Lost foam casting is also called dry sand or negative pressure full mold casting, and its casting principle is shown in the figure. The foamed beads were made of pre foaming, aging and foaming molding, and the foam shape similar to the shape and size of the casting was produced. The shape and shape of the complex foam pattern were made by manual or mechanical bonding. The foam pattern or pattern cluster is coated with fire-resistant coating and dried, then put into sandbox and filled with dry sand without binder. The sand is compacted into mold by vibration and negative pressure. The molten metal is poured into the mold, and the foam pattern meets the high temperature molten metal rapidly softening, melting, gasification and decomposition. Under the negative pressure, the decomposed product is discharged into the mold through the paint. The molten metal is filled into the mold and completely occupies the position of the original plastic pattern. After solidification and cooling, the required castings are formed.
The filling and solidification process ofis not only related to the success or failure of casting forming, but also has an important impact on the internal quality of castings. For example, the common defects in production, such as carburization, wrinkle, slag porosity, insufficient pouring, mold collapse and so on, are often formed in the process of casting filling. Compared with the traditional , the filling process of lost foam casting is more complex, and there are complex physical and chemical reactions in the flow front of liquid metal. The foam pattern is softened, melted, vaporized and burned under the thermal action of the hot metal liquid at the front of the flow. All kinds of liquid and gaseous pyrolysis products are discharged into the mold through the coating. Therefore, the influence factors of mold filling process in lost foam castings are more. Besides the influence of gating system shape and size and metal static pressure head in traditional , it is also influenced by pouring temperature, foam pattern density, negative pressure, paint and molding sand permeability, such as schematic diagram of metal filling process.
Up to now, there is no final conclusion on the determination of the element size of the gating system in lost foam casting, and the research is not enough. There are great differences in the opinions of each family. The process design of lost foam casting is lack of systematic theory and empirical formula. At present, the design method of traditional sand casting process is mostly used in lost foam casting process design. The existence of energy, mass and momentum transfer process at the front of liquid metal flow makes it more difficult to produce and control casting defects. It is difficult to obtain high-quality castings by using the process design theory of ordinary sand casting.
Generally speaking, for the newly put into production lost foam castings, the parameters such as pouring temperature, pattern density and coating selected by engineers and technicians based on experience and the designed pouring system and riser scheme must be tested and modified repeatedly before the final optimization and determination of the casting process can be completed. The trial production often takes several months or even longer. Even so, many castings can not be successfully produced by lost foam casting, which brings great risks to the foundry enterprises using lost foam casting technology.
The nodular cast iron shell produced by lost foam casting technology in a factory adopts the traditional “experience + experiment” method for process design, which consumes a lot of manpower and material resources. The products always have shrinkage cavity and porosity defects, which makes it difficult to ensure the quality of castings.
By using the numerical simulation technology of lost foam casting, we can have a clear understanding of the flow pattern of molten metal and the distribution of temperature field in the solidification process of nodular iron shell casting. On this basis, we can analyze the causes of shrinkage porosity and shrinkage cavity, and then guide the production. Through repeated “computer trial pouring” and comparative analysis, the pouring temperature, vacuum degree, pouring system and riser scheme are improved to obtain a reasonable production process.
Numerical simulation of lost foam casting not only shortens the trial production cycle of new products, saves human and material resources, but also improves the casting quality and process yield, makes the products more competitive in the market, and brings great economic and social benefits to the casting industry.