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Do any of you old timers do fine casting face coatings? I came back the next day and the top layer viscosity went up 7s but the smear weight went down instead. It's a bit hard to explain. Also, yesterday the slurry bucket broke down and stopped three times.

heweifeng fusion casting daily learning 2023-10-20 13:34 Shaanxi

This is a question from a friend in a fine casting group on a voice. I will talk about my opinion. First of all, I have not done fine casting face coating. However, not having eaten pork doesn't necessarily mean you've never seen a pig walk, so that doesn't in any way affect my discussion of this issue. In fact, in my perception, fine casting coating is nothing more than a test of the uniformity of the paste test method.

Let's look at this friend's elaboration first. This friend said that when he came back the next day, the viscosity of the top layer had risen by 7 s. I think that, excluding the reason of measurement error, the increase in viscosity is probably due to the evaporation of water in the top layer of slurry. Other than that, there seems to be no other reason. The thickening of the slurry means that the powder-to-liquid ratio increases, i.e., the powder increases and the silica gel does not change, or the powder does not increase and the silica gel becomes less silica gel. Both cases may lead to the increase of powder-liquid ratio.

Because from this friend's account did not find the situation of renewed material. That is, there is no increase in powder, silica sol becomes less. Silica sol is only silica gel particles and water, which only water can change, so the reason for the rise in viscosity must be the evaporation of free water in silica sol. This is the basis for our frequent request to rehydrate the top layer.

In accordance with the silica sol slurry viscosity increases, the powder-liquid ratio increases, the thickness or compactness of the coating should be increased is not, but this friend to do the top layer of the coating weight instead of down, this is how it is?

There are only two reasons for the decrease in the weight of the coating: the coating is thin, or the density of the coating has decreased. Let's take a look at what happened. At the end of his question, this friend said that the pulp drum broke down yesterday and stopped three times. This may be the root cause of the drop in smear weight.

I don't know if you have this concept, when the pulp bucket is stalled for some reason, what will happen to the silica sol coating? Yes, powder-liquid layering occurs. Specifically, if the top layer of slurry is zirconium powder and silica sol, in the absence of stirring, the zirconium powder will automatically sink because of its own weight. (The density of zirconium powder is 4.5-4.9g/cm3, and the density of silica sol is 1.19-1.21g/cm3). If the stagnation time is long, a complete stratification of the powder may occur, with all silica sol on top and all zirconium powder at the bottom.

In fact, the silica sol coating examines the uniformity of the coating, and to be more precise, the uniformity of the zirconium powder distribution in the silica sol. The top layer of silica sol is actually a suspension of zirconium powder in the silica sol, and its stability determines the uniformity and weight of the coating.

The use of a suspending agent is discussed in the Investment Mold Casting Handbook. A suspending agent is actually a reagent that helps the silica sol coating achieve uniformity. If we don't use suspending agent, the uniformity of the coating itself will not be ideal, but it doesn't affect the use. A very important reason why the slurry drum should be turned all the time is to ensure the uniform suspension of the powder, albeit relatively uniform.

This friend of his pulp barrel stagnation occurred three times in one day, although it can not lead to complete separation, but has destroyed the original uniformity of the suspension of the slurry, so, relative to the complete uniformity of the suspension (such as mixing for more than 24 hours), his slurry is now the condition of the slurry is certainly not reached. In other words, it is an unstable intermediate suspension state, which is not up to the state of the slurry in the usual test. Therefore, the surface layer slurry smear of his test definitely cannot reflect the accurate data.

Another point is that the choice of test timing also determines the test results. As we know, the longer the slurry mixing time, the better the homogeneity of the slurry, then, the more uniform the measured smear is meaningful. As for the drop in the weight of the smear, it is also possible that he tested when the slurry had not fully resolved the delamination, then, the smear is light is a normal phenomenon.

The above is my view on this issue, there may be incorrect places, welcome friends to criticize and correct.