When a mould starts producing parts out of tolerance, the temperature control unit is usually the first suspect and the last thing to be checked. Most TCU problems have recurring causes and can be diagnosed with a few tools: a contact thermometer, a pressure gauge, a flow meter and some method. This guide brings together the most common symptoms, their probable cause and what to check first.
Before touching anything: three measurements
Many faults are solved by measuring before dismantling. Record these three values with the unit running at steady state:
- Supply and return temperature. The difference (ΔT) shows how much heat the mould exchanges and whether the flow is sufficient.
- Supply and return pressure. A pressure drop that increases over time points to blockage or scale.
- Actual flow rate. It is the figure most often assumed and least often measured. A flowmeter on each circuit makes it clear.
As a reference, injection moulds usually aim for a ΔT of a few degrees between supply and return. If it is much higher, there is not enough flow or the circuit is dirty.
Diagnostic table
| Symptom | Probable cause | What to check |
|---|---|---|
| Temperature fluctuates | Poorly tuned control, badly placed sensor or insufficient flow | Control parameters, sensor position and condition, actual flow rate |
| Does not reach setpoint | Faulty heaters, insufficient power or cooling valve letting water through | Heater current draw, contactors or relays, tightness of the cooling solenoid valve |
| Does not cool enough | Cooling water too warm or at low pressure, scaled heat exchanger | Mains water temperature and pressure, condition of the heat exchanger |
| Low flow | Dirty filter, scale, hoses or quick couplings that are too narrow | Filter, pressure drop, hose and connection diameter, pump direction of rotation |
| Limescale and deposits | Untreated hard water | Water hardness, increasing ΔT, deposits in the mould and the heat exchanger |
| Leaks | Seals, hoses or couplings degraded by temperature, pump seal | Maximum temperature of hoses and seals, tightening, condition of the mechanical seal |
| Level alarm | Leak, evaporation or lack of top-up | Leaks in the circuit, fill water pressure, level sensor |
| Pump cavitation | Water close to its boiling point, low level or air in the circuit | Temperature versus pressure, level, air venting, suction filter |
| Degraded oil | Operation above the oil limit, oxidation or ageing | Colour, smell, sediment, maximum fluid temperature according to its manufacturer |
Temperature fluctuates or does not reach setpoint
If the temperature rises and falls around the setpoint, check the sensor first: dirt, a poor connection or a position where it does not read the actual fluid. Then the control parameters, especially if the mould has been changed. A small mould on an oversized unit tends to oscillate.
If it does not reach the setpoint, check that the setpoint is within the range of the unit. A water unit with an open tank does not go above about 90 °C; above that you need pressurised water or oil. This is explained in water or oil temperature control unit.
Low flow: the calculation that makes it clear
The required flow depends on the heat to be removed and the permissible ΔT:
Flow (kg/s) = power (kW) / (specific heat × ΔT)
Example with water (specific heat ≈ 4.18 kJ/kg·K). A mould needs to remove 6 kW and you want a ΔT of 3 K:
- 6 / (4.18 × 3) ≈ 0.48 kg/s.
- That is, about 29 l/min.
If your flow meter reads 12 l/min, the real ΔT will be about 7 K and the mould temperature will not be uniform. With thermal oil the specific heat is roughly half that of water (check the oil datasheet), so you need almost twice the flow for the same ΔT.
Limescale, rust and dirt
Limescale is an insulator: a thin layer in the mould channels greatly reduces heat transfer. We explain it with figures in scale and deposits in the mould circuit. Rust particles, for their part, wear the pump seal and block the solenoid valves. A magnetic filter on the return line captures ferrous particles before they reach the unit.
Cavitation and degraded oil
Cavitation sounds like gravel inside the pump and is usually accompanied by unstable pressure. In water it appears when the temperature approaches boiling for the circuit pressure, or when the level is low or there is air. Vent the circuit, check level and pressure, and do not run water above what the system allows.
In oil units, oil that is dark, smells burnt or contains sediment has lost thermal capacity and can form deposits. Check that the operating temperature does not exceed the limit stated by the oil manufacturer and plan the change according to its recommendations.
Typical mistakes
- Changing the control settings without first measuring the flow.
- Using hoses or quick couplings that cannot withstand the operating temperature.
- Topping up with untreated hard water after every leak.
- Mixing different oils or topping up with an oil not rated for the temperature.
- Ignoring a recurring level alarm: there is almost always a leak behind it.
Frequently asked questions
Why does my temperature control unit not reach temperature?
The most common causes are a faulty heater, a cooling solenoid valve letting water through or a setpoint outside the range of the unit.
How often should the circuit be cleaned?
It depends on the water quality. Watch the ΔT and the pressure drop: if they rise over time, it is time to clean.
What should I do if the pump is noisy?
Check the level, air venting and temperature versus pressure. If it persists, check the suction filter and the pump seal.
When should thermal oil be changed?
When it changes colour, smells burnt or shows sediment, and always according to the oil manufacturer’s instructions.
Still not solved?
If the problem keeps recurring or the unit is undersized for the mould, it may be time to review the selection. See our range of industrial temperature control units, try the equipment selector or talk to a technician.