How Racks are Essential to Effective Metal Finishing

Racks offer us the ability to fixture parts in a stationary manner for plating and other surface treatments, such as anodizing, electropolishing, painting, powder coating, and coloring.

The manner in which parts are fastened can range from simplicity (wiring) to specially designed racks with spring contacts and internal anodes. The manner in which parts are racked, along with the integrity of materials used, have a critical bearing on the satisfactory processing of parts. Racks involve the application of metals, ranging from copper, bronze, copper / phosphorous, aluminum, titanium, and stainless steel. As we should be aware, the prices of these metals have risen appreciably. Therefore, it is important to factor in quality maintenance and correct use of racks.

Racks are usually designed and built with these considerations:

• Parts and their geometry
• Plating bath application and throwing power
• Targeted current density (anodic or cathodic, per the finishing application)
• Production requirements per rack/flight bar
• Cost-effective racking of raw parts and unracking finished parts
• Durability and maintenance
• Compatibility with process solutions in contact with
• Safe exposure of rack tips to chemical stripping solutions

For plating, knowing the number of parts that are racked helps to determine the total amperage by (total surface area) × (targeted current density).  

For electropolishing and anodizing, total amperage is determined by: (total surface area) X (targeted anodic current density)

The Importance of Hooks and Rack Tips

The importance is cross-referenced to the current carrying capacity of the metal used in the rack construction (e.g., copper). Hooks and tips are very important. Without good stability in agitated baths and solid contact to the bus bar, electrical contact can be interrupted or fall short of the required current density. Hot or glowing contacts during a plating cycle are a primary alert to this problem.

Rack contacts, hooks, clips, and tips are designed for particular parts. Considerations include:

• Contact points for optimum current distribution
• Cradling to ensure rigidity or gravity contact
• Contact angles to minimize thieving and rack marks
• Geometric orientation prevents thieving, promotes maximum drainage
• Type of plating bath and application 
• Transfer efficiency

The size and dimensions of materials are based on the current requirement.

The typical current capacity of 1 inch square of pure metals serves as a guide to the selection of current demand on racks and splines, as noted in the following table. 

Contacts, tips, and clips can be made of any of the materials described previously, based on process application considerations.  

Trouble-Free Operation and Working Life

Well-built and maintained racks provide an economic payback with regards to trouble-free operation and working life. Rack tips are opened, closed, and conform to specific holding requirements. Quality connections to tips and splines are usually accomplished with rivets, screws, and welds. This is critical to maintaining desired current carrying capacity.

The simplest rack can be a strand of copper wire, straight or braided. It can be easily formed, recyclable, somewhat labor-intensive, but maintenance-free. The downside is limited throughput capacity of work, limited reuse, and brittleness of the wire after one plating pass. Racks increase production throughput, perhaps to the carrying capacity of the process line. Good insulation of the structure is very important. Polyvinyl chloride is the most widely used insulation coating. Done right, it is mixed, applied, and cured for superior service life. During continued use, the insulation could become cracked or pin-holed, resulting in:

• The buildup of laminating metal deposits
• Robbing current from rack tips
• Holding solutions that carry over into downline baths as contaminants
• Accelerating corrosion of the rack base material

Once in service, racks lose their new luster appearance, taking on a used but hopefully not abused appearance. Rack contact points will eventually become over-plated, possibly leading to breaking. Splines may crack or break. Out-of-commission racks or parts thereof effectively lower the production throughput. The service department or rack supplier can provide scheduled maintenance. Many racks are built to facilitate the replacement of parts, such as contact tips. Rack sections that are connected with bolts can be readily serviced. Cracked or pin-holed insulation can be patched or stripped and replaced. With sufficient racks in house, rotation keeps available racks consistent with the ability for effective service maintenance

Effective Chemical Maintenance

In the case of electroplating, the buildup of the excess deposit will reduce contact efficiency current-carrying load, resulting in thieving blocking intended deposit coverage and inability to properly rack parts. Chemical maintenance in the stripping of rack tips and hooks can be very effective. Every complete process cycle can be supplemented with a strip cycle after finished parts have been removed. It is the best way to minimize plating or other unintended deposit buildup on rack tips. Two treatment options are available: immersion or electrolytic. It is important to match rack tips for chemical stripping maintenance to the type of chemical solution. For example, stainless steel rack tips such as 304 or 316 series conform well to chemical stripping. Operating facts for immersion strippers:

• Typically strip copper and nickel 
• It contains nitric acid, accelerators, and fume suppressants
• Solution cooling to minimize temperature rise

Rapid, efficient stripping keeps the contacts clean and more effective. With respect to aluminum anodizing, the preferred racks are typically aluminum. These must be stripped to remove the anodized coating prior to reuse. Most often, the in-house stripping method of choice is the heated caustic etch. Over time, the active Sodium Hydroxide in the etch will strip the aluminum, degrading it to the point of the rack having to be discarded replaced with a new one. The cycle repeats itself. A means to extend service life with a caustic strip is to use a separate caustic strip offline, which contains an inhibitor to coat exposed aluminum after the anodized coating has been stripped. The inhibitor is then stripped in dilute nitric acid. An alternative is to use titanium racks. The upfront cost can be offset since the titanium is not anodized, thus significantly extending the service life of the titanium rack. By eliminating the stripping step, the throughput of parts to be processed is improved.

Electrolytic strippers may exhibit the following profile:

• They anodically strip many metal deposits, such as copper, nickel, chromium, alloys, brass, zinc, tin, and solder
• The primary oxidizer is an anodic current density. 
• Secondary chemical oxidizers supplement the reverse current.
• Buffers maintain bath pH close to neutral.
• Operating temperature ranges from warm to 150°F (66°C).
• Stripped coatings form insoluble metal hydroxides.

The most common strip for chrome off-rack tips is by either the reverse current electro cleaner or the hydrochloric acid dip, both of which may be in the process line. Chromate coatings would strip in the alkaline cleaner. Appropriate maintenance is recommended to prevent stripped metals from detrimentally affecting the intended processing of parts. This is typically accomplished by instituting a bath dump cycle.

Filtration of electrolytic stripping baths permits the removal of the metal sludges. If the sludge is of a consistent assay, it could be recycled by a smelter. Operating temperature encourages evaporation, providing the ability to maintain chemical additions. These factors can position the electrolytic stripper into a no-dump bath.

Many shops and plating installations have specific, designated storage areas for racks. It is important to keep racks off the plant floor, preventing damage from entanglements and being dropped. Racks are only as good as their intended use, design, and application. The materials of construction and proper maintenance cannot be overlooked. This is especially true when dealing with ever-rising costs of metals detrimentally affecting profit margins.

Stephen F. Rudy, CEF, is president of Chem Analytic and has written extensively about the finishing industry. Visit www.chemanalytic.com or call him at 917-604-5001.