Why pouch seals fail — weak, leaking, and contaminated seals
A pouch seal fails in one of a few predictable ways — it peels open, leaks through a channel, burns through, or comes apart entirely. This is how to read which mode you have, fix it, and tell whether the problem is your spec or the line that ran it.
THE SHORT ANSWER
Most pouch seal failures trace to the seal triad — heat, pressure, and dwell time — being outside the film's window, or to product getting into the seal area before the jaws close. A seal is a controlled melt: the two films' inner sealant layers must melt and flow together under pressure long enough to fuse. Get that wrong, or contaminate the interface, and the seal never locks. Diagnose by mode:
- A weak or peelable seal usually means too little heat, pressure, or dwell.
- A channel leak usually means a wrinkle or product trapped across the seal.
- Burn-through means too much heat; a cold seal means too little.
- The wrong sealant film won't lock no matter how you set the machine.
- Find the actual leak with a test before you change anything else.
What you're seeing: how a seal fails
Before you change a machine setting, name the failure. A seal that peels open cleanly is a different problem from one that looks sealed but still passes air, and the two have opposite fixes. This article is part of our packaging problems diagnostic library. Start by matching what you see to a mode.
| Failure mode | What it looks like | What it usually means |
|---|---|---|
| Weak / peelable seal | Opens with a light pull, no fiber or film tear | Under-processed triad, or the wrong sealant for a lock seal |
| Channel leak | Seal looks closed but passes air or liquid | A wrinkle or product trapped across the seal |
| Burn-through | Holed, brittle, or distorted along the jaw line | Too much heat or too long a dwell |
| Cold seal | No bond, or a bond only in patches | Too little heat, pressure, or dwell |
| Delamination at the seal | Layers lifting or tunneling beside the weld | A laminate bond failure, not the seal itself |
A quick field test separates most of these: pull the seal apart by hand. A properly locked seal tears the film before it opens; if it peels apart clean and intact, the two surfaces never fused, which points at the triad or the film. If it holds everywhere except one spot, that spot is a localized channel, and your evidence.
The seal triad: heat, pressure, and dwell
Every heat seal is made by three variables working together — the seal triad: temperature, pressure, and dwell time. The jaws bring the inside surfaces to the sealant's melt range (temperature), press them together hard enough to make full contact and push out trapped air (pressure), and hold them there long enough for the polymer to flow and fuse (dwell). Change one and you usually have to change another: speed the line up and dwell drops, so you need more heat to compensate; drop the pressure and even the right temperature won't close a wrinkle.
The catch is that the correct window belongs to the film, not the machine. Each structure has a seal window specified by the film or sealant supplier — a range of temperature and dwell where it fuses without distorting — set by the sealant layer inside the laminate, the layer that actually melts.
This matters for diagnosis because two of the most frustrating failures aren't triad problems at all — they're structure problems. If the inner sealant is the wrong resin, too thin, or not a lock-seal grade, no amount of heat, pressure, or dwell gives you the seal you want; you just burn the outer layers instead. That's why verifying the film is a step apart from dialing the machine — our stand-up pouch spec guide covers choosing the structure and sealant.
Ranked causes: why the seal failed
Here are the causes we see most often, roughly in the order worth checking — the cheap, common ones first, the structural ones last.
| Likely cause | Why it happens | How to tell |
|---|---|---|
| Triad too low (cool / short dwell / low pressure) | The sealant never fully melts and flows together | Seal peels open clean and intact; weak or stringy, no film tear |
| Product in the seal area | Powder, oil, moisture, or fibers contaminate the interface | A visible channel, fisheye, or dull streak in the seal; leaks under pressure |
| Wrinkle or gusset overlap in the seal | Folded material leaves an unsealed path through the weld | Leaks trace to a corner or gusset; a visible pleat in the seal band |
| Triad too high (burn-through) | Excess heat or dwell thins, distorts, or holes the sealant | Brittle, wrinkled, or holed seal; film puckered at the jaw line |
| Wrong sealant / film structure | The inside layer isn't compatible, thick enough, or a lock grade | Seal never locks even at correct settings; peels every time |
| Delamination near the seal | The laminate layers separate rather than the seal opening | Layers lift or tunnel beside the weld; the seal weld itself may look fine |
| Contaminated or aged film / worn tooling | Slip additives bloom, film ages, or jaws/Teflon degrade | Seals drift worse over a run or between lots at the same settings |
Two deserve a flag. Delamination masquerades as a seal failure but is really a bond failure between layers — the fix lives in the converter's lamination process, not the sealing station; if the layers come apart rather than the seal opening, read delamination in flexible packaging next. And a seal that drifts worse across a run means a variable is moving: a jaw creeping, a film lot changing, or slip additive blooming, not a wrong baseline.
How to fix failing pouch seals
Work these in order. Each step either fixes the seal or rules a cause out, so you're not changing five things at once and guessing.
- Dial in the seal triad. Set temperature, pressure, and dwell to the film's specified seal window, change one variable at a time, and confirm with a seal-strength pull before committing to a full run. If you sped the line up, add heat to recover the lost dwell.
- Keep product out of the seal zone. Add a clean seal margin above the fill line, control dosing and dust extraction, let powders and aerated fills settle, and wipe or air-knife the seal band so powder, oil, moisture, or fibers never land where the jaws close.
- Verify the sealant film is right for the job. Confirm the inner sealant layer and the total laminate structure match the product — lock vs. peelable, gauge, and compatibility with the fill. If the film is wrong, no triad setting will save it.
- Rule out delamination near the seal. Check whether the layers are separating instead of the seal opening; if they are, it's a lamination-bond issue — see delamination in flexible packaging and take it to the converter.
- Find and confirm the actual leak. Locate the exact point of failure with a leak test before you change anything else, and re-test after each fix to confirm it closed. Use the pouch-leak troubleshooting method to isolate seal vs. film.
If you'd rather triage the symptom first, run it through the packaging symptom checker — it maps what you're seeing to the likely causes and the guide for each.
How to find the actual leak
A seal that "fails" often isn't leaking where you think. A leak test converts a vague "it leaks" into an exact location and mode — a channel in the top seal, a pinhole in a gusset corner, and a product puncture look the same on the shelf but have different fixes. Common methods, from simplest to most sensitive:
- Visual and touch. Inflate the pouch gently and look for the escape point; run a finger along the seal band for a channel you can feel.
- Submersion / bubble test. Press a lightly inflated pouch under water and watch for a bubble stream — the cheapest way to pinpoint it.
- Dye penetrant. Draw a dye into the seal area to reveal a capillary channel that's too fine to see dry.
- Vacuum decay or burst. Instrumented tests that quantify seal integrity for QA sign-off rather than just locating one defect.
Once you have the location and mode, the fix follows the tables above — triad for a weak seal, contamination control for a channel, film choice for a puncture-prone product. The full method is in pouch leaking: finding the leak.
Is it your fault or your supplier's?
This is the question that decides who fixes it, so be honest about the split. It usually falls into one of three buckets.
- It's a spec problem (yours to fix). If the wrong film structure or sealant was chosen for the product — a peelable seal where you need a lock, or a gauge too light for the fill — that traces back to the spec, and it'll fail on any line. Fixing the machine won't help.
- It's a process problem (the co-packer's or line's to fix). If the film is correct but seals are weak, contaminated, or drifting, the sealing process is the variable — triad settings, worn or misaligned jaws, product overshooting into the seal, or line speed. This is the most common bucket, fixable at the machine without changing anything on paper.
- It's a material/lot problem (the converter's to fix). Delamination, inconsistent sealant, slip additive blooming, or seals that fail only on one lot point at the film's manufacture. Pull the lot numbers and hand it back with samples.
The fast tell: does it fail the same way on every unit, or only sometimes? A consistent failure at correct settings is almost always spec or material. An intermittent failure is almost always process: contamination, a wrinkle, or a variable drifting. Keep the failed samples and lot numbers either way; they turn a stalled supplier conversation into a fix.
When to escalate to your converter
Handle the machine-side causes yourself first — the triad and keeping product out of the seal are on the line. Escalate when the evidence points at the material or the structure:
- Seals won't lock even inside the film's stated seal window, on more than one machine.
- You see delamination — layers separating rather than the seal opening.
- Failures track a specific film lot, or appeared after a structure or supplier change.
- An instrumented test (vacuum decay, burst) fails a batch you can't fix at the line.
When you escalate, bring the failed samples, the lot numbers, the settings you ran, and a photo of the failure mode. That lets the converter reproduce it instead of guessing, the difference between a re-run and a return.
How PackOS heads off seal failures
The cheapest seal failure is the one that never ships, and most are decided at spec time — the sealant layer, the gauge, the gusset and seal geometry, whether the structure suits the fill. PackOS reconstructs the pouch spec from your file, surfaces the laminate structure and seal areas, and prices it against the real geometry, so the sealant and structure are a deliberate choice, not a default you discover on the line. See how it reads and rebuilds a flexible spec on the technology page, or run a real pouch file through Quick Quote and get the structure, dieline, and price back in about a minute.
Frequently asked questions
Why is my pouch seal weak or peeling open?
A seal that peels open cleanly, with no film or fiber tear, almost always means the seal was under-processed: not enough heat, pressure, or dwell time for the sealant to melt and fuse. The other common cause is the wrong sealant film — a peelable grade where you needed a lock seal, or a sealant too thin for the job. Confirm the film first, then dial the triad and re-test with a seal-strength pull before a full run.
What is a channel leak in a pouch?
A channel leak is a small unsealed path through a seal that otherwise looks closed. It usually forms when a wrinkle, gusset overlap, or trapped product — powder, oil, or moisture — sits across the seal band when the jaws close, leaving a capillary the seal never bridged. The pouch can look perfect and still pass air or liquid, which is why a leak test is the only reliable way to find it.
What's the difference between a weak seal and delamination?
A weak seal is the two inner sealant surfaces failing to fuse, so the seal itself pulls apart. Delamination is the bonded layers of the laminate separating from each other — tunneling, blistering, or edge lift — while the seal weld may look fine. They have different fixes: a weak seal is corrected at the sealing station by adjusting the triad or the film, while delamination is a lamination-bond problem the converter has to fix.
How do I keep product out of the pouch seal area?
Design and run for a clean seal zone. Leave a seal margin above the fill line, control dosing so fills do not overshoot, extract dust and let powders or aerated products settle before sealing, and wipe or air-knife the seal band on the line. On a form-fill-seal line, product bridging into the seal is one of the most common causes of channel leaks, and it is a process fix rather than a spec change.
Is a failing pouch seal my fault or the supplier's?
It depends on whether the failure is consistent or intermittent. A consistent failure at correct machine settings usually points to the spec or the material — the wrong film or a bad lot — which is the converter's or your spec's problem. An intermittent failure, where some pouches are fine and some are not, usually points to the sealing process on the line: triad settings, worn jaws, or product in the seal. Keep the failed samples and lot numbers to settle it either way.