What should be paid attention to in the pre-plating treatment of zinc alloy die castings in zinc alloy die casting plants?

Zinc alloy die casting is a one-time die casting with high production efficiency and low processing cost. It is especially suitable for parts with less stringent tolerance requirements and complex geometric shapes. It has been widely used to replace copper, copper alloy and steel materials. For structural parts and decorative parts with less force and complex shapes, the surface treatment process is also in a mature stage. Dongguan zinc alloy die casting factory needs to pay attention to the following points:
1. Understand the structural characteristics of zinc alloy die castings

The surface of the zinc alloy die casting is very similar to the surface of steamed buns, with a layer of 0.02-0.10 um thick, smooth and dense metal layer, and a loose and porous structure below it. Therefore, during mechanical polishing, it is strictly necessary to prevent the smooth and dense layer from being thrown through, and to avoid the exposure of loose and porous inner materials, resulting in blistering and peeling of the coating.

2. Master the chemical properties of zinc alloy die castings

In the die-casting process, the influence of process factors will cause zinc-rich and aluminum-rich parts in the same zinc alloy die-casting part. Strong bases preferentially dissolve the aluminum-rich phase, while strong acids preferentially dissolve the zinc-rich phase. If strong alkali or strong acid is used in the process of degreasing or etching, it will cause foaming and peeling of the coating during electroplating, which will seriously affect the yield. For this reason, zinc alloy die castings are not suitable for degreasing and corrosion in strong acid or alkali.

3. Meet the size requirements of zinc alloy die casting pre-coating

Zinc alloy die castings should be pre-plated and primed after the pre-plating treatment is completed. Generally, copper is plated in a cyanide copper plating bath without NaOH, or nickel is plated in a neutral nickel plating solution. Tubular parts should be deposited in a pyrophosphate bath.

The thickness of the coating should not be less than 6pm, otherwise the very thin copper layer will quickly diffuse into the zinc alloy, forming a layer of extremely brittle copper-zinc alloy, which will affect the quality of the coating. The thinner the copper layer, the faster the copper will diffuse into the zinc alloy.

4. Accelerate the connection speed between the pre-plating treatment of zinc alloy die castings and the electroplating into the tank

The pre-plating treatment of zinc alloy die castings is to remove the oil stains on the surface of the parts and the oxide film formed by the oxidation of oxygen after the parts are left in the air for a long time, so that the metal ions in the plating solution can be directly deposited on the pure surface. After the pre-plating treatment is completed, the parts should be immediately plated in the tank. It is not allowed to stay in the atmosphere or in the water tank for a long time, so as to avoid the surface being oxidized again or forming hydrates on the surface, which will affect the bonding force of the coating. If the part has a slight oxide film or hydrate, it can be immersed in 10 g/L boric acid solution, and then transferred to 10 g/L NaCN solution for activation to restore the surface to purity.