Sealing Defects in Li-ion Pouch Cell Manufacturing (Most Critical Stage)
Sealing Defects in Li-ion Pouch Cell Manufacturing (Most Critical Stage)
Heat sealing is the operation that turns a formed aluminum laminate pouch into a closed container. Heated sealing bars press the inner plastic (sealant) layers of the laminate together until they melt and fuse. This is done along the top edge, where the tabs come out, along the sides, and at the corners. The result is a thin plastic joint that has to hold electrolyte in and keep moisture out for the whole life of the cell.
In the line, sealing comes after the jelly roll has been placed in the formed pocket and the pouch has been folded. Some seals are made before electrolyte filling, and the last one is made after formation and degassing. A seal that looks acceptable but is weak will usually show its problem much later, as leakage, swelling or capacity fade. This is why many engineers call it the most critical quality gate in assembly.
Why this stage matters
The aluminum foil inside the laminate is an excellent barrier, but it cannot bond to itself. Only the seal does that job. If the seal is open, thin, or has a channel through it, three things go wrong over time:
- Moisture from the air enters and reacts with the electrolyte, which produces acidic by-products and extra gas.
- Electrolyte creeps out through the channel, leaving dry areas and a messy, hazardous surface.
- Gas that should be collected and removed at degassing escapes in an uncontrolled way.
A leaking cell found at the end of the line costs the full value of the cell. A leaking cell found in the field can be a safety event. That is why the sealing process is controlled so tightly and why every seal is inspected.
How the sealing process works
- Pouch positioning. The pouch with the jelly roll is located in the fixture so that the seal line falls in the correct place, clear of the roll and the tab area.
- Tab and laminate check. The tabs, already carrying their tab film, are confirmed to be straight and centred. The laminate edge is held flat.
- Bars close. The upper and lower sealing bars close together and apply force along the seal line.
- Heat and dwell. The bars hold the set temperature for the set dwell time. The sealant melts, flows slightly and wets the opposite layer.
- Release and cooling. The bars open and the polymer solidifies. Some machines use a cooling bar or a short hold so the seal does not get pulled apart while still soft.
- Inspection. The seal is checked by camera or by eye for width, wrinkles, burns and contamination. Sampled cells go to the laboratory for strength tests.
Key process parameters
Typical settings depend on cell design, materials and equipment supplier, so the values always come from the cell maker's specification and the laminate supplier's data sheet. The table lists what each parameter does.
| Parameter | What it controls | How it is checked |
|---|---|---|
| Sealing temperature | How fully the sealant melts and bonds | Thermocouple readings on the bar, periodic surface-temperature check with a calibrated probe |
| Sealing pressure | Contact between layers and how much polymer is squeezed out | Pressure gauge or force sensor, pressure-sensitive film test |
| Dwell time | Heat soak into the laminate | Machine timer log, recipe lock |
| Bar parallelism and flatness | Even pressure across the full seal length | Pressure film print, visual check of the seal imprint |
| Bar alignment and position | Where the seal lands relative to tabs and pocket | CCD measurement, go/no-go gauge |
| Laminate and tab-film condition | Cleanliness and bonding surface | Incoming inspection, visual check before sealing |
| Cooling after seal | Seal stability and flatness | Visual check, peel or burst test results |
Typical defects, causes and corrective actions
Many of these defects share causes, so troubleshooting means looking at temperature, pressure, time, alignment and cleanliness together rather than changing one setting at random.
| Defect | Common causes | Detection | Corrective action |
|---|---|---|---|
| Seal wrinkle | Pouch off position, high fixture clamp force, roll too close to seal area, heavy tab folding, uneven laminate tension, particles | Camera and visual check | Re-centre pouch, reduce clamp force, correct roll position, clean seal zone |
| Weak sealing | Bars not parallel, uneven or low pressure, temperature too low or too high, short dwell, contamination | Peel or burst test, dye test | Re-level bars, review temperature, pressure and time, clean |
| Over sealing / burn marks | Excess heat, force or dwell, wrong calibration | Colour change, thin seal under microscope | Lower settings, recalibrate heaters and gauges |
| Sealant overflow | Too much pressure or heat, unsuitable bar geometry | Visual, width gauge | Reduce pressure, check bar edges and profile |
| Incomplete sealing | Uneven pressure, low temperature, worn bar, contamination | Camera, leak test | Replace or regrind bar, clean, correct settings |
| Seal crack | Handling stress, wrong laminate properties, over-stretched laminate, poor cooling | Visual, bend check, leak test | Gentler handling, verify laminate lot, allow cooling |
| Pin hole after sealing | Metal particles, earlier laminate damage, excess pressure | Leak or vacuum test, high-voltage insulation test on laminate | Remove particle sources, screen incoming laminate, reduce pressure |
| Seal contamination | Dust, fibres, metal particles, oil or grease | Visual, magnified inspection | Improve cleaning, check gloves and wipes, review air handling |
| Seal misalignment | Bar parallelism problem, position drift, mechanical looseness, pouch placement error | CCD dimension check | Re-align and lock bar position, inspect fixture wear |
| Corner seal damage | Uneven corner pressure, wrong corner-tool shape, over-stretched laminate | Visual, microscope | Correct tool, balance pressure, review forming depth |
Quality checks and measurements
- Seal width measured on every cell by camera or on samples by gauge, against the drawing tolerance.
- Seal strength by peel test on cut strips, or burst or pressure test on sample pouches. The acceptance value is set by the cell maker.
- Visual and CCD inspection for wrinkles, burns, contamination and alignment.
- Cross-section of a sealed sample under a microscope to confirm that the sealant layers have fused fully and the foil has not been pushed through.
- Leak checks on finished cells, such as vacuum-hold or helium methods where the plant has them, and visual leakage checks after aging.
- Machine checks at shift start: temperature verification, pressure film print, and bar condition.
Safety and handling notes
- Sealing bars are hot. Use guards and wait for cooling before cleaning or adjusting.
- Pinch points exist where the bars close. Follow lockout and tagout before maintenance, and do not bypass light curtains or interlocks.
- The seal made after filling sits close to electrolyte. Solvent vapour is flammable and irritating, so keep ventilation on and keep ignition sources away.
- A cell that has been punctured or crushed during sealing can short internally. Set it aside in a designated tray and follow the plant's procedure for suspect cells.
- Work with clean gloves and clean tools in the dry room so that fibres and oils do not reach the seal area.
Interview questions
1. Why is the sealant layer needed in the laminate?
The aluminum foil cannot be heat sealed. The plastic layer on the inside melts and bonds, which closes the pouch.
2. What three main parameters control a heat seal?
Temperature, pressure and dwell time. Bar alignment and surface cleanliness decide whether those settings act evenly.
3. A seal looks fine but fails the peel test. What do you check first?
Bar parallelism and pressure distribution, then temperature calibration, then contamination on the sealing surface.
4. How can too much heat or pressure be harmful?
It thins the sealant, pushes polymer out of the joint and can bring the foil close to the surface, so the seal becomes weaker, not stronger.
5. How does a wrinkle in the seal cause leakage?
The fold leaves a tiny channel where layers did not fuse. Electrolyte or moisture can travel along it.
6. How do you confirm a new sealing setup is acceptable?
Run trial cells, check width and appearance, do peel or burst tests, cut a cross-section and compare with the cell maker's specification before release.
FAQs
Is sealing done only once?
No. The top and side seals are made during packing, and the final seal is made after formation and degassing. Each has its own inspection.
Can a weak seal be re-sealed?
Only if the plant procedure allows it, and usually only before electrolyte is present. Re-sealing a wetted area is generally not accepted because electrolyte contaminates the interface.
Why are tabs a special case at the seal?
The seal must bond to the tab film on a metal strip, which is a different joint from plastic-to-plastic, so tab position and film condition affect it strongly.
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