May 13, 2008

Cleaning of Plating on Zinc

Cleaning cycles include three vital steps:
  1. The bulk oils, grease, soil, or buffing compound must be removed by pre-cleaning because prolonged contact in the subsequent alkaline electro-cleaner will cause etching or smutting.
  2. To ensure the good electroplate adhesion, electro-cleaning must be used to complete the removal of oil, grease, and soil.
  3. An acid dip must follow alkaline cleaning to remove zinc oxides and hydroxides and any alkaline compounds carried over from the electro-cleaning operation.

Die castings are pre-cleaned by:
  1. Solvent Degreasing
  2. Power Spray alkaline washing
  3. Emulsion cleaning, or
  4. Alkaline soak cleaning.

Buffed die casting should be pre-cleaned as soon as possible after buffing, preferably within 24 four hours, before the compound hardened and becomes difficulty to remove by conventional cleaning.

Solvent degreasing utilizes trichloroethylene or perchloroethylene solvent in facility equipped with heating and condensing equipment. Die castings usually are sequentially immersed in the heated solvent, dipped in cold solvent, and suspended in the heated vapors. Solvent spraying also is included in the cycle if buffing compound has been hardened on the die cast surfaces. The solvent should be inhibited to prevent the formation of hydrochloric acid caused by decomposition. The cost of the chlorinated solvents has encourage the use of alternative pre-cleaning methods. However, a diphase system consisting of a layer of solvent such as trichloroethylene, perchloroethylene, or methylene chloride and an upper layer of water is sometimes adopted to reduce costs associated with the evaporation of solvent. Diphase cleaning is generally followed by power-spray washing.

Power spray alkaline washing is accomplished with conveyorized units usually equipped with washing, draining, rinsing and draining sections and is the most popular method for pre-cleaning. Solution heated to about 80 oC is sprayed with a pressure of 1.8 to 2.1 kg/cm2 through nozzles on 20 to 30 cm centers in the washing area.

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