In mold cooling, heat leaves the melt through three paths, and every mold engineer should know where each one dominates. Conduction is the workhorse: heat moves from the hot polymer through the mold steel to the cooling channel walls. The governing factors are the steel's thermal conductivity and the distance from the cavity surface to the cooling line. That is why beryllium copper inserts or copper-alloy cores are used in hot spots — raising conductivity from roughly 30 W/m·K for P20 to 150-250 W/m·K can cut local cycle time noticeably. Conduction also explains why cooling channel placement matters more than flow rate beyond a certain point: if the heat cannot reach the channel, no amount of turbulence helps.
Convection takes over at the channel wall, where the coolant carries heat away. The key variable is the convective heat transfer coefficient, driven by flow regime. Keep water in the turbulent range — a Reynolds number above roughly 4,000 — and you get a coefficient several times higher than in laminar flow. In practice, this means maintaining adequate flow velocity (often 1.5-2.5 m/s) and avoiding long series-connected circuits that starve downstream channels. For baffles and bubblers in deep cores, the same rule applies: turbulent flow across the entire channel length, not just at the inlet. Radiation is usually the smallest contributor in a closed mold, but it matters in hot-runner zones and open-die or large-part cooling, scaling with the fourth power of absolute temperature.
For toolmakers, the takeaway is to design for conduction first (steel selection, insert geometry, channel-to-surface distance), then verify convection with a proper flow and Reynolds check, and treat radiation as a secondary factor except in high-temperature niches. A balanced approach across all three mechanisms keeps mold surface temperature uniform and cycle time predictable. For more mold sourcing and tooling information, visit MoldSo at www.moldso.com.