When we design a cooling system, the numbers that matter most come from three heat transfer modes: conduction, convection, and radiation. Conduction through the mold steel follows Fourier's law, q = -kA(dT/dx), where k for P20 mold steel sits around 29 W/m·K and H13 lands near 24 W/m·K. That difference alone can shift cycle time by several seconds on a thick core. Convection at the waterline is governed by q = hA(Ts - Tf), and for turbulent flow in a 10 mm cooling channel, h typically ranges from 5,000 to 10,000 W/m²·K. Keep your Reynolds number above 10,000 and you stay in the turbulent zone where heat transfer actually performs. Drop below 4,000 and you are wasting pump pressure. Radiation, q = εσA(Ts⁴ - Tsur⁴), usually contributes less than 5% in a closed mold, but on large automotive tools with exposed cores, ignoring it can throw off your thermal balance by 8–10°C.

On the shop floor, these formulas translate into concrete decisions. A 1.5 mm diameter baffle in a deep core rib increases local h by roughly 40% compared to a straight drilled line, because it forces turbulent flow into dead zones. Manifold blocks with beryllium copper inserts push k up to 105 W/m·K, cutting hot spots that cause warpage. When we troubleshoot short shots or sink marks, we first check flow rate: 4 L/min per circuit is a baseline for a 12 mm channel, but high-cavitation tools often need 8–10 L/min. If your mold temperature controller shows a 15°C delta between inlet and outlet, your convection coefficient is too low—either the channel is scaled or the flow is laminar.

These calculations are not academic. They decide whether a 30-second cycle becomes 24 seconds, or whether a warped part becomes a profitable one. Every mold engineer should run these numbers before cutting steel, not after. For more mold sourcing and technical resources, visit MoldWorld at www.moldw.com.