Importance of Rack Design
Rack design is critical to proper anodizing. The rack must handle the maximum weight load and still be capable of carrying the required electrical current. In other words, racks should be well constructed with plenty of cross sectional area. Points of contact need to be selected with care, preferrably in positions that are hidden from sight. Other factors to be considered include suitability for the process solutions being used, simplicity of loading and unloading, density of component loading for highest production, adequate drainage of the work and cost effectiveness.
Titanium is a silvery gray, lightweight metal with a high strength to weight ratio, good electrical conductivity, high heat transfer efficiency and excellent corrosion resistance. Racks fabricated from chemically pure titanium should be used. The titanium racks do not require stripping and are resistant to attack by most solutions used in the anodizing process. Titanium, being higher in price than the alternative, aluminum alloy, is primarily utilized in high volume, long run production lines. Compared to aluminum alloy, titanium reduces rack maintenance costs and is far more economical in the long term.
Current Carrying Capacity
A number of conflicting values appear in the literature regarding the current carrying capacity of titanium. It is obvious that the maximum safe current will vary with the medium and its ability to remove the heat generated. Thus, current capacity in air is substantially lower than that in liquid media and well agitated systems can handle more current than stagnant electrolytes. The rule of thumb for design of aluminum racks is 1000 amperes per square inch (ASI). The comparable value for solid titanium is 350 ASI. In applications where higher current capacity is desired, while maintaining the benefits of titanium, a titanium-clad aluminum rack can utilize a design factor of 850 ASI.
Types of Racks
A number of different rack designs are available as off-the-shelf standard items. Splines are typically square or circular rods. The most common rack design is the single spline finger rack, also known as a “fir tree” or simply a utility rack. It is especially suited to the racking of small parts. Disc or “umbrella” racks are an adaptation of finger racks in which the fingers are arranged radially and bent to form an umbrella shape. The umbrella shapes are utilized in pairs, either in tension with concave facing surfaces or in compression with convex facing surfaces.
Box racks consist of a frame comprising two vertical splines. These are the workhorses of the industry and permit full use of tankage. Custom rack designs are available to match the rack to the work.
For all racks, tight contact is essential and titanium clips, tips, pressure clamps and bolts are available for this purpose. Titanium fasteners should be utilized in rack construction. Multiple contact points are suggested for higher current capacity.
Titanium and titanium-clad aluminum racks require little maintenance compared to aluminum alloy racks. A general inspection should be performed before racks are placed into service and after cleaning. Check that hooks are tightly secured to the spline, that there is no evidence of metal corrosion and that they are free of oxide. Over time, the titanium will develop an anodic oxide coating which should be removed. A blue or purple coloring is the best indication that removal of the film is needed. Inspect the splines and stations and tighten all fasteners, repair or replace bent or broken fingers and confirm proper alignment.
When racks do require cleaning, this can be accomplished using either mechanical or chemical methods. Mechanical cleaning consists of bead blasting to remove any oxide or scale followed by wire brushing or filing, if necessary, of contact points. The fastest method for chemical cleaning is immersion in a solution of of hydrofluoric acid or ammonium bifluoride until gassing occurs, usually within 15 seconds. The rack should be pulled from the solution as soon as gassing begins and rinsed. Hydrochloric acid can also be used, but it is much slower. In this case, the racks are immersed for 2 hours or more, with periodic inspections to visually determine when the oxide is removed. The rack is then rinsed. Another easy and inexpensive cleaning method utilizes the anodizing tank with the current off. This is conducted overnight. The racks are then rinsed and returned to service the following morning.