Although lost foam casting has been successfully applied in the actual production of iron castings, the cognition and understanding of our liquid metal filling process and filling law are far from enough. At present, the research on the filling process of lost foam casting process mainly focuses on the exploration and experiment of alloy casting, but there is little exploration and Research on the filling process of iron castings produced by lost foam casting. At present, there are few studies on the influencing factors of lost foam casting.
1.Influence of foam pattern casting on mold filling
Foam pattern is mainly influenced by foam material composition on casting mold filling. The heat required for decomposition and the product after decomposition are determined by the density and composition of the foams. The pyrolysis products and the heat absorbed during the pyrolysis of foam materials increase with the increase of foam density. The higher the pyrolysis temperature of the foam material, the faster the temperature drop of the mold filling front is, and the higher the air gap pressure between the metal liquid and the foam pattern, thus reducing the filling speed of the liquid metal. Because the density gradient of the foam pattern increases gradually from the inside to the inside of the foaming process, the shape of the filling front of the metal liquid is raised.
2.Effects of coating properties and coating thickness on mold filling of lost foam castings
The permeability of the coating affects the outward velocity of the gas in the gas gap at the front of the filling of the molten metal, thus affecting the filling performance of the molten metal. When the exhaust capability of the coating is poor, the velocity of gas removal through the coating to the cavity is much less than that of the foam pattern decomposition and gasification, thus increasing the air gap pressure of the metal filling front, resulting in the reduction of the velocity of the filling front of the liquid metal and hindering the filling process. If the air permeability of the coating is too low, it is easy to cause box collapse defects; On the contrary, if the coating has good air permeability, its filling process resistance is small, the filling speed is fast, and the liquid metal is easy to fill the cavity.
The thickness and strength of the coating layer are also one of the main factors affecting the liquid metal filling. The thicker the coating, the stronger it is. The main factor affecting the filling is the permeability of the coating. With the increase of the thickness, the permeability of the coating decreases gradually, and the air gap pressure at the front of the filling increases gradually, resulting in the decrease of the liquid metal filling ability; For the pattern with small coating thickness, the main factors affecting the liquid metal filling are the strength and thermal insulation of the coating. Appropriately increasing the thickness of the coating layer can improve the thermal insulation of the coating, so as to improve the liquid metal filling ability.
3.Influence of metal static head on casting filling
The influence of metal static head on liquid metal filling ability is realized by controlling the gating system. With the increase of the height of sprue, the liquid metal static head will also become larger and the filling speed will be accelerated. At the same time, it is also conducive to the discharge of gas between air gaps through the coating. But in the lost foam casting, the gating system is usually foam material. Therefore, before the foam is filled, the foam must be melted down and the foam will disappear. The increase of the runner height will lead to the decrease of the temperature at the front of the mold filling, which will affect the mold filling. Under different conditions, the contradictory positive and negative aspects of the sprue have different effects on the mold filling of the casting, so that the setting of the metal static head depends on other process conditions. In the mold filling process, when the liquid metal temperature is high and the superheat is large, the front effect of the sprue is the main; On the contrary, the negative impact is dominant. The existence of vacuum has a great influence on the filling of liquid metal. Therefore, in the presence of vacuum, the size of static head can not be used as an important standard for the process design of pouring process. Its design should be based on the fact that the sprue can ensure the sand consumption of the model.
4.Influence of negative pressure on mold filling capacity
Negative pressure is a method used to stabilize dry sand mold; It directly affects the escape speed of pattern degradation products in the process of lost foam casting filling, and can appropriately reduce the back pressure at the front of liquid metal filling. During the production process, the filling speed of high temperature molten metal is affected by the back pressure generated by the foam pattern gasification. In order to ensure the smooth filling of the metal liquid, the gas in the gas gap at the front of the mold filling can be continuously discharged through the molding sand. If the sand box is sealed and vacuumized during pouring, a positive pressure gradient will be generated from the inner surface to the outer surface of the molding sand, which is conducive to the escape of the gas generated in the mold cavity and reduce the gas back pressure at the front of the mold filling, so as to accelerate the filling speed of the liquid metal and improve the filling capacity of the liquid metal.
5.Effect of pouring temperature of molten metal on mold filling process of lost foam casting
The pouring temperature can determine the type of pyrolysis products, the number of various types and the discharge mode of gas. When the temperature is low, there are more liquid components in the degradation products. At this time, the main factor affecting the mold filling speed is the discharge speed of the degradation products, which is easy to cause defects such as cold lap, slag inclusion, wrinkle and so on; When the temperature is higher, the metal liquid has good fluidity and foam filling degradation is sufficient, which can effectively shorten the filling time. However, the temperature of molten metal can not be too high. When casting iron, the temperature of the foam is mainly gas, the temperature of the molten metal rises, and the formation of gas will also increase. However, the permeability of the coating is not affected by temperature. The speed of gas escaping has not been raised correspondingly, which will lead to the increase of air gap back pressure, and finally impede the continuous filling of molten metal. Moreover, it may cause back spray during pouring, slag inclusion, air gap and other defects in the casting. Therefore, the pouring temperature design needs to be determined according to the casting requirements and on-site production conditions.