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In recent years, with the development of science and technology and the requirements of industrial production, the quality requirements for casting alloys have become increasingly strict. Some castings not only require qualified chemical composition and mechanical properties, but also excellent internal and external quality. However, traditional smelting and casting processes can no longer meet these requirements. The main reason is that non-metallic inclusions in the alloy exceed the allowable range. In order to reduce the influence of non-metallic inclusions, strict requirements have been put forward for the raw materials used in alloy preparation, and measures have been attempted in casting processes. Casting filtration technology is a new technology developed in this situation.

Precision castings typically require higher standards. When the casting cavity is filled with liquid, during the solidification process, the slag cannot be discharged, and coupled with the accelerated cooling rate, it has already solidified during the slag floating process, resulting in slag holes and scrap of the casting. Using ceramic foam filter during pouring can significantly improve the purity of molten metal and reduce casting defects. Filtration technology has been widely used in foreign investment casting. Some factories in China use ceramic foam filters made of cast steel. In fact, for castings with high requirements, the economic benefits of using foam ceramic filters (increase of casting output, saving of processing and repair costs, reduction of claims, etc.) far exceed the cost increase of castings produced by filters themselves.

Types and characteristics of foam ceramic filters

According to usage conditions, filters can be divided into three types: cast steel, cast iron, and cast aluminum. We mainly introduce cast steel filters. The foam ceramic filter for cast steel is mainly made of zirconia and carbon materials, both of which have good high temperature resistance and have been verified by years of production practice.

Zirconia ceramic filter material is made of high-purity zirconia as the base material, sintered at high temperatures above 1600 ℃, with stable texture, high strength, and good thermal shock resistance. At the same time, they do not react with any metal solution at high temperatures and can withstand the impact of 1700 ℃ molten steel. Therefore, zirconia ceramic filters are suitable for various steels, including carbon steel, stainless steel, cobalt based and nickel based high-temperature alloys, as well as large cast iron parts. In addition, this type of filter also has good application advantages in investment casting processes.

Selection of foam ceramic filter

When using the foam ceramic filter, the filter aperture (PPI per inch of product) should be determined according to the quality requirements of the casting and the number of inclusions in the liquid metal. The aperture of cast steel filters is mainly 10ppi and 15ppi, while products with specifications of 20ppi and 30ppi are less commonly used. The thicker the filter, the stronger the filter, and the higher the filtering efficiency of the filter, but the higher its usage cost. Considering the strength and filtration efficiency of * * *, it is generally recommended to have a size of 60mm, a thickness of 20-22mm, a thickness of 60-100mm, a thickness of 22-25mm, a thickness of 100-150mm, a thickness of 25-30mm, a thickness of 150mm, and a thickness of 30-40mm.

Placement of filters

Precision castings usually have complex shapes, which pose difficulties to the casting process. When using filters, they are mostly placed at the gate position. In practical applications, the following methods can be used according to different castings and requirements:

Keep a filter stand or use a conical filter in the pouring cup of the casing. After the furnace shell is baked out of the oven, the filter is quickly placed at the gate position, as shown in Figure 1. Alternatively, the same shell can be baked and preheated before pouring to facilitate the passage of liquid metal and achieve better results, as shown in Figure 2.

In order to make the filter easy to place, a filter base needs to be designed, with a gap of 2 mm between the filter and the outer edge of the filter. The filter should be placed at the * * position in the housing, and a base with a width greater than 5 mm should be left on the support surface, as shown in Figure 3. The filter can also be made into a circular shape that matches the gate. It can be wrapped in fiber cotton and placed directly at the gate position, as shown in Figure 4.

The filter is placed in a special ceramic sprue cup. When making the shell, the pouring cup is connected to the casting tree. When pouring, pour the filter into the pouring cup, as shown in Figures 5 and 6.

Place the filter in the pouring system, make a pouring system housing with the filter in advance, and reserve openings in the corresponding positions of the housing so that the filter can be placed on the mounting seat. The shell making table is placed inside the base, sealed with water glass or silica sol, cured and dried, and then subjected to pouring test.

In addition, the pouring temperature of the molten steel should be controlled within the upper limit of the process range, and the pouring speed should be increased as soon as possible to avoid casting defects such as cold shut and insufficient pouring.