Recently, there was a product with shell bulge, so that the post-processing process needs to increase the amount of grinding, some even caused the casting scrap, what is the reason?

      In FIG. 1, the red line marks the bulging position of the shell

      The product portfolio is shown below. The areas marked in red on the combination diagram are bulging areas.

      Figure 2. Product portfolio diagram

      Let's analyze the causes of the product shell bulge. We'll do it the old-fashioned way. Part of the shell bulge, indicating that part of the shell strength is not good, can not resist the heat and kinetic energy impact of the metal liquid, resulting in bulge. So, why is the red line part of the shell not strong?

      We know that wet strength is bad for molded shells for two reasons. One reason is that the shell is not dry, and comes out of the womb with birth defects. One reason is that there are not enough layers, and therefore not enough strength. Let's break it down one by one.

      The bad drying of shaped shells includes two aspects: poor drying and excessive drying. What kind of situation might exist in this product? Because the swelling part of the shell is located at the bottom of the runner, and near the surrounding opening groove. (See the figure below), there is a sprue on the top. Comparatively speaking, the drying speed of the inner casting is slower, and the opening groove around the casting is the last. This is the drying order of the inner cavity shell of the casting. Then, there may be excessive drying outside, and the plane of the inner cavity is just dry, and the position of the surrounding opening groove is not dry, that is, the wax mold module drying is not synchronized. Because the drying process of silica sol coating is actually evaporation of water, the process of coating shrinkage, due to uneven shrinkage, is also the process of producing shrinkage stress. Once the stress exceeds the wet strength of the coating.

      FIG. 3. Structure of peripheral open slot

      This is likely to produce cracks, which foreshadows the swelling of the rear shell.

      There is also a case of drying too fast, that is, drying excess. Drying too fast, greater shrinkage stress, more prone to crack. Drying too fast for the surface, there are only two cases: one is the surface coating viscosity is too small, the other is the use of wind blowing surface. The actual surface drying is natural drying, so wind blowing is non-existent. The only possibility is that the surface viscosity is low.

      There is also a point to note that the surface layer can be bulged, indicating that the surface layer and wax mold surface combination is not good. We have already talked about this in the previous analysis. There are three points: one is the module without cleaning (according to wax models because the product is two pieces of welding, afraid of cleaning can destroy the wax models, so the cleaning from) the second is the surface wetting agent of serous basically without adding, so the wax models between colloidal silica coating and not very good wetting, three is a big flat coating contraction prone to crack. In this case, the crack is located near the round table at the bottom of the wax mold, which is also the place of stress concentration.

      It is possible for modules to have poor strength due to insufficient layers, but it is unlikely for large areas to occur and will not be described here.

      Then, from the above analysis, it can be seen that the problem basically occurs in the dry stage of the shell, which is wet strength. Is it possible that it was caused by high temperature and low intensity?

      Let's look at the determinants of high temperature intensity. In the case of raw material determination, the high temperature strength of shell depends on the drying degree of shell and shell roasting. That is to say, the shell dry, roast thoroughly, the strength of the shell will be good. Generally, the calcination temperature of silica sol shell is about 1020℃ and the calcination time is 120 minutes. The temperature and time are enough for the 5 and a half shells of this product. Moreover, because the casting is aluminum alloy, the secondary roasting temperature of the shell is about 450℃, and the density of aluminum alloy is about 2.7, and the pouring temperature is 600℃±5℃. Under gravity, the general silica sol shell strength is enough to support the thermal energy and kinetic energy impact of aluminum alloy. Therefore, if the shell bulge, it should not be in high temperature pouring or high temperature pouring just played a role in helping.

      Figure 4. (a) Shell surface layer is partially bulged (b), and (c) the backlayer is ruptured due to bulging, leading to the backlayer coating entering the gap

      There is also a point that we often ignore, that is, dewaxing shell. The wet strength of the shell is tested when the shell is dewaxed. If the shell is well dried and wet strength is high, the shell will not crack when it is dewaxed. On the contrary, the drying is not good, the wax expansion stress is very large, it is likely to lead to the shell surface layer rupture, and then the shell bulge.

      Let's recall our thinking, the intensity is bad, find out why the intensity is bad, and then eliminate non-major factors. From the above analysis we can know that this type casting shell bulge is likely the cause of bad with wax models wetting surface coating, surface coating viscosity caused by inappropriate and dry surface is not synchronized, may crack when dewaxing, crack increased during roasting, the metal liquid into the shell to promote shell burst to bulge.

      Of course, this is just analysis, and it has to be validated. For example, after dewaxing, a damage test is performed on the molded shell to determine whether the above analysis is correct. But apparently, the production section had a discount on executing the process documents. As we know, casting is a process with many and complex factors, in which which factor plays the main role must be verified by practice. On the other hand, every detail in casting is too important to be taken lightly.

      So, it is very intuitive that we come down the product production needs to pay attention to the following matters: surface cleaning should be in place, the wetting agent in the surface coating should be added to ensure the surface wetting effect in the slurry; The surface coating viscosity should be appropriate; Dry the surface layer evenly.

      That's all for today. I'm a casting engineer. Pay attention to me and learn different investment casting.