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

Source: Junying   Author: Junying   Posted: 2017-08-04 16:24:04   Hits: 7

Zinc alloy die casts have a major and recurrent defect, which has plagued die casting practitioners and die casting enterprises. Meanwhile, this defect also affects raw material suppliers, die casting, polishing and electroplating enterprises, bringing the occurrence of the dispute over trifles among their departments.  

Zinc alloy die casts have been mainly used in various decorations, such as furniture accessories, building decorations, bathroom accessories, lighting parts, toys, stickpins, buckles and other metal buckles. Therefore, surfaces of die casts are required to have good quality and surface treatment performance. However, the most common defects of zinc alloy die casts are blisters.

Characteristics of blisters 
There are some blisters on the surfaces of die casts. Some blisters appear after the die casts are casted; some blisters occur after the die casts are polished or processed and others are found after the oil injection or plating process.

Causes
1. Causes of holes
The main reasons are holes and the mechanisms of contraction. The holes are usually circular, while most contraction are irregular.

Causes of holes' formation
A. Gases getting into the molten metal in filling and solidification processes can cause holes on the surfaces and interiors of die casts.
B. The gas penetrates during the painting process. 
C. Alloy liquid has excessive gas content, and the gas will precipitate in the solidification process.   

When gases are not discharged properly in the molds, gases in the cavities, gases volatilizing from coatings and gases precipitating in the solidification process will eventually remain in the die casts and form blowholes.

2. Causes of shrinkage cavity
A. During the solidification process of molten metal, the shrinkage cavity is formed, because the volume is reduced or final solidification parts don't get molten metal feeding.
B. The uneven thickness of the die casts or overheated die casting parts will cause slow solidification,  resulting in concave surfaces on the die casts during the volume reduction.

Due to the existence of the air hole and shrinkage cavity, when we have surface treatment for die casts, water may get into the hole; when we bake the die casts after painting and electroplating processes, gases in the holes will expand after being heated or water in the holes will turn into steam, resulting in blisters on the surfaces of die casts.