Common Laminate & Pocket Defects in Li-ion Pouch Cell Manufacturing

The pouch of a lithium-ion pouch cell is made from a multilayer aluminum laminate film. A typical film has an outer protective plastic layer, a thin aluminum foil in the middle as the moisture and gas barrier, and an inner plastic sealant layer. To hold the electrode stack, a recess called a pocket (or cavity) is formed in the film by pressing it into a die, a process often called deep drawing or cold forming.

Original EngineerX process map: the 11 main steps of pouch cell manufacturing in the order used on this site; the highlighted step is the subject of this article.

This step happens at the very start of the assembly line: the film roll is loaded, the pocket is formed, the film is cut and folded, and the formed pouch is inspected before the jelly roll goes in. Defects created here are easy to overlook, but they stay in the pouch and show up later as leaks, swelling or failed seals.

Why this stage matters

The pouch has no rigid metal can, so the laminate alone must keep the cell sealed for years. The aluminum foil is only a few tens of micrometres thick, so forming stretches it close to its limit. A scratch, a thin spot or a micro-crack in this foil becomes a path for moisture to get in and electrolyte to get out. Pocket geometry matters too: the pocket must be the right depth, shape and position so the jelly roll fits without being squeezed and the seal area stays flat and clean.

Finding a bad pouch at this stage costs a piece of film. Finding it after electrolyte filling costs a whole cell and creates a leakage clean-up task. Early inspection is therefore among the most cost-effective checks on the line.

How the forming process works

Incoming film check. The roll is checked for lot number, width, thickness, surface appearance and storage condition. Film is kept clean and dry.
Roll loading and feeding. The roll is mounted and threaded through rollers that keep tension even. Guide rails keep the film straight.
Cavity forming. A punch presses the film into a die to make the pocket. Pressure, speed and hold time are set so the film stretches smoothly. Some designs use a double pocket for a fold-over pouch.
Cutting and folding. The formed film is cut to length and folded to form the pouch body.
Inspection. Camera systems and gauges check cavity position, depth, wrinkles, scratches and cut edges.
Transfer. Good pouches go to jelly roll insertion. Rejects are removed and recorded with the defect type.

Key process parameters

Typical settings depend on cell design, materials and equipment supplier, so limits come from the cell maker's specification and the film supplier's data sheet.

Parameter	What it controls	How it is checked
Film unwind tension	Flatness of the film, wrinkle formation	Tension gauge or sensor, visual check of film path
Forming force and speed	How the film stretches into the die	Machine settings, first-article check
Forming depth	Space for the jelly roll, wall thinning	Depth gauge, camera or coordinate measurement
Punch and die alignment	Cavity position and corner stress	Alignment gauge, cavity-offset measurement
Punch and die radius and surface	Tearing at corners, scratches	Tool inspection, periodic polishing records
Incoming film quality	Barrier integrity, layer bonding	Supplier certificate, sample inspection, adhesion check
Environment and cleanliness	Particles on the sealant layer	Visual inspection, particle counts in the area

Typical defects, causes and corrective actions

Defect	Common causes	Detection	Corrective action
Laminate scratch	Rough rollers, sharp contacts, dirty tooling	Camera, visual under good light	Clean and polish contact surfaces, add protective guides, reject scratched film
Laminate wrinkle	Uneven tension, misaligned rollers, damaged roll	Camera, visual	Set tension, align rollers, remove damaged roll length
Pin hole in laminate	Film defect, particles pressed into film, handling damage	Backlight check, electrical insulation test of film or leak test after packing	Quarantine the lot, improve cleanliness, notify supplier
Aluminum layer crack	Excess stretching, sharp die radius, wrong forming speed	Magnified inspection, leak test, whitening at the wall	Reduce forming depth or speed, correct die radius, check film grade
Laminate delamination	Poor layer adhesion, moisture or age of film, over-strain	Visual bubbles or lifting, peel test on samples	Check storage and shelf life, return the lot if the bond is weak
Uneven cavity depth	Variable forming pressure, tool misalignment, process drift	Depth gauge, camera	Re-adjust pressure, re-align tool, verify with first-article cells
Cavity wrinkle	Film not held evenly, insufficient hold-down, forming too fast	Camera inspection	Adjust hold-down force and speed
Cavity offset / misalignment	Feed length error, tool shift, loose fixture	Camera, dimensional check against datum	Correct feed, tighten and re-align tooling
Over-stretched laminate	Deep forming, high speed, strong clamp	Thickness check, whitening, cracks under magnification	Reduce depth, slow forming, review design with engineering
Corner tearing	Stress at corners, small die corner radius, brittle or aged film	Visual, microscope	Increase corner radius, lower forming severity, check film lot

Quality checks and measurements

Incoming inspection of film width, thickness, surface, roll labelling and supplier data.
First-article check after every setup or tool change: depth, length, width, corner shape and position are measured before production starts.
CCD (camera) inspection on each pouch for position, wrinkles, scratches and contamination.
Periodic destructive checks, such as cutting a formed pocket to see wall thickness and the foil layer.
Tool condition checks at set intervals, including cleaning and polishing logs.
Reject recording by defect type, so trends point to a tool, a roll or a shift.

Safety and handling notes

Forming presses and cutters have pinch and crush points. Use guards and light curtains, and apply lockout before tool changes.
Film edges can cut hands. Wear cut-resistant gloves when threading rolls.
Keep the film clean. Fingerprints, oils and dust on the sealant side will later affect the seal, so handle only by the edges, with clean gloves.
Store film rolls as the supplier advises, avoiding damp or hot areas, and keep rolls wrapped until use.
No electrolyte is present at this stage, but the area still belongs to a controlled production line. Do not carry food, drinks or loose tools into it.

Interview questions

1. What are the layers of a pouch laminate?

An outer protective plastic layer, a middle aluminum foil barrier and an inner heat-sealable plastic layer, bonded together with adhesive layers.

2. Why is the aluminum layer important?

It blocks moisture and gas, protecting the electrolyte and the cell chemistry.

3. What causes cracks in the aluminum layer during forming?

Too much stretching: pocket too deep, forming too fast, a small die corner radius or a film that is not suited to the draw.

4. How does cavity offset affect later steps?

The jelly roll sits off-centre, so seal lines and tab areas can be misplaced, causing seal and dimension problems.

5. How do you tell a laminate defect from a forming defect?

Laminate defects appear on unformed film of the same roll, such as pinholes. Forming defects appear only after pressing, such as corner tears or wall thinning.

6. What is the value of first-article inspection?

It proves the setup is right before many pouches are made from it, preventing a whole batch of scrap.

FAQs

Can a scratched laminate be used if the scratch is small?

Only if the cell maker's specification allows that size and location. Scratches on the sealing area or deep enough to expose the foil are usually rejected.

Why does the pocket not come out the same on every machine?

Tool wear, alignment, press force and film lot all differ, which is why each machine has its own validated setup.

Does laminate age matter?

Yes. Film has a storage life and storage conditions, and old or badly stored film can lose bond strength.