Heater-zone drift is a gradual change in delivered radiant energy even though the recipe setpoint remains unchanged. It creates slow movement in sag, forming detail, wall thickness, and cycle time.
Element aging, contaminated reflectors, line-voltage changes, loose connections, cooling-fan condition, failed power controllers, and sensor drift can all alter output. A zone may also cycle incorrectly because feedback is taken from a location that does not represent the sheet.
Verification
Thermal mapping should be performed at startup and under steady load. Electrical current or power by zone should be compared with the temperature pattern. A setpoint and a glowing element do not prove that the expected heat reaches the sheet.
Faulty hardware should be repaired before the recipe is rebalanced. Permanent compensation for a declining element reduces the remaining control range and can conceal a developing electrical failure.