[This would be a good opportunity to recommend our new weekly-Halacha class
for advanced readers. This issue was, in fact, discussed quite recently.
Rabbi SZ Auerbach did prohibit metal bottle caps which leave a ring:
>But only caps made out of metal are included in this prohibition. It is
>permissible to unscrew a plastic cap, even if it separates and leaves a
>ring around the bottle neck. This is because plastic caps are functional
>even before they are screwed onto a bottle (as opposed to metal ones which
>- due to technological differences - become operational only after being
>unscrewed from the bottle the first time) (8)
>8 Harav S.Z. Auerbach in Tikunim U'milluim pg.14 and in Meor Hashabbos pg.
>481-482. see further explanation in Binyan Shabbos pg. 94. It is also
>permitted to remove the plastic caps that are opened by tearing a litle
>strip connected to the bottom of the cap - Binyan Shabbos pg. 94 quoting
>Harav S.Z. Auerbach.
The full discussion is located at