Foaming in Water-Soluble Cutting Fluids (Coolants)

Metal Removal Fluids

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Foaming is not an untypical occurrence in machine shops using water-soluble cutting fluids, be it emulsions, synthetics, or semi-synthetics. It is not a desired fluid behavior, since the introduction of air, or bubbles, in the coolant can drastically affect cooling, tooling life, and surface finish.  Moreover, excessive foaming may cause coolant overflow that is messy and a hazard to machine operators.  In worse cases, too much foam can cause pump cavitation, leading to expensive repairs or even pump replacements.

Foaming is avoidable, but one has to understand the possible root causes to minimize its occurrence.

  1. Water quality – water that is too soft (0 to > 100 ppm CaCO3), emulsified with a coolant designed for ‘normal’ water hardness leads to foaming.  To address this, if changing water quality is not an option, is the selection of a metal removal fluid more apt for soft waters.   One can also screen coolants before purchasing, by performing comparative foaming behavior tests with test tubes or graduated cylinders.


  1. Low fluid levels at the coolant sump – in situations wherein the pump suction inlet is not fully submerged in fluid, trace amounts of air is certain to be introduced into the fluid circulation and thus causing foam. To correct this, a machine operator should ensure that the sump is always at the right levels (say at least 80% of capacity), at all times.


  1. Excessive coolant flow, or improperly agitated fluids – a machine operator has to avoid excessive coolant flows and pressures that result in situations wherein the return fluid directly spills and splashes into the coolant sump, causing the formation of air bubbles. Other than lowering coolant flow-rates and pressures, one may use deflectors to ‘soften’ the landing of the return fluid to the sump.


  1. Contamination – in set-ups wherein coolants are filled to machine sumps manually, and when a shop uses more than one metal removal fluid, mistakes in terms of picking the incorrect coolant, and thus topping up an inappropriate fluid may lead to foam. In cases wherein the coolant supplier is also the same as the metal cleaner or degreaser vendor, charging with the degreaser into the sump instead of a coolant can lead to excess foaming behavior, too, owing to the detergency properties of cleaners.   Proper and systematic labeling is a good solution for this scenario.


  1. Concentration is too high – the lack of controls for concentration (refractometers, identified person-in-charge, etc.) will normally lead to excessive concentrations (< 10.0%), leading to the formation of foam. Sticking with the coolant manufacturer’s recommended concentration range, and the concentration control procedures should be in place as part of the standard operating procedures of a machine shop.


If all else fails, one may use tank-side defoamers from your fluid vendors, but this can be considered only a temporary, stop-gap measure that does not address the root cause.

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