In the metalworking industry, metalworking fluids (MWF) are used to cool and lubricate the machining activity. The metalworking fluids ensure a reduced friction between the tool and the workpiece. In addition they prolong the life of the tool, carry away swarfs and protect the surfaces of the work pieces.
The metalworking fluids are a complex mixture, which may contain mineral and/or synthetic oils, additives such as biocides and rust inhibitors etc. as well as contaminants like metal fines, tramp oil and bacteria. When droplets of these metalworking fluids are airborne, it is called oil mist.
Oil mist and oil smoke
Oil mist is caused by metalworking fluids that come into contact with fast rotating tools (e.g. milling and drilling procedures). The oil mist is then dispersed into the air.
In addition to oil mist, oil smoke may appear. These are smaller droplets than oil mist. Due to the high temperatures of the metal parts during machining (e.g. grinding procedures) the MWFs become excessively hot and start to ”burn” and degrade.
The small droplets in the air, to a diameter of approx. 1 μ, are designated as oil mist. Everything with a smaller diameter is considered to be oil smoke.
Recommended exposure limit for oil mist
Due to the different types of MWF’s, the composition of the mist is different in most cases, making it difficult to have a single recommended limit for exposure. Most countries make a difference between oil mist derived from water-miscible and mineral oil-based metalworking fluids.
In the USA, the OSHA has set the Permissible Exposure Limit (PEL) for mineral oil at 5,0 mg/m3 (10hr TWA). And the NIOSH has a Recommended Exposure Limit (REL) for all types of MWF at 0,5 mg/m3 (10hr TWA). In Europe, the IFA (formerly known as BGIA) has set the limit on 10 mg/m3 for water-miscible and non-water-miscible metalworking fluids with a flame point greater than 100°C for Germany. The Institut National de Recherche et de Sécurité (INRS) in France has one of the lowest recommended exposure limits, being 1 mg/m3 of aerosol. Switzerland has a Permissible Exposure Limit for heavy mineral oils with a boiling point of over 350°C of only 0,2 mg/m3.*
Oil mist - Danger to man, machine and environment
Many metalworking processes generate oil mist, like milling, turning, grinding and drilling. The the metalworking fluids form a mist during these processes, the larger drops can enter the nose and trachea and can be swallowed down. The smaller droplets can deposit in the lungs and also cause considerable irritation of the eyes, nose and throat. Oil mist and smoke can even contain metal particles with carcinogenic substances. If the mist settles down on the factory floor there will be an increased risk of slipping.
There are not only possible negative effect on humans, it also has a negative effect on your machinery. If the mist settles on the machines, they might be damaged due to corrosion. The sensitive electronics of the machines can also be affected by the mist. This can lead to high maintenance costs.
If the machines and surroundings are not properly maintained, the settled oil mist might increase not only the risk of a fire hazard but also a bacteria and mold growth which will cause hygienic problems. Therefore, effective detection, filtration and elimination of oil mist is so important!
The amount of oil mist
During machining, oil mist is produced by the interaction of the metalworking fluid with the moving parts. When the MWF hits fast moving parts, it is thrown back and dispersed as fine droplets. The gaseous state of any liquid is formed as molecules leave (evaporate from) the surface of the liquid. The process of evaporation speeds up with a lower boiling point of the metalworking fluid and increased temperature of the fluid. In water based emulsion the evaporation will increase the humidity in the plant. The generation of mists are mainly depending on:
- Higher speed of tool = higher generation of mist
- The volume, rate and point of delivery of metalworking fluid to the cutting edge
- Higher temperatures of the workpiece = higher generation of oil smoke
*) Source: https://www.hse.gov.uk/research/rrpdf/rr1044.pdf (Chapter 1.2)