Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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The most common method of attaching the tip to a spine is with a mechanical connection using a machine screw, lock washer, and nut. Materials vary with each manufacturer, but usually stainless steel, steel, or brass are used. Stainless steel connections are desirable because in the event of rack repair they suffer less corrosion attack than steel. Connecting the Tips to the Spine Fig. 4. Double tip connection. Using copper or steel rivets is faster in assembling, but creates a problem when the rack has to be repaired and tips moved. To connect a tip to a spine, a hole is drilled in the spine that will allow the screw to fit through, with a nut to attach to the screw for secure fastening. Using this type of connection, it might be desirable to solder the tip to maintain strength and corrosion resistance. To maintain corrosion resistance, tip connections should be lead soldered. Silver solder can be used to increase conductivity in tip construction, but will increase the cost. Most tips can be affixed to the spine with a single mechanical connection, but with large parts a double connection should be used. A double connection (Fig. 4) is desirable whenever the racking or unracking gives the operator an oppor- tunity to give the tip a certain amount of torque by constant twisting, pulling, and adjusting, thereby loosening the connection at the spine. A double connection minimizes the chance of this happening. Types of Replaceable Tip Connections Type #1: This replaceable tip (Fig. 5) has a knurler, which bites into the copper spine with a stainless steel stud drawn tight with a plastic cap. This type of tip is normally lead soldered for corrosion resistance and strength. Type #2: This replaceable tip uses a knurled section and threaded stud, which is drawn through a hole and then locked in place with a cap nut. Type #3: This type represents a gravity-type replaceable tip either plain or plastic covered. This unit is threaded directly into the spine or cross bar. COATING OF RACKS AND SPINES The final process in fabricating a plating rack is the coating. This coating is com- monly called plastisol or PVC (polyvinyl chloride) resin. Plastisol is 100% solid material and contains no solvents. Plastisol must be heated and cured at a tem- perature of 375-400O F. Prior to coating, the racks or tips are primed with an adhe- sive cement, which helps the plastisol adhere to the racks. In the curing process, it is important that the oven maintain a consistent tem- perature for an even cure. The oven is vented to remove any curing smoke and plas- ticizers. Even before the rack or tip is cemented, it is necessary to rough up the surface for adequate adhesion. This process is called blasting—normally a pro- cedure using some abrasive-type media such as aluminum grit, sand, or metal shot. The plastisol's primary function is to provide a corrosion protective coat- 656

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