Types of cooling lubricants

What kind of coolants are actually available?

Non-water-miscible cooling lubricants (oils)
Like all cooling lubricants, non-water-soluble cooling lubricants also consist of one or more base fluids, which in most cases are mixed with additives. The base oils used can vary between mineral oil, hydrocrack oil, (GtL) gas-to-liquid, PAO (polyalphaolefin) or ester oils, depending on the type of application. Depending on the type of processing and material, various additives are added to the base oils, which have a positive effect on the process.

Compared to water-soluble coolants, non-water-soluble cooling lubricants often have a better lubricating effect due to their viscosity. They also protect the machine against corrosion; with oil, there is no risk of the machine being affected by corrosion.

Machines on which non-water-soluble cooling lubricants are used must be equipped with the right equipment. This includes an oil mist separator (extraction system) and the fire protection system. Depending on the application (e.g. grinding), the oil is vaporized by the cooling nozzles and the high pressure in such a way that it leads to the formation of a fine oil mist. An extraction system with air filter should also be installed here. This serves to separate oil mist, oil smoke and aerosols which should not be inhaled. The extraction system with air filter makes the extracted machine air breathable again.

As oils are flammable in themselves, unlike water-soluble coolants, the right precautions must also be taken here. An automatic extinguishing system recognizes the danger within seconds, alerts you via an acoustic signal and triggers the CO2 extinguishing system in an emergency. If the machine is designed for processing with oil, this should already be integrated, as should the pressure relief flaps to divert any deflagrations.

Once the non-water-soluble cooling lubricant has been filled into the machine, it often lasts for 10 years or longer. The requirements for this are a proper filtration system for the cooling medium, e.g. grinding oil, no entry of foreign substances such as tramp oil and a high-quality oil (GtL or PAO based). Only the amount of oil carried out by machining, tools, parts, filters and evaporation needs to be topped up.

In addition, a high-quality cooling lubricant is characterized by longer tool life, higher surface quality, faster machining, cleaner machines and foam reduction.

Water-soluble coolants
These types of coolants are the most common on the market and, as the name suggests, can be mixed with water. Unlike many other types, water-soluble coolants are not supplied as a ready-to-use product, but as a coolant concentrate and are mixed with water on site at the machine. A differentiation is made here between semi-synthetics (emulsions) and synthetics (solutions). In their correct mixture, both have a water content of approximately 90-95%, depending on the application. Depending on the process and machinability of the material, a higher or lower concentration is recommended. If semi-synthetics and synthetics have been mixed with water, they are water-mixed coolants. Machining operations with high cutting speeds and high heat generation primarily require a cooling effect. Here, the best results can be achieved with water-mixed emulsions or solutions. The majority of water-miscible coolants are used in the form of emulsions, i.e. as a product with a strong cooling and additional lubricating effect in the machining process.

Semi-synthetics (emulsions)
There are emulsions with different mineral oil contents. They are available with low contents of e.g. only 20% up to high contents of e.g. 60%. A higher proportion of mineral oil is particularly beneficial for heavy machining processes where a lot of lubrication is required. In general, emulsions are used in machining processes such as milling, turning, sawing and drilling.

If you take a look at the composition of emulsions, it can quickly reach 30 different ingredients. The components of a good emulsion include, for example, mineral oil or substitute lubricants, various additives, water and an emulsifier system. An emulsifier system is important to ensure that the ingredients combine permanently (emulsify) and do not separate again.

We differentiate here between the so-called “water in oil” emulsion or “oil in water” emulsion.

An emulsion that we are all familiar with from everyday life is mayonnaise, for example.

Mayonnaise is a thick, cold-produced sauce based on egg yolk and oil. The basis is an emulsion of oil and water, with the lecithin from the egg yolk acting as an emulsifier.

Mayonnaise is an “oil in water” emulsion as opposed to butter, butter is a “water in oil” emulsion.

What happens when a spoonful of mayonnaise or butter is dipped into a glass of water?

Nothing happens with mayonnaise, it stays together. With a glass of water containing butter, a “film of fat” immediately forms on the surface of the water - it dissolves.

The water-soluble coolants is therefore an “oil in water” emulsion.

However, the emulsifiers also ensure that if tramp oil enters the coolant, this tramp oil is emulsified and can therefore no longer be separated from the coolant, which means that the machining process is no longer stable.

Synthetics (solutions)
Compared to semi-synthetics, the list of ingredients in synthetics is significantly smaller. Synthetics contain no mineral oil and therefore no emulsifiers that have to ensure that the ingredients combine. The appearance is often clear and transparent so that the process can be easily observed during various processes, such as grinding. Furthermore, the lubricating effect is significantly lower, but the cooling effect of the product is significantly better compared to semi-synthetics. Other components include anti-corrosion additives.

If tramp oil gets into the synthetic, this can be easily skimmed off with the help of a tramp oil separator or oil skimmer, as the oil entered floats to the top and does not mix, as it can be the case with an semi-synthetic.

In general, synthetics are less vulnerable to bacteria formation and bacterial infestation and on average have a longer service life than semi-synthetics.

Additives
In addition, almost every coolant contains additives that have a positive effect on the individual machining process. These range from EP additives, which provide better wear protection and higher pressure absorption capacity, to corrosion protection additives, which protect your machine from corrosion.