When we sit down to lay out cooling lines, the first thing worth pinning down is the heat flux boundary condition at the cavity surface. In a running mold, that surface sees a heat flux typically in the range of 10^4 to 10^5 W/m², depending on wall thickness and melt temperature. This is not a uniform number — corners, ribs, and gate areas pull more heat than flat sections, and if we treat the boundary as isothermal we will undersize the circuit right where it matters. In practice I map the heat load per zone first, then size each branch of the cooling circuit against that local flux rather than against an average. That single habit prevents most of the hot-spot problems that show up later as sink marks or warpage.
The other half of the equation is the convective heat transfer coefficient on the water side. For turbulent flow in a typical 8–12 mm channel, h usually lands between 3,000 and 10,000 W/m²·K. Reynolds number is the lever here: keep it above roughly 10,000 and the flow stays turbulent, which is where the Dittus-Boelter correlation gives us a usable estimate of h. Drop below that and the coefficient falls off quickly, so a channel that looks fine on a drawing can be thermally dead in production. Baffles, bubblers, and conformal inserts all exist to raise local h or bring coolant closer to the heat, and each has a place — but none of them compensate for a circuit that was sized on average flux instead of peak flux.
What this means on the shop floor is simple: measure flow rate per circuit, not just inlet and outlet temperature. A low ΔT across a circuit with high flow tells you the channel is doing its job; a high ΔT with weak flow tells you the pump or the manifold is the bottleneck, not the mold design. Keep the turbulent margin, check the heat flux map against the part geometry, and the temperature controller will have something real to work with. For more mold sourcing and cooling design references, visit MoldSo at www.moldso.com.