Meaning
Characterisation of fluid resistance under variable shear conditions defines how thermoplastic melt moves through the gating system of a mould. In polymer processing, an apparent viscosity drop describes the reduction in flow resistance that occurs as shear rate rises, even though temperature remains constant. This behavior is characteristic of pseudoplastic materials such as polypropylene or polycarbonate where chain entanglement decreases under stress.
As these long molecules disentangle and slide past one another, the resistance to flow drops, which facilitates the rapid filling of complex cavities.
Velocity Variation
Molten plastic experiences high shear gradients as it passes through the restricted areas of the runner and gate. As velocity rises, the apparent viscosity drop accelerates flow, allowing the cavity to fill rapidly before the material begins to freeze. This relationship allows designers to use thinner walls than would otherwise be feasible.
Thermal Consequence
Increased velocity generates internal friction within the moving polymer stream. The resulting heat can cause a localized temperature increase, which further promotes the apparent viscosity drop and reduces the injection pressure required from the machine. If this heating becomes excessive, it degrades the molecular weight of the resin and leads to mechanical failure in the finished part.
Tooling Requirement
Precision tooling relies on predictable flow behavior to ensure uniform cavity pressure across multi-cavity moulds. Moulders must calibrate the apparent viscosity drop using capillary rheometer data rather than simple melt flow index values. This step ensures that the process stays within the calculated pressure limits of the press during high-volume production.
