Drill cracks usually begin at entry or breakthrough, where the part is least supported and stress concentration rises sharply. Acrylic, polycarbonate under chemical or residual stress, and other rigid sheets require particular control.
Standard metal-drill geometry may grab the plastic or generate excessive wedging. Dull tools, high heat, poor chip clearance, unsupported walls, and holes placed too close to a formed edge make failure more likely.
Corrective action
The part should be supported on both sides where possible. A drill geometry intended for plastic, controlled feed, low runout, and regular chip removal reduce stress and heat. Pilot-hole strategy should be validated because it can help one material and worsen grabbing in another.
Cracks that occur only in highly stretched zones indicate a forming-stress problem. Changing the drill will not fully correct a part that already contains severe residual stress.