The basic characteristics of precision casting with silica sol are the use of fusible materials as the mold pattern and refractory materials as the casting mold. Before pouring, the mold pattern is melted to form the mold cavity. As early as 3000 years ago, this process was already used to cast handicrafts. During World War II, due to the needs of the military industry, countries such as the United States and the United Kingdom used the lost-wax casting method to produce the stationary blades of turbojet engines, thus pushing this process into the industrial field and enabling it to continuously develop and improve over more than half a century. The production process of silica sol lost-wax casting is complex, including wax molds, mold shells, pouring, and cleaning, which form a closely connected chain. Any problem at any stage will directly affect the shape and quality of the final castings, and special attention needs to be paid to the control and research of the process.
I. The Importance of Shell Manufacturing Process
Among all the production processes, wax mold manufacturing and shell mold manufacturing are the two key technological steps that reflect the unique characteristics of investment casting. These two processes require special attention in the process research. In recent years, the wax mold manufacturing aspect of investment casting technology has made significant progress worldwide. Producers can ensure the dimensional accuracy and surface quality of the wax mold by choosing appropriate mold materials and adopting modern process equipment. Meanwhile, compared with the subsequent manufacturing process of investment casting, wax mold manufacturing is relatively independent. Unqualified products can be screened out through appearance inspection and size measurement, avoiding further production and increasing losses. Entering the shell mold manufacturing stage, information related to surface quality and dimensional accuracy that are crucial for the final quality of the casting is hidden until the casting is removed. The changes in the cavity quality of the shell can be regarded as a "black box" before the casting is removed. The manufacturing process cannot directly observe the changes in size and quality. Only by having a clearer understanding of the relationship between the shell manufacturing process and defects can the controllability of the entire production process be ensured. More importantly, as the shell is the direct mold cavity for casting, its performance ultimately affects the forming quality of the liquid metal. Therefore, people pay great attention to the shell-making process of investment casting. At the important international investment casting conference - the American Investment Casting Institute's technical conference held annually, shell research has always been a hot topic of attention. About one-third of the papers are related to shells, indicating the importance of shell manufacturing technology to investment casting. In the commonly used shell-making process of investment casting in the international community, silica sol shell molds have dominated due to their environmental advantages, but they also need to face the challenge of intense market competition: on the one hand, they need to adapt to the quality requirements of larger, thinner, and more complex castings proposed by aerospace and military fields; on the other hand, for a large number of civilian products, shortening the production cycle and improving market response capabilities have also become an urgent task.
II. The development of shell technology imposes requirements for the research and development of new types of silica sols.
Meet the requirements of silicon sol type molds for complex investment castings
To manufacture large, thin-walled, and complex castings, on one hand, the problem of shell manufacturing capability needs to be addressed, such as equipment suitable for large shell operations, including shell manufacturing robots, wax removal equipment, etc. On the other hand, the final shell has higher requirements in terms of strength, anti-deformation ability, and dimensional accuracy, especially the strength and anti-deformation ability of the shell, which is the foundation for casting large mold castings. Only by meeting the performance requirements of the shell, ensuring the correct formation of the casting, can the issue of casting dimensional accuracy be further addressed. The strength of the silica sol shell can be classified into normal temperature strength, high temperature strength, and residual strength, depending on the thermal effect it undergoes. Normal temperature strength is to ensure the integrity of the shell during shell manufacturing and wax removal processes. High temperature strength is to ensure that the shell is not damaged during baking and pouring processes. Although high temperature strength is important, experimental determination shows that after being baked at temperatures above 950°C, the strength of the silica sol shell can reach 7-14 MPa, exceeding 6-8 MPa of the ethyl silicate, which can fully meet the requirements of the mold casting process. However, as the high temperature strength increases, the residual strength also increases, causing difficulties in removing the casting shell and requiring an appropriate reduction. Compared with ethyl silicate, the weakness of the silica sol shell lies in its relatively low normal temperature strength, which makes it prone to cracking or deformation during shell manufacturing and wax removal when the shell is large and complex, affecting the final surface quality and dimensional accuracy of the casting. Therefore, improving the normal temperature strength of silica sol has become an important task for promoting and developing the silica sol shell process and an important goal for researching new types of silica sol.
2. The requirements for the development of silica sol in improving the efficiency of investment casting
Compared with large and complex thin-walled castings, the quality requirements for castings in civilian products are relatively low. However, for the latter, the issues of shortening the production cycle and improving production efficiency have become more prominent. The gelation process of ordinary silica sol mainly relies on the dehydration and drying of the silica sol, which takes longer than that of the silicate ester used for chemical hardening. The silicate ester shell can be hardened in about 2 hours by using ammonia for each layer, while the final hardening of the silica sol generally requires more than 12 hours. For some difficult-to-dry parts such as deep holes, it may take even longer. At the same time, since the mold shell for lost-wax casting needs to be manufactured in layers, each layer requires sufficient drying to ensure that the coating for the lower layer does not cause re-solubilization and detachment when it is immersed in the coating. The coating itself will also absorb moisture from the already dried mold shell, thus causing a longer overall drying cycle. This is a schematic diagram of the production cycle of lost-wax castings using silica sol shells under normal circumstances. As can be seen from the figure, the time for shell manufacturing accounts for more than 50% of the entire casting production cycle. To shorten the delivery time of the product, shortening the shell manufacturing cycle is the core issue. The key factors for shortening the shell manufacturing cycle can be divided into internal and external factors. The internal factor mainly refers to the properties of the binder, while the external factor refers to the drying conditions.
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