Precision Casting with Silica Sol Shell

The sol-gel process belongs to precision investment casting, a casting technique characterized by minimal or no machining. It is an excellent technology widely used in the casting industry. It is not only suitable for casting various types and alloys, but also produces castings with higher dimensional accuracy and surface quality compared to other casting methods. Even complex, high-temperature-resistant, and difficult-to-machine castings that are challenging to cast using other methods can be produced through precision investment casting.

Precision investment casting originated from ancient lost wax casting. As an ancient civilization, China is one of the earliest countries to use this technology. Hundreds of years ago, ancient Chinese laborers created this lost wax casting technique to cast various products such as bells, tripods, and vessels with intricate patterns and inscriptions, such as the Zeng Hou Yi burial vessel from the Spring and Autumn period. The base of the Zeng Hou Yi burial vessel features multiple intertwining dragons, connected end to end and interlaced vertically and horizontally, forming a multi-layered cloud pattern with a hollow center. These patterns are difficult to produce using conventional casting techniques. However, with the lost wax casting process, the characteristics of low strength and ease of carving of the wax material can be utilized. Craftsmen can carve wax replicas of the Zeng Hou Yi burial vessel using ordinary tools, and then proceed with the investment casting process by adding gating systems, applying coatings, dewaxing, and casting, resulting in exquisite replicas of the Zeng Houyi burial vessel.

The practical application of modern investment casting in industrial production began in the 1940s. At that time, the development of jet engines in aviation required the manufacturing of heat-resistant alloy components such as blades, impellers, and nozzles, which were complex in shape, precise in size, and required a smooth surface finish. Because heat-resistant alloy materials were difficult to machine and the components had complex shapes, they could not be manufactured or were difficult to manufacture using other methods. Therefore, there was a need to find a new precision forming process. Drawing inspiration from the ancient lost wax casting technique, and through improvements in materials and processes, modern investment casting methods were developed on the basis of ancient techniques. Thus, the development of the aerospace industry drove the application of investment casting, and the continuous improvement and refinement of investment casting also created favorable conditions for further enhancing the performance of the aerospace industry.

China began to apply investment casting to industrial production in the 1950s and 1960s. Since then, this casting process has undergone significant development and has been widely used in various manufacturing industries such as aviation, automobiles, machine tools, ships, internal combustion engines, turbines, telecommunications equipment, weapons, medical devices, and tools. It has also been employed in the production of artistic and craftworks.

The investment casting process, simply put, involves creating meltable patterns (commonly known as wax patterns or models) using fusible materials such as wax or plastic. These patterns are coated with several layers of specialized refractory coating. After drying and hardening to form an integral shell, the model is melted out from the shell using steam or hot water. The shell is then placed in a sand box, and dry sand is filled around it for molding. The mold is then placed in a furnace for high-temperature firing (in the case of high-strength shells, molding may not be necessary, and the demolded shell can be fired directly). After firing, the mold or shell is poured with molten metal to obtain the casting. Investment castings typically achieve high dimensional accuracy, generally reaching CT4-6 (compared to CT10-13 for sand casting and CT5-7 for die casting). However, due to the complexity of the investment casting process and various factors affecting dimensional accuracy, such as material shrinkage, deformation of the mold, linear changes in the shell during heating and cooling, alloy shrinkage rate, and casting deformation during solidification, the dimensional consistency of ordinary investment castings still needs to be improved (the dimensional consistency of castings using medium to high-temperature wax materials needs to be significantly improved).

During the molding process of investment casting, high-quality molds with smooth cavity surfaces are used, resulting in relatively high surface smoothness of the investment pattern. Additionally, the shell is made of refractory coating prepared from high-temperature resistant binders and refractory materials, which are applied to the investment pattern. The cavity surface in direct contact with the molten metal has high surface smoothness. Therefore, the surface smoothness of investment castings is higher than that of general castings, typically achieving Ra 1.6-3.2μm. The advantage of investment casting lies in its high dimensional accuracy and surface smoothness, which reduces the need for machining. Only a small machining allowance is required on parts with high requirements, and in some cases, castings can be used without machining, with only polishing allowances left.

As evidenced, the utilization of investment casting methods leads to significant savings in machine tool equipment and processing time, resulting in substantial reductions in metal raw materials. Another advantage of investment casting is its capability to produce complex castings of various alloys, particularly high-temperature alloy castings. For instance, the blades of jet engines, with their streamlined external contours and internal cooling passages, are nearly impossible to shape using conventional machining processes. Employing investment casting techniques not only enables mass production and ensures casting consistency but also mitigates stress concentrations caused by residual tool marks from machining.