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Precision casting is a casting method relative to the traditional casting process. It can obtain relatively accurate shapes and high casting accuracy. The more common practice is to first make an electrode of the desired blank (which can be left with very little or no margin), and then erode the mold with the electrode to form a cavity. Then the wax is cast by casting to obtain the original wax mold. The wax mold is then brushed with a high temperature resistant liquid abrasive in one layer. After obtaining sufficient thickness, it is dried and then heated up to dissolve the internal wax mold to obtain a cavity consistent with the desired blank. Iron is then cast in the cavity and the shell is stripped off after curing to obtain a precision manufactured product.

Modern investment casting methods

Lost wax casting, now called investment casting, is a kind of casting process with less cutting or no cutting, and is an excellent process technology in the foundry industry, which is very widely used. It is not only applicable to various types and alloys of casting, but also produces castings with higher dimensional accuracy and surface quality than other casting methods, and even complex, high temperature resistant, not easy to process castings that are difficult to be cast by other casting methods can be cast by investment casting.

Investment casting is developed on the basis of ancient wax casting. As an ancient civilization, China is one of the countries using this technology earlier, as far as several hundred years BC, China’s ancient working people created this lost wax casting technology, used to cast with a variety of fine patterns and words of the bell and the vessel and other products, such as the Spring and Autumn period of the tomb of Zeng Hou B Zun plate. The base of the tomb of Zeng Houbei is a number of intertwined dragons, which are connected at the beginning and end and interlaced at the top and bottom, forming a multi-layered cloud-like pattern hollowed out in the middle. These patterns are difficult to make with ordinary casting techniques, and with the lost wax casting process, the wax can be used to make use of the characteristics of no strength and easy carving, and the wax can be carved with ordinary tools to make crafts made of the same wax as the tomb of Zeng Houbei to be obtained, and then Additional pouring system, paint, dewax, pour, and then you can get the exquisite tomb of Zeng Houyi zun plate.

Modern investment casting method in industrial production was practically applied in the 1940s. At that time, the development of aviation jet engines, requiring the manufacture of complex shapes like blades, impellers, nozzles, etc., the size of precise and smooth surface heat-resistant alloy parts. Due to the heat-resistant alloy material is difficult to mechanical processing, the complex shape of the parts, so that can not or difficult to use other methods of manufacturing, therefore, the need to find a new precision molding process, so drawing on the ancient handed down lost wax casting, after the improvement of materials and processes, modern investment casting methods in the ancient process based on the important development. Therefore, the development of the aviation industry to promote the application of investment casting, and the continuous improvement and perfection of investment casting, but also for the aviation industry to further improve the performance of the creation of favorable conditions.

Pressed melt

China is in the fifties and sixties of the last century began to apply the investment casting in industrial production. Since then, this advanced casting process has been developed tremendously, successively in the aviation, automotive, machine tools, ships, internal combustion engines, gas turbines, telecommunications instruments, weapons, medical equipment and tools and other manufacturing industries are widely used, but also for the manufacture of arts and crafts.

The so-called investment casting process, simply put, is made of fusible materials (such as wax or plastic) fusible model (fusible mold or model), coated with a number of layers of special refractory coating, after drying and hardening to form a whole shell, and then use steam or hot water from the shell to melt off the model, then the shell in the sand box, filled with dry sand modeling around it, and finally the mold into the roaster after High-temperature roasting (such as the use of high-strength shell, can be modeled without the shell directly after the mold roasting), the mold or shell after roasting, in which the molten metal is poured and the casting.

Investment casting process

The dimensional accuracy of investment casting is high, generally up to CT4-6 (CT10-13 for sand casting, CT5-7 for die casting), of course, due to the complex process of investment casting, there are more factors affecting the dimensional accuracy of the casting, such as the shrinkage of the mold material, the deformation of the molten mold, the change in the linear volume of the shell in the heating and cooling process, the shrinkage of the alloy and the deformation of the casting in the solidification process, so ordinary Although the dimensional accuracy of molten die castings is high, its consistency still needs to be improved (the dimensional consistency of castings with medium and high temperature wax materials should be improved a lot).

When pressing the molten die, the cavity surface finish is used to press the mold with high surface finish. In addition, the mold shell is made of high temperature resistant special binder and refractory material formulated with refractory coating hanging on the molten mold, and the surface finish inside the cavity in direct contact with the molten metal is high. Therefore, the surface finish of molten die castings is higher than that of general casting parts, generally up to Ra.1.6~3.2μm.

The biggest advantage of investment casting is that due to the high dimensional accuracy and surface finish of investment casting, so it can reduce the machining work, only in the parts of the higher requirements of the parts can leave a little machining margin, and even some castings only left grinding, polishing margin, without mechanical processing can be used. Thus, the use of investment casting method can save a lot of machine equipment and processing hours, substantial savings in metal raw materials.

Another advantage of the investment casting method is that it can cast a variety of alloys of complex castings, especially high-temperature alloy castings can be cast. Such as jet engine blades, its streamlined outer contour and cooling with the cavity, with mechanical processing process can hardly be formed. Production with investment casting process not only can achieve mass production, to ensure the consistency of the casting, and avoid the stress concentration of residual knife pattern after mechanical processing.

The sprue cup is a basic component serving the casting technology, especially with the development of the steel industry has an extremely close relationship. The gate cup is one of the constituent parts of the smelting process equipment.

The gate cup is mainly used as a channel to guide the liquid metal into the cavity. If the arrangement of the gate cup is not reasonable, it may make the casting produce defects such as porosity, trachoma, iron bean, slag eye, shrinkage, crack and under-pouring. Therefore, it is very important to choose the correct structure and size of the gate cup and to place it reasonably on the casting.

Note: CT is the meaning of “casting tolerance”, general machining is not IT7, IT9 or something, that is the machining tolerance, CT is another set of data, the same as IT, is to the casting with it.

CT is the casting tolerance, IT is the ISO international dimensional tolerance, JT is the Chinese Ministry of Machinery dimensional tolerance

Casting process method Alloy Type Tolerance class (CT)
Sand hand shaping Cast steel 11-13
Sand hand shaping Grey cast iron 11-13
Sand hand shaping Ductile Iron 11-13
Sand hand shaping Malleable Iron 11-13
Sand hand shaping Copper Alloy 10-12
Sand hand shaping Light metal alloys 9-11
Sand machine molding and shell shape Cast steel 8-10
Sand machine molding and shell shape Grey cast iron 8-10
Sand machine molding and shell shape Ductile Iron 8-10
Sand machine molding and shell shape Malleable Iron 8-10
  1. Surface roughness

The surface of the part should indicate the grade of roughness. If more surfaces have the same surface roughness grade, it should be concentrated in the upper right corner of the drawing, and add the word “remaining”.

The choice of roughness grade, generally according to the work requirements of each surface and dimensional accuracy grade to decide, under the condition of meeting the work requirements, shall not arbitrarily improve the grade.

Casting process method Casting process method Alloy type Tolerance grade (CT) Sand hand shaping Cast steel 11-13 Sand hand shaping Gray cast iron 11-13 Sand hand shaping Ductile cast iron 11-13 Sand hand shaping Malleable cast iron 11-13 Sand hand shaping Copper alloy 10-12 Sand hand shaping Light metal alloy 9-11 Sand machine shaping and shell type Cast steel 8-10 Sand machine shaping and shell type Gray cast iron 8-10 Sand machine shaping and shell type Ductile cast iron 8-10 Sand machine shaping and shell type Malleable cast iron.

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