After the molten metal is poured into the mold, it usually produces volume shrinkage in the process of solidification and cooling, which may lead to shrinkage porosity and shrinkage cavity defects in the parts of nodular cast iron that solidify late, and greatly reduce the mechanical properties of nodular cast iron. The most common method of feeding nodular cast iron is to set riser. Use the riser to store the molten metal, and use its own gravity to make the molten iron replenish to the later solidified part to prevent shrinkage porosity and shrinkage defects. In addition, the riser also plays the role of exhaust and slag collection. The basic principles for designing risers are:
(1) The solidification time of riser shall be later than that of nodular cast iron;
(2) It can provide enough liquid metal for feeding;
(3) During feeding, there is always a channel between the part of nodular cast iron that needs feeding and the riser;
(4) The feeding pressure is large enough to ensure that the nodular cast iron is always in a negative pressure state during solidification.
The automobile hub support studied is a small nodular cast iron in wet sand mold. Nodular iron has large liquid and solid shrinkage in the solidification process, and has strong paste solidification characteristics. It is easier to produce shrinkage porosity and shrinkage cavity defects than gray iron castings. According to the previous practical production experience, the self feeding effect of nodular cast iron for hub support is poor, so it is very necessary to feed with the help of riser.
There are many types of risers, which can be divided into top riser and side riser according to the location of the riser; According to whether the top of the riser is in contact with the atmosphere, it is divided into open riser and concealed riser. Considering the structure and hot spot distribution position of nodular cast iron, the side concealed riser with convenient modeling, higher feeding efficiency and convenient later cleaning is selected to feed nodular cast iron. According to the principle of determining the riser position: make the nodular cast iron meet the sequential solidification, and use as few risers as possible to feed one nodular cast iron or even multiple hot joints on multiple nodular cast iron. A total of three concealed risers are set to feed two nodular cast iron. The molten iron is injected into the thick part of nodular cast iron through the riser, so that the riser not only has the effect of feeding, but also has the effect of slag collection and slag retaining.
The shape of the riser is spherical, cylindrical, waist cylindrical and so on. The shape of the riser directly affects its feeding effect. Generally, the smaller the surface area of the riser, the slower the heat dissipation speed and the longer the solidification time. Therefore, the most ideal shape is the spherical riser. However, the spherical riser will greatly increase the difficulty of mold lifting, and the hot joint at the top of nodular cast iron is strip, so the waist cylindrical concealed riser with lower cost is selected, and the schematic diagram of waist cylindrical concealed riser is shown in the figure. In order to keep the riser neck unblocked for a long time and enhance the feeding efficiency, the length of the riser neck should be shortened as far as possible and the section size of the riser neck should be gradually reduced to the direction of nodular cast iron.
According to the characteristics of paste solidification of nodular cast iron, the riser size is calculated by empirical proportion method. Since two side risers and one middle riser are designed in the scheme to feed two nodular cast iron, the size of a single riser does not need to be too large. The middle riser feeds two hot joints, so its diameter should be 1.05 ~ 1.3 times larger than that of the side riser, so it is taken as Dr = 55 mm and HR = 60 mm; Dr side = 50 mm, HR side = 70 mm. The size of riser neck is too large, which will produce new contact heat joints on nodular cast iron, making feeding difficult; If the size of the riser neck is too small, it will affect the feeding capacity of the riser. Therefore, a wide and thin riser neck is adopted. The length of the riser neck is L = 18.5 mm and the sectional area of the riser neck is 188mm2.