Mold Cooling Channel Reynolds Number, Flow and Heat Removal Calculator
Cooling time is usually the largest part of the molding cycle, and it only works if coolant flow in the channels is turbulent. A laminar circuit can carry a fraction of the heat. This calculator checks Reynolds number for your flow and diameter and tells you the minimum flow to get turbulence.
Your numbers
Measured with a flow meter on the circuit. 1 US gpm = 3.785 L/min.
Typical mold channels are 8 to 14 mm. A 7/16 in channel is 11.1 mm.
Water viscosity falls as it warms, which raises Reynolds number for the same flow.
Outlet minus inlet. Good practice keeps this at 2 to 3 deg C.
Your results
Engineering estimate for straight round channels with water. Real circuits have bends, baffles, bubblers and fittings that change pressure drop and turbulence. Glycol mixtures have higher viscosity. Verify with a flow meter and mold temperature measurements.
Reynolds number vs flow per circuit
Sweeps Flow per circuit from 1 to 30, holding your other inputs fixed.
Cooling circuit audit checklist
Optional: talk through how to log flow and delta T readings per mold in your ERP maintenance module.
Optional. The tool and every export work without this. No spam.
How to use it
Measure flow in each circuit with a flow meter, or take it from the temperature controller specification divided across the circuits in parallel. Enter the channel diameter, the coolant temperature and the temperature rise you see between inlet and outlet. The primary results are the Reynolds number and the minimum flow needed to hit your target. If actual flow is below the minimum, check for serial circuits, undersized hoses and quick disconnects before touching process settings.
The engineering behind it
Reynolds number is density times velocity times diameter over dynamic viscosity. Flow in a round pipe is laminar below about 2,300 and turbulent above about 4,000, with a transition region between. Mold designers commonly target 5,000 or more for a margin. Heat removed is mass flow times the specific heat of water, 4.18 kJ per kg K, times the coolant temperature rise. Pressure drop uses the Darcy equation with the Blasius friction factor for smooth turbulent flow, or 64 over Re for laminar flow.
- Re = 997 x v x d / viscosity (water near room temperature)
- Laminar below 2,300, turbulent above 4,000
- Heat (kW) = kg/s x 4.18 x delta T
Reading the result
With the defaults, 8 L/min in an 11 mm channel at 20 deg C gives about 1.4 m/s and a Reynolds number above 15,000, well into turbulence, and removes about 1.7 kW at a 3 deg C rise. The chart shows how Reynolds number scales linearly with flow, so a circuit at 2 L/min would be near 3,800 and transitional. A large temperature rise with low flow is the field symptom of a circuit that is carrying too little heat.
Common mistakes
Putting cooling channels in series so the last one gets warm coolant, using small hose barbs that choke flow, running chilled water at 10 deg C where viscosity is highest, and assuming the controller's rated flow reaches the mold. Scale and rust reduce effective diameter over time, so flow checks belong on the preventive maintenance plan.
- Prefer parallel circuits of equal length
- Keep the inlet to outlet rise within 2 to 3 deg C
- Descale or flush cooling lines on a schedule
Frequently Asked Questions
What Reynolds number do I need for mold cooling?
Flow is fully turbulent above roughly 4,000, and designers typically aim for 5,000 to 10,000. Between 2,300 and 4,000 the flow is transitional and heat transfer is unreliable. Turbulence matters because heat transfer can be several times higher than in laminar flow.
Does this work for water and glycol?
It assumes plain water near room temperature. Glycol mixtures are more viscous, which lowers the Reynolds number for the same flow. For a mix, increase the viscosity and density to match the supplier's data. The select list offers water only.
How do I convert gallons per minute?
One US gallon per minute equals 3.785 liters per minute. A common 3 gpm flow through a mold circuit is therefore about 11.4 L/min. Enter liters per minute in this calculator.
Why is pressure drop a lower bound?
The formula covers only a straight smooth round channel. Real circuits have bends, baffles, bubblers, O-ring steps and quick couplings that add losses, often doubling the total. Use it to compare options rather than to size a pump exactly.
Netray can help tie mold flow checks and cycle time data from your MES into your ERP maintenance records, so a cooling restriction shows up as a work order before it shows up as scrap.
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