Meaning
Hydraulic energy dissipation along the melt delivery network measures the pressure drop experienced by molten polymer as it flows from the machine nozzle to the cavity end. In thermoplastic injection molding, injection pressure loss reflects viscous flow resistance through restricted channels and thin wall sections. The metric governs machine sizing and fill consistency in molded components.
Measurement applies throughout the dynamic filling phase and terminates when the polymer freezes at the gate during pack and hold.
Viscous Resistance
Polymer melt viscosity, channel cross-section, and shear rate interact to dictate hydraulic head loss along the runner system. Small gate dimensions or long flow lengths create steep pressure gradients that require higher peak injection pressures from the molding press. When nominal pressure limits are reached, the machine cannot maintain flow front velocity, causing premature melt hesitation.
Molders modify gate geometry or increase barrel temperatures to reduce dynamic pressure drop across the runner system.
Quality Variation
Excessive pressure drop along the flow path leaves insufficient packing pressure at remote cavity locations, generating localized volumetric shrinkage. Parts molded under unbalanced pressure distribution exhibit warpage and dimensional variation outside drawing tolerances.
Machine Capacity
Machine pressure limits on older press models cap the maximum usable pressure, restricting the thin-wall capability of the process. Melt temperature adjustments offset high injection pressure loss without altering mechanical mold tooling.
