Original technology and existing problems of rapid casting of tractor gearbox shell

1. Casting process

In order to ensure the smooth filling of molten iron and prevent the occurrence of defects such as porosity and shrinkage porosity, a closed gating system is adopted. The cross-sectional area of each gating system is shift F, straight shift F, transverse shift F, in = 1.63 ° 1.5 ° 1. The casting has a large flange higher than the overall plane. The pouring process is simulated by CAE simulation software. It is easy to have shrinkage porosity and shrinkage cavity at the thick part of the casting and the higher part at the top of the large flange. Therefore, three feeding open risers are placed at the part where shrinkage porosity and shrinkage cavity occur during simulation.

The length of the casting is long, and the maximum length dimension can reach 949 mm. The total weight of the sand core blank required in the production and manufacturing process is about 750 kg, and the sand core structure is complex. Some processing areas are deep (the depth is 114 mm), which makes the ball head tool (the tool length is 100 mm) for finishing unable to reach its processing depth. In order to solve these problems of machining sand core, the “segmented core” parting scheme is adopted to divide the sand core into three parts. The parting scheme designed on UG nx8.0 software is shown in Figure 1. The movable block and patch produced by parting design are formed by laser sintering machine.

The core of the gearbox shell is prepared as a blank sand block with acid hardened furan resin, and then the front and back sides are processed on the five axis digital Moldless precision molding machine to finally process the sand core required for molding (Fig. 2). The upper and lower molds of the gearbox shell are molded with water glass mixed sand to prepare the blank sand block, and then processed on the three-axis digital Moldless precision molding machine to finally process the upper mold and lower mold required by the group (Fig. 2). Most of the movable blocks and patches produced by parting have multi-faceted structure, and very few even need six faceted processing, which is difficult for material reduction manufacturing. Therefore, these movable blocks and patches are uniformly formed by laser sintering molding machine. This production and manufacturing mode of simultaneous operation of five axis digital modelless precision molding machine, three-axis digital modelless precision molding machine and laser sintering molding machine reflects the advantages of short cycle and quick effect of rapid casting in the casting production process to the greatest extent. The lack of operation of digital modelless precision molding machine or laser sintering molding machine will greatly affect the cycle of mold processing, Then the production cycle of rapid casting will also be delayed. After all the sand molds and sand cores are processed, group the mold, lower the core, check the skin thickness and buckle the box. The sand mold is positioned with positioning pins, surrounded by sand boxes around the periphery, and fixed and reinforced with pressure iron at the top. The pouring temperature is 1420 ℃, and the pouring time is 84 s.

2. Problems after mold formation and casting forming

As the “segmented core” parting process scheme is adopted to divide the sand core into three parts, and some movable blocks and patches that need to be bonded to the sand mold or sand core in the later stage are produced in the parting design, the assembly accuracy of the parts with assembly relationship and the parts that need to be bonded during core assembly is inappropriate. The sand mold made of reduced materials is detected by the coordinate measuring instrument The dimensional error of (core) is about 1.5 mm. After bonding, the cumulative dimensional error of sand mold (core) is expanded to about 2 mm, which may lead to the final dimensional error of casting of 1.5 ~ 2 mm.

(a) External mold (b) Sand core

After shot blasting, the casting does have dimensional error, with the error value of 2 mm, which affects the qualified rate of the product to a certain extent. After further detection, there are many pores on the top surface of the casting and the surface quality is poor.