Bore sealing, orifice control & assembly
What a plug does that a flat liner cannot.
A plug fitment works inside the neck rather than across the top of it. A short skirt presses against the bore while a flange sits under the cap, which gives a second sealing surface in a completely different plane from the liner disc. Where a bottle must not weep on its side or must meter what comes out, that is the difference.
The bore seal survives what the rim seal misses
A liner disc bears on the top land of the finish, so any chip, mold seam or product residue on that land is a leak path. A plug seals against the inside wall over several millimeters of contact, well below where product splashes during filling. On bottles that travel on their side, that is usually the difference between a clean carton and a stained one.
Orifice diameter is the dosing decision
Where the plug carries a central bore, that bore is what meters the product. A narrow opening breaks liquid into countable drops for tinctures and flavorings; a wider one gives a controlled stream for sauces and oils. It is a molded dimension, so it cannot be adjusted after tooling, and it wants sampling with the real formula at the real temperature.
Low-density polyethylene is chosen for its recovery
The plug has to deform going in and then push back against the bore for years. LDPE does that better than stiffer resins, which take a set and slacken. Polypropylene versions exist where heat resistance matters more than long-term spring, and they need a looser interference to avoid splitting the skirt on insertion. The two are not drop-in replacements.
Glass bore tolerance governs, not the fitment
Molding tolerance on the plug is far tighter than the bore tolerance of a glass neck, so the fit range comes almost entirely from the bottle. A plug specified against one sample bottle can be tight on one cavity and loose on another from the same mold set. Retention checks belong on bottles drawn across cavities, not on a single reference piece.
Insert after filling, and think about the line
Plugs are pressed in on a dedicated head after the bottle is filled, before capping. That adds a station and a change part per neck size, and it puts an operation between two others that were previously adjacent. Small runs often insert by hand, which is workable but slow, and it is worth knowing which of the two a program is planning for.
Removal is a design choice with consequences
Most plugs are meant to stay for the life of the bottle, and a customer who levers one out with a knife usually damages the neck. Where refilling is intended, a tabbed or flanged version can be lifted deliberately. Deciding this early avoids the situation where a refill program is announced on a pack whose fitment was never designed to come out.
