The choice of silica sol coating will directly affect the surface roughness and dimensional accuracy of investment castings. Silica sol coatings generally can directly select silica sol with a mass fraction of silica of 30%. The coating process is simple and the operation is convenient. At the same time, the casting mold shell produced by using the coating has high strength, and the shell-making cycle can also be shortened.
Silica sol is a typical water-based binder with a silicic acid colloid structure. It is a polymer colloidal solution in which highly dispersed silica particles are soluble in water. The colloidal particles are spherical and have a diameter of 6-100 nm. The process of investment casting to make the shell is the process of gelling. There are many factors influencing gelation, mainly electrolyte, pH, sol concentration and temperature. There are many types of commercial silica sols, and the most used is alkaline silica sol with a silica content of 30%. The process of making silica sol shell is relatively simple. Each process has three processes: coating, sanding, and drying. Each process is repeated many times to obtain a multilayer shell of the required thickness.
There are generally two methods for producing silica sol: ion exchange and dissolution. The ion exchange method refers to the ion exchange of diluted water glass to remove sodium ions and other impurities. Then the solution is filtered, heated and concentrated to a certain density to obtain silica sol. The dissolution method refers to using industrial pure silicon (the mass fraction of silicon ≥ 97%) as the raw material, and under the action of the catalyst, it is directly dissolved in water after heating. Then, the solution is filtered to obtain a silica sol.
Technical Parameters of Silica Sol for Investment Casting |
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| No. | Chemical Composition (mass fraction, %) | Physical Properties | Others | |||||
| SiO2 | Na2O | Density (g/cm3) | pH | Kinematic Viscosity (mm2/s) | SiO2 Particl Size (nm) | Apperance | Stationary Phase | |
| 1 | 24 - 28 | ≤ 0.3 | 1.15 - 1.19 | 9.0 - 9.5 | ≤ 6 | 7 - 15 | in invory or light green colour, without impurity | ≥ 1 year |
| 2 | 29 - 31 | ≤ 0.5 | 1.20 - 1.22 | 9.0 - 10 | ≤ 8 | 9 - 20 | ≥ 1 year | |
The castings obtained by the silica sol shell making process has low surface roughness, high dimensional accuracy and long shell making cycle. This process is widely used in casting high-temperature heat-resistant alloys, heat-resistant steels, stainless steels, carbon steels, low alloys, aluminum alloys and copper alloys.
The silica sol precision lost wax investment casting process is suitable for repeatable production of net shape components from a variety of different metals and high performance alloys. Although generally used for small castings, this process has been used to produce complete aircraft door frames, with steel castings of up to 500 kgs and aluminium castings of up to 50 kgs. Compared to other casting processes such as die casting or sand casting, it can be an expensive process. However, the components that can be produced using investment casting can incorporate intricate contours, and in most cases the components are cast near net shape, so require little or no rework once cast.
The main components of the wax coating of the investment casting process are:
Surface layer silica sol adhesive. It can ensure the strength of the surface layer and ensure that the surface layer does not crack;
Refractory. It is generally high-purity zirconium powder to ensure that the coating has sufficient refractoriness and does not chemically react with molten metal.
Lubricant. It is a surfactant. Because the silica sol coating is a water-based coating, the wettability between it and the wax mold is poor, and the coating and hanging effect is not good. Therefore, it is necessary to add a wetting agent to improve the coating and hanging performance.
Defoamer. It is also a surfactant whose purpose is to eliminate air bubbles in the wetting agent.
Grain refiner. It can ensure the grain refinement of castings and improve the mechanical properties of castings.
Other appendages: suspending agent, drying indicator, sustained release agent, etc.
Correct selection of the proportion of each component in the silica sol coating is the key to ensuring the quality of the coating. The two most basic components in coatings are refractories and binders. The ratio between the two is the powder-to-liquid ratio of the coating. The powder-to-liquid ratio of the paint has a great influence on the performance of the paint and the shell, which will ultimately affect the quality of the casting. If the powder-to-liquid ratio of the coating is too low, the coating will not be dense enough and there will be too many voids, which will make the surface of the casting rough. Moreover, an excessively low powder-to-liquid ratio will also increase the tendency of the coating to crack, and the shell strength will be low, which will eventually cause leakage of the molten metal during casting. On the other hand, if the powder-to-liquid ratio is too high, the coating will be too thick and the fluidity will be poor, making it difficult to obtain a coating with a uniform thickness and suitable thickness.
Coating preparation is an important part of ensuring the quality of the shell. When formulating the coating, the components should be uniformly dispersed and fully mixed and wetted with each other. The equipment used for paint formulation, the number of additions and the stirring time will all affect the quality of the paint. Our investment casting shop uses continuous mixers. In order to ensure the quality of the coating, when all the components of the coating are newly added raw materials, the coating must be stirred for a long enough time.
The control of the properties of silica sol coatings is an important quality control step. The viscosity, density, ambient temperature, etc. of the paint must be measured at least three times a day, and should be controlled within the set range at any time.
Post time: Jul-25-2022