Meaning
Unequal flow behavior of molten polymer across different branches of a runner system arises from non-uniform shear and temperature distributions in the melt stream. This rheological imbalance causes the cavities of a multi-cavity injection mould to fill at different rates and pressures. The variance leads to parts with different physical properties.
Flow Non-uniformity
Non-symmetric shear heating in the runner splits the hot and cold material unevenly. During a run, this rheological imbalance means that some cavities receive hotter, less viscous polymer, while others receive cooler, thicker melt. This difference occurs even in geometrically balanced runners, as the material on the outer walls of the runner is hotter due to shear.
Mould Defect
Cavities that fill faster or pack under higher pressure will produce heavier and denser parts. The presence of a rheological imbalance results in dimensional variations, warpage, and different shrinkage rates across the same mould run. This variation makes it impossible to hold tight tolerances across all cavities.
Process Solution
Optimizing the runner layout or using flow rotation technology is necessary to distribute the shear energy evenly. Correcting the rheological imbalance ensures that the melt entering each cavity has the same temperature and viscosity. This adjustment leads to uniform part weights and consistent mechanical performance, improving the overall yield of the moulding operation.
It reduces scrap rates significantly by preventing the occurrence of short shots and flash on the same shot.
