Rigid Molds

• Molds for concrete are sometimes made out of rigid materials such as fiberglass or resin if the finished product has a simple shape such as a vase. A rigid mold has the advantage of being able to keep its shape but needs careful construction to avoid undercuts. An undercut is an area where the mold and concrete can lock together and is the the biggest reason why rigid molds are not often used to cast rigid materials.
  
  

Flexible Molds

• Flexible molds are typically made of latex rubber, urethane, tin-cure silicone or platinum-cure silicone. They can peel off omore complicated forms that would otherwise lock into a rigid mold. Flexible molds usually cannot hold their shape and so require a rigid shell. They are more complicated to make and usually more expensive but are also much more user-friendly.
  
  

Number of Pieces

• Occasionally you can build up a mold over a simple object such as a pyramid. This is called a bucket mold, because the pyramid is pulled straight out of the mold and you pour concrete in as you would into a bucket. More often you must divide molds into multiple pieces that are strapped, bolted or clamped together. The number of pieces depends on the object being molded. For example, a mold of a head will usually consist of front half and back halves, divided down the sides of the head and over the ears.
  
  

Solid Casting

• For a solid cast, you make a large batch of concrete and pour it into the mold until it is full. Because this pouring can trap air pockets within the mold, tap the sides of the mold with a rubber mallet. This vibration causes the trapped air to rise to the surface as bubbles and they pop once on the surface. Concrete molded in this way will remain in the mold anywhere from 6 to 24 hours before demolding and will typically require several days to completely cure.
  
  

Hollow Casting

• A more lightweight and less expensive alternative to a solid casting is a hollow one. You mix a smaller batch of concrete and pour it into the mold. In the case of bucket molds you use a paintbrush to cover the inner surface with concrete. You rotate larger multi-piece molds to evenly coat the inner surface. Add additional batches of concrete to build up thickness, allowing each layer to cure for several hours before adding the next.