What's inside each one
A semi-synthetic coolant contains mineral oil in a proportion of 5–25%, emulsifiers, corrosion inhibitors, biocides, and extreme pressure additives. The mineral oil provides lubricity, making it particularly effective in high-friction operations: deep drilling, tapping, and broaching.
A synthetic coolant contains no mineral oil: it is formulated solely from organic and inorganic compounds in water. It offers greater transparency (the cutting point is visible), better cooling, and lower bacterial proliferation as there is no organic substrate for microorganisms.
Performance by material
When machining carbon steels and low-alloy steels, semi-synthetic has the advantage in high-friction operations due to its greater lubricity. With aluminium and its alloys, synthetic usually performs better: mineral oil can generate deposits on the workpiece surface and make subsequent finishing processes (anodising, painting) more difficult.
In grey cast iron, synthetic with appropriate EP additives is the predominant choice: it does not stain, facilitates visual inspection of the part, and resists the microbiological load generated by cast iron better.
For difficult-to-machine materials (Inconel, titanium, austenitic stainless steels), the selection depends more on cutting speed and tool geometry than on the coolant base type. In these cases, the specific formulation matters more than the category.
In-machine service life and maintenance
Synthetic coolant, having no mineral oil, has a lower tendency toward bacterial degradation. Under equivalent maintenance conditions (concentration, pH, and temperature control), a well-formulated synthetic coolant can last 6–12 months in the machine, compared to the typical 3–6 months of a semi-synthetic under the same conditions.
The determining factor is not the coolant type but maintenance discipline. A well-managed semi-synthetic (weekly concentration control with refractometer, pH adjustment, periodic tank cleaning) can outlast a poorly managed synthetic.
Concentration control is mandatory. A coolant below 5% concentration loses anti-corrosion protection. Above 10%, it increases foam, skin irritation, and cost without proportional benefit.
When to use each: decision criteria
- Semi-synthetic: steels, cast iron, tapping and deep drilling, high-friction operations, cost-sensitive installations with good maintenance.
- Synthetic: aluminium and light alloys, grey cast iron, environments where cut visibility is critical, installations with high microbiological load, workshops with limited capacity for ongoing maintenance.
- Neither will solve problems caused by incorrect tool geometry or inadequate cutting speeds. The coolant improves the process; it does not correct it.
The real cost: beyond the price per litre
A quality synthetic coolant can cost 20–40% more per litre than a standard semi-synthetic. However, if its in-machine service life is double and it generates fewer rejects due to corrosion or poor surface finish, the total cost per machined part may be lower.
The correct calculation includes: fluid cost, change cost (downtime, cleaning, waste), corrosion rejects, quality control incidents, and preventive maintenance hours.
Semi-synthetic remains the most widely used option and delivers excellent results when maintenance is consistent. Synthetic wins on aluminium, in high-cleanliness environments, and when service life and transparency are priorities. The best choice is the one that considers the complete process, not the drum price. If in doubt, we can analyse your process and recommend the right formulation.
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