Glass transition temperature.

The temperature at which the amorphous regions of a polymer loosen and its chains start moving, which is the condition sublimation depends on. Below it, polyester is closed and dye in the gas phase has nowhere to go; above it, the fibre opens and disperse dye lodges inside, held there by physical attraction rather than by a chemical bond. That is why a press setting is two conditions and not one: the platen has to carry the fibre past its glass transition and hold it there long enough for the dye to move in, which a cold shop, a damp blank or a heavy garment all work against. It is also what ties this to dye migration: take the same fibre back through a cure tunnel or under a second press and the garment's own disperse dye moves again, this time up into the ink lying on top of it. Cotton offers none of this. Cellulose does not soften at press temperature the way polyester does, so there is nothing to open and nothing to close again around the colour.