TECHNOLOGY FOCUS

UNDERSTANDING CONTAMINATION IN METALWORKING FLUIDS

PART I - INTRODUCTION AND CLASSIFICATION OF CONTAMINANTS

by Joe Eppert

December 2000:

Introduction:

It is widely accepted that the various forms of contamination introduced to the MWF system from the machining operation and improper MWF management practices are the prime determinants of the useful life of the fluid.  If we wish to extend this life as long as possible, it is important that we first understand the various types of contaminants typically found in MWF systems and the effects they may have over time.  The first portion of this series, Introduction and Classification of Contaminants, will serve to set the stage for future discussions on the effects of particular contaminants found in MWF systems.

The Machining Process from the MWF's Perspective:

From the point of view of the MWF (and removing recycling processes from consideration) the machining operation can be thought of as four distinct phases.  The fluid begins its cycle in what we will call Phase 1, storage in the sump.  In this phase, the fluid is allowed to interact with whatever environment it is exposed to, typically with the surrounding air.  It is also given ample opportunity to interact with itself, until some sort of equilibrium is reached.  From Phase 1, we proceed with Phase 2, which is delivery of the fluid from the sump to the machining operation.  The purpose of Phase 2 is to make the fluid available at Phase 3, which is the actual utilization of the fluid as a coolant and lubricant (among other functions) at the machining zone.  Upon use, the fluid is delivered from the machining operation back to the sump.  This final delivery to the starting point in the cycle can be labeled Phase 4 and for general purposes can be thought of as the final phase in the reoccurring cycle. 

At each of these phases, the fluid is exposed to various forms of contamination, which act to disturb the condition of the fluid.  These slight disturbances over time eventually show themselves as a large change in functionality (or efficiency) of the fluid at the machining operation.  In Phase 1, the fluid is able to pick up contamination from various external sources including any impurities in the air or any substances that are accidentally put into the sump.  Additionally, evaporative losses, which alter the fluid composition, occur in Phase 1.  Assuming a fairly closed system, contaminant input at Phase 2 (delivery to the machining operation) should be quite low.  Possible sources of contamination include removal of biofilm from the interior of unclean delivery lines.  Potential forms of contamination at Phase 3 include machine-tool leak oil, metal chips, any operator inputs (bacteria on hands, food, spit, etc.), residue oils or cleansers from previous operations found on the workpiece, and any evaporative losses caused by the high temperatures encountered at this stage.  Phase 4 contamination is similar to Phase 2, and the fluid begins the cycle again.

Classification of Contaminants in MWFs:

The contaminants typically found in MWFs can be divided into two classes, primary contaminants and secondary contaminants.  Primary contaminants include those that are widely encountered in MWF use and can be listed as tramp oil, microorganisms, machining particulate, and water related.  Secondary contaminants are not as widely encountered, or are thought of as external contaminants (not being related to the MWF system or machining operation) and can be listed as floor sweepings, detergents, trash, food, spit, cigarettes butts, etc.  The majority of our future discussions will be on the primary contaminants, with brief talk on the secondary.  In summary,

            Primary Contaminants:
                * Tramp Oil
                * Microorganisms
                * Machining Particulate
                * Water Related
            Secondary Contaminants (External):
                * Floor Sweepings
                * Workpiece coatings
                * Detergents
                * Trash
                * Food
                * Spit
                * Cigarette Butts

Assessing Contaminant "Controllability":

If our interest is to maintain the fluid in the "fresh" state for as long as possible, we must learn how to appropriately control the contaminants listed above.  If we consider the relative difficulty of controlling each contaminant (again, not incorporating recycling measures or reactive measures such as biocides into our thought process), we can classify the contaminants based on how controllable their introduction to the system is.

            Controllable:
                    * Water Related
                    * Some External
            Moderately Controllable:
                    * Tramp Oil
                    * Some External
            Uncontrollable:
                    * Microorganisms, Machining Particulate, Some External

Although we cannot control evaporation, any water related impurities that are introduced to the MWF system from make up could be controlled with some investment of capital.  If we are interested in removing the effects of ions found in tap water, deionized water (or any other type of controlled water source) can be utilized.  In this sense, the water related contaminants are quite controllable, although there are financial considerations involved as well.  Various external sources that originate from the worker (spit, food, cigarette butts, etc.) can be controlled if the workers are educated as to how to appropriately manage the fluid.

Tramp oil can be considered moderately controllable, since methods to control machine-tool leakages could be incorporated.  However, since these leaks are a result of lubricant requirements of the machine-tool system, their control may be difficult in some cases.  Again, various external contaminants can be thought of as moderately controllable.  In example, incoming workpieces could be washed of any residues prior to the machining operation, although this solution may not be feasible in all cases.

Microorganisms can be termed uncontrollable since their addition to the system is so difficult to manage.  As it is a function of MWFs to flush metal chips from the machining operation, we would expect the introduction of these chips to the system to be completely uncontrollable.  Again, various external contaminants may be present in this category.

The MWF Management Process:

What this leaves us with is a systematic (and efficient) process for minimizing contamination in MWF systems.  First, and most importantly, we control those contaminants for which we have the ability to do so.  Second, we incorporate recycling techniques to remove those contaminants that we really can't do anything about.  Third, we combat those contaminant sources that we cannot control or remove through recycling by incorporating various reactive measures (most notably biocides).  Although difficult in some cases, it is possible to choose recycling measures such that the need for reactive measures in third stage is drastically reduced.  If we are able to control the contaminant inputs without such reactive measures, we can be assured that we are managing the MWF system to the best of our abilities, and should expect to see a lengthened useful life of the fluid.

 - Joe Eppert