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.
Thermal Resistance Layer
Meaning
Barrier to heat flow created by surface roughness, air gaps or specialized coatings that exists at the interface between the polymer melt and the mould. A thermal resistance layer acts as an insulator that slows the transfer of energy from the hot plastic to the cooling water in the tool. This resistance is the sum of several individual factors, including the contact resistance between the surfaces and the conduction through any films or residues on the tool.
Its presence is the main reason why parts do not cool instantly and why cycle times are longer for thicker sections. Minimizing this resistance is a primary goal in the design of high performance moulding tools for the automotive and electronics industries.
Insulation Mechanism
Heat transfer at the interface is hindered by the lack of perfect physical contact between the polymer and the metal. The thermal resistance layer is formed primarily by the microscopic gaps filled with air or other gases that have very low thermal conductivity. Even when the melt is under high pressure, these gaps persist at the microscopic level, creating a bottleneck for the heat flow.
The thickness of this layer can be increased by the presence of mould release agents, moisture or outgassing from the resin. If the tool surface is highly textured, the effective thickness of the insulating layer is greater than on a polished surface. This explains why matte finish parts often require slightly longer cooling times than high gloss parts made from the same material.
Gradient Formation
Presence of an insulating layer at the surface leads to the development of steep temperature differences within the part. The thermal resistance layer causes the skin of the polymer to stay hotter for longer than it would if it were in perfect contact with the steel. This slows the formation of a frozen layer, which can affect the flow of the material during the filling phase.
It also means that the core of the part loses its heat more slowly, which can lead to non uniform shrinkage and internal stresses. Engineers use thermal simulations to account for this resistance and to predict how it will affect the warpage of the final product. By managing the tool temperature and the packing pressure, moulders can influence the magnitude of this resistance and achieve more consistent cooling.
Solidification Delay
Impact of the interface resistance on the total production time is particularly noticeable in high volume manufacturing. A larger thermal resistance layer extends the time the part must stay in the tool before it is rigid enough for ejection. This delay directly reduces the number of parts that can be produced per hour, increasing the overall cost of the project.
Sourcing virgin resins with consistent thermal properties helps in managing this resistance, as the material will behave predictably in every cycle. Regrind can introduce impurities that may deposit on the mould surface over time, increasing the resistance and forcing a slowdown in production. Regular cleaning and polishing of the mould surface are necessary to keep the interface resistance at its lowest possible level and maintain the efficiency of the moulding process.