Usually in the inner surface or hot spot of the casting, such as the water jacket cavity of the cylinder head, or in the inlet and outlet passage, due to the effect of high temperature molten iron during pouring, the transformation expansion of the silicon sand in the sand core will cause cracks on the surface of the sand core, and the liquid metal will penetrate into it, resulting in the defect of flying bulge of the casting, namely “vein pattern”. Once the veins appear, it is difficult to clean them. When there are veins in the water jacket cavity, the light will affect the cleanliness of the inner cavity, the heavy will affect the flow of cooling water, thus reducing the cooling effect on the engine, even causing serious consequences of “cylinder burning” and “cylinder pulling”. When there are veins in the air channel, it will affect the vortex characteristics of the air channel, and ultimately affect the overall performance of the engine.
The results show that the tendency of veins in cold core process is slightly higher than that in shell core process.
1.1 as mentioned above, the root cause of vein pattern is the expansion crack of silica sand caused by high temperature molten iron acting on the sand core.
1.2 the sand core material does not have the property of low expansion, or it can not absorb the expansion caused by heating itself.
1.3 the toughness or high temperature strength of the sand core is not enough to overcome the expansion stress leading to cracks
1.4 the materials used shall not cause expansion cracks in the sand core at high temperature.
1.5 the molten iron fails to solidify and crust before the sand core cracks, so as to prevent the formation of veins.
For the listed reasons (or the mechanism of vein formation). Obviously, the following measures should be taken;
2.1 under the condition that the mold filling temperature of the molten iron can be ensured to be perfect without defects such as pores, the lower pouring temperature shall be adopted as far as possible to reduce the degree of thermal expansion of the sand core; at the same time, the faster pouring speed shall be adopted to avoid the possible expansion cracks of the sand core caused by high temperature baking for a long time.
2.2 for the core sand that is easy to produce vein sand core (such as water jacket core, air inlet and exhaust channel core), the original sand shall be pre treated to eliminate phase change expansion, or some auxiliary materials shall be added to the sand core material to reduce the thermal expansion rate of the sand core material; and then the particle composition of the original sand shall be graded by three or four sieves, so that the sand core material can absorb the expansion deformation itself.
2.3 if necessary, a certain proportion of non quartz series sand (such as olivine sill sand, zircon sand, etc.) shall be used in the core material. The first one has a very small expansion rate, and the second one has a good thermal conductivity, which makes the time of iron shell formation earlier than the time of phase transformation expansion and cracking of the core.
2.4 improve the toughness and high temperature strength of sand core materials.
2.5 use sintered coating with excellent strength, toughness and thermal conductivity to enhance the ability of anti expansion crack on the surface of sand core.
The above measures are used for cold core sand and hot covering molding sand (shell molding sand). It can be seen that the main measure to prevent or reduce vein defects is to improve the expansion performance of sand core.