Grease II - True Grit
(Still Not the Musical)
Barbara Kanegsberg
Avoiding unexpected changes in process fluids can make the cleaning process
simpler and more economical. However, even without unexpected formulation
or product changes, some process fluids are a headache to remove. Vexing
examples are polishing and lapping compounds. Such products contain both
a lubricant matrix and grit. The particulate material serves a useful purpose
in polishing the surface to achieve the appropriate characteristics. Such
products are not restricted to metal working applications. For example,
polishing compounds can be a challenge to remove in optics applications.
In an article in
a recent issue of Clean Source, we explained how changes in the metalworking
fluid can adversely impact surface cleanliness and product
quality Metalworking
fluids might be changed in response to performance, customer requirements,
or real/perceived environmental regulatory issues. We discussed how to
determine if there has been a change and how to use advance notification,
education,
and communication to avoid the cleaning problems that come with unnecessary
product changes.
Sometimes, however, the appropriate product for the job is inherently
difficult to remove because it leaves both a thin film residue (the
lubricant matrix)
and a particulate residue (the grit, polishing compound, or rouge).
By residue, we mean contaminants, undesirable residue - materials that
will interfere with functioning of the product. Where you have grit
and a matrix,
there is particulate and thin film contamination, so the cleaning
process has to be planned and researched carefully.
The right force
Typically, you will not be able to dissolve the grit in the cleaning
agent. The grit is, after all, a particle. In addition, the lubricant
matrix is
often exceedingly viscous. Therefore, typically, it is necessary
to use cleaning force.
But how much force? That’s where testing comes in. With polishes
and rouges, it is crucial to achieve just the right amount of
cleaning force.
With inadequate cleaning force, residue of the particles and the
surrounding matrix remain. If adherent particles dislodge at a later processing
or assembly
step, they may
mar the product
surface. Ultimately, the presence of thin film or particulate
residue means that coatings may not adhere; product performance
may be
compromised.
Too much cleaning force can also be damaging. In an uncontrolled
process, polishing compounds can mar the same optical surfaces
that were just
polished. High pressure spray can result in grit particles
becoming embedded in soft
substrates like aluminum. Sometimes, the cleaning force is
sufficient to break up the grit, resulting in smaller particles.
Small particles
can
be exceedingly adherent (See article “Gecko Feet” in this issue).
Ultrasonics can be useful in such situations. You can adjust
the ultrasonic parameters to remove the grit without damaging
the components.
Two
basic parameters include the frequency or pitch of the sound
and the amplitude
or “loudness” (of course, you can’t hear ultrasonics).
All other things being equal, a higher frequency results in more gentle
cleaning. A lower amplitude means more gentle cleaning. Ultrasonic cleaning
is a complex
topic; we can provide you with additional information. Call us; or send
us an email.
The right process
Consider the potential impact of the three steps in the cleaning
process: washing, rinsing, and drying. Where grit residue
can re-contaminate the surface, several rinse steps may
be essential.
Time and temperature
also
have to be
considered. For example, a longer process time sometimes
results in
more effective removal of soil. However, with high force
combined with longer
process times, you increase the potential for embedding
particles in the substrate.
The right cleaning agent
You cannot ignore the cleaning agent. It must have the
solvency characteristics appropriate to the soil, particularly
the
matrix. Some matrices can
be readily dissolved or suspended in water-based materials;
particulate grit
will be
displaced at the same time.
Some very heavy lapping and polishing compounds have
a matrix that will not dissolve in water. Too often,
trying
to switch
to water-based
cleaning
agents
in such instances becomes an exercise in futility.
These are best removed by organic (carbon-containing) compounds.
The right soil
If you cannot remove the polishing or lapping compound;
and if you or your management feels that you absolutely
cannot
use an
organic
solvent, your
option is to change the soil (the polishing compound).
Some lapping compounds are readily removed with water.
In such
cases, you
have to coordinate with
the people doing the polishing; and extensive testing
on non-production parts may be required. As with
cleaning, the polishing compound
has to be optimized
to the application at hand. It is not uncommon to
test half a dozen alternative polishing compounds before
you find one
that
works.
Removing lapping and polishing compounds
In summary, the combination of particulate with a
viscous, adherent matrix make removal of lapping
and polishing
compounds an exercise
in “True
Grit.” Actually, it’s an exercise in true finesse. The cleaning
force has to achieve a balance between removing particles and either
driving them into the substrate or pulverizing them to smaller, even
more adherent
particles. The cleaning agent and process have to be carefully designed.
You might want to consider change in the soil for easier clean-up.
So you use only water soluble lapping compounds?
That’s good, but you
may be trading one cleaning problem for another. Coming soon to Clean
Source: “Grease
III - The Blob Returns”
