Melt fracture in its mildest form reduces surface gloss, though the roughness itself may not be visible to the naked eye. More severe cases show clearly visible transverse-direction zigzag lines. Interfacial instability occurs between layers of coex film and assumes short-wave or long-wave patterns. Short waves are like melt fracture but occur inside the film and may appear as uniform haze or as visible waves.

Melt fracture is caused by two distinct phenomena: pulsations in melt pressure and skin rupture. Pulsations in melt pressure are caused by a slip/stick phenomenon in the die. When polymer sticks and then breaks loose from metal surfaces in the die, melt pressure goes up and down. Pulsations are transmitted through the melt as it exits the die, much as earthquake tremors (also caused by a slip/stick release of pressure in the earth’s crust) ripple out from an epicenter.


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