Improving the strength of naturally formed cores in precision casting shells

Improving the strength of naturally formed cores in precision casting shells

What is natural core formation in investment casting? The so-called natural core formation refers to the core formed naturally through the shell making method, which is used to form holes or grooves on the casting. We know that there are many ways to form casting holes or grooves, including using mold cores, soluble cores, ceramic cores or gypsum cores, and so on. Among them, the holes and grooves formed by molds and easily soluble cores are generally naturally formed through shell making methods. Of course, there is another method, which is to first shell the local holes and grooves of the casting, dry them, and then feed them into the wax mold or mold as the forming core, reducing the drying time in casting production and improving production efficiency. This is a unique approach in investment casting.

Among these core forming methods, the strength of ceramic cores is relatively better to ensure, and the quality of natural core forming depends on the shell production process, actual operation, and drying conditions. The main topic we are discussing today is how to ensure and improve the strength of natural core formation. In the previous push, a friend's shell experienced insufficient shell strength leading to fracture, as shown in Figure 1. The strength of natural core formation mainly depends on two aspects. On the one hand, it is the quality of shell making, which mainly refers to whether each layer of coating is uniform, whether it is firmly adhered, and whether there are missing layers or bridging situations; On the other hand, it refers to the dry condition.

1、 In terms of shell making quality, it is important to pay attention to the uniformity, firmness, and bridging phenomena mentioned above. Operators need to determine the viscosity of the coating and the size of the sand particles to be used when operating the holes and grooves. For small holes and grooves, it may be necessary to make multiple layers of low viscosity and small particle sand. Before and after production, the floating sand in the holes and grooves should be cleaned in a timely manner, and compressed air should be used if necessary to ensure that each layer is achieved. The drying strength and firing strength can be increased by 40% to 50%. Of course, this is also one way to improve the strength of shell making.

2、 Another supporting condition for shell strength is the degree of dryness. Due to the relatively harsh drying conditions inside the core, the drying process is very difficult compared to the surface, and cannot be treated equally according to conventional drying conditions. The situation mentioned in the previous tweet is basically this kind of situation. So, it is necessary to perform special drying on the local shell of the core in order to lay a solid foundation for the strength of the shell of the core. As we mentioned earlier, this strength is hereditary. If the quality of drying or shelling cannot be guaranteed, the wet strength and high temperature strength in the later stage will be greatly reduced, which can also be called a sequela.

So, what should we do if we suspect or intend to enhance the strength of natural core formation? Here, I will introduce several methods to you:

1. Add metal short wire method.

Precision machining