When sizing cooling channels for an injection mold, the first thing I calculate is heat flux at the cavity surface. For a typical PP part running a 25-second cycle, peak heat flux during filling and packing often lands between 15,000 and 40,000 W/m². That number drives everything downstream. If you skip it and just space baffles every 50 mm because "that's what we always do," you'll get hot spots, warpage, and a mold that never hits the quoted cycle time. The heat flux gives you the driving potential; the boundary condition tells you how that heat actually leaves the steel. Get the boundary condition wrong and your Reynolds number, pressure drop, and channel diameter calculations are built on sand.

In practice, I use three boundary condition types depending on what I'm solving for. First, constant wall temperature — useful for rough sizing when turbulent flow dominates and the coolant side coefficient is high. Second, constant heat flux — this matches the filling stage best, where the polymer delivers a near-steady thermal load before the part solidifies. Third, convective boundary condition — the realistic one, coupling coolant temperature, flow rate, and channel geometry through a Nusselt correlation. For a 10 mm diameter channel with 8 L/min water flow, you're looking at roughly 12,000–15,000 W/m²·K on the coolant side. That convective coefficient, not the steel's conductivity, usually limits how fast you can pull heat. Mold builders who understand this stop guessing on baffle placement and start matching channel layout to the actual heat flux map from Moldflow or similar simulation.

The payoff is concrete: cycle time drops, scrap rate falls, and you stop arguing with the customer about why the mold "runs hot" on one side. If you're quoting a new tool, build the heat flux estimate into your cycle time assumption before you send the price. For more mold sourcing and technical resources, visit MoldWorld at www.moldw.com.