Polycarbonate sheet annealing runs hotter and drier than the acrylic cycle. A converter making 6 mm polycarbonate guards had a puzzle: parts machined on Monday measured flat, and the same parts measured 2 mm out of flat by Friday. Nothing in the process had changed. The sheet was relieving stress from extrusion and machining, slowly, at room temperature.
Polycarbonate sheet annealing is the answer to that class of complaint, and it is a different cycle from the one used on acrylic. Polycarbonate needs more heat, drier air and a longer cool.
Polycarbonate sheet annealing: temperature, hold and cool
- 3 mm: 1 hour hold at 120 C, cool 5 C per hour
- 6 mm: 2 hours hold, cool 4 C per hour
- 10 mm: 3 hours hold, cool 3 C per hour
- Above 15 mm: 6 hours hold, slow cool overnight in the oven
The hold time is set by the centre of the sheet reaching temperature, not by the clock on the wall. A 10 mm panel in a forced-air oven needs roughly 1.5 hours just to soak through; adding hold on top is what makes the cycle effective.
Ramp rate in is 10 C per hour, and out is slower. A part pulled from 120 C into 20 C room air will quench the surface and lock in a new stress pattern, often worse than the one it started with.
Moisture, support and the warp check
Polycarbonate is hygroscopic. Sheet that has been stored in a humid warehouse can hold enough moisture to haze when heated. Dry the oven air, or pre-dry at 90 C for two hours before the anneal. The visible symptom is a faint milky surface that will not polish out.
Support is the second control. The panel must lie on a flat, clean bed, free to expand. Any restraint, a clamp or a weight, becomes a warp as the sheet relaxes against it.
After cooling, check flatness on a granite table with a feeler gauge. A 6 mm guard should sit within 0.5 mm over 600 mm. If it does not, the cause is usually insufficient hold time rather than the temperature.
Annealing is not always worth it. Thin flat sheet used as delivered does not need a cycle, and parts that will be thermoformed afterwards carry their own stress history that the forming step resets. It earns its cost on machined parts, bonded assemblies and anything that will see heat in service.
For the forming step that often follows, the polycarbonate drying guide covers the pre-dry schedule, and the machine guard sheet guide sets out thickness and fixing for guard panels.
Discuss annealing and machining sequence with oversales@rpplexiglass.com before your parts are cut.
Frequently Asked Questions
At what temperature is polycarbonate annealed?
Typically 120 C to 125 C, well below its glass transition near 147 C. Hold time scales with thickness, then cool at 5 C per hour or slower to avoid reintroducing stress.
Why does polycarbonate need dry oven air?
Polycarbonate absorbs moisture, and heating damp sheet can produce surface haze and fine bubbles. A dry-air oven or a nitrogen purge keeps the panel clear through the cycle.
Can annealing remove warp from a formed part?
It can relax stress that causes later warp, but it will not flatten a part that is already distorted. Warp is corrected in the fixture, not in the oven.