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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.
Holes, Seams, Threads, Recesses
and Tubing Assemblies
Improve Drainage to Minimize Contamination
Fact Sheet #6
Recessed areas on a part can accumulate
process oils and fluids. During cleaning and plating
steps, substantial amounts of trapped fluid can leak
out and contaminate baths. Or worse, as the part contracts
and expands as it changes temperature, trapped fluids
can seep over clean surfaces. Contaminated surfaces
prevent plating from bonding to the metal surface. The
worst contributors are narrow, blind holes and lapped
sheet metal seams. Finishing shops are well equipped
to remove surface films of most machining fluids. But
unless alerted or the fluids are obvious, most metal
finishing job shops assume no pools of fluid contamination
remain. Many problems can be avoided by eliminating
these troublesome features during product design.
Improved Part Design Reduces Fabrication
and Plating Costs
True Story
An original equipment manufacturer
(OEM) contracted with a sheet metal fabricator to form
and assemble a family of parts constructed from lightweight
steel. The parts had numerous bends, with spot welds
on lapped and folded seams. The fabricator brought the
parts to a plater to have an electroless nickel finish
applied to improve corrosion and wear resistance. The
family of parts represented over $100,000 in plating
sales annually.
After a trial run, the plater quoted
a cost 25% higher than the fabricator had budgeted.
The quote was accepted because other platers processing
these parts had experienced 70% reject rates, with costly
delays and increased costs. The primary reason for the
high reject rates was the spotty appearance of parts.
Reject Causes
Because appearance had become
the critical criteria for acceptable parts, it was necessary
to duplicate exact inspection procedures at the manufacturer,
fabricator and plater.
Visual inspection of critical surfaces
was done at arms length, without tilting or rotating
the part, and under identical lighting. Using these
standard procedures, all parties were able to compare
incoming defects against rejects to establish accountability.
After keeping meticulous records,
three causes of rejects were identified which led to:
- Reducing oil use on a straightline
sander;
- Designing
more efficient packaging that was reusable and
more protective; and
- Running
smaller plating lots to speed drying and minimize
water stains.
With the process in control, rejects
were reduced to 5%.
Design Changes
Even though product quality
was now good and parts were delivered on schedule, plating
costs were still significantly higher than the OEM had
budgeted. Recessed areas, spot-welded seams, and the
absence of racking holes contributed to difficult parts
processing and, ultimately, to the cost.
At a meeting of all three companies,
inspection and process time documentation was invaluable
in demonstrating how part design increased the cost
of plating. The OEM decided design changes were necessary
to cut costs:
- Racking holes were
positioned to promote drainage and to protect critical
surfaces.
- Airways were created in
recessed areas.
- Drain holes were added,
reducing cycle time. On one part, two additional holes
(3/16 inch I.D. [inside diameter]) cut drain time
in half, reducing cycle time proportionately.
- Endtabs replaced full length
seams. This greatly reduced the volume of fluid trapped
in seams, which leaked during subsequent operations.
In the end, the plater was able to
reduce its plating charges by 10% over the next two
years. In addition, relocating the spot welds to the
end tabs allowed sheet metal fabricating steps to be
combined. This reduced manufacturing costs by more than
enough to bring the overall project within budget. With
improved part design, quality was reliable, delivery
schedules improved, labor was reduced and profits increased
for all three companies.
Problem Sources and Solutions
In this case, the source of the
problem was small amounts of process fluids remaining
in seams. A number of design features--such as blind
holes, small diameter holes, threads, surface texture,
porous metal structures and recesses in general--can
cause process fluids to be carried into later processing
steps.
In the following section the most
troublesome of these features will be addressed and
a number of possible solutions proposed. Work with your
metal finisher to decide how and in whose facility to
implement a solution. Start evaluating at the design
stage.
WELDED TUBULAR FRAMES
Enclosed areas in these products
are generally assumed to be completely sealed, isolating
exterior from interior surfaces. In most cases, pinhole
leaks are present, created by air escaping as the part
is heated during welding. Even if no contaminants are
present when the product leaves the welding station,
parts undergo expanding and contracting cycles in many
finishing operations. This first sucks process chemicals
into the tube interiors, and then bleeds them out, contaminating
surfaces being plated or finished.
Design Solutions
- Specify drain holes
at the top and bottom of each cavity. Use hole diameters
consistent with the volume of the cavity: a minimum
of 3/16 inch for small assemblies; 3/8 inch minimum
for 2-inch tubing; 1/2 inch minimum for 4-inch and
larger tubing. Hole size is important to produce drain
times that are in seconds rather than minutes. If
needed, these holes are easily sealed with rubber
plugs for subsequent operations or with Allen-head
bolts for a permanent seal.
- V-notch tube ends prior
to welding as an alternative to drilling holes.
- Add racking holes so assemblies
are oriented to promote drainage. Try to combine racking
holes with some of the drain holes.
- Check with the plater during
the design phase to assure that tanks are sufficiently
large to allow the desired racking orientation.
- Add holes to provide pressure
relief during welding to reduce weld defects.
BLIND HOLES
If plating is required inside
holes, plating thickness cannot be guaranteed at depths
greater than the length of the hole diameter without
special procedures. Blind holes that open downward trap
air, preventing plating solutions from entering. In
addition, holes collect and retain cutting fluids which
tend to drain out slowly when exposed to heat and strong
plating chemicals. This contaminates product surfaces,
showing up as drip marks and circles around holes, and
also contaminates plating baths. Threads, surface roughness,
surface texture, grooves or keyways increase the difficulty
of dealing with soils in holes.
Design Solutions
- Specify a through hole
or a bleed tap
where possible so air can get behind any residual
liquid. Shaking or trying to drain liquid plugs from
deep narrow holes creates a vacuum behind the plug,
which works to prevent drainage. Bleed taps break
this vacuum to allow drainage.
- Specify minimum hole diameters
of greater than 0.125 inch (1/8 inch) for smooth bores
and greater than 0.1875 inch (3/16 inch) for threaded
bores if at all possible. Larger is better for finishing
operations.
- Specify hole depth no greater
than necessary for the minimum strength requirement.
Ideally the depth to diameter ratio is less than two
to one (2:1). Remember that deep holes cost in terms
of fabricating time and tool wear, in addition to
creating processing difficulties.
- Add chamfers to threaded
holes to: 1) promote drainage; 2) avoid over plating
the first thread; and 3) protect threads from damage.
Procedural Solutions
- Apply cutting fluids by
drip, mist or through the tool
to minimize the machine fluids present. Avoid flood
application. Use air or vacuum to remove chips.
- Use low viscosity, low surface
tension fluids that will drain easily.
- Avoid silicone-based fluids
and high sulfur content fluids.
- Specify that holes be plugged
or masked.
- Bore and thread holes after
parts have been plated. This requires careful fixturing
and handling of parts to avoid damaging plated surfaces.
There are cases where reduced inspection requirements
have more than offset the added costs. For parts fabricated
in clean rooms, boring after plating gives more control
over the amount and type of soils likely to be present.
Remember, the insides of small holes generally do
not get well plated without special and expensive
procedures.
- Store parts so fluids drain
completely away. Avoid situations where parts on the
bottom of a tray are submerged in drained fluids.
Cleaning Solutions
If holes are unavoidable, remove contaminants as
soon as possible.
- Fabricate a fixture to direct
compressed air, vacuum or cleaning fluids into each
hole. Ideally, such a fixture would be part of the
machine tool so no extra steps are needed.
- Manually
blow out residual liquids. Compressed air nozzles
are commonly used. An alternative is a vacuum gun
that operates off compressed air combined with a filter
or capture device. One available model combines vacuum
with a compressed air blast. The vacuum gun alternative
also reduces oil mists in the workplace and the related
facility cleaning needs.
- Have
the metal finishers manually blow fluids out of
recesses. Generally, finishers will not be able to
set up fixtures for more automated removal of bulk
liquid contaminants.
SHEET METAL SEAMS
Folded or lapped spot-welded
seams form thin recesses which draw substantial volumes
of liquid into them due to capillary forces. When exposed
to heat, which expands the liquid and air volumes, or
to harsh chemicals, this residual liquid bleeds out
contaminating part surfaces and baths.
Design Solutions
- Eliminate lapped seams
where possible. Use butt welds or reduce the number
of pieces in an assembly.
- Construct lapped seams with
a gap of greater than 0.002 inch so liquids can
be removed, or make the seam tight with a gap less
than 0.0001 inch so liquids will not bleed.
- Minimize the lapped seam area
by reducing its length or width. Use tabs or end tabs
in place of full-length seams and locate spot welds
there.
- Minimize the number of spot
welds and create a dimple at them to minimize
the area of tight clearance.
Procedural Solutions
- Use evaporative stamping
lubricants that will not remain on the parts.
Carefully evaluate and avoid lubricants that contain
a silicone oil.
- Use the minimum volume of
lubricant needed for quality forming and adequate
tool life.
- Apply lubricant to the tool
wear points rather than the part to minimize the
part surface area coated.
- Apply the lubricant precisely
where metal on metal movement and stress will occur.
- Dry parts in an oven to
force residual liquids to evaporate.
Cleaning Solutions
- Direct a high pressure
spray of cleaning
solution at the seam to force oils out. Dry with high
pressure air to displace liquids. Or use vacuum drying
to aid evaporation. Hot air drying can be done, but
a long drying time is generally required.
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