February 4, 2008

Jigging

Parts which have been polished with polishing pastes are degreased using organic solvent, such as benzine or dichloromethane, and are dried in air until the traces of the solvent is disappeared. They are then mounted on anodizing jigs made of aluminum or clad alloy. The design of the jig requires special considerations. It should ensure a firm contact with both the work and the anode bar and at the same time the area of contact should be as small as possible because this area will remain unanodized. It should be just sufficient to avoid local heating of the metal, which is usually the cause of an attack on the surface of the work during the anodizing process.

 

Before use the jigs have to be freed from the oxide film acquired in the preceding operation. This can be done by filling or by pickling in a 10 percent caustic soda solution. To ensure good contact with the work, the jigs are provided with spring or screw clamping means in the form of two frames in which the parts are clamped between two pieces of aluminum wire netting. Small parts such as washers, bolts and rivets can be anodized in perforated aluminum containers into which they are packed and pressed down firmly by the lid of the container so that they are brought into firm contact with one another.

 

Degreasing the jigged parts are then degreased. For parts having a roughly finished surface, chemical degreasing is suitable using a solution containing 8-12 kg. caustic soda, 40-50 kg trisodium phosphate, 25-30 kg sodium silicate and 1 Klt water at 60-70 oC for 2-4 minutes. Polished parts are degreased in a solution containing 40-50 kg soda ash, 3-4 kg trisodium phosphate, 25-30 kg sodium silicate and 1 Klt water. The parts are introduced into the baths as cathode, steel sheets being used as the anode. After degreasing the parts are washed in hot running water and then in cold running water until all traces of the alkaline solution have been removed. Properly degreased surfaces should be uniformity wetted by water.

No comments: