John Campbell of the University of Birmingham and others have carried out a lot of research work on the design of gating system, and found that smooth mold filling is the basic condition to ensure the quality of sand mold castings, and the core of gating system design is to control the mold filling speed of sprue.
During the filling process of liquid metal, the front end of liquid metal is surrounded by a layer of oxide film. With the advancement of liquid metal, the oxide film continues to move to both sides. The oxide film moving to both sides becomes the skin of. At the same time, new oxide films continue to form to ensure the continuity of the whole process. As shown in the figure, the force balance diagram of liquid metal free surface during mold filling is given.
When the free surface becomes hemispherical with radius r, the surface tension reaches the maximum value of 2 γ/ r。 When the fluid flow velocity reaches a specific value, the internal pressure of liquid metal is balanced with the maximum free surface tension ρν 2/2=2 γ/ r. The flow velocity of liquid metal at this time is called I critical velocity ν c。
Where: ν C is the critical velocity, M / S; P is the density of molten metal, G · cm-3; γ Is the free surface tension of molten metal, N / m.
When the flow rate of liquid metal inis less than the critical speed, the surface tension is greater than the internal pressure of liquid metal, the liquid metal flows smoothly in the mold filling process, and the surface oxide film will not crack, that is, the oxide film will not be involved in the liquid metal to form oxide slag inclusion. When the flow rate of sand casting liquid metal is greater than or equal to the critical speed, the internal pressure of the liquid metal is greater than the surface tension of the liquid metal, that is, the sand casting liquid metal will break through the metal film on the surface, the surface oxide film covering the liquid metal will break and fold, and may be involved in the liquid metal to form oxidized slag. As we all know, the existence of oxidation slag will seriously endanger the mechanical properties of materials, especially the fatigue properties.
Critical velocity of liquid aluminum alloy ν C is 0.4 ~ 0.6 m / s, the critical flow rate of steel and superalloy ν C is 0.5m/s. J. Professor Campbell found that the mold filling speed has a great impact on the quality of sand castings when studying the anti gravity pouring of cylinder head. When the mold filling speed is 0.3 ~ 0.5 m / S (flow rate of inner gate), not only the ideal sand castings can be obtained, but also the mechanical properties of sand castings can be improved. When designing the cylinder block of Cummins n series engine, Zong Zhenhua and others adopted the choke section to control the mold filling speed at about 0.335 M / s, which effectively improved the quality of sand castings. Researcher Li Dianzhong of Institute of metals, Chinese Academy of Sciences pointed out that the mold filling speed of gray cast iron should be limited to 0.5 m / s. The Japanese scholar itamura et al. Studied the mold filling speed of pressure sand casting by experiment and computer simulation. It shows that when the mold filling speed is less than 0.5 RN / s, the ideal sand casting can be obtained. When the mold filling speed is greater than 0.5m/s, the quality of sand casting will be reduced, and defects such as porosity and slag inclusion may appear. A large number of studies show that the critical filling velocity of common liquid metal is about 0.5m/s.