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.
Real Contact Area Ratio
Meaning
Ratio of the actual microscopic area of contact between two surfaces to the total apparent surface area available for thermal or mechanical interaction. The real contact area ratio is a critical factor in determining the thermal conductance between the polymer melt and the mould wall. Because both surfaces have microscopic roughness, they only touch at the peaks of their asperities, leaving much of the interface separated by tiny gaps.
This ratio increases as the injection pressure or the clamping force rises, forcing the asperities to deform and create more contact points. A higher ratio leads to better heat transfer and more efficient cooling of the part.
Asperity Interaction
Surfaces at the microscopic scale are never perfectly smooth and consist of a landscape of peaks and valleys. When the polymer melt is first injected, it only makes contact with the tallest peaks of the tool steel, resulting in a low real contact area ratio. As the pressure in the cavity builds, the soft polymer is forced into the valleys of the steel surface, significantly increasing the amount of contact.
The hardness of the polymer and the roughness of the steel both play a role in how easily this contact is established. If the tool is highly polished, the asperities are smaller and more numerous, which can lead to a more uniform contact area. Conversely, a textured or etched surface will have larger gaps that require more pressure to fill, affecting the overall cooling rate of the part.
Loading Effect
Applied pressure from the injection machine is the primary driver for increasing the amount of physical contact at the interface. The real contact area ratio is not a constant value but changes throughout the moulding cycle as the cavity pressure fluctuates. During the packing phase, the high pressure maximizes the contact area, which is necessary for pulling heat out of the melt as quickly as possible.
As the part cools and starts to shrink, the pressure against the mould wall decreases, and the contact ratio drops. This change is one reason why the cooling rate slows down in the later stages of the cycle. Engineers use this relationship to determine the optimal packing pressure for different resins to ensure the best balance between cooling speed and part quality.
Thermal Conductivity
Energy transfer across the interface depends on the path that the heat takes through the solid contact points and the air gaps. Since heat moves much faster through the metal to polymer contact than through the air, the real contact area ratio effectively limits the thermal performance of the mould. Increasing the ratio by using smoother tool surfaces or higher pressures can reduce the total thermal resistance and shorten the cycle time.
Sourcing resins with a lower modulus can also help, as they deform more easily under pressure and create a larger contact area. However, very high contact ratios can make part ejection more difficult due to the increased adhesive forces. Moulders must find the right balance to ensure efficient production without causing damage to the parts or the tool.