Calculating Cavity Thermal Contact Conductance for Etched Tool Surfaces
Calculated thermal contact conductance on etched tools drops up to 80 percent during cooling, requiring dynamic pressure modeling to avoid warpage and cycle delays.
Solid Spot Conductance
Meaning
Thermal transfer occurring through the microscopic points of direct physical contact between the polymer part and the tool steel during the cooling phase. Solid spot conductance represents the most efficient path for heat to leave the molten resin and enter the cooling system of the mould. Because air is an excellent insulator, the majority of the heat flow happens at these small islands where the surfaces are pressed together by the injection pressure.
The value of this conductance depends on the number and the size of these contact points, which are determined by the surface roughness and the applied load. Managing this variable is necessary for optimizing the cooling rate and maintaining a fast production cycle.
Contact Mechanics
Interaction between the rough surface of the mould and the soft polymer defines how many solid spots are created. In the context of solid spot conductance, the asperities on the steel surface act as the primary contact points that penetrate into the polymer. The mechanical properties of the resin, such as its hardness and its modulus at processing temperatures, determine how much it deforms around these peaks.
High pressure during the packing stage forces the polymer into the valleys of the steel, increasing the real contact area and thus the conductance. As the part cools and its stiffness increases, the ability of the polymer to maintain this intimate contact may decrease. This relationship shows why the early stages of the cooling cycle are the most effective for heat removal.
Heat Flow
Efficiency of the thermal path through the solid contacts is much higher than the path through the air filled gaps. Solid spot conductance is the dominant factor in the overall interface heat transfer coefficient as long as the part is under pressure. The thermal conductivity of the tool material itself also influences this value, as the heat must move from the contact point into the bulk of the steel.
Using high conductivity alloys like copper based inserts can enhance the heat flow through these spots, even if the polymer conductivity remains low. If the tool surface is contaminated or has a thick layer of oxidation, the conductance can be reduced, leading to longer cooling times and potential part defects. Regular maintenance of the tool surface is required to ensure that the solid spots remain clean and effective.
Pressure Sensitivity
Sensitivity of the heat transfer rate to the cavity pressure is a key consideration for process setup and control. Solid spot conductance increases as the pressure rises because the higher force creates more contact area at the microscopic level. This means that a moulder can speed up the cooling of a part by increasing the packing pressure, provided the part can withstand the force without flashing.
When the pressure is released at the end of the cycle, the conductance drops, and the part relies more on the less efficient gas gap conductance. Sourcing resins with consistent flow and deformation properties is necessary to ensure that this pressure response is repeatable. Variations in regrind quality can lead to different levels of contact at the same pressure, causing the cooling rate to drift and affecting part dimensions.