Sealing and capping
Updated
The last operation, and in a pressurized product the component most likely to fail first.
The seal holds the pressure
In a still drink a seal keeps things out. In a carbonated or live one it holds gas in against pressure that rises with temperature.
Which makes the closure a pressure component rather than a lid, and makes the seal check a safety check.
Crown caps
The beer standard, cheap, excellent under pressure, and they need a crimper set correctly.
Too loose and it leaks; too tight and the crown is deformed and the seal is compromised.
The liner does the sealing, not the metal, so a crown with a damaged or wrong liner fails whatever the crimp looks like.
They are single-use, which is a customer consideration and an advantage for tamper evidence.
Screw and ROPP
A screw closure suits a still or lightly carbonated product and is what customers prefer for resealing.
ROPP, roll-on pilfer-proof, is formed onto the bottle thread at capping and gives a tamper-evident band, and it needs a capping head set up properly.
Both depend on torque. Under-torqued leaks, over-torqued distorts the liner and leaks anyway, and in a pressurized product either is a failure that appears days later.
A torque gauge is cheap and it is the difference between a consistent seal and a hopeful one.
Checking the seal
Torque on a sample through the run.
A pressure or leak test on a sample: invert a filled bottle and look, or hold sealed bottles warm and look for weeping.
And keep warm retained bottles specifically to test the seal at the pressure it will actually meet, behind a screen, because that is the condition in which a closure fails.