Critical Cleaning, Precision Cleaning, and Ordinary Cleaning
Barbara Kanegsberg
What’s the difference between critical cleaning, precision cleaning,
and ordinary cleaning? We have heard many definitions. They are all useful,
but only up to a point. The term cleaning or even critical cleaning as applied
to products involves a number of goals that add up to a better product. Once
we understand what cleaning means, it’s easier to put together
a more effective, competitive process.
Cleaning Goals
Some goals and concepts that come under the term cleaning include:
• removal of unwanted surface contaminants
• contamination control
• achieving the appropriate surface quality, characteristics, and attributes
• preparing the surface so that the next step in the process will be successful (this can involve surface modification to allow successful coating)
In theory, it is possible to consider each of these factors separately;
but, usually, when people talk about “cleaning,” they
are including some or all of the above factors.
Therefore, I’m not overjoyed with the term “cleaning” as
applied to products and components. Anyone coming up with an all-inclusive
word, preferably in Greek and ending in “ology,” is entitled
to a grande non-fat vanilla latte (or equivalent).
Critical cleaning
In the meantime, we will continue to talk about “cleaning.” If
soil is matter out of place, then cleaning is the removal of matter out of
place. Critical cleaning has sometimes been referred to as cleaning parts
that already look pretty clean, cleaning parts where you can’t
see any soil on them. This idea works for many applications.
We might think
of removing heavy oils and greases from automotive
parts as general cleaning. The final cleaning step
to particles and fingerprints from
a gyroscope
might
be thought of as precision cleaning or critical cleaning.
Critical cleaning of visible soil
However, let’s consider optics deblocking. When optics are polished,
the individual parts are often set into what
is called a blocking compound such as wax or even pitch (an asphaltum).
The process
of removing
the blocking
compound, is called deblocking. Deblocking is
a cleaning process, because we are removing soil or matter out of place.
The soil
has to be
completely
removed without physically damaging the optics
(the substrate) and without subtle, undesirable surface modification.
Most of
us would think
of optics
deblocking as a critical process. Anyone who
has worked to remove pitch from optics will attest that the components
start
out with visible
soil.
Critical process points
In addition, in our experience, we have found numerous
instances where a critical cleaning step occurs
early in the process.
The part
may show visible
soil; the fabrication area may bear no resemblance
to a cleanroom. If the soil is not removed correctly
at the critical
point, the
rest of
the process
can be compromised. Sometimes only certain
soils have to
be removed at that point. For example, in preparing
surfaces for engineered
coatings (such as
plasma vapor deposition, ion vapor deposition,
and thermal spray), organic
contaminants must be completely removed,
or the coating process will be unsuccessful. Removal of salts
is not as
critical, because there
is usually
an impingement
cleaning step immediately prior to coating.
In fact, we have observed parts with visible but “degreased” fingerprints
that are successfully coated. Even though visible
soil remains, this is
a critical cleaning step.
You probably have a critical cleaning process
Cleaning is a value-added manufacturing activity.
For lean, cost-effective, competitive manufacturing,
I
have adopted
the general policy that
if a cleaning step cannot be eliminated
without product damage, it is
a critical cleaning
step.
About a dozen years ago, I made the comment
that everyone knows what precision
cleaning or critical
cleaning
means: it means “my cleaning process;” and
everyone knows what ordinary cleaning means: it means “everybody else’s
process.” I went on to indicate that when we start to compare notes,
we realize that most cleaning processes are actually precision cleaning;
and that we have a great deal to learn from each other’s
applications.
Precision cleaning or critical cleaning
is in the eye of the beholder, and
rightly so.
If you
think
it’s an important process step, you’re
probably right. If you have questions
about your cleaning process, even if
it is not
conducted
in a cleanroom, do not hesitate to contact
us.
