Meaning
Injection moulding flow analysis must account for the reduction in melt pressure that occurs as the molten polymer passes through the restrictive opening of a sequential valve gate system. This hydrodynamic resistance is called the valve gate entrance pressure loss and it is caused by the sudden contraction of the flow channel at the gate entry. The energy consumed by this restriction reduces the pressure available to pack the mould cavity.
It does not include the pressure drop that occurs along the runner system or within the cavity itself.
Flow Resistance
High shear rates are generated when the polymer melt is forced through the narrow orifice of the valve gate. This shear heating can temporarily lower the viscosity of the resin, but the geometric restriction still creates a high valve gate entrance pressure loss that must be overcome by the injection unit.
Tooling Impact
Tool designers must size the gate pin and orifice to balance the cosmetic requirement of a small gate vestige with the need to minimize flow resistance. A gate that is too small increases the pressure drop, requiring a higher injection pressure to fill the part. Designers use flow simulation software to calculate this pressure loss and optimize the gate geometry before cutting steel.
Moulding Consequence
Excessive resistance at the gate entry can lead to moulding defects such as short shots, sink marks, or voids due to inadequate packing pressure in the cavity. If the machine cannot deliver the required pressure to overcome the valve gate entrance pressure loss, the polymer will cool before the cavity is fully packed. Moulders must often raise the melt temperature to reduce the viscosity and lower this pressure loss, which increases the cycle time and energy consumption of the process.
