Entrance Pressure Loss

Viscoelastic Strain

Viscoelastic properties of the polymer cause the melt to store elastic energy as it accelerates into the contraction. The entrance pressure loss depends heavily on the elongation flow behavior of the resin. When the polymer molecules are stretched rapidly, they resist the deformation and generate a localized pressure peak at the transition.

This peak increases the overall injection force required to fill the mould cavity. In highly branched polymers like low density polyethylene, this elastic storage is pronounced, resulting in a large pressure drop that must be compensated for by increasing the melt temperature.

Processing Penalty

Elevated resistance at the gate raises the melt temperature due to localized shear heating. If the entrance pressure loss is too high, the injection moulding machine may run out of pressure capacity, which leads to incomplete parts or short shots. This penalty is particularly severe when molding thin-walled parts with highly viscous resins like polycarbonate.

Runner Optimization

Tapering the gate entrance reduces the sudden acceleration and minimizes the pressure drop. A gradual transition helps the polymer chains orient smoothly without storing excessive elastic energy. This design adjustment lowers the required injection pressure and reduces the residual stress in the molded component.