Abstract: A plastic insert for sealing caps such as Crown Corks which is capable of forming a seal impermeable to liquid when it is pressed by the cap against the beaded mouth of a container. The insert comprises a central projection directed towards the interior of the container. This projection may comprise a transverse hole and/or a longitudinal hole rendering it possible to apply by means of a drawing pin or a safety pin or sew by means of a needle the insert onto a blouse or a display card board. In this manner the insert may be used as a trimming thus offering a certain advertisement to the manufacturer of the contents of the container.