Meaning
Thermal rate constant quantifies the heat flux transferred per unit area and temperature difference across the contact boundary between molten polymer and mould steel. In injection moulding thermal analysis, interfacial heat transfer coefficient governs how rapidly thermal energy leaves the cavity to solidify the plastic part. The value shifts dynamically across the cycle as pack pressure establishes intimate contact and subsequent cooling shrinkage creates an insulating air gap.
The parameter stops governing thermal transfer once the part separates completely from the cavity walls.
Boundary Dynamic
High melt injection and holding pressures force hot molten resin against microscopic steel surface peaks, maximising contact conductance. During this initial packing phase, heat transfer values reach peak levels exceeding one thousand watts per square metre kelvin. As the resin solidifies and undergoes volumetric shrinkage, contact pressure drops, causing microscopic air gaps to form.
These micro-gaps introduce thermal contact resistance that reduces heat flux by an order of magnitude.
Simulation Accuracy
Mould cooling software models require dynamic contact conductance values to predict cycle times and warpage accurately. Assuming a constant heat transfer coefficient underestimates cooling time in thick part sections and miscalculates differential cooling across core and cavity halves. Semi-crystalline materials like polyoxymethylene release latent heat of crystallization, requiring accurate conductance tracking through the transition plateau.
Warpage simulations fail to predict out-of-plane distortion when asymmetric heat transfer rates across opposing tool halves are ignored.
Tooling Influence
Steel thermal conductivity, surface roughness and tool surface coatings directly modulate heat transfer efficiency at the interface. Polished tool steels exhibit higher conductance than heavily blasted or chemically textured surfaces due to reduced void space. Fast-cycling packaging tools utilise high-conductivity beryllium copper or aluminium inserts to maximise boundary cooling rates.
Processing engineers balance mold temperature settings against boundary heat loss to avoid premature freeze-off in thin flow channels while keeping overall cycle times economically competitive.
