Wheaton W224100-181 Rubber 20mm Snap-On Style Stopper, Gray Chlorobutyl/46 (Case of 1000)

This design provides maximum sealing for vials with a blow-back glass finish, which is a small ridge on the interior of the vial’s neck. The snap-on stopper locks into the blow-back ridge, providing a tight seal and keeping the stopper from backing out. This style is not recommended for vials with a straight-walled finish. The blow-back feature is typically found in vials from North American manufacturers.