Breathability of Precision Casting Shell

Breathability of Precision Casting Shell

The relationship between the permeability of the shell and temperature of the three binders provided in the investment casting manual. Today, precision casting manufacturer Tianyao Metal will talk to you about the breathability of the mold shell.

The low-temperature breathability of the shell is the best. As the temperature increases, the breathability of the shell decreases, and after reaching a valley, the breathability of the shell continues to increase. In fact, the above conclusion was obtained through table tennis experiments. Due to the fact that in this experiment, the ping pong ball test shell was tested after being roasted. Let's see what the standard says (JB/T 4153-1999): Place the sample that has been parked for two days (fully dried) into the calcination furnace and raise the temperature to 180 ℃ at a rate of 8 ℃/min. Keep warm until the ping pong ball completely vaporizes. Subsequently, rapidly heat up to the calcination temperature at a rate of 15-18 ℃/min: the ethyl silicate shell and silica sol shell are heated to 950 ℃ for 1.5 hours; Water glass shell at 850 ℃, insulated for 1 hour. Cool with the furnace and take out the sample for testing.

So, can we conclude from the above figure that the shell with sufficient roasting has the best air permeability. Or conversely, the better the breathability of the shell, the more thorough the shell roasting will be? I feel that there is room for discussion. Why? Since the more thorough the roasting, the better the breathability of the shell, why isn't the second roasting of the shell more sufficient? Why has the breathability decreased? In fact, in the above experiment, the influence of temperature on the viscosity of gas flow was not taken into account first. In fact, the higher the temperature, the greater the viscosity of gas flow (which is different from that of liquids), and the resistance to gas flow will also increase. It seems that the permeability of the shell decreases, but in fact, the temperature intensifies the viscosity of the gas, which is not related to the permeability of the shell or that the permeability of the shell has not changed at all. If we believe that as the temperature rises, the breathability of the shell decreases, this is incorrect. However, as the temperature rises and the gas becomes sticky, the combined result is a decrease in the high-temperature permeability of the shell.

In addition, we must understand all the factors that affect the breathability of the shell in order to truly understand the factors that determine the breathability of the shell. In fact, the permeability of the shell mainly depends on the density of the shell structure, or in other words, the proportion of porosity on the shell wall. The properties of binders, refractory materials (including refractory powder and refractory aggregate), shell making process, and calcination process are the main factors affecting the permeability of the shell. For example, the ratio of powder to liquid, the physical properties of silica sol, the physical properties of refractory materials, the number of layers of the shell, the shell making process, and the roasting process, etc. Therefore, roasting is only one factor that determines the permeability of the shell. Baking alone can fully ensure the best permeability of the shell. It can only be said that, when other factors of the shell are fixed, the more thorough the roasting, the better the permeability of the shell. Of course, perhaps there is a vertex in the breathability of the shell, which I think should be.

Of course, the above views are only personal and there must be some inaccuracies. We welcome criticism and correction. In addition, the breathability of the shell is also something that cannot be seen or touched, and it is difficult to have a specific measurement method. The key is that our shell consistency is too poor, but it does not affect our discussion of shell breathability.

Precision machining