Meaning
Physical mechanism in polymer rheology where the movement of a chain removes the barriers surrounding neighboring chains. Convective constraint release explains how melts behave under high shear rates during injection moulding. It acts as the primary relaxation mode when the flow speed exceeds the inverse of the molecular disengagement time.
Molecular Dynamics
Mathematical descriptions of this mechanism focus on how the relaxation of the tube allows the chain to move more freely. When the melt is sheared heavily, the surrounding chains are swept away, reducing the friction on the center molecule. Flow changes.
This reduction in local resistance allows for the processing of high-molecular-weight resins at lower pressures than static models would predict.
Shear Influence
High shear rates in the gate area of an injection mould trigger convective constraint release to lower the apparent viscosity. This effect allows the material to fill thin walls before the melt freezes against the tool surface. Flow remains stable.
Without this mechanism, the pressure required to move the polymer through narrow channels would exceed the capacity of the machine hydraulics. Engineers utilize rheological models that include this effect to optimize the sizing of runners and gates.
Viscosity Effect
Processing at speeds beyond this limit requires careful management of the heat balance.
