The coating for gray cast iron cylinder block is selected according to the core assembly process and sand core

The gray cast iron cylinder body adopts the traditional horizontal pouring process, the sand core is dipped with coating respectively and then assembled together, and the coating type can be selected according to different conditions. In the pouring process, different cores are subject to different degrees of conduction, convection and radiation of high temperature molten iron, and the performance requirements of coatings are also different. The bending strength and heat storage capacity of thin, small or slender sand cores such as water jacket and oil passage are relatively low. When the sand core is surrounded by high temperature molten iron (during or after pouring), it is easy to deform due to upward buoyancy. In serious cases, it will lead to core fracture or sintering defects in the inner cavity of the casting; The bending strength and heat storage capacity of simple thick sand core such as crankcase or side core are relatively high, and its ability to resist the thermal action of molten iron is relatively strong, and the requirement for high temperature resistance of coating is relatively low. Most foundry factories choose water-based coating with high cost performance.

With the development of light weight and thin wall of automobile cylinder body, in order to effectively solve the quality control difficulties in horizontal casting process, the integral core vertical casting process has been successfully applied to the thin-wall and high-precision gray cast iron cylinder body (as shown in Figure 1). The sand core of this process can not be dip coated separately, only a single type of coating can be comprehensively considered, Large size of sand core and complex structure of core group are the two main characteristics of integral core group vertical casting process. The coating layer of sand core is required to be relatively thick, uniform and without accumulation, otherwise the thin wall is easy to produce defects such as sand sticking, sintering and core breaking. The coating hanging performance is affected by many factors, among which thixotropy and shear dilution are two important indexes to evaluate the coating hanging performance. The user and the supplier should jointly develop the coating which is in line with the casting production characteristics. The gen3 cylinder body of our company has been verified by several rounds of tests, and the thixotropy rate of the coating is n ≤ 0.05 [n]=( η a0.5- η a10)/ η 5] and the shear dilution ratio m should be controlled between 5 and 7 (M 0= η A6/ η A60)。

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