Induction seal: sealing a bottle under its cap
You unscrew the cap, and the bottle is not yet open: a thin silver foil closes the neck. This seal is applied without contact, in a fraction of a second, by a magnetic field.
By Tecmom editorial team · 2 min read
A foil inside the cap
An induction seal starts life inside the cap. It is a disc made of several layers: most often a board backing, a wax layer, an aluminium foil and a polymer film facing the neck.
The bottle is filled, and the cap is screwed on with its disc. It then passes under an induction head.
How the magnetic field seals
The induction head creates an alternating magnetic field. It does not act on the plastic, but induces currents in the aluminium foil, which heats up instantly. The heat melts the polymer film, which bonds to the rim of the neck.
In two-piece systems, the wax melts at the same time and releases the foil from its backing: the seal stays bonded to the neck, the backing stays in the cap and acts as a liner when it is screwed back on. In one-piece systems, the whole disc bonds to the neck.
What it brings
Two things, essentially.
- A seal until first opening: the contents do not touch outside air, and the bottle travels better.
- Proof of first opening: a pierced or missing seal is visible, like a broken tamper-evident band. See tamper-evident packaging.
Once opened, the bottle relies on its cap alone. See multidose bottle sealing.
What it asks of the neck
A seal only bonds well to a neck rim that is flat, even and clean. An oval neck, flash, or a parting line on the sealing surface, and the bond becomes discontinuous. Power setting matters too: too high, it can warp the cap; too low, it leaves a weak bond.
It is a mould question once again: the neck's sealing surface comes out of injection at size, or it is wrong. See bottle neck finish and what the mould leaves on the part.
An induction seal is seen only once: when it is pierced. Yet that brief moment sums up everything expected of a pack: closed to the end, and opened for the first time by the person who will use it.
Glossary
- Induction seal
- A sealed foil that closes a bottle neck under the cap.
- Induction
- Heating of a metal foil by an alternating magnetic field, without contact.
- Sealing surface
- The flat rim of the neck to which the seal bonds.
- Tamper evidence
- A feature that shows whether a pack has been opened.