Meaning
Temperature distribution within the flowing polymer melt develops a highly localized zone of steep thermal gradients adjacent to the cold mold wall. This thermal boundary layer acts as an insulating barrier that slows the extraction of heat from the molten core of the molded part. The thickness and stability of this layer are influenced by the flow rate, thermal conductivity, and mold temperature.
Viscosity Profile
The large temperature difference between the mold steel and the polymer melt creates a highly viscous skin that clings to the tool walls. Inside the thermal boundary layer, the high viscosity restricts flow, forcing the hotter polymer in the core to slide through a narrowed channel. This shearing action can generate frictional heat, which can lower the viscosity of the core material.
Heat Transfer
Because polymers are poor conductors of heat, the frozen skin layer slows the cooling of the warm interior of the part. The progression of the thermal boundary layer determines how quickly the heat can be extracted from the part core to the cooling water channels. Processors must optimize mold cooling to manage this insulating effect and maintain short cycle times.
Molding Defect
Variations in the thickness of this boundary layer across the part geometry can lead to uneven cooling and residual stresses. A non-uniform thermal boundary layer will result in different levels of polymer shrinkage, which can cause warpage or surface defects like sink marks. Balancing the cooling circuit design helps maintain a uniform thermal profile and avoids these defects.
