During the mold test of the cylinder head, the casting air hole defect on the oil cover shell surface is much more serious than the above situation; During the first and second round of mold test, there are not only a large number of casting pore defects with large cavity size at a and B and C and D bosses shown in the figure, but also a certain number of casting pore defects with small cavity size on the oil hood surface near the middle of the casting. The casting porosity defects of the cylinder head in the whole process of mold test and trial production are analyzed. It is considered that the causes of the casting porosity defects mainly include the following aspects.
(1) The temperature gradient of molten iron in the mold cavity is in a bad state due to the mid injection gating system. At the initial stage of the cylinder head mold test (i.e. the first and second round), tests were carried out on two liquid inlet gating systems on the parting surface: inlet side liquid inlet (mid injection liquid inlet scheme I) and exhaust side liquid inlet (mid injection liquid inlet scheme II). The results show that under the condition of melting molten iron in cupola, the medium injection gating system makes the molten iron in the mold cavity low and high on this kind of cast iron cylinder head, which belongs to poor liquid filling state, which is not conducive to the discharge of gas in molten iron, and it is very easy to have more casting pore defects and large cavity size on the shell surface of cylinder head oil cover.
(2) The top injection gating system makes the temperature distribution of molten iron in the mold cavity unbalanced. During the third to sixth round of trial production of the cylinder head, we changed the gating system of the cylinder head to top injection gating system. Because one of the two sets of riser systems on the crude oil casing surface is changed into a top injection gating system, it is not noticed that the minimum cross-sectional product of the inner gate (i.e. the original riser neck) of the top injection gating system is much larger than the choke cross-sectional area of the gating system. Therefore, the top injection gating system makes the liquid inlet in the middle of the cylinder head cavity very large, while the liquid iron inlet at both ends of the cavity is very small, In this way, the temperature (gradient) distribution of molten iron in the mold cavity is in an unbalanced state of high in the middle and low at both ends. Therefore, it is not conducive to the discharge of gas in molten iron at both ends of the cylinder head oil cover surface, and casting air hole defects are easy to appear at a and B and at C and D bosses as shown in the figure.
(3) The coated sand for water jacket sand core is in poor condition. During the third to fourth rounds of trial production of the cylinder head, the particle size of coated sand for water jacket sand core is appropriate (the main particle size is 40 / 70 mesh). In these two rounds of trial production, there are few casting air hole defects on the surface of oil hood. During the fifth to sixth rounds of trial production, due to various reasons, the coated sand for water jacket sand core of cylinder head is too fine (the main particle size is 50 / 100 mesh), and the gas generation of coated sand is also high (> 15 ml / g). From this analysis, it is considered that during the fifth to sixth round of trial production, the fine particle size and high gas generation of coated sand for water jacket sand core are also one of the main reasons for the defects of air holes in casting.
(4) The air permeability of wet clay sand is poor. The casting production mode of the cylinder head adopts the casting production mode of wet molding clay sand appearance, static pressure molding, hot core box coated sand core, one box and one flat casting. Its main tooling is 1200 borrowed from the original production of six cylinder block × eight hundred × For the 330 (mm) sand box, because all the castings are placed in the lower box, the upper box is obviously high, and the air permeability of the outer mold sand (back sand) is low (< 150) during the fifth to sixth round of trial production, it is considered that this is also one of the important reasons for the occurrence of casting porosity defects.
(5) The quality of molten iron is low. The molten iron used for pouring cylinder head is the molten iron melted by cupola, and experts in the same industry also have a common understanding: the content of gas and slag in the raw molten iron melted by cupola is significantly higher than that melted by electric furnace (power frequency furnace or medium frequency furnace, etc.). During the first to sixth rounds of trial production, no effective degassing and slag removal treatment was carried out before the cupola. Therefore, it is considered that this kind of molten iron with high gas and slag content is also one of the important reasons for the occurrence of casting porosity defects.