Metalworking Fluid Problems & Solutions Guide

Metalworking fluids (MWFs), also known as cutting coolants, play a critical role in machining by providing lubrication, cooling, rust prevention, and chip removal. However, during use, coolant systems often face issues such as odor, foaming, rusting, corrosion, and shortened tool life.
This guide summarizes the most common coolant problems, their causes, and effective solutions to help manufacturers maintain stable machining operations and extend fluid life.

Causes:

• Excessive leakage of slideway oil

• Poor biocide management

• Insufficient antimicrobial performance

• Coolant stagnation for long periods

• Fluid instability causing oil separation

Solutions:

• Remove tramp oil in time (e.g., use an oil skimmer)

• Strengthen pH and concentration management, avoid contamination by food or tobacco

• Replace or increase the dosage of biocides

• Before long machine downtime, increase coolant concentration/pH and add preservatives

• Switch to a high-performance coolant

Causes:

• High working concentration or excessive surfactant

• Excessive flow rate and pipeline resistance

• Defoamer failure

Solutions:

• Dilute with water; add high-performance defoamer

• Reduce flow velocity or system resistance

• Replace defoamer or use low-foam surfactants

Causes:

• Mold/fungus growth

• Coolant degradation, forming sludge with tramp oil and chips

Solutions:

• Replace with new coolant containing antifungal agents (e.g., BIT20, AMP95, B77)

• Identify root cause of degradation before further treatment

Causes:

• Low coolant concentration

• Reduced pH

• Insufficient corrosion inhibitor

• Coolant degradation

• Long-term contact of dissimilar metals

Solutions:

• Add fresh coolant to increase concentration

• Add organic alkanolamines to stabilize pH

• Supplement with rust inhibitors

• Replace degraded coolant

• Avoid long-term contact of dissimilar metals in coolant

Causes:

• Excessive tramp oil

• Hard water

• Incorrect dilution method

• Reaction between chips and coolant additives

• Severe degradation or bacterial contamination

Solutions:

• Install tramp oil separator

• Use water that meets hardness requirements or use hard-water resistant coolant

• Adopt correct dilution method (oil-in-water)

• Remove reactive chips promptly

• Replace degraded fluid

Causes:

• High pH values

• Overuse of bactericides or nitrites

Solutions:

• For iron/steel machining: maintain pH 8.5–9.5; for aluminum machining: maintain 8.5–9.0

• Avoid excessive use of triazine biocides (e.g., BK, which releases formaldehyde)

• Control nitrite dosage to prevent skin irritation

Causes:

• Blackening of die-cast aluminum

• Yellowing or darkening of aluminum alloys

Solutions:

• Reduce or avoid strong alkaline substances (monoethanolamine, diethanolamine, borates)

• Use aluminum corrosion inhibitors (recommended: siloxanone + phosphate esters)

Causes:

• Insufficient coolant supply in cutting area

• Low concentration

• Poor lubrication

Solutions:

• Increase supply volume, adjust nozzle direction or flow

• Add fresh coolant to increase concentration

• Use high-lubricity coolant

Causes:

• Insufficient lubrication

• Poor cooling effect

• Excessive grinding debris

Solutions:

• Increase coolant concentration or use higher lubricity coolant

• Increase coolant supply; reduce cutting speed if necessary

• Add suitable flocculant/settling agent; filter debris regularly

Maintaining stable coolant performance is essential for productivity, tool life, and workpiece quality. By monitoring coolant concentration, pH, cleanliness, and microbial control, most problems can be prevented before they occur.
For severe cases, switching to next-generation high-performance coolants can further improve machining reliability.