Solution to Cracks in Precision Casting Shell

Solution to Cracks in Precision Casting Shell

There are burrs on precision castings, and the location is determined, right on the side of the casting gate. What does this indicate? It indicates that there are significant issues with the casting process design of castings. Generally speaking, when the location of defects is uncertain, it may not necessarily be a process design issue, but in most cases, it is an operational issue; And if the location is determined and the defects are consistent, it can be almost 100% certain that there is a problem with the process design of the casting. Generally speaking, if there are burrs on the surface of the casting, it is definitely due to cracks in the mold shell, whether it is during the dewaxing, roasting, or pouring stages. The mold shell has cracked, and the strength at room temperature or high temperature is low. Therefore, for this specific part, today precision casting manufacturer Tianyao Metal will discuss the solution for you.

Let's first take a look at the wet strength of the shell. This casting is a single gate and not located at the top of the casting, so there may be poor dewaxing after shell making is completed. And especially for cylindrical castings. There is a description in Mr. Shanwu Yangshu's work, as shown in the following figure. And there is also a graph showing the relationship between the drying rate of the shell before dewaxing and the dewaxing index to illustrate the crack occurrence area of the shell. From the figure, it can be seen that the lower the drying rate of the shell and the higher the dewaxing index, the greater the probability of cracks appearing in the shell. Actually, this casting basically conforms to the description above. For cylindrical castings with a single gate and a relatively high dewaxing index V/a, is there any incomplete drying of the mold shell?

The answer is affirmative. Why? In fact, the grouping method of castings determines that it is not only difficult to hang sand on one side of the casting gate, but also not easy to dry. You can refer to the combination diagram shown in Figure 1. Due to the high tree rate of the casting group, there are 24 pieces in one group. The location of burrs in castings, except for the parts on both sides that may be relatively easy to hang sand, is difficult to ensure the quality of slurry and sand on the inner parts, and it is also impossible to check the quality of shell making. In addition, drying is also a problem. The further it goes to the back layer, the harder it is to guarantee the adhesion of slurry and sand. In addition, if employees do not operate carefully and do not clean the floating sand, these areas may even bridge. Think about it, dryness cannot be guaranteed, and the shell is not dense. How can the strength of the shell be guaranteed?

So, the result is: for cylindrical castings and a gate, the dewaxing index V/a is relatively high, and the mold shell cannot be dried well, so there may still be bridging. Therefore, the cracks should have occurred during the dewaxing process, and have little to do with subsequent roasting and pouring.

The next step is how to improve it. Since it is a process design issue, I think it is still necessary to improve the process design. Firstly, it is necessary to make the dense castings thinner, at least to improve the quality of shell making and drying issues; Secondly, increase the number of gates to minimize the pressure of wax discharge. We cannot improve the structure of castings, but we can improve the quality of shell making and the dewaxing factor.

As mentioned yesterday, a wave of rising raw material prices has harmed investment casting enterprises. However, we cannot be at a loss for ourselves. We have to do whatever we need to do. Improving quality is actually the best way to reduce costs.

Precision machining