Shrinkage is the physical nature of the alloy. Under normal pouring temperature, the shrinkage volume of the alloy with fixed composition cannot be changed, but this does not mean that the shrinkage cavity of the casting is inevitable. It has been proved that the compact castings without shrinkage cavity can be obtained if the solidification of steel is controlled reasonably and the sequential solidification is realized. The so-called sequential solidification is to add a series of technological measures, such as riser or cold iron, at the hot spot where the shrinkage may occur (i.e. the thickest part of the diameter of the inner circle), so that the part of the casting far away from the riser solidifies first, then the part near the riser solidifies, and finally the riser itself solidifies. According to this cooling sequence, the solidification shrinkage of each part of the casting can be fed by the liquid metal, and the shrinkage cavity is transferred into the riser. The riser is the redundant part of the casting, which shall be cut off during the casting cleaning. When there is no riser, there is a shrinkage cavity in the thick wall of the upper part of the casting. After adding the riser, the casting solidifies in sequence and the shrinkage cavity is transferred to the riser.
There are many kinds of risers. Ordinary risers, whose surface is exposed on the upper box, are fed by the static pressure of metal. This kind of riser has the advantages of convenient modeling, flexible operation and easy to supplement hot metal during pouring, so it is widely used. However, its feeding efficiency is poor, it consumes a lot of metals, and its use in some parts of castings is limited. In order to make up for the shortage of common risers, various forms of dark risers are often used in mass production. The dark riser has slow heat dissipation and high feeding efficiency, and it is also convenient for feeding the side or lower part of the casting. In order to further improve the feeding efficiency, measures can also be taken to improve the internal temperature or pressure of the riser, such as heating riser, atmospheric pressure riser, etc.
In order to control the solidification of the casting, cold iron can also be placed at the hot spot of the casting. The role of cold iron is to increase the cooling speed of the thick part of the casting and prevent shrinkage, but the cold iron itself does not play the role of feeding. Cold iron is usually made of cast iron or steel.
The possible shrinkage at the hot spot when there is no riser; the right side of the figure shows that after adding riser and chill, the casting realizes sequential solidification and prevents shrinkage. Placing riser and implementing sequential solidification can effectively prevent shrinkage, but riser wastes metal, consumes labor time, increases casting cost, and increases internal stress of casting, which is easy to produce deformation and crack. Therefore, it is mainly used for alloys with large solidification shrinkage and small crystallization interval, such as cast steel, high grade grey cast iron, nodular cast iron, malleable cast iron, brass, etc.