Drying of surface coating for investment casting

Drying of surface coating for investment casting

In the previous articles, we discussed the shell making of investment casting surface layer separately. After the shell making is completed, the next issue is drying. We know that the surface layer is very important for the entire shell production, mainly because it not only needs to withstand the heat of high-temperature metal liquid, but also its raw material price is the most expensive in the shell making process. With the continuous increase in raw materials, many companies are using polycrystalline sand and fused corundum powder to replace zircon sand powder. Based on these, the surface layer not only needs to be well made, but also needs to be well dried to ensure the overall quality of the shell. Today, precision casting manufacturer Tianyao Metal will talk to you about the drying analysis of investment casting surface coatings.

The longest process in the shell making process is drying. When using silica sol as the binder, 70% of it is water, and the thickness of the first layer of the surface layer is about 0.15mm, which is a very thin coating. In order to prevent cracking and peeling of this very thin surface layer, forming a solid surface layer, it is necessary to strictly control the room temperature and humidity of the surface layer coating room. This temperature is generally recommended to be 24 ± 2 ℃, and the lower limit should be used for the surface layer as much as possible, so 22-24 ℃ is more suitable; The humidity should be as high as possible, and it is recommended to be 60 ± 5%. Due to the thin thickness of the surface layer, no matter how complex the shape of the casting is, it is necessary to strive to maintain the same drying speed in all parts. The use of forced air drying or humidity less than 50% can easily cause coating cracking and peeling. Try not to exceed the above range for room temperature. Usually, the drying time for the first layer is 2-3 hours. The drying rate of the surface layer can be expressed by the dehydration rate, and the limit of drying degree is approximately 70% of the dehydration rate.

Due to the thin surface layer, no matter how complex the product shape is, its drying speed is still relatively fast, and the thin thickness cannot be expected to have high shell strength. In addition, due to the dense density of the surface layer, the dehydration rate after several hours of drying under certain conditions can reach over 90%, which is often prone to cracking and peeling. This phenomenon often occurs during the first and second shell making stages. Therefore, it is best to dry the surface layer at high humidity and low temperature, and avoid intense drying. The method of mastering the degree of drying is to first roughly use the drying time as the approximate standard, and at the same time, scratch and check the surface of the most difficult to dry part of the product (generally speaking, if there is a damp feeling, it is definitely not dry, and if there is hardening, it should be basically dry). There are many ways to dry the mold shell, and as mentioned in previous articles, if you are interested, you can go back and read it. Due to the significant differences in drying conditions between the surface layer and the back layer of the shell, the shell making site must be clearly separated, which is very important. Many domestic factories do not pay attention to this point. We have also mentioned before that for products with complex shapes, the drying conditions of the surface layer must be met, and even local small areas can be created inside the surface layer to meet the drying requirements of the product surface layer. It should be noted that the pulp barrel should be separated from the dry environment as much as possible. Drying requires a larger area of control and requires a higher air conditioning power. Regarding the inspection of drying conditions, some factories check the temperature of air conditioning control. Personally, I believe that more dry and wet thermometers should be set up at multiple points to display data through actual intervals to detect the actual situation of air conditioning operation. After all, constant temperature and humidity are necessary conditions for shell drying.

Precision machining