Permeability of Precision Casting Shell

Permeability of Precision Casting Shell

The permeability of precision casting shell is an important performance indicator of the shell. The permeability of the mold shell has a significant impact on precision casting castings. For example, it is often found that insufficient pouring of castings (including incomplete shape and rounded corners at right angles) and suffocation may be caused by poor permeability of the mold shell. Another point is that the breathability of the mold shell is also an important factor affecting the quality of the mold shell dewaxing. Today, precision casting manufacturer Yufeng Metal will talk in detail about the breathability of precision casting molds.

1、 Under normal circumstances, molten metal enters the mold cavity from the sprue cup through the sprue, runner, and inner gate. During this process, it is possible that the pouring method may be incorrect, resulting in gas entrapment or poor refining of the metal liquid with its own gas, or gas generated by the reaction between the metal liquid and refractory impurities in the mold shell. These are common problems encountered in investment precision casting. Some of these gases are avoidable, while others are unavoidable, and they must have a way out. Either the mold shell or the casting.

2、 The air permeability of the mold shell is not good, and the gas cannot escape, so it can only find space from the casting or outside the casting. In castings, invasive pores are formed; Outside the casting, there is a phenomenon of insufficient pouring. When dewaxing the mold shell, steam transfers heat from the outside of the mold shell to the wax mold module. The thermal conductivity of the mold shell is good, and the breathability of the mold shell is also important. Otherwise, the wet strength of the mold shell cannot resist the thermal expansion of the wax mold, leading to cracks in the mold shell and affecting the quality of the mold shell and castings.

3、 The general flow of metals is the same as that of other liquids. Where there is little resistance, flow there. Therefore, the locations where insufficient pouring occurs are usually at the end (top or bottom) of the metal flow or in areas with thinner wall thickness. Therefore, these areas should often be the focus of attention for process personnel. The permeability of the mold shell is an important indicator of its performance, but it cannot be predicted in general. You can only make predictions based on the recent production situation of castings. So, a good craftsman should first consider the breathability of the mold shell, and then design the casting process based on this.

4、 What factors are related to the breathability of the mold shell?

In fact, the mold shell is composed of binders and refractory materials (including powder and granular materials), so it is closely related to the physical and chemical properties of binders and refractory materials. Taking coatings as an example, if the viscosity of the coating is high and the powder to liquid ratio is high, the coating will become dense, resulting in poor air permeability; On the contrary, if the viscosity of the coating is low and the powder to liquid ratio is low, the coating will not be dense and the permeability will be improved. For the surface layer, the use of silica sol with small particle sizes results in a low powder to liquid ratio and good air permeability. For the transition layer and back layer, using coarse powder and sand with small mesh size also has a good effect on the permeability of the mold shell. In addition, the particle size distribution of sand and powder also has a significant impact on the permeability of the mold shell. The wide or narrow particle size distribution of sand and powder is not good. It is recommended to use sand and powder with bimodal distribution in the data. The roasting of the mold shell has a significant impact on its breathability. After the shell is baked, the breathability of the shell will be greatly improved.

5、 So, how can we solve the breathability of the mold shell?

1. To put it inappropriately, wool is produced on sheep, and the breathability solution must first be controlled from the raw materials. Suppliers can be required according to relevant standards, and then verified through incoming inspection and trial use.

Precision machining