In mold cooling design, everyone talks about waterline diameter and pitch, but the real starting point is the boundary condition on the plastic-to-steel interface. Heat flux q" across that interface governs how fast the part solidifies, and it is driven by the temperature difference between the melt and the mold wall divided by the interfacial thermal resistance. In practice, a well-designed circuit that ignores this boundary condition will still miss its cycle time target. Before you open a cooling analysis, define the melt temperature, ejection temperature, and the contact resistance at the cavity surface — these three inputs anchor everything downstream.
The second element is the convective heat transfer coefficient between the cooling water and the channel wall, h. For typical turbulent flow in a 10 mm waterline at 2–3 m/s, h lands in the range of roughly 5,000–10,000 W/m²·K, and it drops sharply if flow goes laminar. That is why Reynolds number matters more than nominal pipe size: a 12 mm line running at 1 m/s can underperform a 10 mm line at 3 m/s. In cores and sliders, where space is tight, baffles and bubblers are common, but remember their effective heat transfer area and flow path differ from a straight-through drilled line — model them as such, not as a simple cylinder.
The third element is the mold steel's thermal conductivity and the distance from cavity surface to waterline. H13 at about 25 W/m·K behaves very differently from a copper alloy insert at 300+ W/m·K, and moving a waterline from 15 mm to 25 mm off the surface can add seconds to the cycle. Balance all three — interface heat flux, water-side convection, and steel conduction — and your calculated cooling time will match the press. For more mold sourcing and design reference information, visit MoldSo at www.moldso.com.