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So in this video(https://www.youtube.com/watch?v=v1z829NF9PA) at around 2:35 it says that the alluminium ingots for alluminium foil, after being heated, are cooled down with a mixture of 95% water, 5% oil. Why not simply use 100% water? Because I looked it up online and it seems most oils have a lower specific heat than water, so would be worse for cooling. Is it that oil is somehow cheaper, or some other reason? This episode was in 2001 btw, on Jan 6(so before 9/11)

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That mixture concentration sounds the same as what is used for CNC coolant to me.

When machining metal we want to use water as a coolant because of its high specific heat capacity but water causes rust, and so it is mixed with a water soluble oil at a concentration of about 5%.

You might then ask "Rust? But it's aluminum." Guess what: aluminum corrodes as well. The oxide layer is not as easily visible, and doesn't lead to as deep a failure as with iron, but aluminum oxide tends to fall off as a power. Not good in clean environments. All the equipment handling that aluminum is also made of steel.

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  • $\begingroup$ Ah okay. But why 5%? Is that arbitrary, or is there some calculations involved to determine that? Or is it trail-and-error? $\endgroup$
    – MeltedStatementRecognizing
    Sep 3, 2022 at 16:03
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    $\begingroup$ Different water soluble oil formulas work best at at different concentrations. Too low and it cannot sufficiently prevent rust. Too high and the cooling capacity is reduced but lubricity is increased (and it leaves a thick nasty oily film when it dries). 5% is typical concentration across many coolants but you could run much higher if you want (some people run at 15%). The coolant I use at work is supposed to be a minimum of 5% concentration but as the water evaporates it goes up to 8% or higher and then I top it off with more water because I don't want that nasty film. $\endgroup$
    – DKNguyen
    Sep 3, 2022 at 16:04
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    $\begingroup$ And many of these water soluble oils also are bio-degradable (i.e. rot) so that's another reason you don't want the concentration too high because higher concentration means more stuff to rot. But higher concentrations also prevent rust better (if you let water-based coolant dry the steel won't rust immediately but if you leave it long enough some rust will still form and higher concentrations reduce that). $\endgroup$
    – DKNguyen
    Sep 3, 2022 at 16:17
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    $\begingroup$ @MeltedStatementRecognizing Aluminium corrodes just like steel rusts… $\endgroup$
    – Solar Mike
    Sep 3, 2022 at 20:44

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