Gating system (running system) is the general name of the channel of liquid metal flowing into the mold cavity in iron mold sand coated casting mold. The gating system generally includes three parts (as shown in the figure): the gate cup, also known as the outer gate, mainly plays the role of receiving the metal liquid poured in by the ladle: the sprue is the part connecting the gate cup and the transverse sprue, which is used to introduce the metal liquid from the outside of the iron mold sand coated casting mold into the inside of the mold; The transverse sprue, which connects the sprue and the inner sprue, is used to distribute the metal flow from the sprue; The inner sprue connects the transverse sprue and the iron mold sand coated casting cavity, and the metal liquid enters the iron mold sand coated casting cavity. The function of the gating system is to control the mold filling speed and time of liquid metal, make the liquid metal enter the iron mold sand coated casting mold smoothly and quickly, avoid eddy current and turbulence, reduce the flushing of liquid metal flow on the iron mold sand coated casting mold, and prevent slag or other inclusions from entering the mold cavity.
Sand coated casting of iron mold is a very complex process, which can be divided into three parts: mold filling, solidification and cooling. The filling process has the greatest influence on the quality of iron mold sand coated castings. Because the flow and cooling process of liquid metal in the cavity are carried out at the same time, change at any time and affect each other, it is difficult to judge the changes of internal temperature field and flow field by intuitive experience. Research at home and abroad shows that most casting defects such as shrinkage cavity, porosity and porosity are caused by unreasonable design of gating system.
For iron mold sand coated casting, the iron mold has good cooling conditions and rigidity, which can make full use of graphitization expansion to form self feeding, so as to reduce shrinkage cavity and porosity defects. However, even so, it is difficult to avoid shrinkage cavity and porosity defects in some iron mold sand coated castings, such as thick wall castings. In recent years, people have done a lot of research from the perspective of the mechanism of shrinkage porosity. However, due to the very complex solidification and shrinkage characteristics, various influencing factors, and the complex and changeable material of iron mold sand coated casting, it is difficult to form a unified and comprehensive understanding of the formation mechanism of shrinkage porosity. An equilibrium solidification theory is widely used to solve the problem of shrinkage cavity and porosity, and has achieved good results in practical production. Balanced solidification is a solidification principle that makes use of the dynamic superposition of shrinkage and expansion and takes technological measures to make the shrinkage and feeding, shrinkage and expansion in proportion per unit time. This theory emphasizes the effect of white feeding, supplemented by external feeding, so as to reduce or eliminate the defects of shrinkage cavity and porosity. Shrinkage cavity and porosity are formed in the stage of liquid shrinkage and solidification shrinkage. Therefore, the application of equilibrium solidification theory can be realized on the premise of mastering the variation law of flow field and solidification field in the mold filling process of iron mold sand coated castings.
Gating system design has a great influence on the quality of iron mold sand coated castings and is one of the key factors to control the quality of iron mold sand coated castings. An excellent gating system should ensure the rapid and stable filling of molten metal; It is conducive to the concentration and elimination of slag and gas in molten metal; The temperature field distribution in the mold cavity is adjusted to make the iron mold sand coated castings cool and solidify according to certain solidification rules, which is conducive to the densification of iron mold sand coated castings. If the gating system is not designed properly, there may be defects such as air hole, slag hole, shrinkage cavity, crack, sand inclusion and slag inclusion, which directly affect the quality and yield of iron mold sand coated castings. Most iron mold sand coated casting defects are formed in the tens of seconds to a few minutes of pouring. J. Campbell et al. Studied the flow of liquid metal in the gating system, and the results showed that most of the casting defects were caused by the improper design of the gating system. At present, the research direction of gating system, on the one hand, it is necessary to analyze the flow state of liquid metal during mold filling and after entering the mold cavity, and eliminate severe turbulence and eddy current flow by optimizing the design of gating and riser, so as to avoid alloy oxidation and reduce the impact and erosion of liquid metal on the mold wall of iron mold sand coated casting; On the other hand, the temperature changes of liquid metal and mold during mold filling are analyzed, and the casting defects caused by pouring problems such as cold shut and insufficient pouring are predicted.