FEF (First Edge Folding) in Lithium-Ion Pouch Cell Manufacturing
FEF (First Edge Folding) in Lithium-Ion Pouch Cell Manufacturing
First Edge Folding, shortened to FEF, is a mechanical finishing step in which the sealed edge of the pouch is bent over in a controlled way. It follows degassing and the final seal in the flow described here. The folded edge is shorter, tidier and better protected, and the cell takes its near-final outline.
No chemistry happens in this step. It is purely a forming operation done with folding tools, guides and a press. Even so, it works on a sensitive area, the seal and its corners, so it needs the same care as the sealing step itself. Different plants may arrange folding steps slightly differently, and a second fold may follow, so always refer to your own process sheet.
Why edge folding is needed
After degassing, the extra pouch portion is removed and the cell is resealed. The seal edge now sticks out as a flat flange. If it were left like this, it could catch on trays, bend by itself or be struck during handling, and the seal corners would carry concentrated stress.
Folding turns this flange down toward the cell body. The edge is less exposed, the cell becomes narrower to fit a module slot or a holder, and the stiffness around the seal improves. Folding also makes later automated handling smoother because there are fewer sharp or flapping edges.
On the other hand, a fold is a bend in a multilayer film. If it is made too hard, too sharply or in the wrong place, it can crease or crack the aluminium barrier layer or hurt the seal. That is the reason for tight control of force, angle and alignment.
How the FEF process works
A typical sequence on an automatic folding station looks like this.
- The cell is moved to the folding station and held in a fixture with the body supported and the seal edge exposed.
- Cameras or mechanical stops align the edge so that the fold line is at the right distance from the cell body.
- A folding blade or roller lifts the seal edge and bends it through the programmed angle.
- A pressing tool holds the fold under controlled force for a short time, so that it keeps its shape.
- The tools retract and the folded cell is released without dragging the fold open.
- The fold is inspected by camera or gauge for angle, height, straightness and any damage, and the cell is passed or rejected.
Key process parameters
Values depend on cell design, materials and equipment supplier, and actual limits are given by the cell maker's specification. The main parameters are listed below.
| Parameter | What it controls | How it is checked |
|---|---|---|
| Folding pressure or force | Shape retention without damaging the laminate or the seal | Pressure gauge or load reading, trial folds, visual check for creases |
| Folding angle | Final edge shape and cell width | Angle gauge, vision measurement, comparison with the drawing |
| Alignment accuracy | Fold line position relative to the seal and the cell body | Camera offset readings, fixture condition check |
| Hold time | Whether the fold stays closed or springs back | Observation of rebound after release, recipe check |
| Tool surface and radius | Smooth bend without scratching or creasing | Visual and touch check, tool wear record |
| Fixture support | Absence of body deformation and tab bending | First-piece inspection, cell thickness check |
| Cell temperature, where used | Flexibility of the laminate and its behaviour in bending | Process sheet and temperature sensor if fitted |
Typical defects, causes and corrective actions
The following defects can arise at this stage. Several are linked to the three main parameters: force, angle and alignment.
| Defect | Typical causes | Detection | Corrective action |
|---|---|---|---|
| Uneven or misaligned fold | Poor alignment; fixture wear; cell moved in the clamp | Vision check of fold line and height | Realign guides and camera; replace worn fixture; adjust clamping |
| Crease or crack at the fold | Excess force; very sharp angle or bend radius; brittle or cold laminate | Visual and magnified check; leak or dye test on samples | Reduce force; use a larger radius or softer tool; confirm laminate and temperature condition |
| Seal damage near the fold | Fold line too close to the seal; high force on the seal area | Seal inspection after folding; peel or burst check on sampled cells | Correct fold line position and force; review seal quality upstream |
| Springback or open fold | Force or hold time too low; material memory | Fold height measurement a few seconds after release | Increase hold time or force within limits; adjust the angle setting |
| Wrinkle or buckle on the cell side | Uneven pressure; edge not aligned; body not supported | Visual check, camera outline comparison | Balance tool pressure; improve support; re-teach alignment |
| Tab bending or tab-area damage | Fixture touches tab; folding too close to the tab zone | Visual check of tabs, tab angle gauge | Clear the tab area in the fixture; adjust fold start position |
| Dimension out of tolerance | Angle or position drift; tool wear | Gauge or CCD measurement of width and thickness | Recalibrate; replace worn parts; sort the batch since last good check |
A fold defect found early is only a forming problem, but one found in the field may be a leak path. This is why many plants use a camera on every cell after folding and keep a record of which tool and recipe was used.
Quality checks and measurements
Sampling plans and limits vary with the cell maker, and the following checks are common.
- First-piece approval after changeover, tool replacement or a long stop.
- Camera inspection of every folded cell for fold line, straightness and visible creases.
- Gauge check of fold height, cell width and thickness at set intervals.
- Close visual inspection of seal corners and fold lines with magnification on sample cells.
- Verification of tool force and position through periodic calibration.
- Sampled leak or seal-integrity checks as the control plan requires, and a record of any rejected cell with the tool number.
Safety and handling notes
At this point the cell has usually been through formation and degassing, so it contains electrolyte and holds some charge. Treat every cell as live.
- Avoid any contact between the two tabs or between a tab and metal parts. Use insulated fixtures and non-conductive tools, and keep metal items such as pens and rings away.
- Wear gloves and eye protection when handling cells, since electrolyte is irritating and flammable. Ventilation must stay on.
- Do not press, bend or fold a swollen or leaking cell. Separate it in the marked quarantine container and inform the supervisor.
- Keep guards and interlocks closed, and use lockout and tagout before tool change or jam clearing, because press tools can trap fingers.
- Use the electrolyte spill kit and follow the plant procedure if a pouch leaks. Do not use water on electrolyte spills without checking the plant instructions.
- Know where the fire extinguisher and emergency exit are, and report any heating, smoke or smell immediately.
Interview questions
1. What is FEF?
First Edge Folding is a controlled bending of the pouch seal edge after degassing, to protect the seal and shape the cell.
2. Why do we fold the edge?
To protect the seal area from damage, reduce stress at corners, narrow the cell and make handling safer.
3. Which parameters control the fold?
Folding pressure, folding angle, alignment accuracy, hold time and tool condition.
4. What happens if the fold force is too high?
The laminate may crease or crack, the aluminium layer may be damaged and the seal near the fold may be weakened.
5. What is springback and how is it reduced?
Springback is the fold partly opening after release. It is reduced by suitable hold time, force and angle settings.
6. What safety rule is key when handling cells at FEF?
Never short the tabs. The cell is charged and contains electrolyte, so use insulated tools, PPE and quarantine damaged cells.
FAQs
Does FEF affect cell capacity?
Not directly, as it is a mechanical step. Poor folding can however damage the pouch and lead to a leak or reliability problem.
Is FEF done before or after degassing?
In the flow described here it follows degassing and resealing. Always follow your plant's own sequence.
Can a badly folded cell be re-folded?
Only if the specification permits rework. Repeated bending can crack the laminate, so many plants do not allow it.
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