Read the failure before you change anything
An induction seal fails in a predictable way, and the pattern tells you where to look. Before you touch the sealer, take a failed liner off a bottle and look at it in good light. Is the weld ring complete or partial? Is the foil wrinkled, lifted, or torn? Is the backing still in the cap or stuck to the bottle? Each pattern points to a different fix, and changing settings blind is how a small problem becomes a whole shift of rework.
The sections below go symptom by symptom. Work through the one that matches what you are seeing.
What a good seal looks like
Before diagnosing a bad one, know what good looks like. A healthy induction seal has a continuous, even weld ring all the way round the mouth; the foil sits flat and tight with no wrinkles, bubbles or lifted edges; and the membrane separates cleanly at the intended layer when opened. If a sealed pack fails any one of those three, it is a warning sign even if it is not yet leaking.
Keep a few good sealed samples from a run that worked. When something goes wrong later, having a known-good reference next to the failure makes the difference obvious, and it is the fastest thing to show us when you send a diagnosis.
The four inputs behind every failure
Every induction seal problem traces back to one of four inputs: the liner structure, the bottle mouth, the cap and the sealer. When you can name which input is responsible, the fix is usually obvious; when you cannot, you change things at random and burn a whole shift.
- Liner structure. The heat seal layer must match the bottle polymer, and the cut diameter must cover the sealing land. A mismatch here fails every bottle in the same way.
- Bottle mouth. Flatness, flash, warp and ovality decide whether the weld can form a complete ring. A geometry problem fails some bottles, or one side of the ring.
- Cap. Torque and squareness press the liner flat while the weld forms and sets. A cap problem shows up as intermittent or off-centre failures.
- Sealer. Power and dwell govern how much heat reaches the foil. A setting problem usually shows up as uniform under- or over-sealing across the whole run.
The order above is also the cheapest order to check. You do not buy new liners to fix a sealer setting, and you do not re-tool a sealer to fix a bottle-mouth problem.
| Symptom | Most likely cause | First check |
|---|---|---|
| Leaks but looks closed | Mouth not flat, or wrong seal layer | Weld ring for gaps; polymer match |
| Foil lifts or falls off | Wrong seal layer or low settings | Polymer match, then power/dwell |
| Foil wrinkles or puckers | Too much heat or pressure | Lower power or dwell |
| Weak seal | Low settings or wrong structure | Burst test at each setting |
| Hard to open or tears | Over-sealing | Lower power or dwell |
| Wrong piece comes off | Separation layer or mouth weld imbalance | Confirm build and settings |
| Partial weld ring | Centring, cap squareness or warp | Line alignment and cap torque |
Symptom 1 — The seal leaks, but it looks closed
This is the most dangerous failure because it passes visual inspection. The foil is present and the cap is on, but contents still escape, or the pack loses pressure and the product degrades.
Likely causes
- Mouth not flat. An uneven lip, flash, or a moulding line leaves gaps where the weld cannot form. The seal holds where it touches and leaks where it does not.
- Wrong seal layer. If the heat seal lacquer is not matched to the bottle polymer, it never truly welds — it only sits on the lip.
- Cap torque too low. The cap is not pressing the liner flat against the mouth during sealing, so the weld is patchy.
- Sealer power or dwell too low. The foil heats enough to look bonded but not enough to melt the lacquer across the full ring.
What to do
- Check the bottle lip for flatness and flash. A quick way to expose the problem is to seal a batch and inspect the weld ring for gaps.
- Confirm the heat seal layer matches the polymer — PE bottle, PE grade; PET bottle, PET grade; and so on.
- Raise torque to spec, then re-test before touching the sealer.
- Increase power or dwell in small steps and re-test, rather than jumping straight to maximum.
Symptom 2 — The foil lifts or falls off the mouth
The liner comes away from the bottle lip either immediately or after a short time. In severe cases the foil simply drops into the pack or stays in the cap.
Likely causes
- Wrong seal layer for the polymer. The lacquer has nothing to weld to, so there is no bond at all.
- Sealer settings too low. The foil never reaches the temperature the lacquer needs.
- Contaminated lip. Product residue, dust or release agent on the lip blocks the weld.
What to do
- Re-verify the bottle polymer and the seal layer grade. This is the first thing to rule out.
- Raise power or dwell in controlled steps and re-test.
- Make sure the filling line is not splashing product onto the lip before capping; a dry, clean lip welds far more reliably.
Symptom 3 — The foil wrinkles or puckers
The foil surface is no longer smooth — it shows wrinkles, ripples or a puckered centre. This looks bad and usually weakens the seal.
Likely causes
- Too much heat. Overheating softens the foil and the laminate, which then deforms as the cap and head press on it.
- Too much pressure or torque. Crushing the liner before or during sealing distorts it.
- Liner not sitting flat. A liner that is slightly too large for the cap, or cocked at an angle, wrinkles as it is pressed down.
What to do
- Reduce power or dwell first — wrinkles are more often over-sealing than under-sealing.
- Check the cut diameter against the cap; confirm the liner drops into the cap flat, not cocked.
- Confirm cap torque is within the range the liner is specified for.
Symptom 4 — Weak seal, low burst strength
The seal holds under a gentle press but fails under pressure, in transit, or when the pack is squeezed. This is a barrier and weld-strength problem, not just a cosmetic one.
Likely causes
- Sealer settings too low. The weld is under-developed and never reaches full strength.
- Wrong structure for the contents. An aggressive fill can attack the seal layer or adhesive and weaken it over time.
- Too little foil coverage. If the cut diameter is too small, the foil does not cover the full sealing land.
What to do
- Raise power or dwell in steps and run a burst test on each setting until the seal meets spec.
- Tell us the contents — oils, solvents and acids may need a more resistant structure.
- Confirm the cut size covers the sealing land without overhanging into the threads.
Symptom 5 — Seal is too strong or tears on opening
The opposite failure: the seal is so aggressive that the consumer cannot open it cleanly, or the foil tears and leaves fragments on the mouth.
Likely causes
- Over-sealing. Too much heat or dwell over-bonds the lacquer and can melt the structure beyond the intended separation point.
- Release wax not separating. In a two-piece liner, the membrane should separate cleanly from the backing; if the wax layer is wrong, the whole thing tears.
What to do
- Reduce power or dwell in steps and re-test opening torque.
- Confirm you are using the right build for the opening experience you want — a two-piece liner with a resealing backing, or an easy-open pull-tab for a cleaner peel.
Symptom 6 — The wrong piece comes off, or the backing stays on the bottle
With a two-piece liner, the foil membrane should stay on the bottle as tamper evidence while the paperboard backing stays in the cap. When the separation happens in the wrong place, the pack looks wrong and the reseal is lost.
Likely causes
- Separation layer out of position. The release wax is meant to sit between foil and backing; if the bond there is stronger than the seal to the bottle, the whole liner lifts instead.
- Seal too strong at the mouth. If the mouth weld is weaker than the wax bond, the liner peels off the bottle and stays in the cap.
What to do
- Confirm the build is two-piece and the release wax is specified correctly.
- Check the sealer settings — an over-strong mouth weld can invert the intended separation.
Symptom 7 — Partial or intermittent weld ring
The weld ring is present on one side or in patches, and the result varies bottle to bottle. This is usually a mechanical or alignment problem rather than a material one.
Likely causes
- Bottle not centred under the sealing head. The field is uneven across the mouth.
- Uneven cap pressure. A cocked cap presses the liner harder on one side.
- Warped mouth. An oval or tilted lip welds only where it contacts.
What to do
- Check the bottle centring and the sealing head alignment on the line.
- Confirm caps are applied square and at consistent torque.
- Inspect a run of bottles for warp or ovality at the mouth.
Sealer settings without the guesswork
We deliberately do not hand you one universal "correct" power number, because there is not one. The right setting depends on the bottle diameter, the cap, the foil and how fast your line runs. What matters is understanding the direction of each adjustment, so you tune deliberately instead of poking at dials.
- Power is the strength of the induction field. Larger diameters need more power because there is more foil to heat and a longer sealing land to weld; smaller diameters need less.
- Dwell is the time the cap spends in the field. It is usually measured in milliseconds, and small changes matter because the foil heats quickly.
- Pressure comes from the cap torque and holds the liner flat while the weld forms and then sets on cooling.
Treat them as three dials that trade against each other. An incomplete weld usually means more power or dwell; wrinkles or an over-aggressive seal usually mean less. Change one dial at a time, re-test, and write the result down. A sealer that "worked last month" and fails today changed for a reason, and the record is how you find it.
Storage, transport and shelf life
Not every failure happens on the filling line. A seal that leaves the factory sound can still fail in the field, and those failures are the most expensive because they reach the customer.
- Storage temperature. Keep liners within their specified range. Extreme heat can soften the wax or adhesive; extreme cold can make the layers brittle and harder to peel cleanly.
- Moisture. Paperboard backing absorbs moisture, which changes its stiffness and how cleanly the membrane separates on opening.
- Transport and stacking. A weld that survives a gentle bench test can fail under sustained vibration or weeks of stacking in a hot container.
If failures only appear after shipping, test a sealed pack the way it actually travels — temperature-cycled, vibrated and stacked — rather than judging it on the line alone.
A quick field test for a suspect seal
Before you call anyone, run a sealed pack through three checks. They take minutes and tell you whether the problem is the weld, the structure or the line.
- Look at the weld ring. A good induction seal leaves a continuous, even ring where the lacquer melted. A broken, thin or patchy ring points to geometry or settings.
- Squeeze or pressurise the pack. A hermetic seal holds pressure without weeping at the lip. Any liquid at the edge means the weld is not complete.
- Peel one open. The membrane should separate cleanly at the intended layer. If it tears, leaves fragments, or comes off in the wrong piece, the structure or the separation layer is wrong.
Do all three on several bottles, not one. A single good bottle is luck; a run of consistent results is information.
A systematic order to diagnose any failure
When you are not sure which section applies, work in this order. It goes from the cheapest checks to the most expensive, so you do not replace a liner when the problem is the line.
- Confirm the material match — bottle polymer against the seal layer grade.
- Check the mouth — flatness, flash, warp and ovality.
- Check the cap — torque, squareness and whether the liner sits flat.
- Check the sealer — power and dwell, then adjust in small steps and re-test.
- Check the contents — aggressive fills may need a different structure.
- Check the cut size — full coverage of the sealing land without overhang.
Nine times out of ten, the answer is in the first three. If you have worked through the list and the seal still fails, we can help — and a photograph of the failed seal tells us most of what we need to know.
When the same failure keeps coming back
A one-off failure is an event; a failure that returns every run is a process. Treat them differently.
- One-off. Look for something that changed for a single batch — a new bottle delivery, a different cap lot, a maintenance job on the sealer, or a fill that ran hotter than usual.
- Recurring. Look for something structural — a mouth geometry that has always been marginal, a seal layer that is a near-miss rather than a true match, or a torque that is set at the edge of the liner's range.
For a recurring failure, the fix is rarely a dial adjustment. It is usually a change to the liner structure or to the bottle. That is exactly the kind of decision we can help you make, because we see the same failure modes across many filling lines.
How to photograph a failed seal for diagnosis
If you send us a failed seal, a few well-chosen photographs are worth a page of description. Take them in good light, with the liner lying flat.
- The foil face. Shows wrinkles, puckers, burn marks or print damage.
- The weld ring. Turn the liner over and photograph the side that touched the mouth — a complete ring versus a patchy one tells us most of the story.
- The cap and backing. Shows where the separation happened and whether the backing ring stayed in the cap.
- The bottle mouth. A side-on shot of the lip reveals flash, warp and moulding lines.
Add your bottle material and sealer settings, and we can usually narrow it to one or two causes within the day.
Prevention in one checklist
Prevention is cheaper than rework, and it follows the same four inputs. Run this at the start of every new run and you will catch most failures before they leave the line.
- Lock the liner structure to the bottle polymer — PE bottle, PE grade; PET bottle, PET grade.
- Confirm the cut diameter covers the sealing land without overhanging into the threads.
- Inspect a sample of mouths for flatness, flash, warp and ovality before you start.
- Set cap torque to spec and check that caps go on square, bottle after bottle.
- Set the sealer once, write down power and dwell, and re-test before the run.
- Keep a few sealed samples from the start of the run as a reference for later.
- Store liners in their specified range and keep the paperboard dry.
For more background on how the liner is built and chosen, see the induction seal liners page and the FAQ.