Induction Cap Sealer Product

Overview

An induction cap sealer welds an aluminium-foil liner onto the mouth of a plastic or glass container without touching it. The foil sits inside the screw cap when the cap is applied; as the capped container passes under the sealing head, a high-frequency magnetic field induces eddy currents in the foil, the foil heats, and a polymer layer on its underside melts and bonds to the bottle rim. The result is the familiar peel-off freshness seal on sauce jars, pharmaceutical bottles and motor oil — hermetic, tamper-evident, and applied at full line speed with no contact and no moving parts at the seal point.

How it works

The liner is a laminate: pulpboard or foam backing, a wax or release layer, aluminium foil about 20–40 µm thick, and a heat-seal film matched to the container material (PE for HDPE bottles, PET-compatible polymer for PET, special grades for glass). The capper torques the cap down so the liner presses firmly on the land of the bottle finish; the induction sealer then does the welding downstream.

Inside the Induction Sealing Head, the Work Coil — a few turns of water-cooled copper tube — carries 50–500 A of alternating current at 20–200 kHz. The Flux Concentrator ferrites shape the resulting field downward so it threads through the foil disc passing 3–6 mm below, the gap held constant by the Tunnel Guide rails. The alternating flux induces circulating eddy currents in the foil, and the foil's own resistance dissipates them as heat: 50–300 ms in the field takes the seal layer to roughly 180–200 °C. The melted film bonds to the rim under the cap's clamping pressure and freezes as the bottle exits. In wax-bonded two-piece liners the same heat melts the wax, which absorbs into the pulpboard and releases the backing to stay in the cap as a reseal liner.

The field is generated by the Power Unit. The Input Rectifier turns mains power into a DC bus, and a resonant bridge of IGBT Power Module switches chops it at the tank frequency set by the Tank Capacitor and the coil inductance. The Gate Driver Board tracks resonance continuously, because caps entering the field shift the tank's tuning, and the Matching Transformer steps the inverter output down to drive the low-impedance coil. Running at resonance keeps switching losses small, but the coil still dissipates real heat: the Cooling System loop pumps water-glycol from the Coolant Reservoir through the coil and inverter cold plates via the Coolant Pump, rejecting heat through the Radiator. The Coolant Flow Switch inhibits RF if flow stops — an uncooled coil fails in seconds at full power — and the Head Thermostat backs it up. Units under about 2 kW skip the liquid loop and cool with air alone.

Sealing on a moving line

Power and dwell must match. The Bottle Sensor gates the field on only while a cap is underneath, saving energy and keeping stray field down; the Conveyor Speed Sensor tells the controller the conveyor speed so it can scale power as the line ramps — a bottle crawling under the head at startup power would scorch its liner, the most common induction-sealing defect along with its opposite, the cold partial seal that fails ASTM F2029 peel testing. The Foil Detector verifies by eddy-current response that a foil actually passed in each cap, and the Reject Output Module flags missing-foil containers for a downstream pusher.

Setup is geometric. The head hangs from the Head Bracket on the Stand Column, cranked up or down on a Ball Screw to set the air gap per bottle format; the Stand Base parks beside the customer's conveyor. Cap height changes of a few millimetres matter — field strength falls off steeply with distance — so the gap is part of every changeover recipe stored in the controller, set through the LCD Panel and Touch Digitizer.

Why induction

Compared with conduction sealing (a heated platen pressed on the cap), induction never touches the cap, cannot scuff decorated closures, works through the closed cap, and reaches sealing temperature in the foil only, leaving the bottle and product cold. It draws power only during the 100 ms each cap is in the field. Its limits are equally clear: it needs a foil in the cap, metal caps shield the field and cannot be induction sealed, and metallic ink decoration near the bottle mouth can couple and overheat. Capper torque remains the silent partner — a loose cap gives no clamping pressure and no amount of RF power produces a seal.