Thermosol Fixation Dose & Line Speed Limit

The halving rule is an engineering approximation to Arrhenius behaviour and holds only across the working thermosol window of roughly 190 to 215 degrees; extrapolated far outside it the factor becomes meaningless, and above about 220 degrees polyester begins to change in ways that no amount of dwell compensates for. Curing temperature means the temperature of the cloth, which is not the air set point and not the controller reading - a heavy construction entering wet runs cooler than the zone it is in for longer than most operators allow, and the honest way to get this number is a temperature-recording device through the oven rather than off the panel. Heat-up time is treated as wholly unproductive, which is slightly pessimistic because some fixation occurs on the way up; the error is in the safe direction. The dose figure says whether fixation completes, not what shade results: migration, sublimation of the lighter disperse fractions and thermomigration of the fixed dye into finish films all move the shade at constant dose, which is why a line held at exactly the same dose by a different time-temperature pair can still need a shade correction.