Induction Cap Sealing Explained for Engineers: Eddy Currents in a Bottle Cap

Peel the foil off a new bottle of medicine or cooking oil and you are looking at a neat bit of applied physics. That foil was sealed to the bottle rim without anything touching it, through a closed plastic cap. Here is how it works, and what it means if you ever have to integrate one of these machines into a line.

The stack inside the cap

An induction-ready cap contains a liner (the "wad") that is typically a layered stack: a backing layer, an aluminium foil layer and a heat-seal coating that faces the bottle rim.

The physics

1. The sealing head drives an alternating current through a coil, creating an alternating magnetic field.
2. The field passes straight through the plastic cap, because plastic is not a conductor and does not heat up.
3. In the aluminium foil, the changing field induces eddy currents.
4. Those currents meet the foil's electrical resistance and turn into heat (Joule heating, P = I²R).
5. The heat-seal coating melts onto the bottle rim and bonds as it cools, leaving a tamper-evident, leak-proof inner seal.

The cap and the product are not heated directly; only the conductive foil is.

Why seals fail (the debugging checklist)

• Loose cap: the foil is not pressed against the rim, so it heats but does not bond.
• Contaminated rim: oil or syrup on the rim blocks bonding.
• No foil wad: no conductor, no heating. The machine cannot seal a cap without the wad.
• Wrong dwell or power: too little energy gives a weak bond, too much can scorch the liner.

Modeling energy per bottle

On a conveyor sealer, energy delivered per cap depends on head power and how long the cap spends under the coil:

def dwell_s(coil_length_mm: float, conveyor_m_per_min: float) -> float:
    return (coil_length_mm / 1000) / (conveyor_m_per_min / 60)

for speed in (2, 6, 12):  # m/min
    print(speed, "m/min ->", round(dwell_s(300, speed), 2), "s under the coil")
# 2 m/min -> 9.0 s, 6 m/min -> 3.0 s, 12 m/min -> 1.5 s

The coil length here is an illustrative value, but the relationship is the point: double the conveyor speed and you halve the dwell, so power must rise or seal quality drops.

Hand-held vs conveyor

Hand-held units (around 500 W) are held over each capped bottle, while continuous conveyor units (around 2000 W) seal bottles as they pass under the head. If you want real-world specs for both types, this overview of induction sealing machines lists cap ranges and power ratings.