Durable polycarbonate is dishwasher safe and is the best material to stand up to freezing-heating cycles. For the fill line - simply fill Rapi-Kool Plus with water screw on the top and freeze. eliminating the stress caused by overfilling. Helps to rapidly chill food through the temperature danger zone (135 degree C (57 degree C) to 70 degree C (21 degree C) in 2 hours then to 41 degree C (5 degree C) within 4 hours. Patented and NSF Listed. Durable polycarbonate is dishwasher safe and is the best material to stand up to freezing-heating cycles. Patent pending.