The position within the freeze zone at which the bubble radius change is imperceptible is called the frost line. The nip rolls and the bubble inflation combine to create an elongating force on the polymer bubble-tube and the inflating air causes a circumferential tension on the bubble-tube. The resulting biaxial stress can induce further crystallization, an action termed flow-induced crystallization. Although this effect has been included in recent work involving microstructural constitutive relations, it is neglected in earlier models of blown film extrusion.