Analysis of the causes of burrs in precision casting

Analysis of the causes of burrs in precision casting

The reason for burrs in castings during investment casting production is firstly the cracking of the casting shell during dewaxing. The first reason for cracking in the mold shell is due to insufficient wet strength of the mold shell, accompanied by poor wax removal or poor temperature control in the mold shell chamber, resulting in mold shell cracks caused by casting thermal expansion. Another important reason is poor surface permeability, which leads to poor dewaxing. The last possible reason for cracking in the mold shell is the unequal setting of the working parameters of the dewaxing kettle. Today, precision casting manufacturer Tianyao Metal will discuss in detail the causes of burrs in castings mentioned above.

1、 The wet strength of the shell is insufficient. Let me give an example of a shell making parameter that produces burrs: the surface layer uses 270 mesh zircon sand powder (graded powder), with a powder to liquid ratio of 4.5:1, the viscosity measured by the 4th cup is about 43 seconds, the temperature is 22 ℃, the humidity is 65%, and the drying time is 6-8 hours; The shell is made of 5 and a half layers, and the drying time for the other layers is also 6-8 hours. From the above shell making parameters, I feel that the powder to liquid ratio of the surface layer is a bit high, with a powder to liquid ratio of 4.5. I consulted with the refractory material supplier, and they said that the powder to liquid ratio is not high because it is a 270 mesh zircon powder. But using graded powder, graded powder can be mixed with high powder to liquid ratio, but high powder to liquid ratio can damage the permeability of the mold shell. According to the drying conditions of the surface layer, it seems that the problem is not significant, and the drying of the surface layer should also be acceptable. However, the drying time of the sealing layer is relatively short. Generally speaking, the viscosity of the sealing layer slurry is the smallest, directly penetrating the first five layers to the surface layer. In theory, it is recommended to dry for more than 24 hours, which is relatively short and may cause lower wet strength.

2、 Design of pouring system for castings. We know that the design of the pouring system also has a wax removal function. If the wax removal is smooth, the probability of cracking defects in the mold shell will be greatly reduced. Generally, a pouring system design similar to top pouring is used, but in reality, this pouring system design is relatively beneficial for wax removal. If the contour of the casting is relatively large, the lower part (top of the casting), and there are fewer wax outlets, it is easy to form a certain degree of poor wax discharge.

Precision machining