(This is part one of a two-part article on Hull cells and their use in plating operations. Part two will appear in the December issue of Products Finishing.)
Most platers use a Hull cell to find out what is wrong when a plating station begins to produce rejects. But Hull cells can do far more than help to get you out of trouble. Properly used, they can prevent problems. You can use them in routine daily maintenance, establishing operating parameters, and in considering modifications or improvements of a plating process.
Because a Hull cell produces a deposit that is a true reproduction of the electroplate obtained at various current densities within the operating range of a particular system, it allows experienced operators to determine multiple process parameters, including:
- Approximate bright range.
- Approximate concentration of primary bath components, such as metal and electrolyte.
- Approximate concentration of addition agents.
- Presence or absence of metallic and organic impurities.
- “Covering power”—the lowest current density at which plate is deposited.
- “Throwing power”—metal distribution.
- Effects of temperature variations.
- Effects of pH variations.
The Hull cell is a time saver. Hull-cell panels are inexpensive when compared with experimenting in a production tank plating actual parts. You can introduce variables quickly and safely in the small tank represented by the cell. And you can see the results of several different tests at the same time.
Hull cells accurately duplicate all the variables present in a production-plating tank size, shape, width, depth and time parts are exposed to the process. The simplest Hull cell is a clear, trapezoidal Lucite chamber that is filled with plating solution and equipped with electrical contacts—essentially, a miniature plating station. Its design allows users to angle the cathode panel so that current flow is directly related to the distance from the anode. The current applied increases as the distance decreases. The clear plastic lets you see what is happening while metal is being deposited, if the plating solution does not block out visual observation.
The cell holds 267 ml of plating solution. At that volume, there are direct correlations of milliliter or gram additions to 100 gal of plating solution, without the need for a math degree. A 2-g or 2-ml addition to the 267-ml cell equal an addition of 1 oz/gal in your operating bath.
Types of Cells
From this basic cell concept, many variations, modifications and auxiliary aids have been developed over the years. For chrome bath evaluations, for example, porcelain is substituted for Lucite, because the Lucite deteriorates with prolonged exposure to chromium plating solutions. There are many other variations in cell volume and configuration, some of which are described below.
The Gornall cell was developed in an attempt to duplicate conditions in a printed-circuit-board plating operation. It consists of two, 534-ml cells welded together so that air and heat can be introduced to both sides of a perforated plastic panel. The larger cell volume allows more tests per bath sample with solutions whose chemistry will be altered too much by plating more than two or three panels (such as bright nickel). Larger volume also allows you to run several test panels before replacing the chemically depleted test solution.
Still larger cells—with volumes to 1,000-ml—can be equipped with heaters and agitation. The larger size allows more precise evaluation of baths.
A hanging Hull cell can be hung in an operating tank anywhere a rack of parts can be hung. Capable of reading from 0 to 50 amps, the cell hangs on a cathode bar below the solution and is an excellent tool for finding “dead” areas, poor electrical contact, no current, reduced current, and other conditions.
The Haring cell is a 1,000-ml cell that is used for more precise evaluations of throwing power. This cell is a rectangle with grooves that are different distances from the
Jiggle cells are mechanically actuated to move the panel up and down via connection to an offset cam. Panels run in this cell are one inch wide by approximately seven inches long, formed on a wooden mandrel that provides expanded areas (approximately one inch square) of various common asf areas commonly looked at in more precision evaluations. The areas are 1-20, 20-40, and 40-60 asf, by mandrel design. Also, right-angle bends in panels provide shelf areas for roughness observations.
The anode in a jiggle cell is (conveniently) a spear from the production bath, and the anode bag is readily made. Jiggle cells also provide hook-ups for air agitation and heat.
While Hull-cell panels can provide a quick and factual picture of what is happening in the production tank, never forget that analysis should be a routine precursor of Hull cell evaluation. Looking at a panel without first knowing the chemistry of the bath can result in very misleading interpretations. Correcting bath chemistry often will eliminate a problem before you run a Hull cell. Having done this, you can count on the Hull cell to give you a much truer picture of what is happening in the large tank.
Please note that if, for example, your supplier suggests a 1-to-10 ratio of zinc metal to caustic, and outlines it as 0.8-1.2 oz/gal of zinc and 8.0-10.0 oz/gal of sodium hydroxide, running the bath at 0.8 oz/gal zinc and 12.0 oz/gal sodium hydroxide is not a 1-10 ratio. And while all around you are screaming for the answer to what’s wrong, remember that no bath is smart enough to cause rejects only on one particular part, or parts only in one area of a rack, or one side of a part, or every other barrel load. If you feel that’s what is happening...go take a walk, come back and start over again!