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.
Mikic Yovanovich Model
Meaning
Theoretical approach for calculating the thermal contact resistance between two surfaces by analyzing the deformation of microscopic peaks under a specific load. The mikic yovanovich model is widely used in engineering to predict how heat flows across the interface where the polymer melt meets the mould wall. It assumes that the surfaces are rough and that contact only occurs at the highest asperities, which deform elastically or plastically depending on the pressure.
By calculating the ratio of the real contact area to the apparent area, the model provides a value for the thermal conductance of the interface. This information is necessary for designing cooling systems that can handle the high heat loads of rapid injection moulding.
Asperity Contact
Surfaces that appear flat to the naked eye are actually covered in microscopic peaks and valleys that determine the nature of the contact. In the mikic yovanovich model, these asperities are characterized by their statistical height distribution and their average slope. When two such surfaces are pressed together, only the tallest peaks make physical contact, creating small islands of solid to solid conduction.
The rest of the interface is separated by a tiny gap usually filled with air or another gas. Heat transfer through these solid spots is much more efficient than through the gas filled gaps. As the clamping force or the injection pressure increases, more asperities come into contact and the existing ones deform, increasing the total area available for heat flow.
This mechanism explains why higher packing pressures lead to faster cooling in the moulding process.
Pressure Dependency
Relationship between the applied force and the thermal conductance is a central feature of this mathematical framework. The mikic yovanovich model predicts that the contact conductance will increase as a power function of the contact pressure. In an injection mould, this means that the heat transfer rate is not constant throughout the cycle but changes as the pressure in the cavity rises and falls.
During the packing phase, the high pressure ensures a high conductance, but as the part cools and shrinks, the pressure drops and the conductance decreases. This drop in heat transfer can lead to a slowing of the cooling rate just when the part still has a lot of internal heat to lose. Engineers use the model to determine the minimum pressure required to maintain an acceptable cooling rate for different material and tool combinations.
Prediction Accuracy
Validating the results of the thermal analysis requires comparing the model predictions with actual measurements from the production line. While the mikic yovanovich model is a powerful tool, its accuracy depends on having precise data for the surface roughness and the mechanical properties of both the mould steel and the polymer. Sourcing high quality resins with consistent hardness and thermal conductivity is necessary for the model to yield repeatable results.
If the resin properties vary, as often happens with regrind, the actual contact area may differ from the predicted value. Surface coatings on the mould can also alter the asperity interaction, requiring adjustments to the model parameters. Despite these challenges, the model remains a standard for understanding the complex thermal interactions at the mould interface.