Cap liners: what seals a closure without a foil seal
A screw cap does not seal thanks to its thread. The thread only tightens; what holds the liquid in happens against the rim of the neck. There you find either a liner or a lip designed into the mould.
By Tecmom editorial team · 2 min read
The thread tightens, the liner seals
Screwing on a cap brings its top down onto the rim of the neck, called the land. The seal comes from this contact: a soft surface or a precise edge must match the land all the way round, without a break.
There are two families of solutions: inserting a liner inside the cap, or moulding into the cap a sealing lip that acts as one. See the bottle neck finish for the neck's geometry.
Inserted liners
The most common is the polyethylene foam liner, compressible, which squeezes against the land and takes up its small irregularities. Its three-ply version, a foam core between two low-density polyethylene films, is known by the trade name F217; it resists taste and odour transfer and water vapour well.
Other liners exist: pulp and plastic film, pulp and foil for dry products, adhesive liners activated by tightening. None of these simple liners is a proof of tamper evidence: that needs a band or a sealed foil.
The moulded lip, a liner with no added part
A cap can also seal without a liner. A thin lip or flange is moulded inside it, which on tightening presses against the land or inside the neck. It is a single part, with no liner to fit, and the material touching the product is the cap's own.
The trade-off is precision: the lip must be designed and held tightly, together with the neck facing it. It is a matter of mould design as much as of material; see tolerances of moulded parts.
Choosing according to the content
The liner touches the product: it must withstand it. Some liners fear oils, others bleach or solvents; see essential oils and plastic. It must also hold up after repeated openings, which matters for a multidose bottle.
For a medicine, the liner is part of the closure system declared in the dossier, just like the cap and the bottle.
The liner is the smallest part of a pack. It is often the one that decides whether the product arrives intact.
Glossary
- Cap liner
- A disc or ring placed inside a cap to seal it against the neck.
- Land
- The top rim of a bottle neck, where the cap closes.
- Sealing lip
- A flexible edge moulded into the cap, replacing an inserted liner.
- Foil seal
- A membrane sealed onto the neck, evidence of first opening.