Determining Polymer Wall Conductance on Smooth Tool Steel
Polymer wall conductance on smooth tool steel peaks above 3500 W/m²K under packing pressure and drops below 600 W/m²K upon thermal gap detachment.
Packing Pressure Phase
Meaning
Pressure stages follow the initial cavity filling to force additional material into the mould as the polymer cools and contracts. During the packing pressure phase the machine maintains a specific force to compensate for the natural volumetric shrinkage of the resin. This prevents the formation of internal voids and ensures that the part takes the exact shape of the tool.
The duration of this stage is limited by the time it takes for the gate to freeze.
Volumetric Compensation
Success in the packing pressure phase depends on the ability to move a small amount of extra melt into the cavity as the density of the part increases. Polymers lose volume as they transition from a liquid to a solid state. Without this additional material, the part would be undersized or show surface depressions.
The pressure must be high enough to overcome the increasing viscosity of the cooling polymer. This prevents the part from pulling away from the walls too early.
Gate Solidification
The end of the packing pressure phase occurs when the material in the narrowest part of the feed system turns solid. Once the gate freezes, no more material can enter the cavity and the pressure inside the part begins to drop independently. Moulders perform gate freeze studies to determine the precise second when this happens.
Applying pressure after this point is a waste of energy and can cause stress at the gate.
Dimension Control
Consistent control of the packing pressure phase is the primary method for holding tight tolerances across a production run where material viscosity might shift. If the pressure varies, the part weight and dimensions will fluctuate.