a. Analysis ofprocess for impeller
The overall size of the sand cast impeller is large, and the outer diameter is Φ 700mm, 130mm in height, 202kg in weight, and the spokes are flat. For this kind of impeller, the hub and spoke thickness are mostly considered from the perspective of Casting modeling and feeding, and it is more appropriate to useprocess for production. However, if you carefully analyze and study the blade part of sand casting impeller, you will find that due to the complex shape, flow line shape and thin wall thickness of a single blade, it is only 5mm; Moreover, the spacing between blades is very small, the number is up to 54, and the technical requirements for dimensional accuracy, surface quality and internal quality are very high.
Firstly, from the perspective of modeling, the whole sand casting impeller adopts sand casting, but the overall shape can not be modeled; Similarly, when adopting the core assembly molding or core pulling molding process commonly used in the current casting production, due to the large number of cores, difficult assembly and positioning, poor accuracy and cumbersome operation, the technical requirements for workers are not only very high, the spacing between blades cannot be guaranteed, but also the rim ovality is easy to exceed the tolerance. Therefore, the surface roughness RA3 of blades cannot be guaranteed by sand2～Ra1. 6. The and dimensional tolerance shall meet the technical requirements of cT4 ~ CT5. Secondly, considering that the sand cast impeller is made of zg1cr18ni9ti, which is easy to oxidize and has poor fluidity, while the blade wall thickness is only 5mm, it is not only difficult to form during pouring, but also prone to casting defects such as sand flushing, air hole and cold shut. Therefore, the sand casting impeller that meets the technical requirements cannot be obtained by using the sand casting process alone.
b. Analysis of investment casting process for impeller
According to the characteristics of investment casting and the structure of investment casting impeller, if the investment casting impeller is produced by investment casting alone, although the shape and quality of the blade can be guaranteed, the strength of the mold shell is obviously insufficient due to the large size and weight of the investment casting impeller. At the same time, it is not easy to set up a complex pouring (riser) injection system for the hub and spoke, Moreover, because the spoke part is a large plane and the mold shell is easy to deform, the investment casting process alone can not cast the investment casting impeller that meets the technical requirements.
c. Selection of investment mold sand mold compound molding process for impeller
Through the above analysis of impeller sand casting and investment casting technology, it can be seen that although sand casting and investment casting technology have incomparable advantages over other technology methods, they also have their own insurmountable shortcomings. Therefore, it is difficult to meet the strict requirements of large size, complex structure, impeller dimensional accuracy and surface quality by using the investment casting method for general production of small and complex impellers or the single process of core assembly or core pulling sand casting for production of large and simple impellers. Therefore, if the advantages of the two processes can be combined, the impeller castings that meet the technical requirements can be obtained.
Through the analysis and Research on the structural characteristics and process characteristics of this kind of impeller casting, as well as the advantages and disadvantages of investment casting and sand casting, it is found that if this kind of impeller is from the inner side of the rim (diameter: Φ 416) if divided, it can be divided into two parts: hub and spoke with simple shape and blade with complex shape and high quality requirements. Due to the similarity of mold materials and the complementarity of process characteristics between sand casting and investment casting, for the blade part with complex shape and high quality requirements, investment casting can be used to first manufacture the outer core of the mold shell of the whole blade part, and for the hub spoke part with simple shape, sand casting can be used to manufacture the sand mold of the corresponding part and then close the mold, The investment sand casting composite mold of the impeller meeting the quality requirements can be obtained.