Mixing slurry for surface coating of investment casting precision casting

Mixing slurry for surface coating of investment casting precision casting

Last week, we introduced several aspects of surface coatings in precision casting. Today, precision casting manufacturer Tianyao Metal will continue to talk about the introduction of slurry in surface coatings. In fact, slurry preparation is the process of fully wetting silica sol and refractory powder. In the book "Practical Investment Casting Technology" edited by Jiang Buju, there is a sentence that can explain the importance of slurry mixing, the standards of slurry mixing, and the influencing factors: the preparation of coatings is also an important part of ensuring the quality of coatings. When preparing, each component should be evenly dispersed, thoroughly mixed and wetted with each other. The refractory powder does not agglomerate and is thoroughly mixed with the binder to ensure excellent coating performance. The equipment, feeding sequence, and mixing time used for coating preparation can all affect the quality of the coating.

The process of preparing pulp is actually a preparation process, but it is obvious that the freshly prepared pulp cannot be used and must go through a process of awakening the pulp, which is very important. Why? Because the newly mixed slurry barrel often contains many bubbles, which are often caused by the uneven surface of the refractory powder. When the binder is mixed with the refractory powder, the binder will replace the air that originally existed between the gaps on the surface of the refractory powder, and these air will form bubbles in the slurry, which will affect the stability and concentration of the slurry. Therefore, it is necessary to drive out the bubbles in the slurry. This is actually a process of fully wetting the binder and refractory powder, and it is also a process of awakening the slurry.

To ensure the quality of the coating, when all surface coatings are prepared with new materials, the mixing time should be greater than 24 hours before use. If some of the materials are newly prepared (also known as renewal materials), the mixing time can be shortened to 12 hours. The mixing time for the transition layer and back layer coatings can be less than that of the surface layer coatings. When all new coatings are added, the mixing time should be greater than 10 hours. When some are added, the mixing time can be shortened to 5 hours.

We can see from the above description that mixing pulp is not a simple task. It may have been included in our process documents and textbooks in less than a hundred words. However, this does not mean that mixing is an unimportant process. Take a look at the point like defects on the surface of the product shown in Figure 1. Is it possible that they are caused by bubbles in the slurry? It's hard to say, so we dare not use them immediately.

One thing to remind is that manufacturers producing in large quantities should not mix coatings in the shell making operation bucket, but should set up specialized coating preparation equipment and continuously stir for 24 hours, so that the coating quality will be better. Some manufacturers also use quick mixers to help with mixing, which are effective ways to solve the quality of the pulp. On the contrary, the method we usually use to mix pulp in the barrel is a lazy approach, just like the daily shell making method. Of course, I just hope that when making shells, you don't use the method of preparing coatings on site, because after all, you are engaged in a high-tech casting industry - investment casting. I hope that you don't dry investment casting into ordinary casting, which is beyond the scope of precision casting.

Precision machining