Meaning
Energy losses experienced by molten resin as it travels from the nozzle to the gate reduce the effective pressure inside the mould cavity. The runner pressure drop is caused by the internal friction of the polymer and the cooling of the melt as it touches the cold walls of the channel. It must be accounted for during the tool design phase to ensure that the packing pressure is sufficient to prevent sink marks.
This drop is most severe in long or thin runner systems.
Flow Resistance
Viscous heating can sometimes offset the cooling effect in high speed injection. However, the runner pressure drop shows an energy loss as the flow path increases. Moulders use rheology software to calculate these losses before the steel is cut.
Part Quality
Inadequate pressure at the end of the flow path leads to dimensional instability and poor surface finish. If the runner pressure drop is too large, the material may freeze before the cavity is completely full. Increasing the melt temperature can lower the viscosity and reduce the pressure loss.
Mould Geometry
Sizing the runners correctly involves a trade off between material waste and flow efficiency. Minimizing the runner pressure drop requires larger diameters but this increases the cycle time and the amount of regrind. Circular cross sections are the most efficient because they have the lowest surface area to volume ratio.
