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Analyses of Blisters on Zinc Alloy Die Casts' Surfaces (Part Two)

Source: Junying   Author: Junying   Posted: 2017-08-24 15:26:58   Hits: 12

2. Causes of water lines, cold shuts and hot cracks

3. Causes of intergranular corrosion 
Lead, cadmium and tin which are harmful impurities in zinc alloy composition will gather in the boundary of crystalline grain and cause intergranular corrosion. The metallic matrix will break due to intergranular corrosion and galvanization accelerates the break. The intergranular corroded part will expand and raise the plating layer, which causes the blister on the die cast' surface.
 
4. Causes of improper mold and die casting product designs 
Changes of pouring speed, pouring direction, coolant system and mold walls

5. Causes of improper after processing technologies
(1) Burnishing 
Roll finish, polarized

(2) Polishing 
Force of polishing: too much force will destroy the hardened layer and too little force can’t have the effect of polishing.

(3) Degreasing and cleaning
During burnishing and polishing processes, the accessory material like wax will remain on the surface of die casting. Therefore, we usually use alkali liquor, acid liquor and electrolytic degreasing to clean the surface of the die cast.

Zinc alloy die casting products are mainly made of zinc and aluminum. During the die casting process, segregation will happen. Zinc accumulation and aluminum accumulation will occur on some parts of die casting products; alkali can make aluminum dissolve first and acid can make zinc firstly dissolve. Pinholes will form on surfaces of die casting products and solutions will remain on surfaces of die casting products; blisters will occur on surfaces of die casts; therefore, it is very important to clean and remove the oil on the surface of the zinc alloy die casting products.

(4) Causes of the process and storage
A. After die casts being manufactured, water absorption and oxidation will happen on the die casting products due to no preservation of them.

B. After burnishing and polishing processes, the surface of the die cast will oxidize if the die cast doesn't have the electroplating process.

C. After cleaning and removing the oil, the surface of the die cast will be eroded if the die cast doesn't have the electroplating process timely.