How Induction Sealing Works: Cap Liners and Heat Generation

Induction sealing welds a foil liner inside a bottle cap using an electromagnetic field - the physics, the liner structure and the line variables.

The core idea of this page: induction sealing welds a foil liner inside a bottle cap by generating heat inside the foil with an electromagnetic field — no contact, no open flame. It produces a hermetic, tamper-evident seal that a heat sealer on film cannot. Choose it when you need a leak-proof, tamper-evident closure on a bottle or jar.

For the machine landscape, see Heat Sealing vs Induction Sealing and Top 10 Sealing Machine Manufacturers.

How induction sealing works

The foil liner

The cap contains a multi-layer liner: a foil layer, a heat-seal layer that bonds to the container lip, and a backing. The liner is placed in the cap before capping.

The sealing head

After capping, the container passes under an induction sealing head. The head generates a high-frequency electromagnetic field, which induces eddy currents in the foil liner — heating the foil and melting the heat-seal layer onto the container lip.

The result

The foil bonds to the rim to form a hermetic, tamper-evident seal under the cap. The consumer must break the foil to open the pack — which is the tamper-evidence and the leak-proofing.

Failure modes to design around

Liner or cap mismatch. The liner must match the cap and container material; a liner that is too small or the wrong heat-seal layer does not bond.
Under-sealing. Too little power or too fast a line leaves an incomplete bond — the pack leaks and the tamper-evidence fails.
Over-sealing. Too much power overheats the liner and can damage the product or the cap.
Head alignment. The sealing head must sit at the right distance and alignment over the cap for even heating.

How to verify an induction sealer

Seal-integrity test: apply pressure or a vacuum to sealed containers and confirm no leak.
Tamper-evidence check: confirm the foil tears when the cap is first opened.
Line-speed tuning: confirm the seal holds at your line speed on your container and cap.
Liner compatibility: confirm the liner and heat-seal layer match your container material.

Frequently asked questions

Is induction sealing the same as heat sealing?

No — heat sealing presses heated elements onto film; induction sealing heats a foil liner inside the cap with an electromagnetic field. They close different package features.