Undercuts are part of a cast object that cannot pull out of a mold cleanly without breaking the mold or the cast. For complex shapes more than two piece molds may be necessary. Another way to contend with undercuts is to use a flexible mold material so the mold that can bend around the final cast pieces.

Single use molds can 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. Often an object will have undercuts in one orientation but not have under cuts in a separate orientation. With planning and observation, many undercuts can be avoided to reduce the mold piece count.