Draw Ratio and Natural Draw Ratio
topic
Draw ratio (DR) = output speed / input speed, typically 3.5–5.5× for polyester. Natural draw ratio (NDR) is the ratio at which necking occurs — the neck propagates stably as amorphous chains align. Below NDR: partial drawing; above NDR: over-drawing causing fibrillation. NDR depends on MW, orientation of POY, and temperature. For PET POY at 85°C: NDR ≈ 1.7–2.5×.
Role
Understanding natural draw ratio is fundamental to selecting drawing conditions. Drawing at or above NDR ensures uniform orientation; below NDR results in thick-thin yarn defects that cause dyeing streakiness in fabrics.