1. Casting analysis and sand casting process selection
Figure 1 shows the 3-cylinder block of passenger car. The sand casting material is HT250 and the overall dimension is 356mm × 307mm × 267mm, basic wall thickness 4mm, blank weight of sand casting 32kg. The casting has complex internal structures such as water jacket and horizontal hole. Sand mold core assembly gravity pouring process is selected in the trial production stage.
2. Design of gating system and sand casting process parameters
Combined with the traditional casting process, the sand casting adopts horizontal pouring, and the specific pouring system design is shown in Figure 2. The pouring system of sand casting adopts two linear runner, the middle runner and the lower runner. The runner turns into the flow blocking surface of the bottom runner at the end face of the casting, and then is divided into two runner at the bottom and the middle. Three ingates are led out from the middle runner to connect with the bearing bush seat of the cylinder block, and four ingates are led out from the bottom runner to connect with the crankcase flange surface of the cylinder block. And add a number of ventilation needles on the lap of the upper surface of the sand casting and the height of the side, as shown in Figure 3. The pouring time of sand casting and the specific size of pouring system are calculated according to the formula, and finally determined by computer simulation and optimization.
3. Mold design and sand mold manufacturing scheme design
As shown in Figure 4, referring to the parting method of conventional process, the casting sand mold is divided into outer mold, left and right end face cores, crankcase core, top cover core, water jacket core, oil duct core, etc. The left and right end face core, crankcase core, top cover core and water jacket core form a core group and then run into the lower die. Figure 5 shows the structural diagram of some sand cores.
In the design process of sand mold manufacturing process, the manufacturing process of the company shall be adopted as far as possible. Because the upper and lower outer molds have only one-sided structural characteristics and large volume, the digital Moldless casting process is selected; Other sand casting sand cores can be manufactured by laser selective sintering process from the analysis of structure and volume. However, due to the preliminary test analysis, the high-temperature gas generation of laser selective sintering process sand core is high and the air permeability is poor.
All end face cores and crankcase cores only adopt laser selective sintering process, and the hollow structure design of crankcase before processing is adopted to facilitate the exhaust during sand casting. The hollow structure design is shown in Figure 6. Due to the complex structure and thin wall thickness of water jacket core, top cover core, oil duct core 1 and oil duct core 2, it is not easy to carry out hollow design and processing. 3D inkjet printing sand core with low air generation is adopted.
