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.
Non-Newtonian Flow Index
Meaning
Rheological parameters quantify the degree to which a polymer melt departs from ideal viscous behavior under shear stress. The non-newtonian flow index, often represented as n, determines how readily a material thins as it is pushed through gates and runners. A value of one indicates Newtonian behavior, while lower values denote more pronounced shear-thinning action.
Flow Behavior
Molten plastics usually have an index below one. This value is determined by polymer chain entanglement. As shear increases, the chains align, which reduces viscosity.
Processing Consequence
A lower index allows moulders to fill thin-wall parts at lower injection pressures than would otherwise be required. It also means that melt viscosity is highly dependent on the flow velocity through the gate. If the index drifts due to batch-to-batch variation, it can lead to erratic cavity filling and inconsistent part weight.
Measurement Method
Testing labs calculate this parameter by measuring shear stress across a range of shear rates using a capillary rheometer. The slope of the resulting curve on a log-log plot yields the index value. This parameter is used in mould-filling software to ensure that simulations accurately predict how the polymer behaves during the high-speed filling phase of the cycle.
By utilizing the correct index, engineers avoid specifying oversized injection molding presses, reducing both energy consumption and capital expenditure.