Viscosity of silica sol in precision casting

Viscosity of silica sol in precision casting

In the third chapter of Mr. Jiang Buju's book "Practical Investment Casting Technology", which discusses the structure of the mold shell, there is a description that: upon careful analysis, there are two types of investment casting shell structures: integral shell and layered shell. There are significant differences in the structure of the two types of shells, each with its own advantages and disadvantages, both of which are quite important. The shell structure is related to factors such as coating viscosity and sand particle size. When using coatings with low viscosity and good fluidity, combined with sand with appropriate particle size, a thin and dense overall shell can be obtained. This type of shell is dense and has high strength, but once cracks appear, the cracks will rapidly expand and are prone to fire. Integral shell is commonly used in small and medium-sized casting shells. The use of coatings with high viscosity and appropriate particle size sanding can produce a layered shell, which is not as strong as the overall shell and should be made thicker. However, when cracks appear, they can effectively prevent crack propagation through the gaps in the sanding layer. Therefore, layered shells are suitable for producing large castings. In fact, from this passage, we can also understand some principles of viscosity design for silica sol, which are to use low viscosity or low viscosity for small castings, and high viscosity or high viscosity for large castings.

Mr. Shanwu Yangshu's "Practical Techniques for Precision Casting" discusses the design of shell viscosity for the fork shaped disc core casting of printers. It is written as follows: The quality requirements for this piece are stricter than those for current precision castings, with a spacing of 1.2mm for 24 needles, which is very narrow. In order to fully fill the mold shell with the needle spacing, white zircon fine sand is used for sanding, and compressed air must be used to blow away the loose sand on the surface layer during operation. The viscosity of the surface coating is about 20 seconds for a 4 inch cup. In fact, the spacing between the 24 needles of the printer's forked disc core is very narrow here, so its shell making viscosity should use a smaller viscosity, or a silica sol coating with a smaller powder to liquid ratio. Of course, not only is the viscosity of silica sol coating low, but the size of the surface sand, transition sand, and back sand should be well designed. This also reflects a shell design concept. In foreign countries, multi-layer shells are used, and the multi-layer here may be at least 8 layers or more, while the multi-layer used in China is at least 4 and a half layers. This also shows the accuracy of the product. The fewer layers of the shell, the smaller the size accuracy of the product, the more layers of the shell, the more accurate the product size. The dimensional accuracy is naturally higher.

In investment casting production, products are the first priority, and the coating ratio is the second, serving the products.

Precision machining