Metalworking Fluids – Life Cycle Testing

James Holloway, Fullbrook Systems Limited

Using the correct Metalworking Fluid (MWF) can reduce downtime, and increase tool life, as well as improving the quality of finished items. Keeping the MWF in good condition is vital in keeping the metal working system working at its optimum.

MWFs can be divided into water-miscible fluids (emulsions, semi-synthetics, synthetics) and neat oils.

Neat oils are used to provide excellent lubrication, but have a limited capacity for heat dissipation. In order to provide both lubrication and heat dissipation water-miscible systems are used (emulsions, semi-synthetics, synthetics).

In addition to lubrication and heat dissipation, MWFs also provide chip removal, and can inhibit corrosion. They can also improve machining performance, improve tool life, and yield a better surface finish.

An emulsion is a colloidal system, where one liquid is dispersed and stabilised into another immiscible fluid. In order to provide the correct performance characteristics the MWF is diluted with “soft” water to form an Oil-In-Water (O/W) emulsion. This describes an emulsion where the oil droplets are stabilised in a continuous water phase. The stabilisation is achieved through the use of surfactants (emulsifiers) and additives. The surfactants migrate to the interface between the oil and water and help to create a stable droplet (Micelle).

Soft water is defined as having low concentrations of metal ions especially calcium and magnesium, and can be prepared by taking hard water and artificially removing the ions. This improves the performance of the MWF but can bring foaming issues.


In order to improve the overall performance other additives such as Corrosion Inhibitors, Lubricity Agents, Antifoams, Biocides, Dyes, Wetting Agents and Antioxidants are added.

One limitation of MWFs is that they lose their lubricating properties over time because of their exposure to high temperatures and stresses in mechanical operations. Traditionally the lubricating ability of MWF emulsions has been linked to their “interfacial properties”; and in particular to their stability with respect to creaming.

Creaming is a description of the movement of “oil” droplets to the top of an emulsion. The increase in oil concentration at the top of the sample will eventually become visible to the naked eye. This process can be observed in an everyday scenario when cream forms in a milk bottle.


The rate at which this happens is a measure of “Creaming Stability”. The faster the droplets move the more unstable the system is and the lower the “Creaming Stability”. The creaming rate can be related to the products performance and stability.

Figure 1: Migration – Creaming Stability Continued on page 20

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