Injection moulding machine cooling: the right water for mould and oil

Cooling an injection moulding machine is the job of two separate circuits: water that cools the mould and water that cools the hydraulic oil. Mould water sets the cycle time and the part's dimensional, surface and warpage quality, which is why in most plants the moulds are connected to chiller water at a constant temperature or to a mould temperature controller. The oil heat exchanger and the feed throat make do with warmer water; for these, cooling tower water or a separate warm circuit is usually enough. A good design separates these two needs and gives each the water temperature and flow it requires.

Where does the heat come from, and where does it go?

Plastic granules are melted by the heater bands on the barrel and by the friction of the screw. Most of this heat has to be taken back out in the mould: the part can only be ejected once it has cooled enough to solidify. The second heat source is the hydraulic system; losses in the pump, valves and piping heat the oil. In all-electric and servo-pump machines, the heat coming from the oil drops markedly, while the mould's heat load stays the same.

In an injection moulding machine, cooling water usually goes to three places:

• Mould: where the part solidifies; the most sensitive circuit.
• Hydraulic oil heat exchanger: keeps the oil at its working temperature.
• Feed throat: the zone where granules pass from the hopper into the barrel; it is cooled so the material does not soften early and clump.

Mould cooling: cycle time and part quality

For most parts, cooling time is the longest part of the injection cycle. The temperature and flow of the mould water determine this time directly. When the water arrives unevenly, the operator lengthens the cycle to stay on the safe side, and hourly output falls. If the moulds are cooled with tower water, the effect is most visible on summer afternoons, when the water is warmest.

On the quality side, what matters is that the mould surface has the same temperature at every point and in every cycle. If one area stays warmer than another, the part warps, sink marks form in thick sections, internal stress and dimensional deviation increase, and surface gloss changes. That is why in mould circuits:

• the water temperature is held constant in summer and winter and from shift to shift,
• the flow is chosen high enough to create turbulent flow in the channels,
• the water temperature difference between mould inlet and outlet is kept to a few degrees.

The material determines the temperature at which the mould runs. With materials such as PP and PE the mould is kept relatively cool; with engineering plastics such as PC, PA and POM it runs markedly warmer. The material supplier's data sheet gives the right range. In fast-cycle work such as thin-wall packaging and PET preforms, a low and very stable water temperature matters as much as capacity.

Hydraulic oil and the feed throat

Hydraulic oil is usually held in a 40–50 °C band. As the oil heats up it thins; seals and the oil itself age faster, and the machine's repeatability suffers. Oil that is too cold, on the other hand, flows sluggishly and slows the movements. Reaching this target does not require chiller water at the oil heat exchanger: tower water at around 30 °C or a separate warm circuit does the job. Spending 7–10 °C water on the oil heat exchanger means tying up chiller capacity in work the mould does not need and raising the risk of condensation on the pipes.

The feed throat is likewise cooled with warm water. The aim is to stop the granules from softening and bridging before they enter the barrel. A throat kept colder than necessary, however, can collect condensation, which causes a separate problem with moisture-sensitive materials.

Tower water or chiller water?

An open cooling tower can cool water to a few degrees above the air's wet-bulb temperature. In most of Türkiye this means water at around 30 °C on summer days. That temperature is sufficient for oil heat exchangers, feed throats and some moulds that run warm. We explain what temperature a tower can deliver in summer in our article on cooling tower selection.

If the mould needs water below the outside air temperature and constant all year, a chiller is required. That is why most plants install two circuits side by side:

• chiller circuit: water at a constant temperature, usually between 8 and 15 °C, for the moulds,
• warm circuit: tower or dry cooler water for the oil heat exchangers and feed throats.

Sending open tower water straight into the mould leads to scale and dirt building up in the narrow cooling channels; as a channel narrows, cooling becomes uneven. The mould circuit is therefore kept closed, with a plate heat exchanger between it and the tower water where needed. In regions with cold winters, the free-cooling coil on industrial units stops the compressor when the outside air is cold enough and cools the mould water with outside air.

When is a mould temperature controller needed?

A mould temperature controller (thermolator) is a small unit placed next to the machine, with its own pump, heater and cooling heat exchanger. It heats the mould to the required temperature and, during production, keeps the temperature constant by transferring surplus heat to chiller or tower water. It is needed when:

• the mould must run markedly warmer than the chiller water,
• moulds connected to the same circuit need different temperatures,
• the mould should be brought to working temperature quickly at the start of production.

Water units usually go up to around 90 °C, pressurised types higher; for higher mould temperatures, thermal oil units are used. These units do not replace the chiller: they pass the heat they take from the mould on to a cooling circuit. Their load is added to the total when the chiller capacity is calculated.

How is the chiller capacity found?

The most reliable method uses the total flow of the mould circuits and the supply-return temperature difference. The practical formula, which follows from water's ability to carry heat, is: Q (kW) ≈ flow (m³/h) × ΔT (K) × 1.163.

Example: assume that in a ten-machine shop a total of 12 m³/h of water goes to the moulds and returns 3 K warmer. Q = 12 × 3 × 1.163 ≈ 41.9 kW. The load of the mould temperature controllers and any other consumers on the same circuit is added on top. If not all machines run at the same time, diversity is taken into account; if new machines are planned, a margin is left.

If the flow is unknown, the machine manufacturer's mould cooling data or a calculation from the material side is used: the amount of plastic processed per hour, the difference between melt temperature and ejection temperature, and the material's thermal properties give the load. You will find the method in detail, along with the other selection criteria, in our chiller selection article.

Per machine or central?

For a single machine or a cell of a few machines, a mini chiller (5–60 kW) with its pump and tank inside the casing is practical. It sits next to the machine, and its water temperature is set for that machine's mould; when the mould is changed, the other machines are not affected. For work that needs ±0.5 °C precision, the model with an inverter compressor is chosen.

For systems that serve a whole shop from one central point, the industrial chiller class (300–2,000 kW) is the place to look. In the process version, the leaving water is held constant to ±0.5 °C; several units can be installed with N+1 redundancy in a master/slave arrangement, so that the line keeps running when one unit stops. For sizes in between, the choice is made in the project according to the machine layout and the expected redundancy.

As an example of a process unit installed per machine, we can point to Viskon Rubber Factory: for its production process, a mini chiller with two refrigeration circuits, a tank and a pump was assembled and tested in the workshop, then installed on a galvanised frame on site and commissioned.

What we need for a quotation

• number of machines, clamping forces and materials processed,
• mould water temperature and flow per machine,
• load of the oil heat exchangers; any existing tower or dry cooler,
• number of mould temperature controllers,
• operating pattern (shifts or 24/7) and expected redundancy,
• where the unit will stand and the summer temperature of the region.

Send this information through our quotation form, and we will work out the right unit and circuit layout for your shop together.