Rheological Verification of Recycled High Density Polyethylene in Multicavity Tooling
Verifying recycled HDPE in multicavity tools requires capillary rheometry beyond standard melt index to map shear thinning and prevent cavity filling imbalance.
Cavity Fill Imbalance
Meaning
Multi-cavity injection moulding tools require uniform distribution of molten polymer, but flow differences between the individual paths often compromise consistency. Run-to-run consistency drops when cavity fill imbalance occurs, leading to localized flash or short shots. The defect originates from shear-induced melt temperature differences in the runner branches rather than from runner geometry alone.
Flow Variance
Viscosity differences develop within the runner system because material flowing closer to the wall receives more shear heating. This thermal asymmetry creates uneven pressure drops in subsequent runner splits. When polymer enters the cavity gates, the hotter melt flows faster, filling some cavities prematurely.
Product Impact
Parts from the slower-filling cavities suffer from poor packing and dimension shrinkage. The weight of the moulded products varies, which breaches customer quality standards. Under-packed parts often exhibit sinks, while over-packed cavities produce flashed parts with high molded-in stress.
This variation alters the mechanical properties of the finished components, causing premature part failure in the field. When the moulded pieces are assembled, mismatched dimensions create weld-line defects and structural weakness across the production batch.
Mitigation Option
Tooling engineers modify runner paths or adjust gate dimensions to balance the flow. Variable melt temperatures are corrected by incorporating melt flippers that reposition the sheared layers before the next runner split. Dynamic monitoring of cavity pressure provides the necessary feedback to confirm that flow balance has been restored.