In die casting, the die receives metal at several hundred degrees in every cycle and, seconds later, a spray of release agent that cools it abruptly. That thermal swing decides part quality and die life. A temperature control unit for die casting is not there to heat "a little": it is there to keep the die always in the same temperature window, from the very first shot. This guide explains why oil is used, which temperatures are usual and how to avoid the mistakes that shorten die life the most.
Working temperatures by alloy
Usual industry values. Every part has its own setting: wall thickness, metal flow length and finish requirements shift the set point.
| Alloy | Metal temperature | Usual die temperature |
|---|---|---|
| Aluminium | 640–700 °C | 180–280 °C |
| Magnesium | 640–690 °C | 200–260 °C |
| Zamak | 400–430 °C | 150–200 °C |
With the die below its window you get cold laps, flow marks, shrinkage and incompletely filled parts. Above it, the metal tends to stick to the steel (soldering), the cycle gets longer and ejection becomes harder.
Why oil and not water
Water transfers heat better, but at atmospheric pressure it boils at 100 °C. Keeping it liquid at 200 °C takes more than 15 bar, and at 250 °C about 40 bar. That is not reasonable in a circuit with hoses and quick couplings next to a die casting machine. Thermal oil works up to 300 °C at low pressure, which is why it is the usual fluid for aluminium and magnesium. We explain it in detail in oil temperature control units up to 300 °C.
In zamak, with lower set points, some installations use pressurised water when the die temperature does not exceed about 160–180 °C. If you are unsure between the two fluids, see water or oil temperature control unit.
Thermal shock and thermal fatigue
With every shot, the cavity surface rises by several hundred degrees in milliseconds and drops again with the release agent. This repeated expansion and contraction creates stresses in the surface layer of the steel. Over thousands of cycles a network of fine cracks appears, known as thermal fatigue or heat checking, which is one of the main causes of the end of life of a die casting die.
The amplitude of that cycle matters. The colder the body of the die, the bigger the difference between the hot surface and the core, and the faster cracks appear. A die kept at a stable temperature with oil suffers less than one that runs cold and is heated only by the metal.
Preheating: do not heat with metal
Heating the die with the first shots is the most expensive practice: scrap parts, wasted molten metal and maximum thermal shock on cold steel. The right way is to bring the die to its working temperature with the temperature control unit before the first shot.
- Heat gradually and evenly. A large die may need one or several hours to reach temperature through its whole thickness.
- Do not start with the die at room temperature if you can avoid it: cold die steel is more brittle under thermal shock.
- If you change dies often, an off-machine preheating station reduces downtime. We describe it in mould preheating.
Circuits: each zone at its own temperature
A die casting die does not have a single temperature. The gate area and thick sections receive a lot of heat; distant zones and thin walls tend to cool down. That is why the usual approach is:
- Separate the fixed and moving halves, each with its own circuit or its own temperature control unit.
- Use several circuits or several units on large dies, to set zones to different set points.
- Combine oil with spot cooling (for example, with water in cores or hot spots) where a lot of heat must be removed from a small volume.
In production, the same unit that heated the die starts removing heat. That is why the cooling capacity of the unit is as important as its heating capacity.
Flow rate: oil needs more
Thermal oil has a specific heat of about 2.2–2.6 kJ/kg·K at working temperature, compared with 4.18 for water, and a density of about 800 kg/m³. To remove the same heat it needs more flow.
Example: a circuit that must remove 12 kW with a 5 °C difference between inlet and outlet.
- With oil (cp ≈ 2.4): 12 / (2.4 × 5) = 1.0 kg/s, about 75 l/min.
- With water: 12 / (4.18 × 5) = 0.57 kg/s, about 34 l/min.
More than twice the flow for the same job. In addition, oil viscosity is much higher cold than hot, so start-up is the critical moment for pumps, hoses and fittings. Size the pump for the real pressure drop of the die and do not throttle circuits to "balance" them: a well-designed circuit is better than a half-closed valve.
Typical mistakes
- Starting cold and heating the die with scrap shots.
- Choosing the unit only by its heating capacity and forgetting its cooling capacity.
- Using hoses and couplings not rated for oil at 250–300 °C: a leak next to molten metal is a fire hazard.
- Not analysing or changing the oil: at high temperature it oxidises and degrades, and loses heat transfer capacity.
- A single circuit for the whole die when zones have very different needs.
Frequently asked questions
At what temperature is an aluminium die controlled?
Usually between 180 and 280 °C, depending on the part. Many processes run between 200 and 250 °C.
Can water be used in die casting?
In zamak and in specific zones, yes, with pressurised water up to about 160–180 °C. For aluminium and magnesium, oil is the norm.
How long should the die be preheated?
Until it reaches working temperature through its whole thickness, not just at the surface. Depending on die size, it can take from one to several hours.
How often should the thermal oil be changed?
It depends on temperature and operating hours. It is advisable to analyse it periodically and change it when the analysis says so.
Let’s talk about your die
Tell us the alloy, the die size, the number of circuits and the working temperature, and we will propose a configuration. You can see our solution for die casting, the range of industrial temperature control units, try the equipment selector or talk to a technician.