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Cleaning vs Material Removal
By A.F. Kenton
Nova Finishing Systems Inc.
Where does cleaning begin and material removal take over, or is
there a difference?
Does it really
matter? Well, yes it does. Cleaning is the material removal process
of foreign matter from parent material. It can also be considered
the removal of mostly organic compounds from inorganic materials;
however, it can also be foreign inorganic contamination. These
organic compounds or contaminates are considered a secondary
substance which does nothing for the part, but can and does
eventually effect the primary parent materials fit, form, or
function.
What constitutes a contaminate depends mostly on the end use or the
environment in which that parent material part will work and/or for
how long a period of time will it safely perform its function. That
is, dirt or contaminates are subjective to arbitrary conditions
imposed by the end user of the part and based upon acceptable levels
of engineering reliability. Under microscopic conditions, all
materials will still maintain some form of contamination, it is just
a matter of how much is acceptable.
Because of the nature and physical
properties of organic materials, cleaning can become a material
removal process when the parent material and contamination bond to
one another. This is a condition where the parent material is porous
or has a rough surface finish. It can also be a result of a chemical
reaction caused by contamination or environment working conditions,
or if there are adhesion problems. Foreign matter can not always be
dissolved and removed with a liquid system alone and surface
finishing may be required to do proper cleaning. In some cases we
are talking about oxidation rather than organic material deposits
and/or their removal.
There are a lot of factors controlling a cleaning or material
removal process. Most cleaning systems use chemicals to dissolve,
attract, or dislodge foreign matter. Cleaning can also be a
molecular bonding or electrical conveyance process. It helps to know
exactly what that foreign matter is composed of so that an
appropriate cleaning action system and equipment can be selected to
remove the contamination. Once a part has been successfully cleaned,
the cleaning process itself creates another problem. Kind of like
good news, bad news. In a sense, the original contaminates serve as
a protective coating against oxidation and by cleaning the part, one
opens up the porosity of the parent material to oxygen in the air
which starts to deteriorate the part and the contamination process
all over again.
So, when is a part clean? I can't
answer that question and I'm not sure anyone else can either. A lot
has to do with the usage of the part and or its working environment,
or in short, specifications that someone has determined will serve
the life of the part in its working environment. Most company's just
specify the measurement dimensions of the part and its surface
finish. The final part finish can not only specify measurements, but
preparation steps and sealant coatings. Rarely does an engineer or
company specify the amount of time between processes and/or
environmental conditions for holding that part until the final
surface finish is achieved and/or applied. Normally, the part design
and materials used to make the part exceed most working conditions
and time lapses in manufacturing are not a problem. Fortunately,
common sense prevails in most manufacturing processes and few people
question anything about the finished part, except fit, form, and
function
Now, getting back to our problem of
foreign contamination matter. We know that liquid systems work well
on most organic materials and normally achieve accepted cleaning
results. However, if a liquid can not get into or at the
contamination problem area it can not remove the foreign matter
without the use of more physical means or greater pressure systems.
Liquid systems can be improved by adding heat to the cleaning
process or a pressurized flow can achieve material removal by force,
within limitations. At too high a heat a part can be distorted or at
too great a force or not enough movement of the part, a pressurized
system can actually cut the part. So, there are limitations to
liquid systems, but they also cover a greatest range of cleaning
capabilities and achieve more acceptable results on most
contaminates than other system.
Earlier we mentioned non-soluble
problems caused by porosity, reactions, and adhesion. Now, these
problems may be all solved, or at least diminished, by liquid
systems, but solid mechanical abrasive systems will also work and
maybe a more cost effective process, especially where surface and
cleaning requirements are important. Most people are familiar with
abrasive wheel and blast systems for material removal and cleaning,
but mass finishing systems are normally better for smoothing the
surface profile of the part. A part's surface profile is very
important and determines the proper cleaning method to use, because
this is where and how contaminates can be held in place. The
smoother the surface profile, the easier it is to clean a part.
The difference between cleaning and
material removal can be considered the amount and type of material
being removed. Again liquids primarily dissolve contaminates and
abrasive methods use a solid to solid method of grinding away and
smoothing of all surface materials of the abrasive, the contaminate,
as well as the parent material to achieve cleanliness. However and
because abrasives are solids and involve a particle break down
process of both the medium and the part, a liquid cleaning system is
normally still a recommended secondary procedure to insure all loose
material removal and/or cleanliness of the part.
Unlike liquid systems, mechanical
abrasive systems use a number of equipment variations or versions
to achieve material removal. All of these differences involve the
use of how the energy or pressure is applied to the medium to
achieve the end results. Each system has its own advantage and
disadvantage depending on the size of the part, the material, and
the configuration of the part. We will not go into these differences
and/or applications in this article; however, for single or small
quantities of parts, the most common method people use or prefer is
the wheel or blast method for cleaning parts. Blast systems are
still the best process for parts that will eventually receive a
heavy sealant coating. However, if the surface finish is critical or
the metal coating is a treatment rather than a coating, this may not
be an option. Smoother, more uniform thin plating or coatings can be
better achieved using mass finishing methods. Another advantage is
that larger volumes of parts can be better handled more efficiently
and cost effective by mass finishing systems.
Increasingly, the medical and high
tech industries are requiring smoother, cleaner surfaces because of
bacteria and harsh environmental working conditions. Machining
processes and liquid systems can only do so much and that may not be
enough to prevent microscopic particles from becoming a potential
problem. So, that means that a mechanical material removal process
is required and/or that means the need for abrasives or greater
mechanical energy systems. Without the use of strong chemicals,
liquid systems will not produce the results that can normally be
accomplished using abrasive methods.
Most machined parts and raw
materials have a surface finish of about a 35 RMS prior to any
secondary process. About the best surface finish one can achieve
using blasting methods is about a 35 RMS or higher, meaning rougher.
By creating a rougher surface feature, one is increasing the amount
of surface area and this surface finish is desirable for coatings to
adhere too. A 35 RMS surface finish is actually about the starting
point for most mass finishing systems and they can get down to a 2
or 4 RMS. Abrasive wheel systems are used a lot on small parts and
they produce results anywhere in between these two methods, but the
surface finishing results from wheel systems will never be rougher
or greater than what the surface RMS profile starts with.
The biggest visual difference of
a blast finished part to a mass finished part initially is its
physical appearance. Both processes produce a more uniform surface
texture and both processes do remove the oxidation and contaminates
that cover a part; however, as mentioned, the rougher texture of the
blast process does improve molecular bonding for thick coating
materials. For precision dimensional purposes a smoother surface
profile of the parent material is more desirable for thin uniform
plating treatments and this can best be achieved using mass
finishing systems.
To do proper plating or chemical
treatment of metal parts, a surface finish of from 12 to 18 is about
the best range to apply a uniform plating, sealant coatings, or
treatments and this surface finish can easily be accomplished by
most mass finishing systems. Finer surface finishes can also be
achieved using mass finishing, wheel systems, or lapping methods;
however, such surface finishes are not normally required and are not
cost effective for the over all need of the part. Any reflective
surface finish is normally for the aesthetic appearance of the part,
but can be desired for medical cleanliness purposes.
The bottom line here is that
cleaning processes may have to include material removal and the
best, most effective method to do this in volume is by using mass
finishing systems that use abrasives in a mechanical process. The
trend in industry is to improve surface finishing standards and/or
cleaning for safety reasons and that usually means smoother surface
profile finishes. * ( Hopefully, this is the first article in a
series to introduce mass finishing systems to those of you who need
cleaner smoother parts.)
This is an optional sentence and depends on you
•
Nova Finishing Systems Inc., manufactures small, heavy-duty bowl
finishers that stack up to most of the big equipment on the market,
but cost much less. Nova series vibratory equipment also comes with
the same warranties of the larger machines. Form more information
on this equipment line, contact:
Nova
Finishing
PO
Box 185, Hatboro, PA 19040 * 1610 Republic Rd. Huntingdon Valley,
PA. 19006
215-942-4474
* 800-444-4159 * Fax 215-953-1342
novasales@novafinishing.com
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