Drying of surface layer of precision casting mold shell

Drying of surface layer of precision casting mold shell

In silica sol precision casting, it is well known that the surface layer determines the surface roughness of the casting, while the back layer determines the dimensional accuracy of the casting. For the mold shell, drying is the only way for the mold shell to achieve wet strength. Therefore, the drying of the mold shell is a very important process in the shell making process. For silica sol molds, the drying of the surface layer is particularly important. Because the surface layer not only determines the surface quality of the casting, but also withstands the thermal shock of high-temperature metals. Today, precision casting manufacturer Tianyao Metal will talk to you about the drying of the surface layer of the mold shell.

Zirconium sand and zirconium powder are generally used for silica sol surface, of course, corundum and fused silica are also used. Due to the use of fine refractory powder on the surface layer, it is generally 300-350 mesh; The refractory sand is about 100 mesh. Therefore, the thickness of the surface layer is generally very thin, about 0.2-0.3mm. It is also very easy to dry. So for the surface layer, for general products, if there are no complex structures such as deep holes and narrow grooves, they will dry in a few hours. However, for deep cavity castings, drying may be slower. For example, valve castings have a relatively complex structure. The three necessary conditions for mold shell drying are temperature, humidity, and wind speed. In the surface layer, only the first two are used, temperature and humidity. Because the surface layer is relatively thin and easy to dry. If the surface layer is dry, it cannot dry quickly, and the pleasure will peel off. When you work on the second layer, as soon as the module enters the slurry, the surface layer may have peeled off. Therefore, the surface layer emphasizes slow drying, low temperature and high humidity. In addition, uniform drying, also known as uniform rate drying, synchronous drying, is also important. Due to the relatively complex structure of castings (investment castings are generally not processed by other processing methods or have high processing costs), the internal and external drying are not synchronized. Some areas have been dried, while others are still in the drying process, so when the second layer is dried, peeling or dissolution may occur. So we can't underestimate this thin surface layer, there is a lot of knowledge inside! Furthermore, the powder to liquid ratio of the surface layer is also important. A high powder to liquid ratio results in low surface roughness of castings, while a low powder to liquid ratio results in high roughness. Of course, it's not that a high powder to liquid ratio results in a low roughness. When the powder to liquid ratio reaches a certain level, it does not have a good effect on the surface roughness of the casting, but instead affects the breathability of the mold shell. The use of silica sol for the surface layer is also important. Generally, manufacturers use a type of silica sol, but in reality, it is recommended to use silica sol with smaller particle diameter in the upper layer textbook. Because this type of silica sol is easy to mix into low powder to liquid ratio surface coatings, it not only takes into account breathability but also has high wet strength.

Generally, factories may adopt a one layer per day approach. For simple castings, this is a waste. It is completely possible to execute the process of two layers per day. However, for products with complex shapes, deep cavities, narrow grooves, etc., it is necessary to ensure low temperature and high humidity.

The recommended humidity for the general surface layer is 50% -60%. If it is too high, it is not easy to dry. If it is too low, it will cause excessive drying and peeling. Another thing to note is that the cleaning of the module must be done properly, otherwise the coating may not wet well on the surface layer and may peel off during adhesive shrinkage.

Precision machining