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08.02 Molds

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A mold is a negative space container that is used in Casting .

Material is poured into the mold and then removed after it solidifies.

Types of Molds

  • One piece mold
  • Two part mold
  • Multi part mold
  • Mother mold

Molds can either be designed to be reusable or just for a single use. All molds have a finite lifespan but with care can be made to make numerous castings.

One Part Mold

A one part mold is the simplest type of mold. It can be easily made with some wet mud and your hand. Simply press your hand into the wet mud and remove it. You can then pour plaster or other material into the negative space left by your hand and let it set. This would give you a copy of the positive form of one side of your hand.

One part molds are often used in the making of plaques, signs and other flat planer and decorative objects.

The downside of one part mold is that the back side of the casting will be dictated by how the material settles into the mold rather than buy a defined shape as in a two-part mold.

Two Part Mold

A two-part mold is made of two sides that join together in a specific way. Keys or other shapes that fit together help align the pieces of the mold. Often a material is used as a gasket in between the two halves of the mold to keep the mold from leaking. Straps or rubber bands can also be used to hold the two sides tightly together.

And opening in the mold allows for the casting material to be poured in. Then the two halves are separated and the casting is removed.

Usually there is a thin or a flange of the casting material that follows the shape of the joining seam of the mold. This material needs to be removed from the casting in a process called chasing .

Multi Part Mold

Multi-part molds have more than two pieces. These molds are used for casting complex shapes that may have undercuts and other details that cannot be accommodated with a single part mold or a two-part mold.

A special type of multi-part mold is a mother mold. This is a mold that has a soft inner structure that is supported by a rigid outer structure called a mother mold.

Mold Design Considerations

Mold making is a technical process that can be improved by considering the materials and intent of the final casting. A single use mold is much different than a mold designed to make hundreds of castings.

Undercuts

When making any mold, it is important to pay attention to undercuts. Undercuts are part of that cannot pull out cleanly without breaking the mold. For complex shapes more than two piece molds maybe necessary. Another way to contend with undercuts is to use a flexible mold material for the mold that can bend around the final cast pieces.

Single use molds can also be used and broken away from the final casting as is often done in metal casting.

Complex shapes can also be broken up into smaller shapes that do not have undercuts and then joined together after all the cast pieces are made.

Mold Keys

Keys can help align and register the different pieces of multi-part molds. This is very important, so the final shape of the casting is accurate.

Mold keys have many different designs, but should be made to simply snap together and align the shape and be easily decipherable by the mold maker and caster.

Simple Cheap Molds

  • Paper Tape Molds use paper tape wrapped around a form similar to Paper-Maché . After enough layers are added the paper can be cut along desired seams and removed from the form. This is a way to make a custom sized body form for use in designing patterns and garments for Sewing .
  • Hot Glue Gun Molds can make quick detailed molds of objects that can withstand the heat of the glue. Hot melt glue may have air pockets if not applied carefully in the first layer.
  • Silicone Tube Molds use 100% silicone caulking to make an inexpensive rubber mold. Silicone needs exposure to moisture or a way for evaporation to take place to cure. If it is applied too thick then the out later will cure and not allow the inside to cure. Mix with a bit of acrylic paint to dry thick pieces.

Elaborate Expensive Molds

  • Ceramic Shell Molds are used in metal casting and can make complex shapes molds with high levels of detail.
  • Multi-part Rubber Molds can help with small undercuts since the rubber is more flexible than the material being cast and can pull out of the undercut without breaking the mold or casting.