What Reynolds number should my mold cooling circuits reach?

At least 4,000, which Autodesk Moldflow treats as fully turbulent; Moldflow recommends 10,000 as its analysis setting. It advises against aiming higher, since heat extraction barely improves once flow is turbulent.

How do I estimate the heat load without a mold-flow cooling analysis?

Multiply resin throughput (kg/hr) by its enthalpy drop from melt to ejection temperature (kJ/kg), then divide by 3,600 to get kW.

Why does chilled water need more flow to stay turbulent?

Cold water is more viscous, and Reynolds number falls as viscosity rises. NIST puts water at 1.31 mPa·s at 10 °C and 0.55 at 50 °C, so the circuit needs about 2.4 times the flow.

Why did the required flow jump when I added parallel circuits?

Each parallel circuit needs its own turbulent minimum, so once turbulence governs, total flow grows with circuit count. Circuits joined in series reach turbulence on less total flow, at a higher pressure drop.

Injection Molding calculator

Cooling Water Flow Calculator

Find the water flow that carries a mold's heat and keeps every parallel circuit turbulent. You need the heat load, allowed rise, water temperature, circuit count and channel bore.

What this calculator does

Water flow a mold needs to carry its heat load at your allowed rise, with every parallel circuit turbulent.

Formula used

Mass flow (kg/s) = heat load (kW) ÷ (cp × allowable rise); heat-load flow (L/min) = mass flow ÷ ρ × 60,000
Reynolds number per circuit = 4 × (mass flow ÷ circuits) ÷ (π × bore in m × μ in Pa·s)
Turbulent minimum per circuit (L/min) = target Re × π × bore × μ ÷ 4 ÷ ρ × 60,000
Required flow = larger of heat-load flow and circuits × turbulent minimum per circuit
Rise at required flow = allowable rise × heat-load flow ÷ required flow

Inputs explained

Heat Load to Remove From Mold: From mold-flow cooling results, or kg/hr × enthalpy drop (kJ/kg) ÷ 3,600.
Allowable Water Temperature Rise: Outlet minus inlet; Moldflow advises no more than 2 to 3 °C.
Water Supply Temperature: Temperature controller or chiller setpoint; colder water is more viscous.
Parallel Cooling Circuits: Circuits fed side by side from the manifold; series loops count once.
Cooling Channel Bore: Drilled channel diameter from the mold drawing; use the largest bore.
Target Reynolds Number: 4,000 is fully turbulent per Moldflow; its recommended analysis setting is 10,000.

How to use the result

Best suited to setting flow on a new mold's manifold, sizing a temperature controller pump for a mold.
Pure water, 5 to 95 °C; glycol mixes are more viscous and need more flow. Checks flow, not pressure drop; confirm the pump delivers it through baffles and hoses.

Current U.S. benchmarks

The producer price index for plastic resins and materials stands at 280.569 (BLS, Aug 2026), up 6.6% from a year earlier. Quotes priced off last quarter's material cost miss this move.
The U.S. has 9,635 plastics product manufacturing establishments employing about 677,302 workers (Census County Business Patterns, 2023).

Common questions

What Reynolds number should my mold cooling circuits reach? At least 4,000, which Autodesk Moldflow treats as fully turbulent; Moldflow recommends 10,000 as its analysis setting. It advises against aiming higher, since heat extraction barely improves once flow is turbulent.
How do I estimate the heat load without a mold-flow cooling analysis? Multiply resin throughput (kg/hr) by its enthalpy drop from melt to ejection temperature (kJ/kg), then divide by 3,600 to get kW.
Why does chilled water need more flow to stay turbulent? Cold water is more viscous, and Reynolds number falls as viscosity rises. NIST puts water at 1.31 mPa·s at 10 °C and 0.55 at 50 °C, so the circuit needs about 2.4 times the flow.
Why did the required flow jump when I added parallel circuits? Each parallel circuit needs its own turbulent minimum, so once turbulence governs, total flow grows with circuit count. Circuits joined in series reach turbulence on less total flow, at a higher pressure drop.

Last reviewed 2026-10-01.