Silica sol investment casting process

Silica sol investment casting process

The basic feature of silica sol precision casting is to use fusible materials as the mold, refractory materials as the mold, and melt out the mold before pouring to form a mold cavity. As early as 3000 years ago, this technique had been 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 investment casting to produce the stationary blades of turbojet engines, thereby pushing this process into the industrial field and continuously developing and improving it for over half a century. The production process of silica sol investment casting is diverse, from wax mold, shell casting, pouring, to cleaning, which is a tight chain. Any problem in any link directly affects the final forming and quality of the casting, and requires special strengthening of process control and research.

1、 The Importance of Shell Making Technology In all production processes, wax mold manufacturing and mold shell manufacturing are two process steps that reflect the unique characteristics of investment casting, and special attention needs to be paid in process research. In recent years, investment casting processes worldwide have made significant progress in wax mold manufacturing. Producers can ensure the dimensional accuracy and surface quality of wax molds by selecting appropriate mold materials and adopting modern process equipment. Meanwhile, compared with the subsequent manufacturing process of investment casting, wax mold manufacturing is relatively independent and can screen out non-conforming products through visual inspection and size measurement, avoiding further production and increasing losses. Entering the manufacturing process of the mold shell, the surface quality and dimensional accuracy related to the final quality of the casting are hidden. Until the casting is cleaned, the changes in the quality of the inner cavity of the mold shell can be seen as a "black box", and the changes in size and quality cannot be directly observed during the manufacturing process. Only by understanding the relationship between the manufacturing process of the mold shell and defects more clearly can the controllability of the entire production process be ensured. More importantly, as the direct cavity for casting forming, the performance of the shell ultimately affects the forming quality of liquid metal. Therefore, people are very concerned about the shell making process of investment casting. At the annual technical conference of the American Investment Casting Association (ICI), an important international investment casting conference, shell research has always been a hot topic of attention, with about one-third of the papers related to shell, indicating the importance of shell manufacturing technology development for investment casting. In the internationally common investment casting shell making process, silica sol shell is dominant 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 proposed by the aerospace and military industries; On the other hand, for a large number of civilian products, shortening the production cycle and improving market responsiveness have become urgent tasks.

2、 The development of shell technology and its requirements for the development of new silica sol

1. Meet the requirements of silica sol shell for complex investment castings

To manufacture large, thin-walled, and complex castings, it is necessary to address the issue of shell manufacturing capabilities, such as equipment suitable for large-scale shell operations, including shell making robots, dewaxing equipment, etc. On the other hand, the final shell has higher requirements in terms of strength, deformation resistance, and dimensional accuracy, especially the strength and deformation resistance of the shell are the foundation for casting large investment castings. Only by ensuring the performance requirements of the shell and correctly forming the casting can the issue of dimensional accuracy of the casting be further addressed. The strength of silica sol shell can be divided into room temperature strength, high temperature strength, and residual strength according to its different heating effects. The room temperature strength is to ensure the integrity of the shell during the shell making and dewaxing processes. High temperature strength is to ensure that the shell is not damaged during the roasting and pouring process. Although high temperature strength is important, experimental measurements have shown that after high-temperature calcination above 950 ℃, the shell strength of silica sol can reach 7-14 MPa, exceeding that of ethyl silicate by 6-8 MPa, which fully meets the requirements of investment casting technology. On the contrary, with the increase of high temperature strength, the residual strength also increases, making it difficult to clear the shell of castings and requiring appropriate reduction. Compared with ethyl silicate, the weakness of silica sol shell lies in its relatively low room temperature strength. Therefore, when the shell becomes larger and more complex, it is easy to cause cracking or deformation of the shell during shell making and dewaxing, which affects the final surface quality and dimensional accuracy of the casting. Therefore, improving the room temperature strength of silica sol has become an important task for promoting and developing silica sol shell forming technology, and is also an important goal for researching new types of silica sol.

2. The requirements for the development of silica sol to improve investment casting efficiency

Compared to large and complex thin-walled castings, civilian products have lower requirements for casting quality. However, for the latter, the issue of shortening production cycles and improving production efficiency has become more prominent. The gelation process of ordinary silica sol mainly relies on dehydration and drying of silica sol, which takes longer than the gelation time of chemically hardened ethyl silicate. The ethyl silicate shell can be hardened in about 2 hours per layer using ammonia drying, while the final hardening of silica sol generally takes more than 12 hours. For some difficult to dry areas such as deep holes, it takes longer. At the same time, due to the layered manufacturing of investment casting shells, each layer needs to be fully dried to ensure that the lower layer of the shell will not cause the problem of dissolution and peeling when immersed in the coating. However, the water in the immersed coating itself will immerse into the already dried shell, resulting in a long overall drying cycle. This is a schematic diagram of the production cycle of silica sol shell investment castings in general. From the graph, it can be seen that the shell making time accounts for more than 50% of the entire 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 for shortening the shell making cycle can be divided into two aspects: internal factors and external factors. The internal factors are mainly the characteristics of the binder, while the external factors are the drying conditions.

Precision machining