Quality Control of Silica Sol Coatings in Investment Casting Precision Casting

In the process of ensuring the quality of castings, in addition to formulating a reasonable and scientific manufacturing process, it is more important to strictly control the quality of silica sol coatings to ensure the quality of cast iron silica sol investment casting rotor wheel. Through strict control of coating parameters, coating quality and drying quality, the quality of precision castings can be further improved to ensure that enterprises can produce qualified products.

1. Relevant characteristics of silica sol

Silica sol is a milky white translucent colloidal solution, which is stable alkaline in most cases and acidic in a few cases. Silica sol has low viscosity, good dispersibility, can fully enter the solid material to be filled, and can ensure the smooth surface of the solid material. There are silanol hydroxyl groups and adsorbed water on the surface of the silica sol particles, which can effectively improve the wettability of the silica sol. In addition, silica sol also has good adhesion and reactivity, and drying and sintering after using silica sol can form a solid film, and once formed, this film will not dissolve again in water and other substances. After the silica sol is homogeneously mixed with the organic resin, it can effectively improve the mechanical and chemical properties of the mixed substance. At the same time, silica sol also has good stability, and once solidified, it will not dissolve in water and deteriorate.

2. Silica sol coating and its preparation

In the process of  silica sol precision casting, the preparation of silica sol investment casting needs to select an appropriate preparation method according to the actual situation. Silica sol coating is divided into surface coating and back coating when manufacturing mold shells. The top coating will be in direct contact with the molten metal, which is also an important factor determining the surface quality of castings. It is required that this coating have good heat resistance, viscosity, and lubricity. The main components of this coating are: refractory materials, wetting agents, binders and defoamers. The back coating does not need to be in direct contact with the metal liquid, it mainly ensures the strength, rigidity and other comprehensive mechanical properties of the shell, the composition of this coating is basically the same as that of the surface coating, but the viscosity of the adhesive is lower, can avoid the accumulation of paint. In addition, there is a transitional coating between the top coat and the back coat, which is mainly to make the top coat and the back coat bond together well, thereby ensuring the coating quality of the transition coat and the overall shell shape. the quality of. The composition of this coating is basically the same as that of the above two coatings, except that the viscosity of the binder is higher, so that the surface layer and the back layer can be tightly bonded together. When preparing silica sol coatings, first add a certain amount of silica sol to the batching tank, then add a certain proportion of lubricant, and stir. During the stirring process, you need to add refractory powder, and finally add defoaming agent.

3. Quality Control of Silica Sol Coatings

There are many factors that affect the quality of investment precision castings, and the shell mold manufacturing process and production equipment can affect the quality of castings. However, among the factors affecting the quality of investment precision castings, the quality of silica sol coating is the main factor affecting the quality of castings. Therefore, it is extremely important to control the quality of silica sol coatings to ensure the quality of castings. In the process of quality control of silica sol coatings, it is necessary to understand the problems that are prone to occur in the quality control process, and take corresponding measures to solve them, mainly from the following aspects.

1. Control of silica sol coating parameters

The control of silica sol coating parameters mainly includes the control of SiO2 content, pH value, viscosity, and other parameters. It is difficult to control the SiO2 content, because the moisture of the binder in the silica sol coating is evaporating all the time, which will lead to changes in the composition of the coating binder, which in turn will increase the SiO2 content in the coating.

2. Quality control of coating coating

In order to ensure the coating quality of the paint, the module needs to be cleaned before the coating is applied, and the surface of the module needs to be slightly etched, which can ensure that the paint is tightly bonded to the module. In addition, it is necessary to conduct a comprehensive inspection of the module during painting and hanging to ensure that the module has no quality problems before painting and hanging the paint.

3. Control of paint drying quality

The drying of silica sol paint requires different forms of drying according to its type. The drying of the surface coating needs to be dried without wind, and the drying time is guaranteed to be about 8 hours; the drying of the overcoat and back coating should be done with ventilation, but the wind speed needs to be controlled at 8m/s, and the humidity needs to be controlled at 50 %, the drying time generally needs to be about 6 hours. In addition, it is necessary to ensure that the previous layer of paint is completely dry before the next layer of paint can be applied and dried.