Minimize & Control Bacterial or Fungi Growth
Clean System Before Introducing Fresh Metalworking Fluid
It is important to clean the machine tool’s MWF delivery system; otherwise, you are exposing the new fluid to the same conditions that forced you to change the fluid in the first place. This is particularly true in the case of bacteria and/or fungi contamination. By draining the sump only, you are disposing of the majority of the bacteria/fungi, but as long as there is some residual MWF in the system, there will be some residual bacteria/fungi. These bacteria/fungi consume the organic components (oil and other additives) present in the metalworking fluid. By allowing them to come into contact with fresh fluid, you are providing them a free food supply. Due to the abundance of food, they will rapidly multiply and within a short period of time, you will find yourself pumping out the machine tool sump again.
Existing bacteria and fungi should be killed by the proper addition of biocide, and then the coolant pumped out and discarded. Any accessible colonies should be physically removed, a suitable cleaner circulated through the system, the cleaner removed, and the system well rinsed before refilling with fresh MWF.
Operate System at Correct Concentration
All water based metalworking fluids are designed to be operated at a given concentration dissolved or emulsified with water. The correct concentration is important to provide the cutting operation with optimal lubricity and cooling, corrosion protection, and resistance to bacteria and fungus. Operating a system at a low concentration may result in decreased tool life, bacteria and/or fungus problems, possible corrosion and eventual downtime. Operating a system at too high a concentration may result in dermatitis, foaming, and heavy residues.
Proper mixing procedures are critical to the attainment of long metal removal fluid life and economical use of metalworking fluid concentrate, as well as to the elimination of metalworking fluid concentration related problems. Premixing the MWF concentrate with pure water at the MWF manufacturer’s recommended concentration is important for initial charge. Actual concentration in machines must be checked frequently and adjusted as needed with pure water, concentrate, or premixed fluid as appropriate to maintain the recommended range.
Ensure Makeup Water Is of Adequate Quality
The quality of makeup water is very important. Water used for making MWF mixtures should be as pure as possible for the most economical and trouble-free use. Minerals in metal working fluid water can corrode machine tools and machined parts, can aggravate deposition of residues on machine tools, and can increase the rate at which bacteria and fungi grow in the metalworking fluid. It is also essential that the proper water miscible MWF be selected.
Water that contains certain dissolved ions such as calcium and magnesium is termed “hard” because they will form scale upon evaporation and will form insoluble soap scum when mixed with many MWFs. Other minerals such as sulfates are detrimental because they promote the growth of sulfate-reducing bacteria that produce a “rotten egg” odor. Some, including sulfates and chlorides, are corrosive to metal and contribute to rust. Minerals are thus very detrimental to the performance of MWF mixtures. The more concentrated these minerals are, the faster they build-up and cause adverse effects to appear. Therefore, the purer the water for making MWF mixtures is initially, the longer the fluid can be used before problems occur. One method of removing minerals is to run it through a zeolite softener followed by a reverse osmosis filter. Purified water can also be produced by deionization, which removes most of the dissolved minerals thus producing a high quality process water.
The incorporation of effective biocides is also helpful in preventing or retarding degradation caused by bacterial action. These compounds may be incorporated as components in formulated MWFs or may be added to MWFs before and during use. Biocidal activity should be broad enough to suppress the growth of a highly diverse contaminant population. Over time, chemical and biological demands may consume the biocides and cause the concentrations to fall below those needed to inhibit microbial growth. Biocides should be added judiciously to prevent microbial growth or to arrest modest growth. Some biocides that function very well in clean products can actually serve as food for the various types of bacteria found in water miscible fluids that are so easily contaminated. Grossly contaminated fluids should be treated if necessary with biocide just prior to pumpout as part of the overall cleaning procedure, but this should be done after operators have ceased working with the fluid (i.e., offshift). Conscientious monitoring and prevention of microbial growth is the best approach for preventing the buildup of endotoxins and other hazardous biological substances and for preserving fluid quality and function.
To avoid problems related to bacteria and/or fungi growth a good filtration system should be in place. A metalworking fluid is subjected to the metal chips and fines of the process, airborne contamination from cascading fluid over a part and the machine, machine leakages, residues left on the part from previous operations, water, operators, and other factors. Whenever possible, these contaminants need to be removed (IAMS 1996).
The build-up of chips and metal fines in the metalworking delivery system provide an excellent “nesting” area for bacteria. In large systems, these chip beds many extend for many yards in sluices and pipes. The associated biomass will be too large for simple treatment with biocides to be effective. The periodic removal of this debris minimizes the potential for bacteria growth and extends MWF life.
Tramp oil is non-emulsified oil that is mechanically entrained in a MWF in large droplets. Tramp oil often results from machine tool hydraulic or way lube systems leaking oil into metalworking fluids. Tramp oil damages MWFs by extracting key components, by providing food for microbes, and by providing an area of reduced oxygen which promotes the growth of anaerobic bacteria. Consequently, all possible steps should be taken to reduce oil leakage.
In some cases it is not possible to avoid tramp oil. Oil is applied to the ways of machine tools to insure proper movement of the workpiece during the machining operation. As the MWF comes into contact with the ways or the oil drips off the way, tramp oil is introduced into the MWF. This should be minimized by applying the required amount of way lube and no more, and by making sure that way lubricators run only when the machine tool runs.
The amount of tramp oil in the system should be minimized through hand skimming or by the use of skimmers, separators, or other devices. Since tramp oil separates and floats when agitation ceases these devices are particularly effective when the system pumps are not running, as on weekends and off-shift. Using system quiet time to facilitate skimming will help prevent problems. In addition, finding a MWF and way oil that are compatible will also help.
It is important to maintain good housekeeping by teaching your company’s employees not to use machine tool sumps as trash receptacles. Paper cups, uneaten food, cigarette butts, and other trash should not be seen floating in the MWF. These not only introduce bacteria into the sump but provide nutrients for bacteria. Trash should go in trash containers even if it means the employee has to walk away from the machine tool.
Source: OSHA https://www.osha.gov/SLTC/metalworkingfluids/metalworkingfluids_manual.html