1. Improvement of gating system
1.1 The intermediate injection water inlet is improved to the top injection water inlet
At the initial stage of cylinder head mold test, that is, after the first and second rounds of trial production, after analyzing and studying the causes of a large number of casting air hole defects on the surface of cylinder head oil hood, we have taken the following process improvement measures: ① put the casting process scheme into (out of) the original design as shown in Figure 2 The gas side medium injection liquid inlet gating system is changed to the oil hood shell surface top injection liquid inlet gating system shown in Figure 1; ② Remove the original mid injection liquid inlet pouring (pouring) system on the air inlet (exhaust) side (Note: when the air inlet side is used as the mid injection liquid inlet pouring system, its structure similar to the pouring system on the exhaust side will be used as the riser system; vice versa).
During the trial production of the third and fourth wheel cylinder heads with the top injection liquid inlet gating system shown in Figure 1, although the casting pore defects on the oil hood shell surface have not been completely overcome, qualitative changes have also taken place. Firstly, the severity of the casting porosity defect on the oil hood shell surface is roughly reduced by 70% ~ 80% compared with that in the first and second round of trial production; Secondly, the casting porosity defects on the oil hood surface show a regular state of casting porosity defects only at both ends of the oil hood surface of the casting (at a and B shown in Fig. 2) and at two bosses C and D; Third, the five cylinder heads cast in one furnace are in good condition without casting pore defects.
Therefore, it is considered that the liquid inlet of the top injection pouring system can make the cylinder head cavity present a benign distribution state of high and low, which is conducive to the overflow (and discharge) of gas (and slag) in molten iron. Therefore, the top injection pouring system is an effective way to overcome the defects of casting pores under the production mode of Wet Mold clay sand casting (and flat casting) of this kind of cylinder head.
1.2 Balanced liquid inlet of ingate
As mentioned above, during the third to sixth rounds of trial production, although the medium injection liquid inlet gating system was changed to the top injection liquid inlet gating system, the problem that the minimum cross-sectional product of the inner gate (i.e. the original riser neck) of the top injection gating system was much larger than the choke cross-sectional product of the gating system was not noticed, and the casting pore defects appeared only at both ends of the oil hood surface of the casting (at a and B shown in Figure 2) and the regular state at the bosses C and D.
After analyzing and studying the above phenomena and problems, we adopted the minimum cross-sectional area of the inner gate (i.e. the original riser neck)（ Σ F) reduced to equal to the choke cross-sectional area of the gating system（ Σ F resistance). Through production practice, this process improvement measure makes the inlet liquid of the inner sprue balanced in the whole cylinder head cavity, so that the temperature gradient distribution of the molten iron in the whole cavity is in a more uniform benign state of high up and low down, which is conducive to the reduction of casting pore defects.
2. Correct selection of coated sand for water jacket sand core
After analyzing and studying many casting porosity defects when the coated sand is too fine (the main particle size is 50 / 100 mesh) in the fifth to sixth round of trial production, we restore the coated sand with coarse particle size (the main particle size is 40 / 70 mesh) and require the coated sand to have a low gas generation (< 15 ml / g).
The practice shows that using coated sand with coarse particle size and low gas output (and appropriate strength) to make cylinder head water jacket sand core is also one of the effective process measures to overcome the defects of casting pores.
3. Optimize sand mold permeability
After analyzing and studying the casting porosity defects in the fifth to sixth round of trial production, it is considered that the low air permeability of molding sand (back sand) is also an important reason for the casting porosity defects of the cylinder head. In order to effectively overcome the casting porosity defects of the cylinder head, we have taken measures to improve the molding sand (back sand) The air permeability of molding sand is optimized to the appropriate range of 150 ~ 200.
In order to effectively overcome the casting air hole defects of cylinder head, the exhaust of sand core and mold cavity is required to be unblocked, that is, five to six 12 ~ 16 mm through casting air holes are drilled at the appropriate position of the upper sand mold on the high side of the sand box, so as to strengthen the exhaust channel of sand core and mold cavity, which is conducive to the reduction of casting air hole defects of cylinder head.
4. High quality treatment of molten iron
After analyzing and studying the characteristics of blowhole defects in the first to sixth rounds of trial production, it is considered that the lack of effective degassing and slag removal treatment in front of cupola is also one of the more important reasons for blowhole defects. Therefore, we have adopted the following main purification and modification of molten iron in front of cupola.
4.1 Degassing and purification treatment of molten iron
0.05% of “NC” purifying agent is added to the raw molten iron melted in the cupola to carry out better degassing and purification treatment for the molten iron, so as to reduce the gas content of the molten iron and improve the purity of the molten iron as much as possible, so as to reduce the objective factors of casting pore defects.
4.2 Modification of molten iron
0.3% ~ 0.4% rare earth ferrosilicon alloy modifier is added to the molten iron melted in cupola to better modify and further purify the molten iron. The addition of 0.3% ~ 0.4% rare earth ferrosilicon alloy (with rare earth content of about 30% and silicon content of about 40%) in front of the furnace can not only improve the material strength (and hardness) of the modified molten iron It can not only improve greatly, but also have a better purification effect on its molten iron.