The filling process ofis different from the traditional filling process of hollow sand mold gravity casting. Complex physical and chemical actions will occur at the front of liquid metal filling, including heat transfer, flow, chemical reaction, solidification and so on. The filling process of liquid metal is more complex. The mold filling characteristics of lost foam casting mainly show in several aspects:
1) The mold filling mode and liquid metal flow pattern of lost foam casting are obviously different from those of cavity casting. In cavity casting, liquid metal filling is greatly affected by gravity and fills the cavity from bottom to top; In the process of lost foam casting, the front of the liquid metal flow is always radiating from the inner gate to the far end, forcing the foam to disappear at one level and finally reaching the far end of the gate. And for the dry sand negative pressure lost foam casting, due to the existence of negative pressure, it may cause the metal liquid to move along the mold wall in the mold filling process, that is, the wall attachment effect. Too high negative pressure may cause serious wall attachment effect, and then lead to defects such as pores and slag inclusion, which needs to be paid attention to.
2) The fluidity of liquid metal in the filling process of lost foam casting is significantly different from that of cavity casting. Due to the existence of foam pattern, foam degradation will take away a lot of heat, deteriorate the fluidity of molten metal, and then lower the filling ability of lost foam casting liquid metal, resulting in defects of insufficient casting and wrinkling. In view of the characteristics of lost foam casting, we can take measures such as raising pouring temperature and negative pressure, decreasing foam density, selecting high quality coatings, optimizing gating system and so on to improve the filling ability of molten metal.
3) The factors affecting the filling speed of lost foam casting are very different from that of cavity casting. The mold filling ability only reflects the pouring result, and the mold filling speed is directly related to the pouring process. Understanding the pouring process is convenient for us to directly design and adjust the pouring system. The process factors affecting the filling speed of lost foam casting mainly include pattern material, coating, pouring temperature, metal static head and negative pressure, etc
Due to the existence of pattern material, the filling speed of molten metal is limited by the degradation speed of pattern, which is much lower than that of cavity casting. The density and type of the pattern, the total amount of foam and even the different manufacturing processes will affect the filling speed of the lost foam casting.
The coating is not only the skeleton of the stable pattern, but also the escape channel of the degradation products of the pattern. The permeability of the coating has a great influence on the filling speed. The appearance determines the quantity and property of degradation products, and the coating determines the escape mode and speed of degradation products. The interaction between the two is the dominant factor affecting the filling of lost foam casting. The existence of negative pressure may weaken the influence of the latter due to its different types and properties.
The pouring temperature directly affects the degradation degree of the pattern. When the temperature is low, there are more liquid components in the degradation products. At this time, the filling speed is mainly limited by the escape speed of the degradation products; When the temperature is high, the fluidity and filling ability of the molten metal are improved, the foam degradation is also sufficient, and the gas products are more. However, there is an air gap between the liquid metal and the mold at the front of the filling mold. The increase of gaseous products leads to the rapid increase of the air gap pressure, and finally hinders the continuous filling of the molten metal. In lost foam casting production, when the gating system is not designed properly, it is easy to have “back spray” phenomenon, which is caused by too much instantaneous gas generation at the beginning of mold filling. The adjustment of the size of metal static head is the most direct way to affect the filling speed by adjusting the gating system. Appropriately increasing the static head has a beneficial effect on improving the filling speed of lost foam casting.
Negative pressure exists in the lost foamfirst because it is used to stabilize the dry sand mold by using V-method casting for reference, but it also directly affects the escape speed of pattern degradation products in the lost foam casting filling process, which can relatively alleviate the flow blocking effect of air gap pressure at the front of the filling process. Comprehensively considering the existence of negative pressure can improve the filling speed of lost foam casting.
The filling process of lost foam casting is complex and difficult to observe. Scholars at home and abroad have done a lot of basic research to reach the above consensus, but the further understanding of lost foam casting is still very important. In the long process of theoretical exploration, numerical and modeling treatment of complex practical problems is a good way to solve these problems. Therefore, lost foam casting simulation technology came into being and is favored in the basic research of lost foam casting process characteristics. Casting simulation software is used to assist in the heat transfer and The research on mass transfer and solidification characteristics will become the trend in the future.