The vaccum process is said to be more precise so there is a gain in productivity, The structure of the iron is more homogenous.
Nevertheless, it is used for the less expensive models, as the quality of the iron in the end is less good.
A plastic film is employed to create the shape of the caul (I dont know if it is then mixed with the iron)
The sand process is providing more moisture in the mold to the iron, and the structure of the metal have thinner grains (micro crystalline structure)
but less homogenous.
First, let me say that I’m not a metallurgist. Nor am I a casting or foundry expert. What I’ve learned about iron casting has come from spending a lot of time in foundries over the past few decades, asking a lot of questions of the people actually doing the casting and studying much of the available literature on the subject.
It’s also come from designing a few frames (plates), having them cast and building the pianos. And then sometimes redesigning the frames, having new castings made and building more pianos. It can be a brutal school and what you learn doesn’t always conform to what all that nice piano marketing stuff would have us believe. Or all those nice theoretical books, for that.
That said, I will repeat what I wrote earlier, the quality of the iron is most often the same in both process!
Vacuum-cast frames are used by the manufacturers of production pianos for all the reasons that have already been discussed. Primarily, they are more precise and they can be less expensive. It is inaccurate and misleading to say they are only used for “less expensive” models. Manufacturers such as Kawai and Yamaha use them in their smaller pianos—regardless of price—because they are good frames.
Have you ever actually been in a foundry using the vacuum process? It’s a fascinating process. The pattern is placed in a “sandbox” and covered with a thin, plastic film so the sand never actually comes in contact with the pattern. A vacuum is drawn pulling the plastic film very tightly against the pattern. Loose, dry sand is dumped over the patter and the whole thing is sealed. (The sand is damp in the green-sand casting process; that is the meaning of the phrase “green” sand.) A cover comes down over the sandbox and a vacuum is drawn over the whole thing. The vacuum pulling against the plastic film holds the sand in place. That is the purpose of the vacuum. When the sandbox is raised it all comes up as a unit.
The pattern is flipped over and repeated for the bottom of the pattern. Then the two are joined and iron is poured into the cavity. Since the temperature of the iron is upwards of 1,200° C (≈ 2,200° F) the plastic film vaporizes as the molten iron enters the mold. The poured frame is then left to cool in the normal manner. The cooling is not forced or accelerated in any way.
The advantages are that finer sand can be used which means that the finished casting is smoother and more detailed. And, of course, even though the initial cost of the vacuum process line is frightfully high the final cost of the frames can be lower because it is a fairly automated process and, since it is an assembly line process, that cost can be spread over many frames.
Within limits the structure of the iron can be whatever you want it to be depending on the physical characteristics of the finished product; in this case a piano frame. The grain may or may not be finer. In the case of Young Chang/Weber foundry the iron is the same for both vacuum-cast frames and sand-cast frames.
Again what seem to be in cause is the better damping of tone with grey iron obtained with the traditional casting.
That said, some plates are designed to be in vibration and to transduce tone, they are heard in the final tone. they are installed on dowels and the belly have to be massive then.
More massive plates go along with lighter belly, then the case is more filtering the tone and the plate is supposed to be neutral (Boesendorfer for instance)
There is a lot of supposition in there. A lot! Just because a frame is mounted on dowels doesn’t mean that it was “designed” to be in vibration and “transduce” tone. That would be a very inefficient system and the cost would be seen in a rapid rate of decay and an overall short sustain time. I’ve also heard factory representatives go on about this but they tend to get rather confused when asked about the purpose of all those nosebolts.