When the shrinkage of the casting in the mold is only subject to the frictional resistance between the metal surface and the mold surface, it is free shrinkage. If the shrinkage of the casting in the mold is also hindered by other obstacles, the copper casting is called hindered shrinkage. For the same alloy, the hindered shrinkage is less than the free shrinkage.

The basic principle of preventing shrinkage porosity and shrinkage porosity in castings is to formulate the correct casting process according to the shrinkage and solidification characteristics of the alloy, so that good shrinkage conditions can be established during the solidification process of the casting, and shrinkage porosity can be transformed into shrinkage porosity as much as possible.

 

The shrinkage cavity appears in the final solidification of the casting. In this way, a riser of a certain size is placed at the place where the casting finally solidifies, and the aluminum casting makes the shrinkage cavity concentrated in the riser, or the gate is opened in the final solidification place to feed directly. In order to establish good feeding conditions for castings during solidification, the main reason is to control the solidification direction of castings to conform to the "directional solidification principle" or "simultaneous solidification principle".
The introduction position of the gating system has an important influence on the temperature distribution of the casting. Adjusting the pouring temperature and pouring speed of liquid metal can enhance directional solidification or simultaneous solidification.

A riser is set on the thick wall of the casting or at the hot joint. The aluminum casting is the most effective process to prevent shrinkage and shrinkage. The comprehensive use of risers, subsidies and chilled iron are effective measures to eliminate shrinkage and porosity in castings.

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