In North America and Europe market, the quality of electroplating on high-end zinc alloy die castings should meet the requirements of ASTM G58 that pass though non-defects test in acid salt fog for 48 hours (10 Class), bringing huge difficulties to quality control of electroplating on zinc alloy die castings. The following factors can be used to meet the requirements.
Requirements for die casting workblank
1. Buy high quality zinc ingots, and store the zinc ingots in clean and dry environment.
2. Die casting workblank has no starving, deformation, shrinkage and blister, decrustation, no compartment and cracks, pores and trimming.
3. The surface is clean, having no oil stains and collision traces.
4. Die castings should be baked in the hopper whose temperature is 150℃ for one hour, without blister.
5. The depth from hypodermic needle to polished surface must be larger than 0.30mm.
6. The density of polished parts is higher than 6.58g/cm3.
The quality control on polishing
1. Single polishing: the polishing should not carried out in the same place with copper parts. The materials also should be different. Large numbers of copper atoms remained on the surface of workpiece are caused by polishing. Adhesion between copper atom and zinc matrix belongs to mechanical adhesion without spread and mutual capacitance, having influence on the binding force between plating and matrix and tissue structure of plating, weakening corrosion resistance when plating with alkaline copper.
2. Use red cream for rough polishing and white cream for bright polishing without missing polishing and marks of mold are not remained. The surface should be full, having no concavities, no embosses, no pinholes and no dark spots.
3. Waxing should be frequent with few quantities. Strong blushing is not allowed in case of causing dense holes on the surface of workpiece due to high temperature.
4. Excessively polishing is not allowed. Compact layer is not allowed to be polished in case of dense pinholes appearing.
5. The parts should be stored in dry and clean place solely. The surface should not be hit, in case of the surface being oxidized and hydrated. The parts should be electroplated as soon as possible after being polished.
Subcutaneous shallow pinhole is the main defect having influence on the quality of electroplating on zinc alloy die castings.
The current die casting technology still cannot eliminate pinhole completely while can control small, dispersed and deep pinholes. Hypodermic pinholes have poor detectability, which examined by naked eye after polishing. That requires that inspectors should have extremely high specialization and eyesight. When hypodermic pinholes are dispersed, having smaller dimensions and high depth, the pinholes have less impact on the quality of electroplating. When hypodermic pinholes are larger, the pinholes can be polished again after being electroplated with acid copper. Cast repairing is available on concavity. Eliminate rough tissue on the surface, then, nickel or chromium, which also can pass through the test in acid salt fog for 48 hours, improving the utilization of die castings, forming refractory.
Cooper plating on basement uses the technology that cyanide copper plating, pyrophosphate copper plating and sulfate bright copper plating.
Use semi bright nickel, bright nickel and nickel sealing.
Chromium plating: for the bright appearance and protective decorative chromium layer having good abrasion resistance, zinc alloy die casting having complicated shape should have electric tank.
Because of the existence of hypodermic pinholes commonly exist on zinc alloy die castings, microporous chromium plating is the only choice for the electroplating of zinc alloy die casting.
Besides above factors, the design of plating hangers is also important.