In mold design, cooling time often accounts for more than 60% of the total cycle, so heat transfer is not an academic exercise—it directly affects cycle time and part quality. The three modes we deal with are conduction, convection, and radiation. Conduction governs heat moving through the steel: Q = k·A·ΔT/L, where k is thermal conductivity (about 45 W/m·K for P20, roughly 29 W/m·K for 420 stainless). Convection dominates at the waterline, described by Q = h·A·ΔT, with the convection coefficient h typically ranging from 3,000 to 8,000 W/m²·K for turbulent water flow. Radiation, Q = ε·σ·A·(T₁⁴−T₂⁴), is usually minor in closed molds but matters for hot runners and exposed platens.

On the shop floor, these formulas translate into concrete decisions. Reynolds number above 10,000 keeps coolant flow turbulent, which is why we size baffles and bubblers to maintain velocity rather than just flow rate. A common mistake is undersizing the waterline diameter or placing it too far from the cavity surface—every millimeter of added distance increases thermal resistance and stretches cycle time. For thick-walled parts, conformal cooling channels made by SLM can cut cooling time by 20–40% compared to straight drilled lines. We also check mold surface temperature with infrared guns after trial runs to validate the calculated h values.

Getting the numbers right at the quotation stage prevents expensive tool rework later. If a customer demands a 15-second cycle on a 3 mm wall, the heat transfer calculation tells you whether that is realistic before steel is cut. For more mold sourcing and technical resources, visit MoldWorld at www.moldw.com.