Whether the structure of the die casting is reasonable or not has a great influence on the quality of the casting and the feasibility and simplicity of the production process. The structure of the die casting is equivalent to the production characteristics of the investment casting, so it is an example of the reasonable structure of some investment casting. In order to ensure the quality of the investment casting in the production process of die casting, it is often necessary to set the process rib and hole on the investment casting according to the needs, so the reference size of the process rib design is the application example of the process hole. Zinc die casting parts can cast very complex parts. In order to improve the production efficiency and accuracy in the process of use, several parts assembly parts and weldments originally produced by others can be directly cast into one investment casting after a little structural improvement. In the process of casting, the pouring temperature of metal mold casting should be controlled properly. If the pouring temperature is too high, the cooling will be slow gradually, the crystallization of the product is coarse, and its mechanical properties are low, so it is easy to form pinholes, blowholes and other defects. If the pouring temperature of metal mold casting is too low, it will directly lead to defects such as insufficient pouring and cold shut. Therefore, the casting temperature is controlled according to the structure of the casting and the casting process characteristics, but the casting limit is reduced to a low level. In the casting process of die casting, the casting process will directly affect the quality of the casting, so in the casting process design and mold design, we should set the shrinkage rate in advance, and constantly modify the size of the metal mold.

The formation of shrinkage cavity and porosity in die casting is as follows

If the volume reduction caused by liquid shrinkage and solidification shrinkage can not be supplemented during the subsequent cooling and solidification process of the liquid alloy poured into the mold, some holes will be formed in the post solidification part of the casting. Among them, the larger volume is called shrinkage cavity, and the smaller and dispersed pores are called shrinkage porosity.

1. Shrinkage cavity
Generally, it appears in the upper part of the casting or the post solidification part, and its shape is mostly inverted cone, with rough inner surface, which is usually hidden in the inner layer of the casting.

The narrower the range of crystallization temperature, the more inclined the casting alloy is to solidify layer by layer, and the easier it is to form shrinkage cavity. First, the liquid alloy fills the mold. Because of the cooling effect of the mold, a layer of liquid alloy near the surface of the mold solidifies quickly, but the inner part is still in the liquid state; With the continuous decrease of casting temperature, the thickness of the shell is continuously thickened, and the volume of the internal liquid alloy is reduced due to its liquid shrinkage and the solidification shrinkage of the supplementary shell, resulting in the decrease of liquid level and the appearance of voids in the die casting. In this way, the casting solidifies layer by layer until it is completely solidified, forming shrinkage cavity in the upper part; Further cooling to room temperature, solid shrinkage will make the overall dimension of the casting slightly reduced.

2. Shrinkage porosity
Shrinkage porosity is the dispersed and fine shrinkage cavity caused by the failure of liquid alloy in the solidification area of the casting.

According to the distribution of porosity, it can be divided into macro porosity and micro porosity

(1) Macro shrinkage refers to the small holes that can be seen by naked eyes or magnifier. It usually appears below the shrinkage cavity.
(2) Micro porosity refers to the micro pores distributed among dendrites, which can only be seen under the microscope. The distribution of shrinkage porosity is large, even throughout the whole section of casting, which is difficult to avoid completely. For general castings, they are not treated as defects, except for some castings with high requirements for compactness and mechanical properties.

In a word, alloys that tend to solidify layer by layer, such as pure metals, eutectic alloys or alloys with narrow crystallization temperature range, tend to form shrinkage cavity and are not easy to form shrinkage porosity; However, some alloys which tend to be mushy solidification, such as alloys with a wide range of crystallization temperature, have a small tendency to produce shrinkage cavity, but are prone to produce shrinkage porosity. Therefore, shrinkage and porosity can be transformed into each other in a certain range.