Crossover Frequency

Viscoelastic Transition

At low frequencies, polymer chains have enough time to disentangle and flow past one another, behaving like a liquid. High frequencies do not allow for this relaxation, causing the material to respond like an elastic solid. The transition point where these two responses balance is the crossover frequency.

Molecular Indicator

This point shifts lower as the average molecular weight of the polymer increases. Sourcing teams use this shift to verify the consistency of resin shipments. Resins with a broad molecular weight distribution will show the transition at a different frequency compared to narrow-distribution materials.

Processing Consequence

Molten plastic processed above this frequency stores significant elastic energy, which leads to die swell during extrusion. In injection moulding, it can create high residual stresses in the finished parts. Molders monitor this property to avoid part warpage and to ensure that resin from different batches behaves identically in the hot runner.

Batches that deviate from the specified crossover point will require adjustments to the cooling time or the melt temperature to maintain dimensional stability.