Some ABS sheet grades and constructions can absorb enough moisture to blister or lose surface quality during heating. The risk depends on grade, storage, sheet structure, exposure history, and supplier instructions; it should not be managed with one universal drying schedule.
Moisture-related bubbles usually increase as the sheet heats and may be widespread. Bubbles that recur at fixed positions suggest extrusion voids or contamination, while fine bubbles near one heater zone point toward thermal degradation. A controlled comparison using properly conditioned sheet is more useful than repeatedly raising drying temperature.
Drying temperature, time, airflow, and sheet stacking should follow the specific supplier data and should be verified at the sheet, not only at the dryer display. Dried material must be protected from re-exposure. If blistering remains after validated conditioning, the heater map, regrind, and incoming sheet should be investigated.