Polycarbonate is hygroscopic, which means it absorbs moisture from the air over time, reaching about 0.2% water content by weight at equilibrium. In thick sheet, that moisture becomes a major defect source because trapped water can vaporize during heating and create bubbles, splay, haze, and loss of optical clarity.

This is why pre-drying is often the line between acceptable output and rejected parts. Even as little as 0.05% moisture can cause bubbles when the sheet is heated for processing.

Storage conditions matter before the sheet ever reaches the oven. A dried sheet can reabsorb up to 0.5% water within 3 to 4 hours on a hot, humid day, and the effect is amplified in 1 inch thickness polycarbonate because the drying cycle must remove moisture throughout a larger mass.

Protective film integrity also matters because torn film can allow contamination and uneven exposure. A clean, dry sheet with controlled handling produces more predictable forming than a sheet that has absorbed ambient moisture for days on the shop floor.

Cooling, Demolding, and Stress Management

Cooling rate controls residual stress, final shape, and warpage. Forced cooling that is too aggressive can lock stress into the part, which may not appear immediately but can later cause distortion or stress cracking in service.

Demold only after the part has enough stiffness to hold shape without distortion. For critical parts, annealing can improve dimensional stability and reduce long-term stress sensitivity, especially after severe forming.