Every cooling problem in a mold eventually comes down to two numbers: how much heat the melt dumps into the steel, and how fast the water can carry it away. Heat flux (q, W/m²) is the bridge between them. In a typical injection cycle, the polymer releases roughly 200–400 kJ/kg as it cools from melt temperature to ejection temperature, and that energy must pass through the mold wall before the coolant ever sees it. If your cycle is running long, the first thing to check is not water pressure but the local heat flux around thick sections and hot spots — that is where the thermal resistance stack (polymer, steel, interface, coolant) is doing the most damage.
On the water side, the boundary condition is convective: q = h·(T_wall − T_fluid). The heat transfer coefficient h depends heavily on flow regime. In cooling channels, keep the Reynolds number above roughly 10,000 to stay in turbulent flow; below that, h drops off and you lose cooling efficiency fast. A practical rule on the shop floor: target a velocity of 1.5–2.5 m/s in the main cooling lines, and check that your channel diameter and flow rate actually deliver it. Also account for the thermal contact between the insert and the mold base — a poorly fitted baffle or bubbler adds interface resistance that no amount of pump pressure will fix. For conformal cooling or deep cores, the same physics applies; just remember that smaller channels raise h but also raise pressure drop, so balance the two.
None of this is theoretical once you are standing at the press. Measure your actual flow, log the inlet and outlet temperatures, and compare them against the heat load you calculated from shot weight and enthalpy drop. If the delta-T across the circuit is too large, the channel is under-sized or the flow is too low — fix the hydraulics before you redesign the mold. For more mold sourcing and cooling design references, visit MoldSo at www.moldso.com.