Silica Sol Investment Casting Process
The basic feature of silica sol precision casting is that it uses fusible material as the pattern and refractory as the casting mold. Before pouring, the pattern is melted to form the cavity of the casting mold. As early as 3000 years ago, this process has been used to cast handicrafts. During the Second World War, due to the needs of the military industry, the United States, Britain and other countries used investment casting to produce the stationary blades of turbojet engines, which pushed the process to the industrial field and has been continuously developed and improved for more than half a century. The production process of silica sol investment casting is various, from wax pattern, shell, pouring to cleaning, which is a tight chain. Any problem in any link will directly affect the forming and quality of the final casting, so it is necessary to strengthen the control and research of the process.
1、 Importance of shell making process In all production processes, wax pattern manufacturing and shell mold manufacturing are two process links reflecting the characteristics of investment casting, which need special attention in process research. In recent years, the worldwide investment casting process has made great progress in wax pattern manufacturing. Producers can ensure the dimensional accuracy and surface quality of wax patterns by selecting appropriate patterns and using modern processing equipment. At the same time, compared with the subsequent manufacturing process of investment casting, wax pattern manufacturing is relatively independent, and unqualified products can be screened out by means of visual inspection and dimension measurement, so as to avoid increasing losses due to continued production. In the process of shell manufacturing, the information related to the final quality of the casting, such as surface quality and dimensional accuracy, is hidden. Until the casting is cleaned out, the change of the quality of the cavity of the shell can be seen as a "black box". The change of its size and quality cannot be directly observed in the process of manufacturing. The controllability of the entire production process can be guaranteed only if the relationship between the manufacturing process and defects of the shell is more clearly understood. More importantly, as the direct cavity of casting forming, the performance of the shell will ultimately affect the forming quality of liquid metal. Therefore, people pay much attention to the shell making process of investment casting. At the international important investment casting conference - the annual technical conference of American Investment Casting Association ICI, shell research has always been the focus of attention, and about 1/3 of the papers are related to shell, indicating the importance of shell manufacturing technology development to investment casting. In the international common investment casting shell making process, the silica sol shell has occupied the leading position due to its environmental advantages, but it also needs to face the challenges of fierce market competition: on the one hand, it needs to adapt to the quality requirements of larger, thinner and more complex castings in the aerospace and military fields; On the other hand, for a large number of civilian products, it is urgent to shorten the production cycle and improve the market reaction ability.
2、 The development of shell molding technology requires the development of new silica sol
1. Meet the requirements of complex investment castings for silica sol shell
In order to manufacture the shell of large, thin wall and complex castings, on the one hand, it is necessary to solve the problem of shell manufacturing capacity, such as equipment suitable for large shell operations, including shell making manipulator, dewaxing equipment, etc. On the other hand, the final mold shell has higher requirements on strength, deformation resistance and dimensional accuracy, especially the strength and deformation resistance of the mold shell are the basis for casting large investment castings. Only by ensuring the performance requirements of the shell and making the castings form correctly can the dimensional accuracy of the castings be further mentioned. The strength of silica sol shell can be divided into normal temperature strength, high temperature strength and residual strength according to the different thermal effects. The normal temperature strength is to ensure the integrity of the shell during shell making and dewaxing. High temperature strength is to ensure that the shell will not be damaged during baking and pouring. Although the high temperature strength is important, the experimental measurement shows that the shell strength of silica sol can reach 7~14MPa after being calcined at a high temperature above 950 ℃, which is 6~8MPa higher than that of ethyl silicate. It can completely meet the requirements of investment casting process. On the contrary, with the increase of the high temperature strength, the residual strength also increases, causing difficulties in shell cleaning of castings, which needs to be appropriately reduced. Compared with ethyl silicate, the weakness of silica sol shell is that the strength at room temperature is relatively low, so when the shell becomes larger and more complex, it is easy to cause shell cracking or deformation during shell making and dewaxing, which affects the final surface quality and dimensional accuracy of castings. Therefore, improving the strength of silica sol at room temperature has become an important task to promote and develop the silica sol shell molding process, and also an important goal to research new silica sol.
2. Requirements for the development of silica sol to improve the efficiency of investment casting
Compared with large complex thin-walled castings, civil products have low requirements for casting quality. However, for the latter, the problem of shortening production cycle and improving production efficiency becomes more prominent. The gelling process of ordinary silica sol mainly depends on the dehydration and drying of silica sol, which takes longer than that of chemically hardened ethyl silicate. The hardening of each layer of ethyl silicate shell with ammonia drying can be completed in about 2h, while the final hardening of silica sol generally takes more than 12h, and it takes longer for some parts that are difficult to dry, such as deep holes. At the same time, since the investment casting shell needs to be manufactured layer by layer, each layer needs to be fully dried, so as to ensure that the problem of back dissolution and falling off will not occur when the lower layer is immersed in the coating, while the coating itself will be immersed in the dried shell, resulting in a long overall drying cycle. It is the schematic diagram of the production cycle of investment castings with silica sol shell under general conditions. It can be seen from the figure that the shell making time accounts for more than 50% of the whole casting production cycle. To shorten the delivery time of products, shortening the shell making cycle is the core link of the problem. The key factors to shorten the shell making period can be divided into internal and external factors. The internal factor is mainly the characteristics of the binder, and the external factor is the drying conditions.