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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.
Particulate
Contamination on Part Surfaces
Eliminate to
Reduce Plating Rejects
Fact Sheet #2
Particulate contamination generally
makes the appearance of plated surfaces rough and uneven.
It can also cause more serious localized defects by
interfering with the bonding between the plate and base
metal. Contamination can be caused if maintenance schedules
are not followed, in-process cleaning is inadequate,
or the electrical grounding of process equipment deteriorates.
Particles can originate from sanding, grinding, machining,
deburring, and buffing operations.
Straightliner
Maintenance Lapse Increases Plating Costs by 40%
True Story
Late in the day-shift, a sheet metal
stamper called its plater to schedule straightlined
parts for zinc plating and a clear chromate finish.
The 400, 9-inch-by-12-foot parts would arrive at 11
p.m. and be needed by 7 a.m. the next day. The job had
been run successfully before so the price was quoted
and the job scheduled for the third shift.
The third shift supervisor noticed
that the parts were coated with a grimy film so he plated
samples. The samples looked dull and cloudy and were
fuzzy--much different than previous jobs. Metal finishers
call this general fuzzy-like coating graphite.
Vapor degreasing removed some of the
oily components of the graphite but caused the remaining
coating to adhere more tightly. This coating proved
impervious to the electrocleaning and acid cleaning
steps.
The choices were to: 1) hand wipe
each part at the cost of $0.50 each, and try to resolve
the cost problems in the morning, or 2) ship the parts
back to the fabricator in the morning for further cleaning
(at perhaps a $0.50 per part), incurring extra transportation
costs and missed delivery deadlines.
Because the plater had a good relationship
with the sheet metal fabricator, they hand wiped and
plated the parts. The job cost 40% more than quoted.
But, if the dirty parts had been processed without manual
cleaning, the job would have been defective. Rework
at the plater to strip, clean and replate would cost
250% over the original quote and delivery deadlines
would have been missed.
The source of the dirty parts was
traced back to one of the fabricator's straightline
sanding machines. Its wash solution had not been
changed on schedule, causing it to contaminate parts
with metal fines, abrasives and oil. Better maintenance
procedures could have prevented a 40% surcharge.
Problem Sources and Solutions
While the problem encountered
above was traced back to a lapse in maintenance procedures,
a number of situations can cause particles to stick
firmly to parts as they undergo further processing.
In the following section, typical causes of particle
contamination are listed and a number of possible solutions
are proposed.
Vapor degreasers have been a primary
precleaning step at metal finishing shops. They work
very well at removing oils and other organic liquid
films that readily dissolve in the solvent. However,
it is a physically mild cleaning method which does not
remove particles from surfaces well, unless the degreaser
has been augmented with ultrasonics, boiling sump immersion
or sprays. Even with augmentation, cleaning and plating
become difficult when small particles adhere to surfaces.
Abrasive Equipment Maintenance
- Solutions
Wide-belt deburring machines
tend to be troublesome sources of particle contamination
when they are not maintained or correctly selected for
the job. Polishing, grinding, vibratory deburring, and
straightlining are other common sources of particulate
contamination. The following steps can help prevent
contaminants:
Follow the equipment manufacturer's
recommendations
for equipment maintenance
and select the appropriate abrasive media.
Clean equipment
at least once a shift
using a coolant spray for wet sanders or a vacuum for
dry sanders. Clean:
- Conveyor belts. Look for belt discoloring
or images transferring from the belt's texture onto
parts.
- Hold-down rolls.
- Squeegee rolls.
Clean heavily oiled parts before
abrasive operations. Dry abrasive use is incompatible
with wet or oily parts. Filtration systems on wet machines
are not designed to remove oil. Oils will circulate
and build up in the system and recoat parts. This may
create a mixed soil that is difficult to remove (see
fact sheet #4).
Replace worn or foul abrasive belts
and media.
Change coolants and process fluids
on abrasive equipment and clean out sumps when fluids
appear discolored or no longer leave clean surfaces.
Maintain equipment cleaning and
dust control features:
- Coolant flow on wide-belt sanders
should be at least eight gallons per minute per foot
of belt width, and sprayed on both the abrasive belt
and the product. Plugged or damaged spray nozzles
should be looked for and repaired.
- Dust exhaust on dry sanders
should be at least 300 cfm per foot of belt width
to remove particles from the work area. Remember that
particles left in the work area shorten abrasive life
as well as soil parts.
Educate operators that their
workmanship not only affects the reject rates from their
own operation but, in many cases, also affects the reject
rates of subsequent processing steps. Try to give operators
feedback on how well their parts fare through the entire
production process.
Prevent metal fines from embedding
into part surfaces of different metals or alloys.
Change grinding and polishing wheels, and abrasive belts,
when jobs change and different metals are used. Or,
schedule jobs-to-be-plated after belt changes and less
critical jobs for the end of a belts life. Longer-lived
abrasive media need to be dedicated to specific metals.
Equipment
Grounding/Magnetic Adhesion-Solutions
Inadequate grounding can induce magnetic fields
in both equipment and the parts being processed. Magnetized
ferrous parts hold particles onto surfaces, preventing
their removal by standard cleaning methods.
Check the grounding of wire
forming machines, wire spot welders, sanders and other
equipment periodically with an ohmmeter. Measure the
resistance from the work fixture, the equipment's metal
frame or housing, and the equipment's neutral and ground
connections to a known building ground connection.
Correct grounding if readings
exceed zero ohms. Most often, grounding problems are
caused by loose electrical connections or broken wires,
caused by equipment vibration. If vibration is excessive,
work to identify and correct those causes.
Demagnetize susceptible parts after
using a magnetic fixture and after spot welding wire.
Tool steels and cast iron alloys are the most susceptible
to retaining induced magnetic fields. Determine if parts
are magnetic by using a gauss meter. Parts with readings
higher than one gauss should be demagnetized. Parts
with readings above two gauss will be very difficult
to chemically or mechanically clean. Some platers have
demagnetizing equipment. Shops with magnetic particle
inspection equipment can generally use it to demagnetize
parts.
In-Process
Cleaning - Solutions
Some fatty oils, coolants and other waterbased process
fluids dry to form a very tough coating which can glue
particles onto surfaces. These coatings can be similar
to varnishes and are best removed by vigorous chemical
or abrasive stripping operations. Like cement on tools,
organic soils and oils are easiest to remove when they
are fresh; cleaning gets increasingly more difficult
with time. Although the soils and oils generally take
longer than cement to set up, they need to be removed
as quickly as possible to avoid cleaning problems.
Wash parts after grinding,
straightlining, wet sanding and deburring. These operations
commonly place small particles on surfaces. Where possible,
machine operators should dip-rinse parts in plain water,
a mild alkaline cleaner or mineral spirits while processing
them.
If rust proofing is needed, check
your rust preventative preference with your metal finisher
Maintain wash baths to control
solids buildup. Look for particle contaminants on parts
surfaces using a wipe test. To reduce the volume of
solutions consumed, try: 1) using a two-stage dip where
the second stage stays relatively clean; or 2) filtering
the bath.
Communication
- Solutions
Where in-process cleaning is not possible, expedite
shipping and finishing of jobs so the finishing
shop an lean the parts before soils set up.
If finishing problems arise, be
prepared to discuss manufacturing processes with
the metal finishing shop. Particularly important will
be changes in:
- Process fluid brands. Also, be
careful of "new and improved" formulations
of your current brand.
- Fluid suppliers.
- Fluid preparation and maintenance
procedures. Changing fluid concentrations, biocides
or the method of coolant recycle can occasionally
cause problems even if the change is within the manufacturer's
recommendations.
- Operators. Especially if they are
allowed to choose or modify the fluids they use.
- Feed rates or cutting and processing
speeds.
- Metal suppliers or alloys.
- Abrasives.
When a problem occurs, consider all
possible sources. Many of the changes described earlier
will seem insignificant and most of the time
they will be. Good metal finishers will review their
shops for similar changes and problem sources. The objective
is to find an economical solution to the problem as
quickly as possible and implement it regardless of where
the implementation occurs.
We would like to thank Timesavers
Inc, Minneapolis, Minnesota for their recommendations
on maintaining abrasive equipment particles from the
work area.
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