MOLDS
Years ago ceramic and glass molds were used, dipping first in hot water then cold water so the casting would pop out. Many molds cracked or shattered using this method.  Advancements have been made to the resin mold by using polyethylene plastic. For molding polyethylene, tiny suction holes must be made in the model to allow suction to form the plastic over the master mold. Sometimes these suction holes are visible on the polyethylene molds. These are not defects. To ensure molds stay in good condition for repeated castings, use mold release and clean your molds with soap and water. A sharp fingernail can make an indentation into a soft polyethylene mold.
You also have the option of making your own molds with RTV silicone or latex. Always test  molds that are not made specifically made for resin by putting a little resin on the back of the mold. Resin will become permanent in most candy molds.