1. Causes of oil deterioration

Water-soluble cutting fluid contains a large proportion of water, so it is very susceptible to the growth of microorganisms (bacteria, yeast, mold) if not properly managed.
The phenomenon of bad odor, black viscosity, precipitation, separation or pH reduction often occurs in the summer or after long holidays, when the circulating solution is stagnant in the sump and lacks circulation.

In the cutting fluid formula, the manufacturer has pre-mixed bactericides, fungicides and pH adjusters, but these substances will gradually be consumed or decomposed over time, especially in high temperature conditions, foreign oil contamination (tramp oil) or metal chip contamination.

When these additives are degraded, the anti-corrosion, sterilization and emulsion stabilization capabilities are weakened, leading to oil deterioration even when the concentration is still high.

2. Daily Solution Addition – Correct Technical Method

During operation, water in the solution often evaporates gradually, causing the oil concentration to increase. If only water is added, the solution will lose balance in the ratio of antibacterial additives and pH stabilizers, reducing the protection ability.

Therefore, the correct addition technique is:

In addition to adding water, the original solution (concentrate) should be added in a reasonable proportion to maintain the chemical balance of the emulsion system.
When diluting, additives such as biocides, fungicides, and pH buffers in the original solution will be re-supplied, helping to prolong the life of the solution.
Recommendation: When adding, add 30–50% of the specified concentration of the original solution each time, combined with clean water (RO or DI water) to reach the standard operating concentration.
Do not just add water daily but forget to add the original solution; increase the amount of addition on weekends or before long holidays to ensure system stability.

3. Regular oil treatment and replacement

Before changing all cutting oil, it is recommended to perform a pre-disinfection step to clean the tank, pipes and nozzles:

3–5 days before changing the oil, add biocide to the circulation tank at the manufacturer's recommended concentration.

Continue normal operation to allow the solution to circulate and disinfect the entire system.
Then drain, clean and refill with new oil.
This way, the entire system is almost completely free of bacteria and mold, helping the new oil to be stable for a longer time and not develop a foul odor soon after changing.

4. Control mold and fungal slime

Mold can grow even in high pH environments. Unlike bacteria, mold does not produce a foul odor, but it produces a layer of slime (biofilm) that can clog pipes, nozzles and sensors.

If the fungus grows strongly, a single oil change is not enough to completely eliminate it — a combination is needed:

Increase the frequency of periodic oil changes (every 3–6 months) depending on the operating environment.

Add anti-fungal additives periodically between oil changes.

In case of severe fungal infection, the entire system should be cleaned with a specialized cleaning solution (system cleaner) before refilling new oil.

5. Technical conclusions

Preventing damage to water-soluble cutting oils depends not only on product quality, but mainly on proper management and maintenance.
Adhering to a reasonable solution replenishment process, maintaining a stable pH, controlling bacterial contamination and performing periodic disinfection will help:

Long-term oil stability, low odor, no stickiness or separation.
Precise machining, clean chips, no rust.
Reduce oil change costs, machine downtime and improve working environment conditions.