Deformation behavior of extruded blown film of high density polyethylene

Extruded blown films of high density polyethylene contain oriented crystalline lamella normal to the extrusion direction. In the elongation perpendicular to the extrusion direction (TD) at -110 degrees C similar to 40 degrees C, the stress suddenly dropped when the film arrived at the yield point and the necking propagated steadily under a constant stress value. TD deformation mechanism is explained by unfolding from one part of the lamellae as proposed by Horio and Kobayashi (silk hat model). On the contrary, in the elongation parallel to the extrusion direction (MD), deformation mechanism is complex and we have proposed an unfolding model. In the MD, stress did not drop even after the yield point, and it maintained a constant value. Consequently the film was uniformly elongated by appearance, but followed by a partial necking. From optical and scannig electron microscopic observations, the lamellae were bended, twisted, or rotated by the stress, and the long axes of lamelae (b-axis) were changed to the drawing direction and the film was elongated by unfolding same as TD elongation.