Investment casting process—the problem of shell delamination

   A few days ago, a friend asked me if I had any materials on shell delamination. I thought he had a problem with some product, but actually, he wanted me to share information on shell delamination. And sure enough, we encountered such a shell delamination issue today. 

     Here's what happened: After firing, when washing the shell of a product, we found that the inner surface layer had severely peeled off. No matter how we washed or poured it, we couldn't clean the shell properly. So, we wanted to see what was wrong inside. After breaking open the shell, we found that not only had the surface layer peeled off, but some sharp corners were also damaged, as shown in Figures 1 and 2 below. This is a typical delamination phenomenon: the surface layer separates from the transition layer and then peels off. So, what causes this shell delamination? Let's try to analyze it.

 First, the separation of the surface layer from the transition layer (or the separation of the first two layers from the back layer, etc.) is clearly caused by a loose connection between the surface layer and the transition layer. We mentioned the cause of this delamination when discussing sand inclusion defects earlier. Due to layering, the local strength of the shell is poor. Under the impact and erosion of the molten metal, it bulges from one place or drifts to another, forming sand inclusions or pinholes.

 We know that investment casting shells have a multi-layered structure. In a high-quality shell, the layers should interlock to form a tightly integrated structure. The layers are interlocked and bonded together by a refractory coating made of silica sol and refractory powder, as well as sand particles that form the shell's framework, ultimately forming a unified shell. Since the silica sol shell is formed by interlocking and bonding between layers, separation between layers must be due to poor interlocking or bonding. Therefore, we should look for the causes in this regard.

 Generally, delamination in silica sol-based coatings can be categorized into the following situations:

  The first type of delamination occurs when the surface layer's abrasive particles are too fine, resulting in a relatively smooth surface after coating. This makes it difficult for the transition layer's abrasive to form a strong embedded structure, leading to poor bonding strength. Furthermore, excessive dust or moisture content (≥0.5%) in the abrasive, coupled with failure to remove loose abrasive particles during the application of the next coating layer, also contributes to poor bonding strength and delamination.

 The second type of delamination occurs when the transition layer coating has too high a viscosity, failing to penetrate the surface layer and resulting in a weak bond between the surface and transition layers.

 The third type of delamination occurs when the surface layer's abrasive is applied too slowly, preventing the abrasive from embedding within the coating layer and instead adhering to the surface, or even when no abrasive is applied at all. The actual bonding between coating layers is inherently weak. Additionally, the lack of pre-wetting between layers also compromises the bonding strength.

 The fourth type of delamination is silica sol re-dissolution. Sometimes, the surface layer is not completely dry when applying the transition layer. If we rush to apply the transition layer, surface layer delamination may also occur.

 Another situation is when the expansion rates of the surface layer and the transition or back layer differ significantly, potentially leading to shell delamination during firing.

     In reality, shell delamination is a complex process with numerous influencing factors, making it difficult to pinpoint the primary and secondary causes. Therefore, when improving shell delamination, we must adopt a comprehensive approach, considering all relevant details and paying attention to every operational detail to effectively mitigate this defect.

     In fact, we don't need to elaborate on the reasons for improving shell delamination; we simply need to reverse the above analysis, and most of our delamination defects will be improved.