You may completely open plastic bottle caps on plastic bottles on Shabbat (even if doing so will leave a plastic ring on the bottle), as long as it is theoretically possible to dispense the liquid without completely separating the cap from the bottle.
Reason Since liquid can be poured with the cap still attached, the sealed bottle does not become a “new utensil”—a Shabbat violation.

However, if you will destroy letters that are printed on the cap, you may not open the bottle.