How vacuum flasks are made

A vacuum flask is two bottles, one inside the other, with the air pumped out of the gap between them.

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The short version

A vacuum flask is two stainless steel vessels, one inside the other, welded at the neck. Sheet or tube is drawn or water-pressure formed into the two shells, then the air between them is pumped out, so heat can barely move by conduction or convection. Facing surfaces may be silvered to bounce back radiant heat.

Form two shells

An inner liner and a slightly larger outer shell are shaped from stainless steel, by deep drawing sheet or by swelling tube with water pressure. Dewar's originals used glass and brass.

Goes in
Stainless steel
Trim and neck the mouth

Edges are trimmed and deburred, the mouth is necked in, and threads are formed for the lid. The two shells have to fit each other closely.
Silver the facing walls

Surfaces facing the gap may be silvered, which reflects radiant heat back to where it came from.

Goes in
Silver
Weld them at the neck

The inner vessel goes inside the outer one, and laser or TIG welding makes the airtight joint. That joint is the main solid path left for heat to travel through.
Pump out the gap

Air is pumped from the space between the walls until it is a partial vacuum, and the opening is sealed. There is almost nothing left to carry heat across.
Fit the stopper or lid

A stopper or cap of polyethylene or cork goes on top. The neck and opening have no vacuum, so that is where heat gets out.

Goes in
Polyethylene

End of the lineVacuum flask

Worth repeating

James Dewar invented the vacuum flask in 1892 while studying the element palladium, and he never patented it.
Two German glassblowers, Reinhold Burger and Albert Aschenbrenner, made it a commercial product in 1904 and named it Thermos.
Large flasks sometimes need spacers to hold the inner vessel, but spacers also let heat leak through.