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.
Gas Gap Conductance
Meaning
Thermal transfer coefficient characterizing the thermal flow through the pocket of air or gas that forms when a shrinking polymer part separates from the mould wall. The value of gas gap conductance drops significantly once the plastic cools enough to pull away from the tool surface. This gap acts as a thermal insulator, slowing the cooling process and extending the time required before the part can be safely ejected.
It is a critical variable in thermal simulations because it represents a major resistance in the path of heat moving from the part to the coolant. Understanding this parameter helps engineers predict cycle times more accurately for parts with high shrinkage.
Gap Formation
Separation between the polymer and the tool occurs as a result of the volumetric contraction of the resin as it changes from a liquid to a solid. The magnitude of gas gap conductance is inversely proportional to the width of this separation. In the early stages of the cycle, high packing pressure keeps the material in contact with the wall, but as the gate freezes and the pressure drops, the part begins to shrink toward its centre of mass.
This creates a void filled with air, or in some cases, outgassed volatiles from the polymer itself. The thickness of this gap is not uniform across the part, as complex geometries and ribs may hold some areas in contact longer than others. This variation leads to uneven cooling and potential warpage if not managed by the tool design.
Thermal Resistance
Total resistance to heat flow is the sum of the conduction through the polymer, the interface resistance and the conduction through the tool steel. Once the air gap appears, the gas gap conductance becomes the bottleneck for heat removal. Air has a much lower thermal conductivity than either the polymer or the steel, so even a microscopic gap can have a large impact.
This insulation effect causes the internal temperature of the part to stay higher for longer, which can lead to sink marks or internal voids. To compensate, moulders may use higher coolant flow rates or lower water temperatures, though these measures have limits. The use of high conductivity mould materials like beryllium copper can help, but they cannot entirely overcome the barrier created by the air gap.
Venting Impact
Management of the gases within the cavity is necessary to maintain a consistent heat transfer environment. While gas gap conductance is primarily about the air between the part and the wall, the presence of trapped air or moisture can also influence the interface. Proper venting allows air to escape as the melt enters, ensuring that the polymer can fully contact the tool surface during the high pressure phase.
If air is trapped, it can form a bubble that permanently reduces the contact area, leading to localized hot spots and poor surface finish. Sourcing resins with low volatile content and ensuring the regrind is properly dried reduces the amount of gas that can accumulate in the gap. Regular maintenance of the vents is required to prevent the buildup of residues that could block the escape of air and gases.