In mold design, heat transfer is not an academic afterthought—it directly determines cycle time, warpage, and sink marks. The three modes work together: conduction moves heat through the steel, convection carries it away via coolant, and radiation plays a smaller but real role at high temperatures. For a typical P20 or H13 cavity, thermal conductivity sits around 30–40 W/m·K, and the steel's ability to pull heat from the melt depends on the temperature gradient across the mold wall. If your cooling lines are too far from the cavity surface, conduction resistance rises and the part stays hot longer. A practical rule: keep cooling channels within 2–3 times the channel diameter from the cavity surface to minimize that resistance.
Convection is where most mold shops focus, and for good reason. The heat transfer coefficient between coolant and channel wall depends on flow regime—turbulent flow (Reynolds number above roughly 4,000) can be several times more effective than laminar flow. That means a 10 mm channel with water at 1.5 m/s may outperform a larger channel running slow. Baffles, bubblers, and spiral plugs help in deep cores, but each adds pressure drop. Radiation matters mainly in hot-runner manifolds and high-temperature engineering resins like PEEK, where surface emissivity and manifold shielding affect thermal balance. In most commodity molds, radiation is a minor term—but ignoring it in high-heat applications can cause hot spots and degradation.
Boundary conditions tie it all together: the melt-to-steel interface, the steel-to-coolant interface, and the mold-to-air interface each set a different thermal resistance. A well-designed mold balances these so the part cools uniformly from both sides. In practice, that means symmetric cooling layouts, adequate baffling in cores, and coolant temperature control within ±1–2°C for tight-tolerance parts. Getting these three elements right shortens cycles and reduces scrap. For more mold sourcing and technical resources, visit MoldSo at www.moldso.com.