I’ve got a clogged sink, and have been reading the labels on a bunch of different products lately. I’d always assumed that they were a strong acid of sorts, which would dissolve whatever sort of material was built up in the pipes. But I’ve noticed that a lot of these products specify that they’re non-corrosive, so I’m not sure what other sort of mechanism may be at play here.
Also, adding heat can really help a drain cleaner work.
I pour a pot/kettle of boiling water down the drain, add the cleaner, wait whatever it says to on the bottle, then dump a second pot of boiling water down to rinse.