Polymer molecules align themselves in the direction of flow. The extent of this orientation depends on the shear rate to which the material is subjected and the temperature of the melt. When the material stops flowing, the induced molecular orientation begins to relax at a rate that is determined by the relaxation time and temperature of the material. If the material freezes before relaxation is complete, the molecular orientation is frozen in.
Frozen-in orientation affects the mechanical properties of the material and also the shrinkage in the direction of material orientation. For a given element, the mechanical properties and the amount of shrinkage are different in directions that are parallel and perpendicular to the direction of material orientation.