What Is Induction Sealing and Why Does It Matter?
Walk a bottling line after a long August shift and you’ll find it on the returns pallet: a few hundred PET bottles leaking through the cap, product weeping down the thread. The cap torque reads fine. The liner is sitting inside the cap exactly where it should. So where is the failure? Almost always, the bottle cap sealing step on that line was never actually creating a hermetic barrier — it was just squeezing a foam disc flat. If you’ve ever asked how induction sealing works on a line like yours, the short answer is that it bonds a foil disc straight to the rim, and it’s one of the cheapest, highest-return upgrades a liquid or powder filling line can make.
Induction sealing is a non-contact method that bonds a thin aluminum foil laminate directly to the rim of a plastic (or specially coated glass) bottle mouth after the cap is applied. The cap still does its regular job — keeping dust out, holding dust caps, providing grip — but the foil disc becomes the real barrier between product and outside world. Once it’s sealed, the bottle is hermetic: no leak path, no wicking, no oxygen migration. And the first person to twist the cap sees a popped, breached foil, which is what gives you tamper-evidence that a simple liner can’t fake.
A hermetic seal you can’t fake
A pressure-sensitive liner and a cap torque set to spec will slow evaporation for a while. They won’t stop liquid wicking past the thread under vibration. We saw a pharmaceutical manufacturer in Jiangsu losing roughly 3% of bottled product to cap leakage before switching to induction sealing — mostly syrups and suspensions wicking past the thread over a six-month shelf life. That 3% sounds small until you annualize it across a 6,000-bottle-per-hour line. The hermetic seal for food packaging and pharma applications is a regulatory expectation as much as a technical one: once foil is fused to the rim, the bottle will bulge on its own before it leaks through the seal.
Where it shows up on the production line
You’ll find it on cooking oil bottles, peanut butter jars, pharmaceutical syrups and tablets, agrochemicals, cosmetics and motor oil — any bottled product that stacks on a pallet. The aluminum foil sealing step sits after capping and before date coding, so it rarely forces a line redesign. Most customers add the sealer over a weekend and run product Monday morning.
The Physics: How Electromagnetic Induction Creates a Hermetic Seal
Here’s what actually happens on the factory floor. The sealing head contains a coil of copper tubing fed by a high-frequency power supply, typically 20–100 kHz. When a capped bottle passes under the coil, the alternating current generates an alternating magnetic field that couples through the cap plastic straight into the thin aluminum disc inside. The aluminum is conductive, so the field induces circulating currents in the foil — eddy currents. Those currents meet the resistance of the aluminum, which converts energy into heat, exactly how an induction cooktop heats a pan without touching it.
Eddy currents and the foil laminate stack
The liner inside the cap isn’t a single sheet. It’s a laminate, usually a three-layer sandwich: a PET (or foam) top layer that protects the foil, a thin pure aluminum foil in the middle that does the heating, and a heat-sealable PE layer on the bottom that touches the bottle rim. The eddy currents heat the aluminum in roughly 0.5–2 seconds. That heat melts the bottom PE layer, which flows and wets the bottle rim while the cap holds it down. Once the bottle leaves the field, the PE cools and solidifies, gluing the foil disc to the rim. This stack is what makes aluminum foil sealing work on non-conductive plastic: the plastic itself never heats up, only the foil does, which is why you can seal a thin PET bottle without deforming the thread.
Why it only takes a second
Total dwell time is short. On a 28–38 mm neck PET bottle running 1,000–2,000 bottles/hour, the coil energizes for 0.3–0.8 seconds. At a 200 mm jar diameter — a gallon of oil or a large supplement tub — sealing time typically runs 1.5–2.5 seconds, because there’s more foil area to heat and more rim to wet. The machine never touches the cap. That’s the point of electromagnetic induction: heat is generated inside the foil itself, contact-free, with no wearing seals and nothing to clean between runs. If you want the deeper materials-science write-up, Packaging World runs a solid technical archive on liner design and coil geometry at packworld.com.
Anatomy of an Induction Sealing Machine
An industrial induction sealing machine has four basic blocks: a high-frequency generator (the power supply), a copper induction coil formed to your cap diameter, a conveyor or indexing mechanism that brings each bottle under the coil, and a controller that sets power, time and head gap. The differences between models are almost all about throughput and how the bottle gets to the coil. SunAura builds both continuous and handheld platforms on the same generator electronics, which makes field upgrades straightforward if a line grows.
Continuous inline systems
Continuous induction sealers mount over an existing conveyor. Bottles pass under a ring or flat coil at line speed while the generator runs continuously — no start/stop per bottle. Power output ranges 1–5 kW for typical continuous models, with high-power units (5–10 kW+) reserved for large jars, thick-wall containers or fast lines above 200 BPM. These are what you bolt onto a filling line that’s already running. Pair them with our conveyor solutions that give you straight, stable, centered travel under the coil — a bottle that wanders laterally under the seal head is a bottle that gets a half-sealed rim. You can browse the full induction sealing machines range on our product page, including the PET bottle induction sealer configured for 28–89 mm caps.
Handheld and bench-top units
A handheld induction sealer is a wand: you hold the sealing tip over a capped bottle, pull the trigger, and the coil energizes for a fixed timer. Power is usually 0.5–2 kW, throughput a few hundred to maybe 1,000 bottles per shift, and the operator moves bottle to bottle. These suit R&D labs, short runs, contract packing, or lines under ~30 BPM. They’re also the right tool if you only need an induction sealing machine for pharmaceutical bottles in a small-fill GMP room and can’t justify a full conveyor, or if you’re sealing a co-packer’s occasional SKU that doesn’t merit inline hardware.
| Parameter | Continuous inline sealer | Handheld / bench-top sealer |
|---|---|---|
| Typical power output | 1–5 kW (up to 10 kW+ for large jars) | 0.5–2 kW |
| Realistic throughput | 30–300+ bottles/min, matched to line speed | ~10–40 bottles/min, operator-dependent |
| Cap / neck range | 20–200 mm, coil swapped per SKU | 20–100 mm with interchangeable tips |
| Line integration | Mounts over existing conveyor; needs centered travel | Bench-top, no integration needed |
| Best fit | Steady runs: food, pharma, chemicals | R&D, short runs, pilot batches |
| Consistency | Repeatable power/time on every bottle | Dependent on operator placement and dwell |
Critical Parameters: Power, Time, and Bottle Material
Most engineers miss this on the first run. They crank the power to max, hit go, and wonder why the seal is either weak (peels off in sheets) or blown (foil melted through, cap top warped). Three numbers decide the result: coil power, dwell time, and the gap between the coil face and the cap top. Get any one of them off and the other two won’t save you.
Power output and gap settings
The starting rule is low and slow. For a standard 28 mm PET cap, a 1.5 kW continuous sealer running at 40–60% power for 0.4–0.6 seconds usually lands in a good window. Set the gap between 3 and 5 mm — too close and you risk arcing and overheating the cap top; too far and the field couples weakly and the seal runs cold. Induction sealing temperature settings aren’t entered directly on the HMI; you set power and time, and the PE layer tells you (by peeling cleanly) whether the liner interface hit 130–180 °C. Build a small DOE — power 40/50/60%, time 0.4/0.5/0.6 s — peel-test five seals per combo, and you’re done inside a half-hour.
When PET, HDPE and glass behave differently
PET seals cleanly because its surface energy is high and it wets molten PE well. HDPE is more slippery and needs a slightly hotter, slightly longer dwell — otherwise the foil peels off in sheets with a shiny, un-wetted rim. PP sits in between. Glass won’t bond to standard PE at all; on glass jars you need a foil liner with a special heat-seal lacquer layer matched to the glass composition, and line speed usually drops because the glass mass draws heat away. If you’re switching resin between product runs, re-run your seal development rather than trusting the old recipe. For pharma lines, keep your seal validation records aligned with ISO 11607 sterile barrier expectations, and confirm liner materials meet FDA food-contact packaging guidance for whatever product you run.
Common Sealing Defects and How to Fix Them
The tricky part is that a lot of defects look like “bad liners” when they’re actually a setup problem. Pull ten bad seals off your reject bin and you’ll usually find one root cause, not ten. Where people go wrong is blaming the liner supplier before checking power, gap, and cap torque.
| Defect | Likely cause | Fix |
|---|---|---|
| Foil peels off cleanly, rim untouched | Under-powered or too short dwell; cap torque too low so liner floats above rim | Raise power 10–15% or add 0.1 s; set cap torque to 8–15 in-lb for 28 mm caps |
| Wrinkled or partial seal around rim | Bottle wandering on conveyor; cap off-center; coil gap uneven | Re-guide conveyor, recenter coil, check star wheel spacing |
| Cap top discolored / foil burned through | Over-powered; gap too small; coil too close to cap | Drop power 10–15%, raise gap to 4–5 mm, shorten dwell |
| Seal looks fine but leaks on drop test | Liner PE resin wrong for your bottle; cold product chilling the rim | Match liner grade to bottle resin; pre-warm product or slow the line |
| Random seals good, random bad | Line voltage sag; loose coil lead; bottle height variation | Check mains stabilizer and coil cable torque; verify neck height consistency |
Frequently Asked Questions
How do I choose between a continuous induction sealer vs handheld?
Run the math on bottles per hour. If your line holds steady at 30 BPM or more for a full shift, go continuous — the payback on leak reduction usually comes inside six months. If you’re sealing under ~1,000 bottles a day, running short SKUs, or validating a new product in R&D, a handheld wand pays for itself in a weekend and can always be retired to lab duty later.
What induction sealing temperature settings should I start with?
You don’t set temperature directly. You set output power (usually 40–70% on a 1.5 kW unit for 28 mm caps) and dwell time (0.3–0.8 s). The liner interface typically reaches 130–180 °C, which is enough to melt the PE seal layer without softening the PET. Confirm the result with a peel test, not a thermometer — a clean peel that leaves a continuous film of PE on the rim is your real target.
Can I induction seal glass bottles?
Yes, but not with a standard PET liner. Glass needs a foil liner with a heat-seal lacquer formulated for glass, and you’ll usually run 20–40% more dwell time because the glass mass draws heat away from the rim. Expect to slow the line slightly. HDPE and PP jars are easier — just confirm the liner grade matches the resin.
How do I verify the seal is actually hermetic?
Three cheap checks cover 95% of the job: visual peel test (foil must come off in one disc with PE fully wetted to the rim), a vacuum or pressure decay test on a sample of bottles, and a drop test from 1.2 m onto a concrete floor. For pharma, log every batch’s power and time settings so you have the validation trail regulators expect. If a bottle passes all three, it’s hermetic.
Next Step: Size Your Induction Sealer With Our Engineers
If you’re sizing an industrial induction sealing machine for your line, our engineers can walk you through the math. Send us your bottle specs — neck diameter, resin, cap type, target bottles per minute, and product temperature — and we’ll come back with a recommended model, a starting power/time recipe, and a realistic SunAura induction sealer price before you buy induction sealer hardware. Every unit is built in our 48,000 m² Qingdao plant, CE and ISO 9001 certified, run-tested 48 continuous hours before shipment, and backed by a 12-month warranty. Drop your specs to [email protected] and we’ll reply within one business day.