A mould can have a perfect temperature control unit and still cool badly. The reason is usually the flow rate: circuits with too little water, poorly distributed or in laminar flow. Mould cooling flow is the variable most often taken for granted and least often measured. This guide explains how to calculate the minimum each circuit needs, how to check the temperature difference and how to measure it on the shop floor.
Two criteria that must be met at the same time
The flow in each circuit must meet two conditions:
- Turbulent flow, so that the water pulls heat away from the channel wall.
- Small temperature difference between inlet and outlet, so that the mould has the same temperature along the whole circuit.
The right flow is the larger of the two. The second one almost always governs, but the first is the one forgotten in narrow channels.
Turbulent flow and Reynolds number
In laminar flow the water moves in layers and the layer touching the wall is hardly renewed: it transfers little heat. In turbulent flow it mixes and heat transfer multiplies. The Reynolds number tells you which regime you are in:
Re = v · D / ν
- v: water velocity (m/s).
- D: channel diameter (m).
- ν: kinematic viscosity of water (m²/s). About 1.0 × 10⁻⁶ at 20 °C, 0.66 × 10⁻⁶ at 40 °C and 0.36 × 10⁻⁶ at 80 °C.
Below 2,300 the flow is laminar; between 2,300 and about 4,000 it is transitional. For mould cooling, a Re of 10,000 or more is recommended, with clearly turbulent flow.
Example: 10 mm channel with water at 40 °C
For Re = 10,000: v = Re · ν / D = 10,000 × 0.66 × 10⁻⁶ / 0.010 = 0.66 m/s. The cross-section of a 10 mm channel is 7.85 × 10⁻⁵ m², so the minimum flow is 0.66 × 7.85 × 10⁻⁵ = 5.2 × 10⁻⁵ m³/s, about 3.1 l/min.
The table gives the minimum flow per circuit for Re = 10,000 by diameter and water temperature:
| Channel diameter | Water at 20 °C | Water at 40 °C | Water at 80 °C |
|---|---|---|---|
| 6 mm | 2.8 l/min | 1.9 l/min | 1.0 l/min |
| 8 mm | 3.8 l/min | 2.5 l/min | 1.4 l/min |
| 10 mm | 4.7 l/min | 3.1 l/min | 1.7 l/min |
| 12 mm | 5.7 l/min | 3.7 l/min | 2.0 l/min |
| 14 mm | 6.6 l/min | 4.4 l/min | 2.4 l/min |
Two practical conclusions: cold water needs more flow to be turbulent, and oil, which is much more viscous, needs even more. For oil circuits, do the calculation with the viscosity of the oil at its working temperature.
Temperature difference between inlet and outlet
The second criterion comes from the heat balance: Q = m · cp · ΔT. For the mould to be uniform, the difference between inlet and outlet of each circuit should not exceed 2–3 °C, and for technical or precision parts it is better to get close to 1–2 °C.
Example: a mould that must remove 6 kW (for example, about 36 kg/h of PP) with four identical circuits. Each circuit removes 1.5 kW. With ΔT = 2 °C: m = 1.5 / (4.18 × 2) = 0.18 kg/s, about 10.8 l/min per circuit.
In a 10 mm channel at 40 °C, the Reynolds minimum was 3.1 l/min. Here the thermal criterion governs: almost 11 l/min is needed, and at that flow the regime is clearly turbulent. Then check that the TCU pump delivers that total flow (about 43 l/min) against the real pressure drop of the mould.
Balancing the circuits
Calculating the flow is useless if it then does not reach each circuit. The usual problems:
- Unequal parallel circuits: water goes where it finds the least resistance. A long circuit or one with many bends gets less flow than a short one.
- Circuits in series: the temperature difference accumulates and the last zone runs hotter than the first.
- Scale and dirt: they reduce the useful cross-section and the flow without anyone noticing. We explain it in scale and deposits in the mould circuit.
- Narrow hoses and quick couplings: a reduced-bore coupling can restrict more than the channel itself.
How to measure it on the shop floor
What is not measured is not controlled. There are three levels:
- Flowmeter per circuit: shows the real flow in each channel and detects blocked or unbalanced circuits. If you are starting out, read our article on the flowmeter and see our industrial flowmeters.
- Measuring inlet and outlet temperature: the real ΔT of each circuit confirms whether the flow is sufficient. It is the basis of delta T control.
- Multi-circuit distribution with monitoring: measures the flow and temperature of each circuit continuously and warns if any of them drifts. See the electronic multi-circuit distribution.
Typical mistakes
- Relying on the pump’s rated flow instead of measuring the real flow in each circuit.
- Connecting in series circuits that should be in parallel, to save hoses.
- Using reduced-bore quick couplings on larger-diameter channels.
- Not checking the circuits until defects appear in the parts.
Frequently asked questions
What Reynolds number does a mould need?
As a reference, 10,000 or more. Below 4,000 heat transfer drops significantly.
What ΔT is acceptable between inlet and outlet?
Between 2 and 3 °C in general, and 1–2 °C for precision parts.
Is more flow always better?
No. Once the flow is turbulent and ΔT is small, more flow only increases pressure drop and pump consumption.
How often should the flow be measured?
Ideally, it should be monitored continuously. At the very least, when each mould is set up and when part quality changes.
We help you with your circuits
Send us the number of circuits, their diameters, the material and the output, and we will calculate the required flow with you. Use the equipment selector or talk to a technician.