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Get It Plated Right
This fact sheet series is produced
by the Minnesota Association of Metal Finishers &
Minnesota Technical Assistance Program for metal fabricators
and their platers.
Cleaning Processes
Capabilities and Limits
Fact Seet #1
Metal finishers clean all parts before
plating. Clean part surfaces are crucial to producing
a uniform, defect-free coating that meets specifications.
Surfaces must be microscopically clean to insure a molecular
bond between the base metal and the plated deposit.
When cleaning is inadequate the plated surface is likely
to be defective.
Precleaning
Changes Save the Job
True Story
A large manufacturer of kitchen cooking
utensils, like spatulas and spoons, had their parts
nickel-chrome plated by a job shop. Parts were formed
from sheet-metal and deburred in very large manufacturing
runs. The plater started the cleaning process with trichloroethylene
vapor degreasing to remove most of the mixture of cutting,
stamping and forming oils found on the surface. Then
parts were plated on an automatic line that included
an alkaline soak, electrocleaning step, acid pickling
bath, followed by nickel and chromium plating baths.
At first the process produced a smooth,
reflective bright nickel-chrome finish with few defects.
However, after a time, an irregular line began to show
through the plate. This cosmetic defect caused a 50%
rejection rate. Because of the quantity of parts on
hand, an improved cleaning method for the remaining
unprocessed parts had to be identified.
A dried on mixture of machine fluids
and deburring solids was the suspected cause. Because
vapor degreasing has been known to cook soils on to
surfaces, the degreasing step was omitted for a short
trial run. This yielded a minor, but promising, improvement.
After several attempts, a revised procedure using a
hot, concentrated caustic soak for an extended period
was implemented in place of vapor degreasing. The lines
did not recur.
For this large job, setting up a customized
cleaning step was effective and economical. But this
incident convinced the manufacturer to 1) minimize the
time that parts sit in inventory--where soils have the
chance to dry on, and 2) try to send a cleaner part
for plating.
Cleaning Issues
and Choices
Soils on parts can generally be placed in one
of three categories:
- Organic soils (lubricants, oils,
grease, resins, fluxes and polymers) tend to be spread
in thin, barely visible films over parts surfaces.
- Inorganic soils (heat and weld
scales, carbon, dusts, rust and other oxides) form
or settle on surfaces exposed to air, sometimes at
high temperatures.
- Mix of organic and inorganic soils
(buffing compounds, fingerprints, shop soils and some
drawing compounds) can be among the most difficult
contaminants to remove.
There is no universal cleaner for
all these soils--rather there is generally a best formula
for each type. Typical types of cleaners include: solvents,
emulsions, waterbased cleaners (acid, neutral or alkaline).
Selecting the proper cleaner is necessary
to remove specific soils efficiently and effectively.
Carefully consider the following factors:
- Soil to be removed.
- Metals processed.
- Surface condition or properties
needed for subsequent operations or the end product
(cleanliness, texture, hardness, etc.).
- Effect of previous processes on
the metal and soils.
- Part size and surface area.
- Susceptibility to corroding.
- Operating and investment costs.
- Environmental impacts and regulations.
- Materials handling methods and
limits.
- Capabilities of existing facilities.
Plating lines have cleaning steps
that are optimized for the types of parts and soils
generally processed, and that are compatible with subsequent
plating chemistries and the wastewater treatment system.
Chlorinated solvents have been widely
used as a first cleaning step because of their ability
to dissolve most organic soils and remove heavy films
of oil and grease. However, increasingly stringent regulations
are pushing industry to look at other cleaning systems
where possible.
Alkaline cleaning has been a mainstay
of platers' final cleaning steps to expose and activate
the metal surface. Nearly every plating line has an
alkaline cleaning step. Most metal finishers also have
additional off-line cleaning capabilities not tied to
a dedicated plating line. Off-line processes typically
require more labor and can include degreasing, hand
wiping, pickling, stripping baths and occasionally polishing
or abrasive blasting.
In some cases, metal finishers can
modify cleaning processes and procedures to solve difficult
problems. They may change cleaner formulas, concentrations,
bath temperatures, soak times or how vigorously the
cleaner is applied. Generally these will increase plating
charges and turnaround time. In extreme cases, part
dimensions, surface texture and surface properties
will be affected by nonstandard cleaning methods. For
other difficult cleaning problems, modifying fabricating
processes makes more sense so that the amount of contaminants
is reduced or the difficult soil is eliminated.
Metal finishers generally have a wide
range of cleaning capabilities. But, few of these cleaning
methods are economical when used on a regular basis.
Using common metal working fluids and fabricating techniques,
and sending out the cleanest parts possible, allows
platers to use standard cleaning methods at their end.
Identify and solve difficult cleaning problems through
cooperative efforts. It's in everyone's interest--saving
headaches, time and money.
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