The design steps of sand casting gating system are similar to those of general iron castings, but they also have their own characteristics. The design contents and steps of general sand casting are roughly as follows:
(1) Selection of casting type and alloy pouring system;
At present, the common gating systems in China are classified into top injection, bottom injection and side injection according to the position of liquid metal flowing into the cavity. A large number of studies have shown that the bottom injection gating system is a more reasonable gating system. According to the cross-sectional area score of each component, it is divided into closed gating system, open gating system, semi closed gating system and closed and open gating system. The closed gating system (a rod > a straight > a transverse > a inner) has good slag retaining capacity, can prevent liquid metal from being involved in gas, and is easy to clean, but the speed of liquid metal entering the cavity is high; The open gating system (a cup < a straight, < a transverse < a self) has stable mold filling, light oxidation and low scouring force, but the liquid metal can not fill the sprue quickly, which is suitable for steel castings and non-ferrous alloy castings; The semi closed gating system (a-inside < a-straight < a-transverse) has stable mold filling and certain slag retaining capacity, but the excessive transverse sprue is filled slowly, which provides space for disorder. It is suitable for gray iron parts and nodular iron castings; Closed and open gating system (inside a < a transverse < a straight) runner is used as the flow control section. The sprue is too large to be filled quickly, and the runner still cannot be filled quickly. It is suitable for iron castings and aluminum alloy castings.
(2) Determination of pouring position;
The pouring position of sand casting refers to the position of the casting in the mold during pouring. The choice of pouring position is an important issue in the process design of sand casting.
(3) Determine the position, number and metal introduction direction of the inner sprue on the sand casting;
The function of the inner sprue mainly includes controlling the speed and direction of the liquid metal entering the mold cavity, distributing the liquid metal, and adjusting the temperature field distribution and solidification sequence in the sand casting. In addition, the molten metal of the gating system can feed sand castings through the inner gate. Therefore, when designing the ingate, attention should be paid to the position, number and metal introduction direction of the ingate on the sand casting, and the flow rate should be controlled to avoid spraying and splashing when the metal liquid flows into the mold cavity, so as to make the mold filling process fast and stable.
(4) Select the position and height of the sprue (generally the height of the upper iron mold).
Sprue height directly determines the size of filling head in gravity casting. The larger the pressure head of liquid metal in the flow direction, the better the filling capacity. If the sprue is too low, the mold filling and liquid feeding pressure is insufficient, and it is easy to have defects such as unclear edges, corners and contours of sand castings, pouring failure, concave shrinkage on the upper surface and so on; The higher the sprue height is, the better. If the sprue height is too high, the filling head will increase, which will lead to certain mold wall migration, affect the dimensional accuracy of sand castings, and cause alloy waste.
(5) Calculate pouring time t;
Pouring time is an important process parameter affecting the quality of sand castings. The determination of appropriate pouring time depends on the structure of sand casting, alloy type, mold process conditions and gating system type. Different sand castings have the most suitable pouring time range under the determined process conditions. The suitable pouring time is a range. When the process parameters of sand casting are in the best combination state, it must correspond to an optimal pouring time.
Fast pouring can reduce the defects of sand inclusion and scab; For gray iron and nodular iron castings, rapid casting can make full use of the self feeding effect of eutectic expansion to eliminate the defects of shrinkage cavity and porosity. The disadvantage is that it has a great impact on the mold wall, which is easy to cause defects such as sand expansion, sand flushing and box lifting; The gating system consumes a lot of metal and the process yield is low.
The metal liquid in slow pouring process has light scouring effect on the mold wall: it can prevent sand expansion, sand flushing, box lifting and other defects; It is conducive to the discharge of gas in the mold cavity and core; The gating system consumes less metal. The disadvantages are: the baking time of the metal on the upper surface of the mold cavity during pouring is too long, which leads to defects such as sand inclusion and scarring: the temperature and fluidity of the liquid metal are greatly reduced, and it is easy to have cold shut, pouring failure and skin wrinkles of sand castings; Reduce the production efficiency of the molding assembly line.