Meaning
Injected thermoplastic melt must be continuously compressed within the mold cavity to compensate for the volumetric shrinkage that occurs during cooling. This active consolidation phase is governed by hold pressure packing, which forces additional material into the cavity as the polymer shrinks. Proper execution of this phase prevents internal voids and surface sink marks.
If the hold pressure is cut off too early, the gate freezes and allows the melt to backflow, causing underpacked regions. Conversely, excessive packing can cause molded-in stress and make part ejection difficult.
Volumetric Consolidation
Shrinkage behavior of polymers dictates the duration and magnitude of the pressure applied after the initial fill. During hold pressure packing, the melt density is increased to ensure the part matches the dimensions of the tool. This step is critical for thick-walled parts where shrinkage is most pronounced.
It directly affects the final weight and density of the molded product.
Cavity Dynamics
Gate freezing acts as the natural boundary for the packing process, since no further polymer can enter the cavity once the gate solidifies. Beyond this point, hold pressure packing has no effect on the part weight or density. Monitoring the cavity pressure curve helps determine the exact moment of gate freeze.
This avoids wasting cycle time on ineffective pressure hold.
Defect Prevention
Incomplete densification leads to cosmetic defects like sink marks or functional defects like internal voids. Moulders optimize hold pressure packing to eliminate these issues while avoiding flash.
