Gypsum mold can be used to make castings with high dimensional accuracy, low surface roughness and residual stress. It has many features that other molds do not have: it can accurately duplicate the pattern, which can make the surface roughness of aluminum alloy castings reach 0.8-3.2 μ m; it has low thermal conductivity, and the thin-walled parts are easy to be completely formed, and the thinnest part can be cast into a thin-walled 0.5mm; it can be inlaid with silicone rubber or vulcanized rubber to make patterns, Make castings with complex shapes. Similarly, gypsum mold casting has its own disadvantages: poor chilling effect of gypsum mold, when the wall thickness difference of the casting is large, shrinkage porosity and other defects are easy to appear in the thick part; poor air permeability of gypsum mold, easy to form porosity, choking and other defects in the casting.
There are three types of gypsum for casting: non foaming gypsum, foaming gypsum and investment casting gypsum. The air permeability of the non foaming gypsum mold is very poor, and the castings with low performance requirements are mainly used . Foamed gypsum mold has certain air permeability, which can be used to produce thin-walled (0.5mm at the thinnest) aluminum alloy castings with curved surface. For example, in the 1970s, Britain developed and patented the technology of silicon rubber mold foamed gypsum mold low-pressure casting, which has been successfully applied to the production of aluminum alloy impellers for supercharger compressors. In the early 1980s in China, The 7th Research Institute of China’s ordnance industry has realized the localization of aluminum alloy castings for compressor impellers, and the product yield has reached over 85% . The basic characteristics of gypsum mold used in investment casting are the same as those of non foaming gypsum mold. The thin-walled, complex and non residual high-quality aluminum alloy integral casting produced by Tec company can reach ± 0.254 m m in size accuracy, ± 0.076 mm in maximum and 0.8 m in minimum.
A foundry in Henan Province introduced a complete set of equipment and technology of gypsum investment casting from Tec company in 1984. Over the past 20 years, through the digestion, absorption and localization of the imported technology, the use of gypsum investment casting technology and laser sintering rapid prototyping technology, the production of complex shape aluminum alloy castings has achieved remarkable results. In the 1990s, Jinan yinlun electromechanical Co., Ltd. replaced vacuum suction casting with pressure regulating casting to improve the mechanical properties of thick and large parts of thin-walled complex aluminum alloy precision castings, The aluminum impeller of marine turbocharger (Φ 180 mm), the aluminum impeller of train turbocharger (Φ 310 mm), the closed exhaust wheel (Φ 509 mm) and the closed compressor of centrifugal refrigerator (Φ 500 mm) are produced.