When we talk about mold thermal balance, three heat transfer formulas do most of the heavy lifting: conduction, convection, and radiation. In practice, conduction dominates inside the mold steel. Fourier's law, Q = kA(ΔT)/L, lets you estimate heat flow through the cavity plate, core insert, and backing plates. For P20 steel, k is roughly 29 W/m·K; for H13, about 25 W/m·K. If you're running a hot runner manifold, the thermal conductivity of the manifold steel and the insulator plates (often 0.1–0.3 W/m·K for titanium or ceramic insulators) directly determines how much heat bleeds into the clamp plate. Get L wrong—say, ignoring the contact resistance between inserts—and your calculated heat loss can be off by 20–30%, which shows up as unstable cavity temperatures on the floor.
Convection governs heat exchange between the mold and the cooling water, and between the mold surface and ambient air. For water lines, Q = hA(T_mold − T_water), where h depends on flow regime. At a typical turbulent flow of 3–5 L/min per circuit and a Reynolds number above 10,000, h for water sits around 3,000–6,000 W/m²·K. Drop below that and you're in transition flow, where h falls sharply and cycle times stretch. On the air side, natural convection gives h ≈ 5–10 W/m²·K, which is why mold surfaces exposed to shop air lose heat slowly and why insulation plates matter. Radiation, Q = εσA(T₁⁴ − T₂⁴), is often the smallest term at mold temperatures under 100°C, but it becomes non-negligible on hot runner manifolds running at 250–300°C, especially where emissivity of oxidized steel (ε ≈ 0.7–0.8) is high.
The real skill is knowing which formula to trust in which zone. For a hot runner system, combine conduction through the manifold and insulators, convection from the nozzle tip to the cavity, and radiation from the hot manifold to the surrounding plates. For a conventional cold runner mold, conduction through the steel plus convection to cooling water usually covers 95% of the balance. Build a simple spreadsheet with these three formulas, plug in your actual steel grades and flow rates, and you'll catch thermal problems before the first trial shot. For more mold sourcing and technical resources, visit MoldSo at www.moldso.com.