In order to obtain high-quality sand castings, both at home and abroad pay great attention to the design of gating system of various alloys, including the structural form, section proportion, introduction position and introduction method of each element, so as to control the flow rate and flow state of molten metal into the mold cavity and reduce sand holes, sand sticking, slag inclusion, pores, cold septum, iron beans and shrinkage cavities caused by improper gating system design Shrinkage porosity, deformation and other sand casting defects. However, how to design a reasonable gating system for different sand castings is still an urgent problem to be solved. In actual production, various quality problems of sand castings are often caused by unreasonable design of gating system. With the development of science and technology and the improvement of the quality requirements of sand castings, the influence of gating system on the quality of sand castings has attracted great attention of sand casting workers, and a lot of research has been carried out, especially the formation mechanism of secondary oxidation slag inclusion in the pouring process of sand castings.
(1) The structure of each component of the gating system shall be simple and smooth to make the liquid metal flow as smooth as possible. Generally speaking, slag retaining measures must be considered for the gating system, such as setting filter screen, but setting too many and complex slag retaining measures can play a role in slag retaining to a certain extent, but it will also aggravate the disturbance and turbulence of liquid metal, produce secondary oxidation slag inclusion, and increase the risk of secondary oxidation slag inclusion defects of sand castings.
(2) Closed or semi closed gating system shall be adopted, and the minimum section shall be set at the connection between transverse sprue and inner sprue. In the early stage of pouring, the pouring system can be closed quickly and play a good slag retaining effect.
(3) The mold filling speed of liquid metal shall meet the requirements of critical mold filling speed as far as possible. When it is impossible to meet the requirements, the reduced pressure inner gate shall be used to make the metal flow fill the mold smoothly, avoid the divergence of liquid flow, spray and splash of metal liquid during mold filling, and minimize the generation of secondary oxidation slag inclusion.
With the improvement of the quality of sand castings, especially the internal quality requirements, the influence of mold filling speed on the quality of sand castings is more and more obvious. The research shows that when the mold filling speed is greater than the critical flow speed of liquid metal (generally 0.5 m / s), the liquid metal produces turbulence, the oxide film on the surface of liquid metal breaks, and the secondary oxide film is involved in the liquid metal, forming internal defects of sand castings, which seriously reduces the mechanical properties of sand castings. Therefore, the filling speed of liquid metal must be strictly controlled in the design of gating system.
When the closed gating system is used for large sand mold castings, it is beneficial to improve the slag retaining effect of the gating system. However, the pressure of molten metal in the sprue is large and the filling speed is fast. The molten metal is easy to diverge and spray, and it is easy to produce secondary oxidation slag inclusion. Therefore, the decompression inner gate must be designed to make the liquid metal fill the mold smoothly.