Lost foam casting process for cast steel valve body

Lost foam casting is a negative pressure full mold casting technology developed on the basis of vacuum sealing molding combined with dry sand full mold casting. Its principle is shown in Figure 1. The foam pattern similar to the size of the casting is shown in Figure, which is made of pre foaming, curing and foaming, and then manually or mechanically bonded. The foam pattern coated with fireproof paint and dried will be put into the sand box, then filled into dry sand, and the sand will be compacted by negative pressure method and vibration. When molten molten metal is poured into the mold cavity through the pouring cup, the foam pattern begins to soften, melt and decompose quickly after contacting molten metal liquid, and its product escapes through the paint under the negative pressure of the sand box. The metal liquid instead of the foam pattern is solidified and cooled to form the desired casting.

The filling and solidification process of lost foam casting has an important impact on the internal and external quality of castings. For example, the common defects such as carburization, insufficient pouring, cold shut, air hole, shrinkage cavity and porosity in actual production are often formed in the filling and solidification process. Compared with the traditional sand casting process, there are complex physical and chemical reactions in the filling process of lost foam casting. Only a single foam pattern starts to soften, decompose and gasified and burn at the instant of contacting with molten metal, and at the same time, the decomposition products of liquid phase, gaseous phase and solid phase are formed. These decomposing products have great influence on the filling process of molten metal. Therefore, there are many factors that affect the quality of lost foam casting, such as the type and size of gating system, the setting of metal static pressure head, the pouring temperature of metal liquid, the foam material and density, the negative pressure, the permeability of coating and molding sand and so on.

At present, due to the deficiency of lost foam casting process design in the whole system theory and empirical formula, most of them refer to the design method of traditional sand casting process [3]. Many process designs rely on a large number of practical experience and tests to select reasonable pouring temperature, negative pressure, coating and other parameters. The design of pouring system and the layout scheme of riser chill can only be determined after repeated tests, which also consumes a lot of time cost and economic cost. The lost foam cast steel valve body studied in this paper is to formulate the casting process scheme according to the previous experience and repeated test method, which costs a lot of cost, but the produced castings still have serious shrinkage cavity and porosity defects, which can not ensure the casting quality.

ProCAST numerical simulation software is used to analyze the flow pattern, temperature field and velocity field of valve body casting during mold filling, find out the causes of shrinkage and porosity defects, improve the original process and carry out numerical simulation experiments to reduce shrinkage and porosity defects and improve the quality of castings. On the other hand, the numerical simulation test pouring by computer can shorten the trial production cycle of new products, save the cost, and promote the high-efficiency actual production of lost foam casting process.

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