When the consumer twists off the cap, a three-layer coextruded foam component separates from the film/paper/foil lamination that is induction sealed to the bottle finish. So the cap retains a liner to provide recloseability without leakage.
Tri-Seal, a Tekni-Plex Co.: Separating liner
The Tri-Gard® Separating cap liner from Tri-Seal (Blauvelt, NY) lets packagers accomplish two goals at once. The multi-layered liner, already inside the cap when supplied to the packaging line, is induction sealed to a plastic bottle to provide a secure seal and tamper evidence.