The sand mold sand core of tractor complex castings adopts the methods of increasing material manufacturing and reducing material manufacturing to realize dieless forming, so as to avoid the prominent problems such as high cost, long cycle and difficult manufacturing in the process of die forming. The following is the determination of forming methods for additive manufacturing and subtractive manufacturing.
(1) Determination of additive manufacturing method. The structure of sand core in complex castings is the most complex, which is suitable for additive manufacturing, and the sand core wall is thin, the size is small, and high strength is required. In this paper, Baozhu coated sand core is made by selective laser sintering machine and SLS method. This method has the disadvantages of slow forming speed and high cost, and has the advantages of high strength of sand core after forming. It can be used for dieless forming of small-size complex sand core. Figure 1 shows the formed coated sand core.
Laser sintering method of Baozhu coated sand core: the model formed by laser sintering needs to be exported to the selective laser sintering forming machine in “STL” format for continuous manufacturing without complex three-dimensional model design. When the bottom structure is scattered and the structure of adjacent layers is greatly different, the base and support need to be designed. If the raw material powder of coated sand is stored for too long, it will lead to insufficient forming strength. Therefore, the quality of raw materials should be controlled to avoid affecting the quality of sand core after forming. A coated sand is selected and tested for many times. The test parameters are determined as follows: in the continuous manufacturing process, the thickness of single layer can be set to 0.3 mm, the temperature of key layer is 75 ~ 80 ℃, the general layer is 70 ℃, and the sintering spacing is 0.15 mm. After the sand core is sintered and formed, due to the poor initial strength, it is necessary to carefully take out the coated sand in the equipment and clean the unformed sand inside and outside. The sand core formed by laser sintering needs to be strengthened to meet the needs of core assembly. After filling with glass beads, it is cured at a temperature of about 200 ℃.
(2) Determination of reduced material manufacturing method. Due to the low forming efficiency and high raw material price of complex sand core made by SLS method, it is also necessary to determine an efficient and low-cost dieless forming method to meet the forming needs of sand mold and large-size sand core. In this paper, the preformed sodium silicate sand block, furan resin self hardening sand block and triethylamine resin cold core box sand block are used as processing blanks, and the sand mold and large-size sand core are made by using mold processing forming machine and NC cutting technology.
Processing and forming method of sand mold sand core: carry out the processing tool path program design, and the processing design between the upper and lower sand molds and various parts of the sand core. After the sand core processing tool path is programmed, the core of complex parts can be processed in the sequence of M16 cavity milling → M8 rest milling → B6 zlevel profile. Generally, no allowance is set. This method has less processing times and high forming efficiency, Meet the processing requirements of parts with various shapes and sizes. Generally, the feed rate of flat head cutter is 2 mm, the feed rate of ball head cutter is 0.2 mm, and the parameters such as tool moving speed and spindle speed are set according to the type of sand blank. After the design is completed, the tool path is checked through 2D / 3D simulation. Figure 2 shows the design and inspection of machining tool path of a product.
Determine the size of the processing blank, and use the molding sand, binder, sand mixer and simple mold to prefabricate the sand blank as the processing blank. Place the processing blank on the equipment, after milling the surface to be processed, milling and forming according to the designed tool path. The processing setting process ensures that the processing starting point of the designed coordinate system is consistent with the actual processing starting point. The milling process of sand core of a product is shown in Figure 3.
After the sand mold (core) is processed, the dimensional accuracy is tested. Measured by the coordinate measuring instrument, the dimensional accuracy of the sand core of the complex casting of the tractor is within 0.5 mm, which meets the casting requirements.