How to Improve Pouch Cell Heat Sealing: Temperature, Pressure and Dwell Time

canrd July 29, 2026 59

Introduction

Pouch cell sealing is a critical step in lithium-ion battery manufacturing that directly affects cell reliability, safety and long-term performance.

Common failures such as electrolyte leakage, pouch swelling, weak seal strength, tab corrosion and laminate cracking are often related to sealing materials, PP fusion quality and heat-sealing parameters.

This guide explains how pouch cell sealing works, why sealing defects occur, and how engineers can optimize temperature, pressure, dwell time and inspection methods to improve sealing consistency and reduce production failures.

Quick Defect Diagnostic Flow

Visible Defect Symptom	First Inspection Priority Areas
Immediate leakage after top/side primary sealing	Seal surface contamination, sealing jaw parallelism, heat-seal temperature, pressure and dwell time
Slow electrolyte leakage after aging or final sealing	Residual electrolyte on seal edges, incomplete PP fusion, damaged aluminum laminate film
Uniform cell swelling after formation cycles	Vacuum drying residual moisture, incomplete degassing, electrolyte impurity-induced side reactions
High self-discharge and corrosion near electrode tabs	CPP tab sealant offset, direct contact between aluminum tab and laminate aluminum layer, oversized unsealed zones
Cracks or localized thinning at pouch pocket corners	Deep drawing mold corner radius, laminate forming ductility, excessive drawing depth
Seal edges separate easily during peel testing	Insufficient thermal input, contaminated sealing interface, worn or uneven sealing jaws

Core Basics: How Pouch Cell Heat Sealing Works

Aluminum laminate film serves as the outer packaging material for soft-pack lithium cells. The training material references an 85 μm laminate sample for structural demonstration, and actual layer thicknesses vary based on cell application and supplier specifications.

In this reference sample, the laminate contains three functional layers:

21 μm nylon outer layer: Blocks oxygen infiltration and provides scratch resistance
33 μm aluminum barrier layer: Isolates external moisture to avoid internal side reactions
32 μm PP inner sealant layer: Melts under heat and pressure to form airtight bonding

The core sealing mechanism relies on thermal fusion of opposing PP layers. When matched upper and lower sealing jaws deliver stable temperature, pressure and dwell time, the inner PP layers melt and integrate into a continuous barrier, blocking electrolyte leakage and water vapor penetration. Nylon and aluminum layers do not participate in thermal bonding, and they maintain mechanical protection for finished cells.

Cross-sectional microscope image of lithium-ion pouch cell aluminum laminated film showing the nylon layer, aluminum barrier layer, and PP heat-sealing layer with measured layer thicknesses.

Cross-sectional structure of lithium-ion pouch cell aluminum laminate film showing the nylon outer layer, aluminum barrier layer, and PP heat-sealing layer used for moisture protection and hermetic cell sealing.

Simplified Pouch Cell Sealing Related Workflow

The full pouch cell manufacturing sequence covers mixing, coating, rolling, winding and encapsulation segments. Only sealing and packaging-related steps are listed below to avoid irrelevant process content:

Aluminum laminate deep drawing (pouch pocket forming)
Jelly roll or electrode stack insertion into formed pockets
Top and side primary heat sealing
Three-in-one internal resistance & short circuit testing
Pre-electrolyte vacuum drying
Electrolyte precision injection
Temporary vacuum sealing of gas bags
Hot and cold cell shaping pressing
Electrochemical formation activation
Degassing and secondary final sealing
Edge folding, visual sealing inspection
Aging, OCV testing and capacity grading

Troubleshooting Top 5 High-Frequency Pouch Cell Sealing Defects

1. Pouch Cell Seal Leakage

Direct Problem Overview

Pouch seal leakage occurs when the inner PP layers fail to form continuous fusion joints. Contamination, improper heat-sealing settings or damaged laminate material are common contributors. Defects split into two categories: immediate leakage after primary sealing, and slow latent leakage detected after long-term aging.

Root Cause Classification

Contaminated sealing contact surfaces

Electrolyte splashes, electrode coating dust and airborne workshop particles can separate PP layers during thermal bonding, creating tiny permeable gaps.
Unbalanced heat-sealing parameters

Insufficient temperature, pressure or dwell time limits complete PP melting; excessive heat and pressure squeeze sealant layers too thin, generating fragile sealing edges prone to rupture.
Aluminum laminate physical damage

Over-stretched pocket corners and micro-cracks generated during deep drawing form hidden electrolyte leakage channels.
Residual internal gas after formation

Trapped gas inside finished cells continuously applies pressure to weak sealing joints and gradually creates leakage paths during storage.

Target Corrective Actions

Install air blowing dust removal stations before all sealing procedures to reduce particulate contamination
Implement hourly sealing jaw cleaning to remove carbonized electrolyte residue
Adjust temperature, pressure and dwell time incrementally, and complete peel strength sampling after each parameter modification
Optimize deep drawing mold corner radii to lower localized laminate stretching stress

Inspection images of lithium-ion pouch cell seal leakage defects, showing pouch corner sealing, edge sealing, and heat-sealed joint failures caused by contamination, improper sealing parameters, or damaged aluminum laminate film.

2. Pouch Cell Swelling After Formation or Aging

Direct Problem Overview

Pouch swelling is driven by two interrelated sets of factors: internal gas generation from electrochemical side reactions, and incomplete gas extraction during secondary sealing. Swelling cannot be attributed solely to sealing flaws; pre-injection drying, electrolyte purity and formation cycling all affect gas accumulation levels.

Dual Root Cause Groups

Pre-sealing process-induced gas generation

Inadequate vacuum baking leaves residual moisture inside electrodes, which reacts with electrolyte to produce gaseous byproducts
Electrolyte with excessive metal impurities decomposes rapidly during formation charging
Improper formation current and temperature accelerate electrolyte degradation and gas output

Sealing process insufficiencies

Secondary sealing vacuum degree or holding time cannot fully extract gas stored in temporary gas bags
Minor slow seal leakage allows atmospheric moisture and oxygen to penetrate cells and trigger secondary gas formation

Standard Inspection Sequence

Verify vacuum oven tightness, baking temperature and holding duration first
Test electrolyte water content before mass injection batches
Extend vacuum holding time during degassing and final sealing
Add 48-hour room-temperature aging sampling to screen latent swelling defects before capacity sorting

Lithium-ion pouch cell swelling after formation caused by gas generation, incomplete vacuum degassing, electrolyte decomposition, or sealing defects, showing typical pouch bulging after aging.

3 Low Pouch Seal Peel Strength

Direct Problem Overview

Low seal peel strength indicates incomplete fusion between opposing PP sealant layers, which can be induced by insufficient thermal energy, surface contamination or degraded laminate inner layers. Acceptable peel strength thresholds vary across different cell projects, and no universal fixed value applies to all pouch cell formats.

Common Contributing Factors

Uneven temperature distribution across the full width of sealing jaws
Residual electrolyte oil film covering the sealing contact zone
Scratched, deformed sealing jaws with poor flatness consistency
Aged aluminum laminate rolls with degraded PP layer thermal fusion performance
Insufficient sealing pressure leading to discontinuous contact between laminate sheets

QC Validation Method

Complete peel strength sampling after every equipment maintenance or parameter adjustment. Use a micrometer to measure cross-section seal thickness to judge uniform PP extrusion status. Grind and re-level sealing jaws once flatness deviation exceeds project tolerance limits.

4. Tab-Area Leakage & Aluminum Tab Corrosion

Direct Problem Overview

Leakage and abnormal self-discharge near electrode tabs occur when CPP tab sealant positioning fails to isolate aluminum tabs from the laminate aluminum barrier layer. Improper sizing of internal and external unsealed zones on top seals creates migration channels for electrolyte, accelerating metal corrosion over cycle storage. This section contains differentiated technical details unique to pouch cell encapsulation training materials.

Key Design & Process Control Points

Offset or undersized CPP tab sealant fails to fully cover aluminum tab metal, creating direct contact paths with laminate aluminum foil
Oversized internal unsealed zones on top seals guide electrolyte toward tab metal regions
Deep scratches penetrating the laminate nylon and aluminum barrier layers allow moisture ingress and tab oxidation

Optimized Process Adjustments

Standardize CPP tape margin control during electrode tabbing procedures; calibrate top sealing jaw tab slot depth to avoid squeezing insulation layers. Use a projector for daily cross-section inspection of top seals to monitor unsealed zone dimensional compliance, following A-E dimension standards marked in encapsulation training drawings.

Lithium-ion pouch cell tab sealing defect showing electrolyte leakage and aluminum tab corrosion caused by improper CPP tab sealant coverage and incorrect top seal dimensions.

Cross-sectional schematic of lithium-ion pouch cell tab sealing structure, showing CPP tab sealant insulation between aluminum tabs and aluminum laminate film to prevent electrolyte leakage and tab corrosion.

5. Pouch Film Corner Cracking During Deep Drawing

Direct Problem Overview

Cracks and excessive localized thinning at pouch pocket four corners originate from mechanical stress concentration during deep drawing. Training FEA stress and strain simulation data confirm pocket corners bear the maximum tensile load, and short-side stress levels exceed long-side stress in standard rectangular pockets.

Contributing Variables

Small mold corner radii create extreme localized laminate stretching
Excess deep drawing depth over-tensions the aluminum barrier layer
Low ductility thermal-laminated film unsuitable for deep-draw 3C cell pocket production
Misaligned laminate feeding generates uneven tension across raw material rolls

Process Optimization Controls

Adjust forming mold corner radii to reduce peak tensile stress; conduct forming performance sampling for every incoming aluminum laminate batch; limit maximum pocket depth according to each film grade’s official technical datasheet.

How Temperature, Pressure & Dwell Time Affect Seal Integrity

Temperature, pressure and dwell time are the three core adjustable variables for all heat-sealing stations. The following table summarizes visible defects induced by parameter deviation and corresponding inspection items. All numerical settings listed below serve only as a project-specific 2Ah LFP pouch cell example and cannot be treated as universal industry standards.

Parameter Deviation & Defect Correlation Table

Parameter Deviation	Typical Visible Defect	Primary Inspection Check
Sealing temperature too low	Low peel strength, discontinuous PP fusion	Multi-point temperature calibration on sealing jaw surface
Sealing temperature too high	Over-thin seal edges, laminate perforation, discoloration	Cross-section thickness measurement of finished seal joints
Sealing pressure insufficient	Uneven fusion, broken seal lines	Air supply pressure inspection, upper-lower jaw parallelism test
Sealing pressure excessive	Complete extrusion of PP sealant layers out of seal zones	Microscopic cross-section observation of seal edges
Dwell time too short	Partial fusion, inconsistent batch peel strength	Sealing machine timing cycle verification
Dwell time over-extended	Thermally degraded brittle laminate and seal edges	Visual inspection for discolored sealing areas

Project Specific Parameter Reference Box

The following settings only apply to our internal 2Ah LFP pouch cell pilot line and cannot be copied for mass production of all cell formats:

Primary top & side sealing: 170–180°C, 0.4 MPa pressure, 2.0s dwell time
Degassing and secondary final sealing: 175–185°C, 0.45 MPa pressure, 2.5s dwell time

Standard QC Inspection Items for Sealing Quality

All inspection standards are extracted from CANRD internal CQP critical quality characteristic documents. Dimensional tolerances and acceptance limits must be customized according to each customer’s cell specification, and no fixed universal tolerance values are applied in this guide.

Seal Appearance Visual Inspection

Check top, side and final sealing edges for wrinkles, trapped foreign particles, discoloration, incomplete fusion lines and edge cracks
Seal Cross-Section Thickness Measurement

Use a micrometer to sample seal thickness; define qualified tolerance ranges per independent cell project requirements
Seal Peel Strength Batch Sampling

Complete weekly batch sampling; match acceptance criteria to customer QC test standards
Pouch Pocket Forming Thickness Check

Measure residual laminate thickness at pocket corners to screen over-stretched thin areas before sealing
Post-Sealing Aging Reliability Test

Conduct controlled temperature aging sampling to detect slow leakage and latent swelling defects that cannot be identified via instant visual inspection
Tab Sealant Dimensional Inspection

Use an optical projector to measure CPP tape margin, internal and external unsealed zone sizes of top seals

Frequently Asked Sealing Defect Questions

Q1: Can pouch film with minor scratches on the outer nylon layer continue production?

A: Scratches limited to the nylon protective layer without penetrating the middle aluminum barrier layer can pass routine inspection. Any scratch breaking through the aluminum foil layer must be scrapped immediately, as long-term moisture and oxygen penetration will trigger tab corrosion and continuous cell swelling during storage.

Q2: Why identical sealing parameters still create intermittent batch leakage defects?

A: Two hidden common root causes lead to unstable sealing results: inconsistent PP layer thickness across different aluminum laminate batches, and accumulated electrolyte residue on sealing jaws between production runs. Increase hourly jaw cleaning frequency and add cross-section thickness sampling for incoming laminate rolls.

Q3: What reliable method confirms complete degassing during secondary sealing?

A: Dual verification standards are used in pilot production lines: 1) Visual inspection for flat cell body without localized bulging after final sealing; 2) 48-hour room-temperature aging sampling, with zero permanent volume expansion as the passing standard.

Q4: What vacuum drying conditions eliminate moisture-induced cell swelling?

A: No universal fixed temperature and time window works for all cell formats. Vacuum temperature, holding duration and target residual moisture ppm must be validated through small-scale trials based on separator heat resistance, electrode active material chemistry and oven heat transfer performance. Avoid applying a fixed 80–100°C temperature range across all production lines.

Q5: What core differences exist between primary side sealing and secondary final sealing requirements?

A: Primary top and side sealing only forms a temporary enclosure with reserved gas bags for electrolyte filling and formation cycles. Secondary final sealing requires complete PP fusion after full gas extraction, with stricter pre-sealing electrolyte cleaning standards to avoid residual liquid contaminating fusion surfaces.

Conclusion

Reliable pouch cell sealing requires precise control of materials, process parameters and quality inspection.

By optimizing PP layer fusion, sealing temperature, pressure, dwell time and tab-area design, manufacturers can reduce leakage, swelling and corrosion risks while improving pouch cell consistency.

Since different cell designs require different sealing windows, engineers should validate parameters based on specific materials, structures and production targets rather than applying universal settings.

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How to Improve Pouch Cell Heat Sealing: Temperature, Pressure and Dwell Time

Introduction

Quick Defect Diagnostic Flow

Core Basics: How Pouch Cell Heat Sealing Works

Simplified Pouch Cell Sealing Related Workflow