Metal Finishing Guide Book


Issue link:

Contents of this Issue


Page 732 of 843

to requirements or specifications. Auxiliary anodes should be made adjustable and easily removable to facilitate loading, unloading and replacement. The auxiliary anodes must be secured to the rack using insulated material so the anode does not cross the cathodic current path with rack. Auxiliary anodes can be made using lead, steel, stainless steel, nickel, or coated titanium, and the choice of anode material is process specific. For nickel/chrome processes, it is best to use auxiliary anodes made of titanium that is platinum plated, platinum clad niobium mesh. During the past few years, platers have had success using an iridium oxide coating on titanium. This material is less expensive than platinum and normally lasts longer before recoating. ROBBERS AND SHIELDS Robbers are used to prevent burning in the high current areas of the part being plated. Robbers also have other names such as "thieves" and "burning bars." Parts with sharp edges or points flush left, no indent at times need special care. The current flow to these areas is typically excessive and the parts have a tendency to burn. By placing a robber, which is connected to the cathode and positioned near the high current area of the part, the robber will pull the plating away from the hardware, resulting in a part that is free of excess plating and burning. Shields are another option to feather the current path to the part. In this case, a shield is used instead of a robber for the parts on the outside border of the rack. A shield can be made of a non-metallic material and is used to cover up high current areas, shielding the plating flow to high current areas of the part. This is an effective and lower-maintenance alternative to robbers. RACK MATERIAL In selecting a material to build a rack, the preferred metal is copper. To carry the necessary current throughout the rack and distribute it to the parts, nothing is more conductive than copper. Some suppliers have been known to use steel in an effort to "price down" their product. This should be avoided at all cost. Steel only carries 12% of the current for the same size in copper. Aluminum has been successfully used even though it only carries 60% of the current as compared to the same size material in copper. Normally when weight is an issue, aluminum is a logical choice. Stainless steel is used throughout the industry as the preferred tip material, and alloys 301/302 and 316 are used. Stainless steel offers adequate current carrying capacity in most cases and is resistant to strippers that are used when the racks are cleaned. MANUFACTURING TECHNIQUES To protect the metallic framework of the rack from the chemistry of the plating line and to insulate the metal rack components from plating, the supplier must coat the rack in a good PVC (plastisol). All plastisol are not created equal. Some rack suppliers cut corners by using bottom-shelf plastisol and this is not obvious when the racks are delivered. The rack looks as good as a higher-priced competing rack. The problem will evolve and is noticeable when comparing the life of the plastisol. Premature degradation of the plastisol will create cracks in the coating, causing solutions to be transferred from tank to tank and contaminating process baths. It will also cause acids to reach the copper framework and 719

Articles in this issue

view archives of Metal Finishing Guide Book - 2013