In machining, cutting and forming, cooling lubricants perform three core functions that directly determine how cost-efficient production is.
New materials, coated tools, higher cutting speeds and stricter environmental requirements have raised the demands on cooling lubricants sharply in recent years. Meeting these demands requires targeted additives in the base formulation - substances that change the physical and chemical properties. Typical additive classes are corrosion inhibitors, extreme-pressure and anti-wear additives, oxidation inhibitors, emulsifiers, biocides, friction modifiers, defoamers, anti-mist additives and adhesion promoters.
DIN 51385 distinguishes non-water-miscible and water-miscible cooling lubricants. Water-miscible cooling lubricants ship as a concentrate. You mix them with water before use - either into an emulsion (milky) or a solution (clear).
Water-miscible cooling lubricants differ mainly in their base oil. The base oil determines the application area, the appearance and the recommended mixing ratio.
The most common choice in machining.
Good machining performance, good corrosion protection. Oil content in the concentrate: 20-50%.
Mainly for grinding operations.
Free of mineral oil. Less suited to classic machining, but produces little foam.
The best machining properties among the emulsions.
Often used as a forming lubricant. Easy to clean off with water.
The base oil in the cooling lubricant largely determines its performance, its ageing behaviour and its impact on people and the environment.
Water-miscible cooling lubricants consist of 90 to 95% water, so its quality directly affects the stability, corrosion protection and service life of the fluid.
The pH value of the make-up water should ideally sit at 7. Values below 6 lower the pH of the emulsion, which weakens the corrosion protection and destabilises the emulsion. Total hardness matters too. Water above 20 °dH tends to form lime soaps. These show as white traces on the emulsion surface and can settle on workpieces and tools. Softening or topping up with fully demineralised water helps here. Below 10 °dH, foam can form - hardness increasers or defoamers help in that case.
For nitrate content, TRGS 611 sets a limit of 50 ppm. This limit prevents the formation of nitrosamines, which can arise from nitrite combined with secondary amines. Nitrite in used emulsions mostly forms through the breakdown of nitrate, so limiting the nitrate content is decisive. The chloride content should stay below 250 ppm, because higher values attack the corrosion protection - water evaporation in particular can concentrate it. Topping up with fully demineralised water helps here as well. Overall, the make-up water should have drinking-water quality, with at most 100 CFU/ml and no pathogens.
Even the best cooling lubricant only works when it reaches the point where it is needed. In practice, two supply methods are in use.
The coolant runs through bores in the tool straight to the contact point. This makes sense above all for tapping, long-chipping materials and deeper holes (from two to three times the diameter). Advantages: optimal chip removal and reduced wear in the cutting zone..
Nozzles direct the coolant onto the machining point from outside. Advantage: no special tool needed. At least one nozzle should sit as close as possible to the tool axis. For stationary drills, an arrangement with two nozzles works well.
A cooling lubricant is only as good as its care. Anyone who monitors the cooling lubricant in use systematically prevents microbial contamination, protects the workforce and extends the service life of the fluid considerably. TRGS 611 makes this care mandatory.
Effective cooling lubricant management monitors and checks the cooling lubricant in use continuously to spot trends early: wrong mixing ratios, loss of quality or bacterial growth. This lowers operating and disposal costs, raises process reliability, cuts the health risks for users and prevents odour nuisance and corrosion problems. It also reliably prevents the growth of microorganisms.
Microorganisms enter the cooling lubricant along four main paths: through liquid contaminants such as tramp oil, through solid foreign matter from the working environment, through mould spores in the ambient air and through low-oxygen conditions when the fluid does not move enough. Filter technology and skimmers counter the first three paths, continuous circulation the fourth.
For the fresh mix and ongoing operation of water-miscible cooling lubricants, DGUV Rule 109-003 requires a test plan. A monitoring sheet for each machine documents the test and care measures and is recommended in addition.
The checks include daily visual inspections and weekly measurements with a refractometer and test strips. A sulphurous smell points to bacterial infestation; a "carpet" on the surface usually points to fungal infestation.
Ingredients in cooling lubricants can affect the health of employees. Consistent care and personal protective measures reduce these risks considerably, however.
Most health damage comes from skin contact with contaminated cooling lubricant - possible consequences are inflamed small wounds, skin rashes and allergic reactions. The respiratory tract can also suffer, above all from biocides, additives and microorganisms such as mould spores. Secondary amines used to form carcinogenic nitrosamines with nitrite; cooling lubricants compliant with TRGS 611 no longer carry this risk.
Water-miscible cooling lubricants in particular are prone to bacteria and fungi, especially after long use or downtime. Microorganisms enter the fluid through make-up water, ambient air, floor dirt as well as sweat and saliva; tramp oil also promotes microbial load and should be skimmed off regularly. A bacterial infestation shows through a strong smell and discolouration. Besides the health consequences, it can clog pipework. Biocides and continuous circulation with oxygen enrichment provide a remedy. Non-water-miscible cooling lubricants are rarely affected.
The most effective protection is avoiding skin contact, supported by suitable skin-protection products such as Cover Skin®. In addition, check the fluid regularly, minimise tramp oil and replace the cooling lubricant in good time when needed.
Spent cooling lubricants count as hazardous waste. With correct care, water-mixed cooling lubricants last from a few weeks to more than two years; non-water-miscible ones last much longer. Once the service life ends, the fluid needs proper disposal.
When tramp oil, fine abrasion or microbial growth bring the cooling lubricant to the end of its service life, it needs proper disposal. For medium-sized metalworking businesses, handing it to a licensed waste disposal or transport company is the usual route. That company issues a waste consignment note, which serves as proof of proper disposal and must be kept. For very large volumes, chemical-physical treatment - such as acid splitting or ultrafiltration - can also make economic sense, separating the oil phase from the water phase.
The German Waste Catalogue Ordinance (AVV) categorises waste by its hazard; it transposes the European Waste Catalogue (EWC) into German law. The safety data sheet lists the applicable waste code in Section 13, "Disposal considerations". The following codes are relevant for cooling lubricants.
Alongside the AVV, observe VDI guideline 3397. Part 1 covers functions, properties and occupational safety measures, Part 2 the correct care and extension of service life, and Part 3 the safe disposal procedure as well as the relevant waste and water law. Spent cooling lubricants rank among the hazardous waste types and can cause significant environmental damage if they enter groundwater.
The guideline is available through the website of the Association of German Engineers (VDI).
