Porosity is usually the most common defect in cylinder block castings, which often takes the first place in the scrap castings. How to prevent porosity is a permanent problem for foundry workers.
The air holes in the cylinder block are mostly found on the outer surface corresponding to the water jacket area of the upper profile (including the periphery of the cylinder head surface), such as the bottom of the air outlet pin (when the air outlet pin is shorter) or the protruding ribs. And the inner surface of the cylinder barrel after processing. In serious cases, due to the large air flow rate of the core and the insufficient exhaust, the upper surface of the core is choked, resulting in large area of holes and irregular sand holes.
In modern production conditions, reactive pores and precipitated pores are relatively rare, and intrusive pores are more common. Now, the invasive pores are analyzed as follows:
1 reasons
1.1 the cavity is not fully vented and the total load area of the exhaust system is relatively small.
1.2 lower pouring temperature.
1.3 the pouring speed is too slow; the filling of molten iron is not stable and gas is involved.
1.4 molding sand has high moisture content, high ash content and poor permeability.
1.5 for engines with dry cylinder liner construction, Improper process of water jacket sand core (such as no exhaust system or incomplete exhaust system is set, or the exhaust channel is blocked due to the loose sealing, so that molten iron is drilled into the exhaust channel during pouring; the sand core is too thin and poorly ventilated; it is not fully dried after coating; the air flow of sand core and coating is too large, or the air flow rate is improper, and the shielding of coating is poor ）Experience has proved that the air hole defects of the cylinder block of the dry cylinder liner are largely related to the technological factors of the water jacket.
1.6 the inoculant is not dried and the particle size is improper; the slag is not fully removed from the molten iron, and the slag is not blocked during pouring, thus causing slag porosity.
1.7 failure to ignite in time during pouring
2 Countermeasures
2.1 a sufficient number of air outlet pins or exhaust plates with appropriate cross-section shall be set at the higher part of the model, and an exhaust cavity shall be set at the core part. The above exhaust system shall lead the air out of the model. Generally, the exhaust cross-section is about 1.5-1.8 times of the total cross-sectional area of the sprue.
2.2 the pouring system should be set according to the principle of semi open and semi closed, and it should have a certain slag blocking function, so that the filling of the molten iron is relatively stable, and it will not fill the mold or generate flying measurement or gas involved. The section size of the pouring system is more appropriate to calculate the pouring speed of 8 ~ 10kg / s.
2.3 the melting temperature of molten iron shall not be lower than 1500 ° C, while the pouring temperature of the final box shall be controlled at about 1400 ° C during manual pouring (it can be adjusted appropriately depending on the casting size and wall thickness). It is better to adopt automatic pouring, and the pouring temperature error shall be within 20 ° C.
2.4 a good sand treatment system suitable for high-pressure molding, the moisture content of molding sand should be controlled at 2.8-3.2%, the actual compactness should be between 36-42, and the wet compressive strength should be 180-220kpa (all refers to sampling and testing at the molding machine). In order to achieve these indicators, it is necessary to monitor the ash content of molding sand, the amount of auxiliary materials, the appropriate original sand size, the temperature of circulating sand and sand mixing efficiency.
2.5 pay attention to slag removal of molten iron, slag ignition during pouring and drying of inoculant.
2.6 for the engine block with dry cylinder liner structure, it is essential to have a very perfect and in place water jacket sand core process:
a. The average fineness of sand used for water jacket core is coarser than other cores, so as to have good air permeability.
b. Set up sufficient interconnected exhaust mesh and make it be able to discharge out of the mold. These mesh can be generated during core making as much as possible, and can also be formed after drilling after forming. For the former, it is necessary to regularly monitor and check whether the hole net is unblocked (be careful that the hole net is easily blocked in case of poor core sand solidification).
c. The properties of sand core should be considered comprehensively, and the strength should not be pursued unilaterally. When the strength is too high, it is bound to increase the amount of resin, so that the air flow rate of core sand is too high from the surface; while when the structure of jacket core is relatively complex, thin sand thickness is not uniform, and the vent network can be opened, it is required that the sand core has high strength, even if the air flow rate is larger, there is no prevention.
d. When the water jacket core has vent mesh, the coating should have better shielding; when the section of the water jacket core is inconvenient to set vent mesh, the coating should have better air permeability, and the particle size of the sand should be coarser.
e. When the water jacket core is provided with an air vent net, and shielding coating is used, the coating liquid shall be prevented from entering the air vent net during the dip coating, and more attention shall be paid to the fire sealing measures (the fire sealing gasket material can be used), so as to prevent the hot metal from entering the air vent net and blocking the air vent during the pouring;
f. The gas emission of the coating shall be low and it shall be fully dried after application.
A mature water jacket core technology can control the porosity rejection rate of the inner surface of the cylinder barrel after processing to 0.3%, or even lower.