Gravity casting is a manufacturing process in which molten metal is poured into the mold and cooled and solidified to obtain the required shape and properties. Casting is a common manufacturing method with low manufacturing cost and great process flexibility. It can obtain complex shapes and large castings. It accounts for a large proportion in mechanical manufacturing. For example, machine tools account for 60-80%, automobiles account for 25% and tractors account for 50-60%.
Several common pouring forms in gravity casting.
1. the top pouring system is short and simple, with small heat loss of molten metal, less metal occupied by the system, and reasonable cavity heat distribution, which is conducive to directional solidification and feeding of castings. However, the mold filling is unstable and easy to splash. With the increase of the casting height, the impact phenomenon is serious, which is unfavorable for exhaust, and the casting is easy to cause slag inclusion and air hole. Therefore, the top pouring system is basically not used in the gravity casting of aluminum alloy.
2. the impact phenomenon of mold filling in the middle pouring system is improved compared with that of the top pouring system, but the heat distribution of the system to the mold cavity is not as reasonable as that of the top pouring system. It is generally used in the production of small castings with a height of less than 100mm.
3. in the bottom pouring system, the metal liquid enters the mold cavity through the system under the action of gravity, and the mold is filled steadily from bottom to top, which is conducive to exhaust and skimming. However, the heat distribution in the mold cavity is extremely unreasonable, and the directional condensation sequence required for gravity casting cannot be realized. Castings cannot be adequately fed. Under the condition of enlarging riser, this method is widely used in small and medium-sized aluminum alloy castings.
When gravity casting is used, in order to reduce the impact, its sprue is often designed into serpentine and inclined bending shapes. In most cases, slag collecting ladle is added to replace the transverse sprue, resulting in too long route for liquid metal to flow, large heat loss, complex structure and high manufacturing cost. The heat distribution in the mold cavity can only make the casting solidify from top to bottom, which is just the opposite to the condensation sequence of gravity casting.
The casting is easy to cause cold shut and blocked feeding. Shrinkage holes are easy to occur in the middle and bottom, and pinholes are easy to occur in the upper part. Once problems occur, it is very difficult to adjust the mold. The common method is to increase the pouring temperature, increase subsidies, and increase the riser, which not only makes it difficult to guarantee the quality, but also causes a large amount of metal consumption. The process yield is generally less than 50%, and the average utilization rate of blank is only 65% - 70%.
