Mold-Making Theory: Is It Hard or Is It Soft?

The first step in any mold-making project is to consider what you want your cast item to be used for. Do you want a solid skull as a base for building up a zombie face with clay and maché? Maybe you want a squishy dismembered hand that looks and feels lifelike. You have to consider what you want your casting to be. You will have to build your mold to the casting material you want to use. This is not absolute, but generally you won't be able to make a hard casting and a soft casting from the same mold. There are ways to try to game the system, but they are compromises to quality and/or ease of use.


Hard molds make soft castings ∞ Soft molds make hard castings

If you want to make a hard, bone-like skull, you will need a soft, flesh-like mold. Inversely, if you want to make a soft, flesh-like hand, you will need to make a hard mold.

Why?

Friction. Why does friction matter?

When you pour casting material into a mold, you are making a  copy of that mold right down to the molecular level. The casting material and the mold have no space between them. If you hadn't used a release agent, they might have even fused together chemically to become one piece.

These parts want to hold onto each other. Imagine you are climbing a rock wall and your hand melts to perfectly fit and grab each rock as you grab it. The mold has the perfect grip on the casting and will not let go if they are made of materials that are too similar in density. The reason you use a hard mold for a soft casting and a soft mold for a hard casting is so that one can give. You need that give to be able to work the casting free from the mold—to peel it away or wiggle it free.

Like I said earlier, there are ways to try to game the system. You could use a "soft" mold that is still harder than the material used for casting. However, as their shore hardness gets closer to one another, it will be hard to separate the casting from the mold without either getting a friction lock or damaging the mold or casting while trying to pry a casting out of a mold so soft you can't get any real leverage to pry the two apart.

Another solution is to increase the complexity of the mold by making extra parts. Instead of having two halves you could have three sections. Or you can break down the original item into smaller pieces and make a series of smaller more simplified molds. While this may solve one problem, it creates others. It slows down your production and your castings now require assembly.

In the next installment of Mold Theory I'll talk about mold making materials that work best for haunting.