Application of foam ceramic filter material in automobile castings

Because of its purification effect on castings, foam ceramic filtration technology has been widely used, and its filtration mechanism, filtration efficiency and application technology have been paid more and more attention. In this paper, the filter principle of foam ceramic filter materials and its application in castings are reviewed.

In the past decade, the proportion of automobile castings in the whole castings has been increasing, and the growth rate is expected to remain at about 10 ~ 15% in 2011. The booming Chinese automobile industry has brought more hope to the casting industry in China, especially the automobile casting industry. Compared with castings in traditional industries, automotive castings are more complex, with higher quality requirements and larger product batch. Therefore, casting equipment and facilities with a certain level of higher production efficiency must be used, and the investment of capital is also higher. With the continuous development of the automobile industry, as well as the requirements of energy saving and environmental protection, the quality of automobile castings has been put forward higher and higher requirements.

Slag inclusion defect is a very important type of automotive casting defects, accounting for more than half of the total defects. The standard of slag inclusion defect control is mainly based on the performance and use of castings. The more precise and harsh the working conditions of castings are, the more strict the restriction on slag inclusion defects. How to reduce the slag inclusion ratio has become an important problem in the production of automobile castings. With the continuous development of casting technology, filtration technology has gradually become an indispensable means to reduce the slag inclusion in automobile castings.

1. Development of filtration technology

As a part of the gating system, the main function of the filter is to prevent the inclusions in the metal liquid from entering the cavity and eliminate the inclusion defects. The use of filter pieces has been reported in the 1950s, but because the overall casting level is backward, the quality and cost of filter pieces and other factors, the dosage is relatively small. With the rapid development of Chinese automobile production enterprises, the output of automobile castings is increasing year by year. The casting quality is developing towards the direction of high quality, high efficiency, high performance and fine, and more and more attention is paid to the filtering technology. From the initial use of pure slag collecting bag to the use of simple coarse prototype porous filter, and then slowly transition to the fiber filter with better filtering effect, and then to the two-dimensional hole structure of straight hole ceramic filter material, and finally developed to the present three-dimensional structure of foam ceramic filter, with its advantages of high filtration accuracy, high through hole rate in the filter technology of automotive castings occupies a pivotal position. At present, there are three kinds of filters commonly used in casting production:

①Glass fiber filter sheet: made of high temperature resistant glass fiber and resin, can be larger than mesh inclusion filter, the effect is general, cheap, simple production process.

②Straight hole ceramic filter: by ceramic material pressing or extrusion molding, filtering effect between fiber filter and foam ceramic filter.

③ Foam ceramic filter material: about 1978 came out, began to be used for non-ferrous alloy, began to be used in 1985 for cast iron, cast steel, its production process to organic precursor as the skeleton, coated with high temperature material slurry, high temperature firing, has a good filtering effect. Moderate strength, the best filtering effect. At present, it is increasingly widely used in this casting industry, especially in automobile castings.

2. Characteristics of foam ceramic filter material

With its unique three-dimensional structure, foam ceramic filter material plays an irreplaceable role in improving casting quality and yield. Its basic functions can be roughly divided into two categories: filtration (removing impurities) and rectification. The diagram is shown in Figure 1.

2.1 Filtering

Filtration is the most important function of foam ceramics, which is mainly composed of three parts.

2.1.1 Floating separation

The molten iron injected from the pouring cup passes through the filter plate installed at the bottom of the runner or pouring mouth. Once the flow rate is slowed down, it will stay in the pouring mouth instantly, and the impurities with light gravity will emerge and separate, which will have the same effect as the bottom infusion and flow into the filter plate.

2.1.2 Surface filtration

No impurities float out of the molten iron. When the molten iron passes through the filter, impurities larger than the aperture will remain on the surface of the filter and be removed. Moreover, the captured impurities themselves also produce the function of a filter layer, which can also capture the impurities entering from behind, improving the filtering effect. However, the possible problem is that the filter layer on this surface will not only catch impurities, but also cause the mesh blockage of the filter, which has a negative effect on the pouring speed and the overflow rate of the filter.

2.1.3 Three-dimensional filtering

This function is the unique function of the foam ceramic filter, is other such as straight hole ceramic and fiber filter does not have excellent filtering structure. That is to say, in other filters on the above two parts can not capture the impurities will flow into the cast, but when the function of the above two parts can not remove the impurities through the tortuous three-dimensional internal space of the foam ceramic filter, and has a larger surface area of the filter skeleton repeated adsorption, will be more effective to block impurities, so that the filtering effect is improved again.