The closing system of a screw caps is important for dispensing the product. The most common closing systems are: plug seal, crabs-claw, liner or insert.
The plug seal is a ring that is designed to seal against the inside wall of the bottle neck finish
The ring angles downward from the inside top panel of the closure and is designed to flex and seal on the top of the neck finish. This system is perfect for PE and PP bottles, less for PET bottles.
The caps are normally supplied with a pre-assembled insert which has a specific opening. When the cap is applied to the bottle the plug closes on the bottleneck and the pin inside the cap closes the plug’s opening.
The closure with the liner presses directly down on the top of the bottle neck finish. The torque of the screw on the bottle neck compresses the foam material and so seals the system