Cavity Surface Texture Effects on Thermoplastic Heat Transfer and Shrinkage
Cavity surface texture increases interfacial thermal contact resistance, delaying resin skin freeze and increasing semi-crystalline thermoplastic shrinkage.
Heat Flux Decay
Meaning
Thermal energy transfer from the molten polymer to the mold steel decreases as the polymer cools and shrinks away from the cavity walls. This heat flux decay occurs because the formation of an insulating air gap between the shrinking polymer and the cold tool face restricts conductive heat transfer. The rate of this decline governs the total cooling time required before a part can be safely ejected.
Cooling Efficiency
Conductive heat transfer dominates the initial phase of mold filling when the molten polymer is pressed hard against the steel. The onset of heat flux decay reduces the rate of heat removal, extending the time needed to reach the ejection temperature. Operators must manage this transition to maintain productivity without risking part deformation.
Gap Formation
Shrinkage occurs as the polymer transitions from a liquid melt to a semi-crystalline or amorphous solid. This volumetric change initiates the air gap that drives the heat flux decay. The thermal conductivity of air is much lower than that of steel, which slows the cooling of the part core.
Cycle Modeling
Accurate mold design software must incorporate transient heat transfer calculations to predict the cooling phase correctly. Overlooking the heat flux decay yields overoptimistic cycle-time projections and leads to inadequate cooling circuit designs. Software simulations adjust for this thermal resistance to optimize the spacing and diameter of the mold cooling lines.