Plating
Rack Design
By
DONALD C. LANG, II
Mitchell-Bate Company
Waterbury, Connecticut
The first plating racks were simple hooks
and wires. This crude start evolved to a configuration of
metal components assembled together. This design became somewhat
more sophisticated with the introduction of plastisol coating.
This evolution of coated racks offered some degree of protection
to the metal components from plate-up and chemical degradation.
This cha in of events took us to the early 1940's and is still
reflected in present day plastisol-coated racks.
Today, rack design concept combines the positive
benefits of both existing and new technology. The rack is
comprised of a mainframe or spline that combines the current-carrying
capability of copper with the chemical resistance of type
316 stainless steel. The framework is shielded by CPVC tubing,
a patented design that prevents deposition on the proprietary
stainless-clad copper.
 |
| 1.
RACK DESIGN using CPVC tube that allows plating solution
to circulate and cool the framework. |
Only the workstations or crossmembers are
plastisol coated and mounted to the outer surface of the CPVC,
which allows for ease of repair and interchangeability. The
plating solution enters through the bottom of the CPVC tube
and seeks its own level. thus cooling the framework, thereby
reducing the resistance, increasing the current distribution
and cathode efficiency (Fig. 1).
This new design eliminates the negative aspects
of racks that are totally plastisol coated.
-
Dedicated design racks are no longer a problem.
Plastisol components can now be removed and replaced with
new ones; an in-house capability.
- Workstation
failure. Ease of workstation replacement maintains
100 pct production capability.
-
High and low current density. Partto-part controlled
plating thickness is maintained through the solution-cooling
effect. Heat distribution and resistance to cathode source
is negated.
This new rack design has more currentcarrying capability than
that of presently used standard racks. This reduces energy
consumption.
Additional
improvements include:
- Reduction
in plating cycles.
-
Increase in number of parts per rack.
- Elimination
of in-house stripping.
-
Will not trap solutions in broken workstations which would
contaminate subsequent baths.
Now
that we have discussed rack design let us review some of the
plating factors that must be considered in rack design.
Basic
Elements of a Plating Cell
Rectifiers are used to convert alternating current (AC) to
direct current (DC). The direct current consists of electrons
flowing continuously in one direction. The electrons enter
the tank at the work piece (cathode) and leave the tank at
the anode. Within the plating solution positive ions (cations)
move toward the cathode; negative ions (anions) move toward
the anode.
Copper conductors usually are used to carry current through
the external circuit outside the plating tank. The electrons
move through the copper. The electrode, which represents the
part being plated, is supplied with electrons from this external
circuit.
The rate of flow of current, expressed in amps is equal to
the pressure divided by resistance. Pressure is expressed
in volts and the resistance in ohms. Hence, the expression
I=E/R (Ohm's Law) governs current flow.
In a series circuit, the resistances are additive. However,
in a parallel circuit each loop is calculated separate then
totaled. Thus, when Ohm's Law is applied the total current
flow is considerably higher in parallel than in a series circuit.
However, other considerations such as dirty connecting joints,
dirty bus bars and rack hooks all contribute to increased
resistances. These added resistances cause decrease or diversion
of current flow which in turn causes variation in deposit
thickness and distribution.
Faraday's Law is also important. It states "that the
quantity of a substance deposited at an electrode is directly
proportional to the quantity of electricity that passes through
the solution and the actual weight of the substance is proportional
to its atomic weight divided by its valence." If time
is measured in seconds, the product of amps times seconds
is called coulombs. Thus, one amp X 10 seconds = 10 coulombs.
Faraday found that it took 96,500 coulombs of electricity
to deposit the equivalent weight of metal.
In plating we are concerned with variations in thickness of
metal on plated parts. In a conventional plating tank, the
workpiece or cathode is normally positioned in the center
with anode close to the side walls. Current flow lines are
closer together near the edges and outside corners of the
workpiece and are relatively far apart on the inside bottom
corners. This thickness variation (current distribution) is
related to plating efficiency.
Factors influencing plate distribution (Throwing Power) include
the following:
- Type
of electrolyte - simple ions or complex ions.
-
Cathode efficiency vs. current density
- Polarization
-
Conductivity
- Anode
basket positioning
-
Part-to-part orientation
Critical data required to design plating racks are depicted
in Fig. 2.
 |
| 2.
ELEMENTS of a plating rack and critical dimensions that
must be considered in designing a plating rack. |
A.
Bar size-The bar size and shape should be designed
with sufficient cross-section to hold the projected load and
conduct current equal to the power source.
Standard material choices include:
- Copper
-
Phosphor Bronze
- Nickel-Plated
Copper
-
Copper-Cored Type 316 Stainless Steel
-
Copper-Cored Titanium
B. Solution Level (free board) considerations
are as follows:
-
The level of solution should be maintained equally in all
tanks.
-
The level should not be allowed to vary more than one inch.
-
Room for ventilation should be allowed for.
C. Thickness, anode to anode - Considerations:
-
Rack with parts should occupy the center 1 3 of total width
available.
-
Allow space for front-to-back agitation stroke.
D. Work area
-
Top of work - The top row of parts should be two inches
below the solution to insure sufficient plating.
E. Bottom tank clearance -
- Bottom
of rack to bottom of tank.
-
Clearance for lug and plumbing should be accounted for.
F. Left to right -
-
Six inches from each tank wall maximum window.
-
Clearance for plumbing and agitation.
G. Overall length -
-
Top of flight bar to bottom of rack.
H. Working area above flight bar -
- Flight
bar supports to support load.
- Hoist
pickup hardware for rack lifting.
I. Working area -
-
Hoist mechanisms should allow equal w window as overall
length plus three inches (clearance).
J. Cathode hook -
-
Conform to work bar.
- Clamp
to work bar.
-
Area of interface between rack hook and flight bar should
be twice the cross section of frame.
K.
Framework -
-
One-inch-square copper carries 2,000 amps under plastisol
in solution.
-
Support the weight of crossmembers.
-
Copper-cored type 316 stainless cooled by solution.
L. Crossmember style
- Horizontal
-
Vertical
- Removable
- Adjustable
- Pivoting
M. Workstation -
- Wire
-
Flat stock
-
Removable
-
Material (stainless steel, phosphor bronze, titanium)
The workstation is one of the most critical portions of the
design. These factors must be considered:
-
Part weight
- Current
required
- Easy
loading
-
Small rack marks
N. Auxiliary anodes - Auxiliary anodes can
be used to enhance the plating in hardto-reach cavities and
contours. Auxiliary anodes should be designed as removable
components to allow for ease of replacement, when needed.
0. Floats -
-
Floats can be used to rock crossmembers allowing solution
in and out of deep cavities.
P. Robbers/Thieves
-
Robbers/Thieves are used on highcurrent areas to reduce
build-up. Robbers can be designed for each part individually
or as one-piece frames. In both cases they should be designed
to be removable for ease of maintenance.
In conclusion, it should be noted that properly designed plating
racks are one of the most important tools used in
the metal finishing industry.
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