Analysis of surface layer detachment in precision investment casting

Analysis of surface layer detachment in precision investment casting

In the production process of precision casting, some shell surface layers may fall off. Today, investment casting manufacturer Tianyao Metal will analyze several reasons for the detachment of the lower layer. The detachment of the surface layer indicates that the surface layer itself has poor strength and cracks; Secondly, the connection with the transition layer is definitely not tight, otherwise it will not fall off on its own. So, the problem lies in the connection between the surface layer itself, the transition layer, and even the back layer.

1、 The surface layer falls off during the immersion of the transition layer. This phenomenon can be divided into two types: one is excessive drying, and the other is inadequate drying. Anyway, it's all due to drying problems. In general, the surface layer is relatively thin compared to other layers, and the drying time is relatively short. It is currently winter, with low air humidity and faster drying of the surface layer. If the original drying time is still used, there is clearly a risk. So, it is necessary to make appropriate adjustments according to the specific situation of the drying room. If the humidity is low and the drying time is long, for example, if the humidity is below 50% and the drying time is longer than 6 hours, it may cause excessive drying and the surface layer may fall off directly during the immersion transition layer. This situation often occurs in single shaped castings such as cylindrical, rod-shaped, and flat castings.

1.1 If the drying time of the surface layer is insufficient, it will directly enter the lower layer for shell making. The surface layer has not yet dried properly, and the silica sol dissolves back, causing the surface layer to peel off. Of course, it is also possible that the wax mold is not cleaned properly, causing the surface layer to separate from the wax mold, resulting in shrinkage and cracking of the surface layer, ultimately leading to peeling off of the surface layer.

The above situation belongs to the insufficient strength of the surface layer itself. Let's take a look at the situation where the connection between the surface layer and the transition layer is not good.

1.2 The strength of the silica sol shell is connected as a whole by the subsequent layer of slurry penetrating the front layer until the surface layer. If the viscosity of the second layer (transition layer) coating is too high, the slurry of the transition layer will be difficult to penetrate into the surface layer. If the two are not tightly combined, the surface layer may detach from the transition layer and peel off.

2、 The silica sol shell actually has strength by interlocking rough surfaces between layers into a whole. If the sand on the surface layer is too fine, it will be difficult to form a interlocking inlay structure between the surface layer and the transition layer, and the connection between the two will not be reliable. Therefore, the probability of the surface layer falling off will be greatly increased. If the dust content and water content in the sanding material are too high, the connection between the surface layer and the transition layer will inevitably become virtual, and the detachment of the surface layer is an inevitable phenomenon. In addition, if the surface layer is sanded too slowly, it may cause the sanding material to fail to embed into the interior of the surface layer coating, float on the coating, or not sprinkle sand at all to form a direct connection between the surface layer coating and the transition layer coating, making it difficult to form a strong inlay structure. The detachment of the surface layer is beyond doubt.

3、 The floating sand on the surface layer is not cleared, so the transition layer shell is directly made. There is no connection between the surface layer and the transition layer, and the transition layer is only connected to the floating sand in a virtual manner. This type of surface layer will definitely fall off.

4、 The main reason for the detachment of the layer after calcination is due to the different thermal expansion coefficients of the two coatings, which causes natural separation between the surface layer and the transition layer, ultimately leading to the fact that the surface layer falls off.

After discussing the possible causes, we need to take corresponding improvement measures. For the two situations of poor drying, we first need to ensure the basic conditions for the surface layer to be dry. For example, the temperature should be kept at 22-24 ℃ as much as possible, the lower limit is best, and the humidity should be kept above 50% to achieve the best 60%. The principle of surface drying is low temperature and high humidity. Another is the drying time, which is actually a relatively difficult to control and master data. Here, I provide a method for everyone to touch it with your hands. Generally speaking, after the surface layer is dry, the surface becomes dry and hard, while before drying, moisture or localized whitening can be felt. Of course, this also requires oneself to frequently touch, feel, and summarize.

Regarding the situation where the connection between the surface layer and the transition layer is not tight, in addition to using pre wetting methods, we must also pay attention to: the viscosity of the transition layer should not be too high; Sanding material on the surface layer should not be too fine; The mass fraction of dust and water in the sand should be less than 0.5%; Sanding should not be done too slowly, and the floating sand on the surface layer should be removed when making the transition layer; It is also important to choose materials with similar thermal expansion coefficients for the surface layer and transition layer, as these are good ways to improve surface layer detachment.

Precision machining