How ceramic sand to improve exhaust manifold casting quality
The exhaust manifold is an automobile accessory connecting the engine cylinder and the main exhaust pipe. The exhaust gas generated by the engine fuel in each cylinder enters the exhaust manifold through the diversion of the exhaust manifold. It can avoid the interference between the exhaust gases of each cylinder, reduce the exhaust resistance and avoid affecting the output power of the engine.
The exhaust manifold is directly linked to the engine cylinder. The exhaust gas temperature of the engine fuel can reach 750 degrees and has been working under the condition of cyclic alternating temperature. At present, the exhaust manifold is generally made of heat-resistant cast steel, with a heat-resistant temperature of 950-1100 degrees. The inner cavity of the exhaust manifold shall be free of casting cracks, cold shuts, shrinkage cavities, sand inclusions and other defects affecting the performance, and the surface roughness of the inner cavity shall not exceed ra25. In addition, there are high requirements for the deviation of pipe wall thickness. The uneven wall thickness and excessive deviation of exhaust manifold pipe have always been the problems that foundry manufacturers try to avoid.
Silica sand is generally used as the core sand in the production of heat-resistant steel exhaust manifolds with traditional resin-coated core sand. However, the refractory temperature and thermodynamic performance of silica sand are not so well. In the production process of exhaust manifold, the casting temperature can reach 1470-1550 degrees. The core sand of silica-based RCS is always deformed, resulting in out of tolerance of pipe wall thickness. Although some foundries make high-temperature phase transformation treatment on silica sand, it will still produce deformation, the thickness of the casting pipe wall fluctuates greatly, and the reject rate is very high. After replacing silica sand with ceramic sand RCS, the air generation of core sand can be controlled. As a result, the dimensional deviation of castings is effectively reduced.
Ceramic sand has good thermal properties with high refractoriness and strong bending resistance. The following table shows the performance indexes of ceramic resin-coated sand AFS55 compared with silica resin-coated sand AFS55:
|Sand Type||The consumption of Resin(%)||Cold Flexural Strength(MPa)||Hot Flexural Strength(MPa)||Gas Evolution|
Adopting ceramic sand as resin-coated sand works excellent. On one hand, the addition of resin is 36% less than that of silica sand resin-coated sand. On the other hand, the flexural strength at room temperature and hot flexural strength increased by 51% and 67% respectively. The gas generation decreased by about 20%. It meets the requirements of high strength and low gas generation of core sand in casting production.
In addition, the key to prevent uneven pipe walls of exhaust manifold is to reduce the deformation and expansion of sand core at high temperature. The thermal expansion rate of ceramic sand is much lower than that of silica sand, which can further reduce the risk of deformation at high temperatures.
After the actual production comparison, the silica resin-coated sand produces the exhaust manifold castings. After the anatomical inspection of the finished product, the wall thickness of the pipe is 3.0-6.2mm. Adopting ceramic sand, the wall thickness of the exhaust manifold is 4.4-4.6mm, which greatly reduces the thickness tolerance of the casting and improves the accuracy of the casting. At the same time, sand-sticking in the inner cavity of the casting is also reduced, which improves the quality and acceptance rate of the casting.