? drum is one of the main mechanical castings of gantry crane equipment. Its material is HT200. The blank dimensions are φ528 ×1 660(mm), the wall thickness is 36 mm, and the weight of a single piece is 950 kg. There is an inner flange 100 mm away from each end. The outer circle, end face, and inside of both ends to the flange need to be machined. The original step gating system was used, and the defects such as sand holes and slag holes were often produced in the lower flange (see Figure 1). These defects are not exposed until the processing is in place, not only the castings are scrapped, but also a large amount of processing hours are spent. To solve this problem. The step pouring system was changed to the top pouring rain pouring system, and good results were obtained.
1 Process plan analysis
The original process is manual modeling, clay sand dry type, axial symmetry parting, flat vertical pouring, step pouring system. The inner runner of the stepped pouring system is divided into upper and lower layers, and the bottom runner is opened in the middle position, so it has the role of middle injection and top injection, that is, the liquid metal can be relatively smooth into the mold cavity, so that it can fill the mold cavity from bottom to bottom, which is conducive to the sequential solidification and self-feeding of the casting. However, due to the high position of the inner runner, the erosion of the sand core is large (because the liquid flow is vertically directed to the sand core), which is easy to cause the surface coating of the sand core to fall off; In the early stage of pouring, the sand mold temperature is low, the hot metal is cooler, and the inclusions are not easy to float. At the inner flange at the bottom of the drum, the scum and inclusions floating to the hot metal surface at the mold wall are easy to "stick" on the mold wall and cool rapidly, thus forming defects such as slag holes and sand holes at the flange.
In the top pouring rain pouring system, the liquid metal first enters the annular cross runner at the top of the casting through the sprue cup and the straight runner, and then continuously falls into the cavity through the dispersed small inner runner holes, the mold is gradually filled from the bottom up, and the higher temperature metal liquid is always on the liquid surface, which strengthens the bottom-up sequence solidification of the casting. In other words, the top injection is conducive to dynamic superposition of expansion and contraction. The molten iron poured in first falls to the bottom of the cavity, and the contraction during cooling is supplemented by the molten iron poured in later from the top. After the completion of pouring, the graphitization expansion generated during the solidification of the lower part can just compensate for the contraction of the molten iron in the upper part, so that the contraction of the molten iron in the upper part is relatively backward, and the graphitization expansion generated by the solidification of the lower part is relatively advanced. The equilibrium point is moved to the end of the pouring time, so that the casting can get a good self-feeding shrinkage. A number of small rain gate is a small section to control the flow, forming a closed pouring system, has a good slag blocking effect, so that the slag particles float in the cross runner and not easy to enter the cavity. The metal liquid is divided into small flow filling cavity, which has little impact on the mold. The continuous falling of small streams makes the liquid level active and plays a stirring role, so that the gas in the hot metal is easy to escape without forming pores, so that the inclusions such as slag and sand are not easy to adhere to the cavity wall, but float up to the riser with the liquid level of the cavity, so as to ensure the quality of the casting.
2 Top pouring rain pouring system design
The rain-poured pouring system is mainly composed of rain-poured inner runner, annular cross runner, straight runner and gate cup, as shown in Figure 2.
2.1 Proportion and dimensions of each component
In order to strengthen the slag retaining ability, a closed gating system is adopted.
In ΣF: F cross: F straight =1∶1∶1.5 In order to prevent high pressure hot metal from spraying into the mold cavity, the empirical formula of in ΣF: F cross =1∶1 is taken according to the rain gate
We get: ΣF =18.49 cm2 considering various factors, ΣF =20 cm2. In order to rationally distribute the flow rate of molten iron, control the temperature distribution during solidification, and prevent local overheating, the number of inner gates is selected to be 12, with a diameter of 15 mm.
The selected ratio gives us that F cross =20 cm2.
In order to improve the slag retention ability of the runner, the cross-section size is 40×50 (mm). F straight =30 cm2, section size is 50×60 (mm).
The production has verified that the top rain pouring system is suitable for the drum parts, and the dimensions of each element of the pouring system designed according to this ratio are correct and feasible, especially the section of the inner runner is similar to that of the cross runner, which reduces the pressure of the liquid flow in the inner runner and prevents the splashing of the metal liquid when entering the cavity.
2.2 Riser setup and size
The top rain pouring system makes the hot metal solidified with a good temperature distribution, the casting has a certain self-feeding, and the risers only need to feed the difference part of the insufficient self-feeding. The annular top riser is selected for feeding and collecting slag, and is also used as the process head. The thickness of the top riser is 71.5 mm(equal to the thickness of the inner flange) and the height is 160 mm. In order to discharge the gas in the mold cavity in time, a side riser of 30×60 mm is opened.
The pool gate type pouring cup is selected during pouring.
3 Conclusion
For long barrel gray cast iron, the top rain pouring system is superior to the step casting system. The cross-sectional area of the top pouring system is equivalent to the total cross-sectional area of the inner pouring system. It can reduce the pressure of hot metal entering the cavity, prevent splashing, and facilitate smooth filling.
In recent years, we have used this process to produce a series of drum castings, and have achieved good results.
