Hello, Teacher! I have a question I'd like to ask. Under what conditions can the back layer achieve proper drying within 4-6 hours?

A friend from Shandong asked a question on WeChat: Their current situation involves a 12-hour drying time for the backup coat. Under what conditions can the backup coat achieve a drying time of 4-6 hours? Given the relatively mature shell-making technology today, this is not a complex issue. However, if traditional shell-making techniques are used, achieving this would still pose certain difficulties. I’d like to take this question as today’s topic of discussion. First, let’s talk about the conventional drying conditions for the backup coat. Due to the low viscosity of the backup coat slurry, if the dipping time is too long, the slurry may penetrate into the previous layers. Therefore, the drying time for the backup coat tends to be relatively longer. If the drying chamber maintains constant temperature and humidity, and considering that the humidity level commonly used for the backup coat in China is 40%-60% (disregarding the impact of constant temperature), the surface humidity of the shell just after slurry dipping and sand stuccoing is significantly higher than the humidity in the drying chamber. As a result, the moisture in the shell undergoes a process of automatic evaporation to reduce humidity until it reaches equilibrium with the overall humidity of the drying chamber. At this point, the driving force for drying due to the humidity difference becomes very small, and the drying rate noticeably slows down.

In Mr. Yamaya Hiroki’s article, the drying process of the shell backup coat is specifically described: Generally, drying can be divided into three stages. In the initial stage of the shell coating layer formation, due to supersaturated moisture, the particles are covered by a water film. Subsequently, under certain drying conditions, the temperature of the shell layer reaches a level balanced with the drying conditions. The period from the start of drying until this temperature is reached is referred to as Stage I, the preheating stage. Afterward, due to the presence of the water film in the shell layer, the temperature of the shell layer remains almost constant, while the moisture content decreases proportionally during this stage. The drying degree at this point can be represented by a sloping line, and this stage is Stage II, the constant temperature and constant speed stage. Then, the drying rate decreases, and when the moisture content reaches a level balanced with the drying conditions, drying terminates. This stage is called Stage III, the deceleration drying stage. For the drying of the backup coat with silica sol, the constant temperature and constant speed stage is very long, so air-drying methods can be used to shorten the drying time.

From the above discussion, we understand that, using conventional methods, it is essentially impossible to complete the drying of the backup coat within 4-6 hours. Additionally, for castings with simple product structures, it might be possible to achieve this with some adjustments. However, for castings with complex structures, such as those with deep holes, blind holes, narrow slots, etc., achieving a 4-6 hour drying time for the backup coat requires abandoning conventional approaches and exploring new methods.

Now, let’s return to the question of how to achieve the goal of drying the backup coat in 4-6 hours. As mentioned earlier, under previous conditions, it might not have been achievable. However, with the rapid development of shell-making technology today, achieving this goal can be said to be easily attainable. Of course, any approach to improving efficiency must be accompanied by technical improvements. As we mentioned earlier, if only ordinary drying conditions are used, achieving a 4-6 hour drying time for the backup coat still falls somewhat short.

Naturally, this friend did not specify whether they meant drying the entire backup coat in 4-6 hours or drying a single backup coat layer in 4-6 hours. If it’s about drying a single backup coat layer in 4-6 hours, it should be achievable with some auxiliary measures without upgrading the equipment. However, if the goal is to dry the entire backup coat in 4-6 hours, then significant modifications to the currently used equipment would undoubtedly be necessary to achieve such a target.

Let’s first discuss the scenario of drying a single backup coat layer in 4-6 hours. We won’t go into products with planar structures but will focus on the drying of products with deep holes, blind holes, and narrow slots. If major changes are not desired, methods such as bottom blowing, localized exhaust drying, or air blowing on areas with deep holes, blind holes, and narrow slots can be employed to improve the local drying rate of the shell. Wuhan Zhongjing has a technical paper discussing the bottom-blowing method, which I find quite reliable. Additionally, some companies have patented methods for improving shell drying efficiency through localized exhaust and blowing techniques. These are all effective ways to enhance shell drying efficiency, and everyone can give them a try.

Now, regarding the scenario of drying the entire backup coat in 4-6 hours, we mentioned that it is entirely achievable today because there is equipment capable of doing so. I’m not sure if everyone has seen Zhongjing’s shell-making equipment, but the entire shell-making process takes only a few hours. Therefore, achieving a 4-6 hour drying time for the entire backup coat is not a pipe dream but a real and feasible outcome. Moreover, vacuum drying can also achieve this goal, but the cost is too high, making it unfeasible for ordinary factories due to budget constraints.

Of course, the purpose of shell-making is to obtain high-quality castings. If improving casting production efficiency and reducing shell production cycles come at the expense of shell quality, such practices are not advisable and could significantly impact the quality of the castings.

In summary: Achieving a 4-6 hour drying time for a single backup coat layer is generally feasible under current conditions with auxiliary measures. However, achieving a 4-6 hour drying time for the entire backup coat requires upgrading traditional equipment.