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Background

About the gold casting process explained here: https://morrisandwatson.com/casting/the-casting-process

The casting metal is melted in a special casting machine that is programmed to insert the metal under vacuum into the flask at the right time, temperature and rate.

Wax tree

Furnace melting

Furnace melting

Tree

Yellow gold tree

Question

About furnace melting and filling the mold with molten gold, does gold enter the mold under pressure or without any pressure?

The metal is under vacuum. So, does it definitely mean the metal is under pressure? I'm a bit confused with pressure, tension, compression, ...

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From a purely physical point of view, being "under pressure" means having a force applied to you. Our atmosphere is constantly applying a force on us from all sides, so objectively speaking we are constantly held under pressure. Our bodies cannot function without that pressure, like in the vacuum of space.

The linked website doesn't explicitly state so, but I assumed the liquid gold would enter the mold without pressure. I've been proven wrong, please check the answer of OP.

My original assumption was that in order to "insert the metal under vacuum into the flask" the molten metal must be in a vacuum as well. It is poured into the mold and flows down by gravity alone. Depending on when the vacuum is released (while the metal is still liquid enough to flow or not), the atmospheric pressure will push the gold into the smallest nooks and crannies of the mold without leaving air bubbles behind.

The opposite of that process would be centrifugal casting, where the mold is spun in a circle and the centrifugal force pushes the material into the mold. You can read more about both casting methods here (Thanks to Allison C. for the hint). I see no indication that this process is used by this specific manufacturer. If you ave questions or concerns about their process, maybe ask them directly for more detailed information.

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  • Thanks :) very helpful answer. I'm going to ask them directly.
    – Megidd
    Commented Nov 22, 2023 at 7:02
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    The "opposite process" I've used in the past to "apply a force" was using centrifugal casting (aka "spin casting") to force the metal into all areas of the mold. Here's a pretty nice summary of both processes I happened on: gemsociety.org/article/lost-wax-casting
    – Allison C
    Commented Nov 22, 2023 at 15:14
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    I do not understand the assumption that the gold enters without pressure. First, why have the vacuum at all in that case? If there is a vacuum in the mold below and the gold is introduced with the weight of the atmosphere (or some fraction of it) above than the gold is being forced into the mold, ie is under pressure.
    – rebusB
    Commented Nov 25, 2023 at 22:00
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As @Elmy correctly suggested, I asked them directly. This is their response.

Thanks for the question.

With our vacuum casting machines, the melting and casting process are done within one large module.

The preheated casting flask is inserted in the bottom ection and the metal is loaded into a crucible above the flask.

The unit is sealed, a vacuum starts and the metal is then melted.

When the vacuum reaches the desired pressure and the flask and metal reaches the required temperature, the now molten metal is dropped into the flask.

Below is a video of a similar unit, which may help you to visualise the process.

Thanks.

https://www.youtube.com/watch?v=mqXk8Trn-Pc

The shared video has an explanation shown below.

..., the green needle is the flask chamber under vacuum, the blue is overpressure pushing metal into the flask.

I guess the difference between overpressure minus vacuum pressure is a decisive factor in deciding if the metal can be considered to be under pressure or not. But I'm still not sure whether I can consider the gold to be entering the mold under pressure or free from pressure. I need to know that as a requirement for numerical simulation of the casting process.

Screenshot

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    Well, isn't this exactly what you need to know? If there's a difference in atmospheric pressure between the flak chamber (where the mold is) and the crucible chamber, then the metal is entering the mold under pressure. I'm sure if you continue reading different sources about vacuum casting machines and the process of vacuum casting, you can get a more definite answer.
    – Elmy ♦
    Commented Nov 23, 2023 at 6:07
  • @Elmy You're right. An available casting simulation tool is able to do the numerical analysis for pressure-free i.e. shape casting. However, the tool is not able to simulate the high pressure. I'm going to research more about the casting machines and about the simulation tool.
    – Megidd
    Commented Nov 23, 2023 at 6:23
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    I don't know much about casting jewelry, but your approach sounds very overcomplicated. Does it really matter how much pressure this tool can simulate? Do pressure differences in the high values even yield different outcomes?
    – Elmy ♦
    Commented Nov 23, 2023 at 9:00
  • @Elmy Right, those are valid points. I have to talk to the developers of the available tool. It's an industrial tool. It's never used to simulate the gold casting before. According to the tool developers, it cannot simulate high pressure casting. I'm not sure what's their definition of high pressure. Maybe what they call high pressure in their industry, is way beyond the pressures of the gold casting process.
    – Megidd
    Commented Nov 23, 2023 at 9:35
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    The vacuum below creates the pressure from the atmosphere above. That's just plain physics. The gold is 100% under pressure. If you are simulating the volume of material involved that would remain consistent with "pressurized" casting via vacuum or drawn in by gravity alone, the gold is not going be be compressed into a smaller space. Velocity of filling the mold would not be the same. I imagine the vacuum molding is more about getting detailed castings than just speed though.
    – rebusB
    Commented Nov 25, 2023 at 21:59
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Not a simple answer because a few processes are used. A major factor is the high density of gold ;

  1. liquid gold pushes in to all volumes.
  2. liquid gold pushes any gases into porous mold material leaving gold filling the pattern.
  3. centrifugal force adds to the density of gold and it pushes into the mold.
  4. (repeat A in vacuum.).

Where I worked, the mold sat on a vacuum table and a partial vacuum helped pull gold down into the porous mold. I am sure a few more options can be visualized.

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  • The point missed by the edit is gold has a very high density ( 19 ,if I remember) and so gravity will push it into any opening in a mold.
    – blacksmith37
    Commented Dec 5, 2023 at 16:58

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