The main reasons are that the hot spot of the precision casting increases and the heat dissipation performance of the precision casting becomes poor due to the plugging after the fine hole shell making, the plugging at the local hot spot of the precision casting, and the poor permeability of the mold shell.
1. Precision castings with elongated holes
Figure 1 shows the casting process of a precision casting with elongated holes. After the shell is made, the thin hole is blocked by the coating, the heat dissipation condition of the precision casting becomes poor, and the cooling speed of the thin wall between the two flanges of the precision casting is fast, resulting in different degrees of shrinkage defects in the hole near the flange. Fig. 2 The thin and long hole of the precision casting is directly plugged and the sprue is placed in the middle of the flange. The increase of the wall thickness between the two flanges of the precision casting makes the feeding channel smooth and solves the shrinkage porosity defect.
2. Shrinkage defect caused by shell making and plugging at local hot spot of precision casting
Figure 3 shows the precision casting of a connecting pipe. The gap at the arrow position in the figure is small. After the shell is made, the hot spot will be increased by completely plugging, and the shrinkage defect of the riser at this position will be more serious. Figure 4 shows the process plan of removing the gate and riser. The shell shall be sprayed with water before pouring the arrow part to improve its heat dissipation conditions and speed up the cooling rate, so as to reduce shrinkage defects.
The poor permeability of the mold shell leads to slow heat dissipation of the precision casting, which will not only cause shrinkage defects, but also cause other casting defects. The permeability of the shell can be increased by adding graphite or wood fiber in the shell coating. Coarse sand is used for medium and large precision castings to improve air permeability.
