The structure of tractor castings is complex. For example, the transmission case shell part of chassis is a thin-walled shell part supporting and containing various internal mechanisms. It has the functions of sealing, dust prevention, heat insulation, sound insulation, storing lubricating oil and protection. It includes gearbox shell and rear axle shell. The structure is complex, with cavity inside and irregular shape. This structure shall be designed according to the following methods when designing the casting process and parting scheme.
1) The casting modeling is carried out by three-dimensional drawing software in order to carry out the CAD design of process and parting. Complex tractor parts generally have structural features such as assembly hole, oil drain hole and assembly surface. When modeling the casting structure, these structural features requiring post-processing shall be filled or machining allowance shall be set, and the model shall be scaled according to the material (cast steel, cast copper and cast iron shall be scaled according to the scale of 2% and 1%).
(2) The complex castings of tractor are generally cast iron, and the thin-walled shell parts are prone to cold shut, so a reasonable pouring system should be designed to fully ensure the mold filling speed, mold filling stability and feeding capacity of the system; Cast iron castings should be cast by side injection and flat casting process. Gate sockets shall be designed at the bottom of the sprue, and cold iron shall be set at the key parts that cannot be fed.
(3) The casting CAE simulation is carried out by using the casting simulation software, and the process is optimized through the analysis of casting defects. Figure 1 shows the process CAD design and CAE casting simulation results of a tractor box.
(4) After the casting process design is completed, reverse the mold and carry out parting design. The process of parting design is to divide the whole mold into sand cores, so as to select appropriate methods to realize dieless forming. At the same time, the interference of each sand core in the assembly process should be fully considered. The parting design of tractor complex castings follows the principle of “less parting and less assembly”, which is generally divided into upper and lower sand mold and internal sand core. According to the needs of equipment forming size, the sand mold and sand core may need to be further divided. Figure 2 shows the parting design scheme of a gearbox shell of a tractor, which is mainly divided into three parts: upper and lower sand molds and large sand core in the middle. The internal radial structure of the complex sand core belongs to the blind area of processing and forming. Therefore, the movable block division of these structures can reduce the difficulty of forming.
(5) Sodium silicate sand and furan self hardening resin sand can be selected for sand mold and thick wall sand core after parting, and triethylamine self hardening resin sand and Baozhu coated sand can be selected for thin-wall complex sand core. Because the poor exhaust of the internal sand core will lead to defects such as porosity and suffocation in the casting, this problem can be effectively solved by increasing the exhaust channel in the parting process. Figure 3 shows the design of the exhaust passage inside the water jacket core of a cylinder head of a tractor.
(6) Fig. 2 positioning module shall be designed during parting, such as reserving positioning holes or positioning with the help of parting features. As shown in Figure 2, the positioning hole structure is reserved for the casting of a tractor box during the parting process, so as to ensure the assembly accuracy during the core assembly molding process.