.

Silicone is great for casting hydrostone. It’s very flexible and strong and captures high detail. It’s more expensive and requires that you have no sulfur present. Many of your oily modeling clays contain sulfur so you have to be careful which clays you use for your models and for sealing around your mold box.

Hopefully this information helps in your selections. Good luck with your future mold making.


Post time: 01-25-2017