Slumping Molds

Am I to fast heating because my mold cracks.
I have a uniform heat studio8 kiln.Tabletop.

Heating too fast is a virtual certainty. You can also have damp molds. You can try them in an oven preheated to 400-500 degrees households. Are you paint them with dish oven separator and not dry enough? You can communicate with people who do the molding material or mold and ask them to speed heating - degrees per minute