Material drying is verified by the dryer's controller, and the verification is usually a temperature and a dew point. The thermocouple's role is indirect but important: the melt temperature at the gate determines whether the residual moisture has been driven off, and a probe that reads inaccurately causes the controller to hold a melt temperature that is wrong for the resin's moisture sensitivity.
The first mechanism is the hydrolysis. Some resins-PC, PET, and nylons-hydrolyze at melt temperature if residual moisture is present, and the hydrolysis rate depends on the temperature. A probe that reads low causes the controller to hold the actual temperature higher than setpoint, and the higher temperature accelerates the hydrolysis. The parts show a loss of molecular weight and a degradation of mechanical properties, and the cause is traced to the dryer when the probe is the actual contributor.
The second mechanism is the venting. Some hot runners have vented sections that allow moisture to escape, and the venting rate depends on the melt temperature. A probe that reads low causes the melt to run hotter, and the hotter melt releases the moisture faster, which can produce surface defects if the venting cannot keep up. The defects appear as splay or silver streaks, and they are attributed to the dryer when the melt temperature is the variable.
The third mechanism is the residence time. A melt that is held at a higher temperature degrades faster, and the residence time that the drying calculation assumes is the residence time at the setpoint. A probe offset changes the actual residence time's effective degradation, and the drying verification is invalid because the melt's condition differs from the assumption.
The fourth mechanism is the dryer's own measurement. The dryer's temperature controller has its own thermocouple, and that thermocouple's accuracy affects the drying. A dryer that reads low runs hotter than setpoint, and the resin is over-dried, which produces its own defects. The dryer's thermocouple should be calibrated on the same schedule as the hot runner's, and the calibration should be recorded.
The practical use is to verify the melt temperature at the gate during the drying validation. The drying curve-the resin's moisture content versus the drying time-is measured at a defined melt temperature, and the melt temperature is the probe's reading corrected for the probe's offset. A drying validation that does not verify the melt temperature is a validation that assumes the probe is correct, and the assumption is the variable that the validation should eliminate.
