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Normally, in investment casting, the mould is preheated before pouring the liquid metal in to reduce thermal shock from rapid cooling.

In a vacuum, the heat transfer due to gas convection is reduced to zero, which will also reduce the thermal shock. Does that mean that, if performing investment casting in a vacuum, I do not have to preheat the mould?

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    $\begingroup$ No direct experience - but I'd expect thermal shock issues to still apply. Temperature gradients can form across mould. $\endgroup$ Jun 27 '15 at 16:17
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Convection, Conduction, Radiation

Of the three modes of heat transfer, only one is affected by the vacuum. As you noted in your question, gaseous convection should be eliminated. That being said, the thermal shock is present as soon as the hot material hits the mold. As soon as the two materials come into contact, the heat transfer will be by conduction. This is the same whether in a vacuum or not.

The mold should still be heated.

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