Fig. 1 shows the temperature field changes of sand castings at 10s, 60s, 130s and 340s after the molten metal fills the mold cavity. According to the change of solidification temperature field in the profile of sand casting in the figure, it can be found that the sand casting conforms to the principle of sequential solidification at the beginning of solidification, the inner gate and riser are in the high-temperature stage, and the metal liquid provided by the riser and inner gate can achieve good feeding. With the cooling of time, the riser part cools rapidly in the air. When the solidification of sand casting occurs within 60s-130s, most of the thick part of the sand casting begins to solidify and form a closed temperature field. At this time, the temperature at the lower part of the cylinder head is high, reaching 550 ℃. At this time, the gating system and riser cannot feed normally. At this stage, shrinkage porosity and shrinkage defects of sand casting are easy to occur.
Figure 2 shows the change of solid rate in the solidification stage of a single riser section under the second scheme. The solidification time of sand castings is 10s, 45s, 125S and 240s respectively. In the initial stage of solidification of sand castings, sequential solidification conditions can be formed at the riser along the riser direction. The risers are in a high temperature state, with good liquid metal flow performance, and sufficient feeding can be obtained at the parts far away from the riser. With the cooling and solidification of sand castings over time, the sand castings gradually form a high-temperature closed area after solidification for 45s, and the riser can not play a good feeding role. After 125S, the section 2 will still produce an isolated liquid phase area, but it is slower than the first scheme. When it reaches 240s, the temperature still shows gradient cooling, and the solid rate of the metal liquid at the bottom of the lower end is only about 70%. It can be inferred that the defects are easy to occur in the isolated liquid phase area.
The positions of shrinkage cavity and porosity of symmetrical top injection casting after solidification are shown in Figure 3. The symmetrical top injection gating system is adopted. After the mold filling, the liquid metal follows the principle of sequential solidification along the direction of inner gate and riser. Due to the smaller dispersion size of inner gate and the larger volume of sand casting, it still can not form a good feeding for the thick part at the bottom. A large volume shrinkage cavity still occurs near the bottom of the sprue cup, which is consistent with the isolated liquid phase region in Figure 2.
The symmetrical top injection type of eight inner gates is adopted to make the liquid metal fill the mold smoothly and shorten the mold filling time. However, if water is fed from both sides, there will be an obvious intersection of liquid metal, which is prone to turbulence and inclusion. Due to the dispersion of liquid metal, the filling time at each horizontal position is inconsistent, which may also lead to liquid metal oxidation, air entrainment and other casting defects. Through the observation of solidification stage, the early solidification is combined with sequential solidification, but the inner sprue still can not adequately feed the thick part of the lower part after 120s. At 240s, it was observed that the thick part had solidified and formed an isolated liquid phase In the relative position, an obvious shrinkage porosity and shrinkage cavity are found. Compared with the first scheme, the shrinkage volume is reduced. Therefore, the second scheme also needs to be optimized and improved.