How are sealed gas cylinders (like in office chairs) filled/pressured?

Question

Like, they don’t have an opening right? How are they sealed?

EDIT: I should rephrase: there has to be some sort of machine that pressurizes, and, in the pressurized environment, seals the opening while not letting out air. How does that look like?

In: Engineering

Answer 1

They have an opening, it’s just one-time filled then pinched and welded shut.

Or the retainer ring can be opened, filled, then pinched and welded shut,