In mold cooling analysis, heat flux density q (W/m²) is the starting point for any meaningful thermal calculation. It represents the rate of heat transfer per unit area and directly governs how quickly a mold surface can shed heat to the coolant. For a typical injection mold running engineering-grade PC at 280°C with a 60°C coolant, the local heat flux at the cavity surface can easily reach 15,000–30,000 W/m² during the initial filling and packing stages. Ignoring this transient peak leads to hot spots, longer cycle times, and warpage. The practical takeaway: always map heat flux distribution across the cavity before sizing your cooling channels. Uniform q means uniform cooling; localized spikes mean you need baffles, bubblers, or high-conductivity inserts such as BeCu or AMPCO alloys.

Convection boundary conditions tie heat flux to coolant behavior. The convective heat transfer coefficient h (W/m²·K) depends on flow regime, channel diameter, and coolant properties. For turbulent water flow in a 10 mm diameter channel at 2 m/s, h typically ranges from 5,000 to 10,000 W/m²·K. That sounds high until you compare it to the mold steel's thermal conductivity (~30 W/m·K for P20). This mismatch means the mold-side resistance often dominates. In die casting, where molten aluminum at 650°C contacts H13 steel, h can exceed 20,000 W/m²·K at the gate but drops sharply downstream. The engineering response is to increase coolant velocity rather than channel count—doubling velocity boosts h by roughly 1.8× in turbulent flow, while adding channels only increases surface area linearly.

From a shop-floor perspective, the boundary condition is never perfectly uniform. Scale buildup, poor manifold balancing, and non-circular drilled channels all degrade h. A practical rule: if your Reynolds number drops below 10,000, you are in transition flow and your cooling predictions will be unreliable. Verify with a flow meter, not a pressure gauge. For more mold sourcing and cooling design resources, visit MoldWorld at www.moldw.com.