Seeking Sustainability through Minimum Quantity Lubrication

Minimum Quantity Lubrication (MQL) holds the potential to improve manufacturing’s reputation as a ‘dirty’ industry, and move us closer to clean and sustainable future in which all stakeholders can thrive. When properly applied, MQL can improve surface finish and increase tool life. Moreover, negative environmental impacts such as carbon footprint, waste, water consumption can be greatly reduced while improving a manufacturing facility’s overall health and safety profile.

Also known as micro-lubrication, micro-dosing, mist coolant, spray cooling or near-dry machining, MQL has been demonstrated to have great potential in machining a wide spectrum of materials. As manufacturers continuously seek to reduce hazardous waste, particularly large volumes of used and contaminated flood coolant, MQL can be a key technology in achieving corporate sustainability goals without sacrificing part cost or quality.

The primary benefit of MQL lies in the dramatic reduction of fluid use. While flood coolant uses between 8 – 16 gal / hour, MQL uses less than one pint / hour. Clearly, this reduction in fluid use also reduces costs, such as manpower and the health and hazardous waste issues associated with managing residual fluid and contaminated chips. On average, a five gallon tank of MQL will allow you to run for the same amount of time as 100 gallons of traditional fluid.

Flood fluid systems need equipment to recirculate, filter, test and treat coolant to keep it viable and prevent contamination from bacteria, tramp oil and swarf. MQL eliminates the spent coolant associated with flood fluid systems and the huge cost of disposal that come with it. As environmental regulations continue to increase the responsibilities of manufacturers to properly dispose of coolant waste, the costs of operation flood cooling systems will only continue to increase. With MQL, the machined workpiece and chips also end up drier and cleaner, making them easier to recycle.

However, despite these huge environmental benefits, challenges in the implementation of MQL remain. Since the very small amount of oil dispensed by MQL systems does not have comparable cooling and chip evacuation abilities as those of high pressure wet machining systems, the technology is seldom used in heavy roughing operations. There are also concerns associated with the consumption of compressed air or other gases at high pressure, the use of which is necessary to carry the microscopic oil droplets to the tool/chip interface. Significant energy and cost is required to compress gas, and the use of compressed gas does increase noise on the factory floor, which clearly has implications for operator comfort and health.

MQL is generally not well suited to deep hole drilling, high localized heat processes such as grinding, special operations like honing, or for difficult-to-machine materials such as nickel based superalloys. Success with MQL requires that careful attention be paid to the combination of tool material, geometry and machining parameters to optimize chip breaking and heat flow. Steel machining requires a different application of MQL than aluminum machining, for example. However, with proper process engineering, many manufacturers have been able to achieve great success in applying MQL to applications commonly assumed to be out of reach of the technology, such as high speed machining of titanium alloys or deep hole drilling in aluminum.

What’s more, although MQL may holistically be better for the factory environment than traditional cooling and lubricating methods, the aerosol produced to aid in the cutting process also disperses as a very fine mist, which can be very difficult to handle when it comes to filtration. There are also additional capital and operating costs associated with the air and gas supply, which should be considered before implementation. Lastly, while the volume of MQL lubricants is significantly lower than that used in flood cooling applications, the cost per unit volume of these high quality fluids is often significantly higher than that of water-based emulsions. Therefore, the consumables may still incur costs on the same order of magnitude as conventional coolant systems.

With all that said, it’s important to recognize that these issues are certainly not insurmountable. In many applications, the benefits still outweigh the challenges, and responsible manufacturers should be looking towards MQL solutions to improve the sustainability of their processes and products to continue to compete going forward.

While existing on the edge of the manufacturing consciousness for a few years, MQL has recently gained momentum in line with the drive towards more sustainable processes. For companies that have implemented MQL machining, the results have certainly been worth the initial investment and learning period in adopting this promising technology. Take for example, Ford, which implemented MQL processes in its Michigan plant on three types of aluminum components, creating a safer, cleaner environment and boosting the plant’s flexibility and efficiency*. Many other manufacturers have likewise reported increased productivity and improved product quality after properly assessing their machining operations to optimize for MQL. To help in this effort, most major tooling manufacturers now offer specialized tools with internal channels designed to deliver the ideal droplet size and gas velocity to enable more effective application of fluid to boost productivity.

While there is still work to be done to make MQL a suitable choice for more manufacturers and in a wider range of applications, MQL clearly provides a cleaner, greener and more sustainable alternative to classic fluid supply. It seems clear that the clean ‘Factory of the Future’ will no longer produce countless gallons of waste coolant, but rely on dry and near-dry processing to reduce adverse environmental, societal and health impacts. As MQL fluids and delivery systems will continue to evolve and as more manufacturers turn towards this sustainable technology, the options available will only continue to grow. Moreover, trends towards near-net shape finish machining rather than heavy roughing will only serve to increase the range of applications in which MQL will offer the most pronounced benefits with regard to cost, product quality and sustainability.

To learn about the practical steps your facility should undertake in considering a switch to MQL, explore our ‘Modern Machining Methods’ eBook now.


About the author

Dr. Julius Schoop

Principal Engineer