The Effect of the Pouring Process on Reducing Casting Defects

There are many types of casting defects, and the factors that affect quality of castings exist in each process of the casting production, as shown in the chart 1. Large castings are characterized by large sizes, that is, large volumes, heavy mass, more molten iron, relatively thick wall thickness, simple shapes and complex shapes. Castings in different fields have different requirements. The main principles to be mastered for the design and pouring of the pouring system of large castings are dispersed bottom pouring (the effect of layered pouring is the best), fast pouring (adding more air risers), and high temperature pouring (enhancing core exhaust).

The dispersed bottom pouring system has advantages of being good for the molten metal to fill the mold steadily, reducing oxidation of the molten metal, having a small impact on the mold and core, preventing sand washing, reducing turbulence and gas entrapment, being beneficial to the discharge of cavity gas and slag removal, avoiding excessive temperature difference in each part, being beneficial to reducing the shrinkage stress of castings, deformation of long and thin castings and preventing cracks. A dispersed bottom pouring system has the following disadvantages: If the filling time is too long, the molten metal will be in contact with the air for a long time during the ascent of the cavity, and the surface will easily form the oxide skin which need to be overcome by quickly pouring. The temperature of the lower part of the casting is high, which is not conducive to feeding. 
 
Casting production processes: improving casting quality, reducing casting defects, casting production processes, raw and auxiliary materials, process design, mold quality, molding sand, molding core making  melting materials, pouring cooling, mold opening and post-processing. 

 
2. Quick pouring
Advantages: molten iron rises fast and is not easy to oxidize; the baking time of molten iron on the cavity is short, reducing the possibility of coating cracking and peeling as well as the occurrence of defects like slag inclusions in die castings; insufficient pouring and cold shut defects are prevented; increase the air pressure in the cavity, forcing the gas to be easily discharged from the mold, and defects like holes of the casting are not prone to happen; the temperature difference of each part of the casting is small so as to prevent cracks.
 
Disadvantages: sand washing defects are prone to appear for sand molds with low strength, and have little impact on sand molds with high strength like resin sand. The cross-sectional area of the gating system has increased, and the casting process production rate has decreased.
 
3. High temperature pouring
Advantages: good fluidity can avoid casting defects that are easy to form due to bottom injection, and castings with clear outlines can be obtained; defects such as cracks, cold shuts, gas holes and pinholes can be avoided. The pouring temperature should be increased appropriately based on wall thickness and structural shapes of the casting, not as high as possible.
 
Disadvantages: defects caused by sticking to the sand and sand washing are easy to occur; a particularly thick piece is easy to cause coarse structure; thick ductile iron parts have a tendency to shrinkage cavity and shrinkage porosity. 
 
The pouring temperature of gray iron castings is shown in the Table 1. According to production practice, the recommended pouring temperature of gray iron castings is shown in the Table 2. For the production of medium and large iron castings, the actual pouring time and pouring speed have a direct impact on the quality of the castings.
 
Table 1 Pouring temperatures of gray iron castings

 
Table 2 Suggested pouring temperatures of gray iron castings

 
Conclusion
For large iron castings with high strength made by casting molds, dispersed and bottom casting methods should be used in the casting process, assisted by methods of appropriately increasing the casting speed and casting temperature at the same time, which can greatly reduce hole defects of the castings, such as gas holes, splashings, crack defects (hot cracks and cold cracks), surface defects (cold shuts, insufficient pouring), structure and performance defects; improve the yield of castings and reduce the loss of waste products and the mechanical processing loss caused by internal defects of the casting in the casting process.