Hot Drawing and Heat-Setting Mechanisms
topic
Drawing above Tg (>67°C) allows chain segment mobility for orientation. Hot-pin or heated plate drawing at 85–100°C (first stage) and 150–220°C (second stage/heat set) induces crystallisation of oriented chains. Heat-setting at 180–220°C under tension (relaxation 5–12%) reduces shrinkage to <1% (from 15–20% in unset yarn). Boiling water shrinkage (BWS) of FDY: 0.5–3%.
Role
Hot drawing and heat-setting are inseparable from the development of polyester's dimensional stability — its key commercial advantage. Heat-setting temperature and relaxation directly determine fabric shrinkage, handle, and dimensional stability in end-use.