Melt Flow Anisotropy

Mechanical Property

Molecular orientation during the filling stage causes the finished part to be much stronger along the flow path than across it. In unfilled polymers, melt flow anisotropy arises from the stretching and freeze-in of the polymer molecules near the cold mold wall. When the molten resin cools too quickly, these aligned chains do not have time to relax back into a random state.

This leads to unbalanced strength, making the part susceptible to cracking under impact loads applied perpendicular to the flow line.

Tool Design

Mold geometry and gate placement dictate how the flowing resin fills the cavity and orientates the polymer structure. Engineers calculate the expected melt flow anisotropy to position gates where the resulting high strength direction aligns with the main stress vectors the part will encounter in use. Proper runner sizing and cavity thickness transitions reduce the sudden shear variations that exaggerate this directional orientation.

Production Defect

Unequal shrinkage across the different axes of a molded component results in severe part warpage and dimensional instability. When melt flow anisotropy is high, the differential shrinkage causes the cooled part to twist or buckle out of its flat profile. This behavior increases the difficulty of holding tight tolerances during long production runs, which leads to high reject rates.

Controlling the mold temperature and slowing the cooling rate can mitigate this defect by allowing the oriented molecules to relax before the material solidifies.