Water quality is the maintenance factor that most affects the useful life of a temperature control unit and the stability of the process. Water that is too hard deposits lime in the mold channels and in the exchanger, and this layer acts as an insulator: the equipment works harder, the cycle lengthens and the part loses repeatability. As a practical reference, a water temperature control unit should work with a pH between 7.5 and 8.5, a total hardness below 15 German degrees and one carbonated hardness below 4 German degrees. Below we explain why those limits matter, how to recognize that fouling is already happening, and what to do to avoid it.
At MARSE we manufacture temperature control units —also called thermoregulators or TCUs— since 1978, and most of the performance incidents that we see in the field do not come from a failure of the equipment, but from the fluid that circulates through it. It is the cheapest breakdown to prevent and the most expensive to ignore.
Why does scale ruin thermal control?
The mechanism is reverse solubility. Calcium carbonate and calcium sulfate dissolved in water become less soluble as the temperature rises, so they precipitate right where it doesn't matter: on hot heat transfer surfaces. The hotter the channel wall is, the faster the deposit grows. Surface temperature weighs much more than fluid velocity on crust formation, so increasing the flow rate helps, but is not a substitute for treating the water.
The problem is that lime conducts heat very poorly. Published measurements place the average thermal conductivity of lime around 1,37 W/(m·K), compared to about 15 W/(m·K) for steel and about 230 W/(m·K) for aluminum. That is, a layer of lime insulates approximately ten times more than the mold steel itself. That is why a deposit that is barely visible when opening a circuit can have a disproportionate effect.
The published research gives an idea of the order of magnitude. A simulation work estimates that one millimeter of lime reduces around one six percent the efficiency of the cooling circuits and doubles the effect that a millimeter of oxide would have on the surface temperature of the mold and on the warping of the piece. The same work concludes that a quarter of a millimeter of deposit barely prolongs the solidification time, but a millimeter clearly penalizes it, and that two millimeters nullify the advantage of conformal cooling, leaving it at the level of a conventional drilled channel. Considering that refrigeration can account for the majority of the cycle time, that percentage is paid on each part.
There is a second, less discussed effect. As the channel section narrows, the equipment needs more pressure to move the same flow, and in the most severe cases the simulations record pressures that exceed what a standard commercial temperature control unit delivers. Long before it gets there, the pump is already working outside its optimal point, consuming more and wearing out prematurely.
Source of simulation data: Zink, Héri-Szuchács, Hajagos and Kovács, “Modeling the effect of scale deposition on heat transfer in injection molding”, Scientific Reports 15 (2025).
What water does a temperature control unit admit: reference parameters
These are the values that MARSE recommends for the cooling water of its equipment. They are not a commercial suggestion: below these thresholds the equipment behaves as designed, and above these thresholds appear corrosion and deposits that are not covered by the warranty.
- pH: between 7.5 and 8.5
- Conductivity: inferior a 150 mS/m
- Total hardness: less than 15 German degrees
- Carbonated hardness: less than 4 German degrees, or less than 20 if stabilized
- Chlorides: inferior a 100 mg/l
- Sulfates: inferior a 150 mg/l
- Ammonium: inferior a 1 mg/l
- Hierro: inferior a 0,2 mg/l
- Manganese: inferior a 0,1 mg/l
- Suspended solids: none
There are three nuances that should not be overlooked. With stainless steel, the chloride limit depends on the working temperature: up to 50 °C it allows 100 mg/l, between 50 and 90 °C it drops to 50 mg/l, and above 90 °C it drops to 30 mg/l. With aluminum, the pH window narrows to a range of 7.0 to 8.0. And below 5 °C, an antifreeze with a corrosion inhibitor must be added, taking into account that glycol should not be used above 120 °C.
To stay within these margins, it is usual to treat the water with an anticorrosive, a non-ferrous metal protector and a hardness stabilizer. In areas with hard tap water, a softener or the use of demineralized water with an inhibitor is usually cheaper than the accumulated cycle loss. You can expand on this point on our page about fluid quality, and the magnetic filters They help retain metal particles that accelerate circuit fouling.
How do I know if I have encrustations? Warning signs
Embedding does not warn all at once: it degrades the process little by little, and that is why it ends up normalizing. These are the signals that should trigger a circuit inspection.
- The equipment takes longer to reach the setpoint or has difficulty maintaining it at full capacity.
- The thermal difference between the flow and return widens with respect to the start-up values.
- The flow rate drops, or the delivery pressure rises to move the same flow rate—a classic sign of an obstructed section.
- The cycle lengthens without anything having changed in the mold or the material.
- Uneven cooling defects appear, such as warping or sinkholes in specific areas of the piece.
- In the worst case, the pump sounds different or the equipment goes into alarm due to temperature.
The reliable way to confirm this is to compare with the reference: record flow, pressure and thermal jump when the installation is clean and measure them again periodically. A temperature control unit with flow monitoring It makes this monitoring much easier, because the drift is seen on the screen before it is noticed on the piece.
How to prevent scale and what to do if you already have it
Prevention is a short and boring protocol, which is exactly what it should be. Analyze the make-up water before starting up and do not assume that the mains water complies. Treat the circuit with the appropriate additives and check their concentration periodically, not just on the day of installation. Check the filters and avoid continuously adding new water, because each new liter brings new salts. Maintain a sufficient flow rate to ensure a turbulent regime, which delays the formation of the deposit. And it records the reference parameters of the installation to be able to detect drift.
If the deposit has already formed, chemical cleaning with a descaler compatible with the circuit materials is the usual route, followed by neutralization, rinsing and replacement of the treatment. It is advisable to do it with the correct product: an acid that is too aggressive can resolve the limescale and create a greater corrosion problem, especially in circuits with aluminum or with sensitive joints. After cleaning, check the condition of the pump, exchanger and probes, because they are usually the components that have suffered the most.
Thermal oil also has requirements
In high temperature equipment the fluid is not water, but the logic is the same: the fluid determines reliability. MARSE requires high quality synthetic oils to prevent premature degradation, with a kinematic viscosity at 40°C of 15 a 26 cSt, and minimum flash point 170°C and a working range of −20 to +320 °C. A degraded oil forms sludge and varnish that dirty the exchanger with the same effect as lime: less transfer and more consumption. If you work above 150 °C, periodic checking of the condition of the oil should be in your maintenance plan.
Frequently asked questions
What water hardness does a temperature control unit accept?
For reference, a total hardness below 15 German degrees and a carbonate hardness below 4 German degrees, or up to 20 if stabilized with an additive. Above these values, the precipitation of lime on hot surfaces accelerates and it is advisable to decalcify or treat the water.
How much does fouling affect cycle time?
It depends on the thickness. Published research indicates that a quarter of a millimeter barely prolongs solidification, while a millimeter already significantly reduces the efficiency of the circuit and two millimeters can nullify the advantage of conformal cooling. Since refrigeration takes up a good part of the cycle, even a small percentage is noticeable in annual production.
Why doesn't my temperature control unit cool like before?
The most frequent causes are incrustation in the exchanger or in the mold channels, a dirty filter, air in the circuit or a worn pump. If the pressure necessary to move the same flow rate has also increased, the tank is the most probable hypothesis and it is time to inspect the circuit.
How often should the cooling circuit be cleaned?
There is no single interval: it depends on the hardness of the water, the working temperature and the hours of operation. The effective thing is not to set a date, but to monitor the drift of flow, pressure and thermal jump with respect to the reference values and act when it appears.
Can I use demineralized or osmosis water?
Yes, and it greatly reduces the risk of scale, but very pure water can be aggressive with some metals, so it should always be used with a corrosion inhibitor and respecting the pH window of the circuit material, which is narrower in the case of aluminum.
Do we check the water in your facility?
At MARSE we have been manufacturing water and oil temperature control units since 1978, and we provide after-sales support to equipment installed around the world. If you suspect that your installation is losing performance due to scale, or you want to check if your plant's water meets the requirements, tell us your process and the values of your make-up water and we will help you interpret them.
Talk to a MARSE technician or scribe to marse@marse.es.
Related readings: water or thermal oil: how to choose the fluid · mold temperature table by polymer · reduced total cost of ownership.