The Problem of Sand Core Deformation of Exhausts
The exhausts of cars work under high temperature cyclic alternating state for a long time, and the oxidation resistance of the material at high temperature directly affects the service life of the exhaust manifold. In recent years, the country has continuously improved the automotive exhaust emission standards. The application of catalytic technology and worm gear supercharging technology has significantly improved the working temperature of the exhaust manifold, reaching more than 750 ℃. With the further improvement of engine performance, the working temperature of exhaust manifold should be increased. At present, heat-resistant cast steel is widely used, such as ZG 40cr22ni10si2 (jb/t 13044), with a heat-resistant temperature of 950 ℃ -1100 ℃. The internal cavity of exhaust manifold is generally required to be free of cracks, cold shuts, shrinkage cavities, slag inclusions, etc., which affect the service performance. The roughness of the internal cavity is required to be no greater than ra25. At the same time, there are strict and clear provisions on the deviation of pipe wall thickness. For a long time, the problems of uneven wall thickness and excessive deviation of exhaust manifold pipe have plagued many exhaust manifold foundries.
Comparison of One Foundry Factory Using Silica Sand and Fused Ceramic Sand
A foundry factory in China was the first to use coated silica sand core to produce heat-resistant steel exhaust manifold. Due to the high pouring temperature (1470 ~ 1550 ℃), the sand core is easy to deform, resulting in the over tolerance of pipe wall thickness. Although the silica sand has been treated by high temperature phase transformation, due to the influence of various factors, the deformation of the sand core at high temperature can not be overcome, resulting in a wide range of fluctuations in the thickness of the pipe wall, which will cause scrapping in serious cases. In order to improve the strength of sand core and control the gas generation of sand core, resin-coated ceramic sand was adopted. The performance tests of silica sand and fused resin-coated ceramic sand were carried out in the laboratory.
The results show that when the resin content of resin-coated ceramic sand is 36% lower than that of silica sand coated sand, its bending strength at room temperature and thermal bending strength are increased by 51% and 67% respectively, while the gas production is reduced by 20%, meeting the process requirements of high strength and low gas production. The key measure to solve the uneven wall thickness of exhaust manifold is to reduce the deformation of sand core (especially high temperature deformation), and reduce the high temperature expansion of small core sand. Because the thermal expansion rate of fused ceramic sand is far lower than that of silica sand, and its coated sand also realizes high strength and low gas generation, it has become an effective solution to solve the uneven wall thickness of exhaust manifold, which has also been proved by the production test in the foundry.
Summary of Fused Resin-coated Ceramic Sand Application
Using Resin-coated ceramic sand for core making, the broken core of the sand core is eliminated, the deformation of the sand core is reduced, the dimensional accuracy of the inner cavity channel of the exhaust manifold is greatly improved, the sand sticking in the inner cavity is reduced, the casting quality and yield are improved, and remarkable economic benefits are achieved.