Based on the DIN 51385 standard, coolant lubricants may be subdivided into the main groups of non-water-miscible and water-miscible coolant lubricants, with the water-miscible types being subdivided again into emulsifiable and water-soluble coolant lubricants.
The non-water-miscible coolant lubricants are supplied as ready-to-use products; the water-miscible ones are usually available as a concentrate. Prior to use, the concentrate is diluted with water to form a water-mixed coolant lubricant. Coolant lubricant emulsions are generated from emulsifiable coolant lubricants; coolant lubricant solutions are generated from the water-soluble ones.
Non-water-miscible coolant lubricants
Non-water-miscible coolant lubricants usually consist of mineral oils. However, hydrocracked oils, polyalphaolefins or ester-based oils are additional options. Such coolant lubricants are primarily used when good lubrication is at the forefront of considerations. Depending on the intended used and on requirements, additives are mixed with the coolant lubricant to optimize the properties.
Non-water-miscible coolant lubricants have some advantages compared to water-miscible types. Thanks to their generally higher kinematic viscosity, they have a good lubricating effect. Besides this, they are characterized by a high pressure susceptibility and good corrosion properties. Another advantage is that with non-water-miscible coolant lubricants no biocides are required for fighting bacteria and fungal attack, as they are virtually germ-free and resistant to bacteria. Their service life is thus considerably longer than that of water-miscible coolant lubricants.
One drawback of non-water-miscible coolant lubricants is that their cooling effect is very slight due to the low specific heat capacity and thermal conductivity compared to water-miscible coolant lubricants. Another problem is posed by oil mist which may be generated when using oils, primarily at high cutting speeds. The aerosols contained within this can lead to health risks in the lungs. Furthermore, oil mist is easily inflammable under certain circumstances, thus rendering adequate safety measures necessary.
Water-miscible coolant lubricants
Water-miscible coolant lubricants may be subdivided into emulsions and solutions. They are primarily used when a good cooling effect is more important than a good lubricating effect. They consist of more than 90 % water and largely have the same physical properties.
Emulsions consist of oil, water, emulsifiers and other additives. The task of the emulsifiers is to disperse the oil in the water, so that after mixing with water a stable oil-in-water emulsion is generated.
Due to their good cooling effect and their low procurement price, coolant lubricant emulsions are widespread in industry.
One drawback compared to non-water-miscible coolant lubricants is their bad resistance to microorganisms. Fungal and bacterial attack, etc. impair the corrosion-protection behaviour of the emulsions and the hygienic boundary conditions for the operating personnel diminish considerably. Furthermore, attack by microorganisms causes the emulsion to lose stability, which can lead to operational disruptions.
In contrast to emulsions, solutions are free of mineral oils. The concentrate usually consists of polymers or salts. As the concentrate is molecularly dissolved in water, no emulsifiers are required for even distribution of the concentrate. To increase performance, additives may be used in the solutions. Solutions are characterized among other things by their good cooling properties. As no emulsifiers are used, foam problems are less likely to occur during use and the resistance to microorganisms is better than with emulsions.
Drawbacks include the bad lubricating properties of the solutions as well as the tendency of the solutions to wash off lubricants and to have an adhering effect.
The properties of coolant lubricants may be changed considerably through the use of additives. The goal is to strengthen the positive properties of the coolant lubricant, minimize the negative properties and thus to generate the desired functional characteristics. Depending on the intended use of the coolant lubricant, the amount and type of additives may be varied. The various types of additives may be roughly subdivided into the following groups according to their function:
- Additives which change the physical properties of the coolant lubricant
- Additives which change the chemical properties of the coolant lubricant
- Tribologically effective additives which change the friction conditions
Coolant lubricants in use
We have made additional items available for the use of coolant lubricants when grinding parts (workpieces): Coolant lubricant supply, grinding burn, errors during grinding, grinding wheels, CNC grinding machines, coolant lubricant filtration systems
Source: Klocke, F.; König, W.: Fertigungsverfahren 1 (2008); Klocke, F.; König, W.: Fertigungsverfahren 2 (2005); Denkena, B.; Tönshoff, H. K.: Spanen (2011); Hipler, F.: Chemie gegen Reibung und Verschleiß (2003); Richtlinie VDI 3397, Blatt 1 (2007)
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A grinding process has to be supplied with lubricoolant, that´s easy to understand. But what are the tasks of the coolant and exactly what are the challenges in the coolant lubricant supply?
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