U-Type vs. Ring-Type Battery Wrapping: Which Should You Choose?

Many metal-cased prismatic lithium-ion cells use adhesive-backed PET insulating film (often called blue film) for external casing insulation. U-type wrapping and ring-type wrapping are two common wrapping configurations for this external insulation. The wrapping route directly impacts insulation coverage, dimensional tolerance, process repeatability and module assembly integration.

This practical guide supports battery R&D and manufacturing engineers to compare both wrapping architectures, diagnose typical defects, select a suitable process and evaluate compatible wrapping equipment.

1. What Is External Battery Wrapping?

External wrapping applies PET insulating film to electrically isolate the conductive metal cell casing from adjacent cells, cooling structures and module hardware. It prevents potential short circuits while reserving clearances for terminals, safety vents and marking areas.

Critical distinction: PET blue film is external insulation, not the primary cell packaging. Aluminum-laminated pouch film forms the main enclosure for pouch cells. They cannot be substituted without full engineering validation.

Material	Core Function	Typical Process
PET blue insulating film	Primary function: electrical insulation for metal prismatic casing; may also deliver surface protection	Film lamination, folding and adhesive bonding
Aluminum-laminated pouch film	Primary pouch-cell enclosure with moisture-barrier and heat-sealing functions	Pouch forming and thermal sealing
Supplementary insulating sheets	Local insulation reinforcement	Die-cutting and precision placement

A wrapping pattern alone does not deliver waterproof performance. Waterproof rating must be validated on the complete cell and module assembly.

2. How U-Type Wrapping Works

U-type wrapping routes film across the cell bottom and two large broad faces, then folds surplus film onto narrow side surfaces. The profile forms a “U” shape and provides continuous bottom insulation as part of the main film layout.

Representative Process Sequence

1. Pre-check cell casing surface cleanliness and dryness against film supplier requirements
2. Align insulating film to reference surfaces of the prismatic cell
3. Apply film to cover the cell bottom and two broad faces, control roller pressure and film tension
4. Fold side sections to form side overlaps while maintaining insulation coverage
5. Inspect corners, edges, clearances and wrapped cell dimensions

Key Engineering Notes

• Continuous bottom coverage integrated within the main film path
• Side folding creates local stacked film layers. The number of overlapping layers varies by blank geometry and folding scheme, with no universal fixed maximum layer count
• Main risks: trapped air bubbles, corner lifting or film tearing if folding precision is insufficient
• Process performance must be compared using identical material, equipment and acceptance criteria during trial runs

3. How Ring-Type Wrapping Works

Ring-type wrapping (loop wrapping) wraps film around the four vertical sides of the prismatic cell. The film ends overlap at a designated seam. Bottom coverage depends on edge-folding design; traditional layouts often adopt supplementary insulating patches, while some designs extend the main film to cover the bottom via folded edges.

Representative Process Sequence

1. Fix and position cell to achieve repeatable film application
2. Wrap film around four vertical faces with controlled tension
3. Form and fix the overlap seam with defined overlap width
4. Fold extended film edges on top and bottom sides as required
5. Apply supplementary insulating patches for uncovered zones when specified
6. Inspect both main wrap and additional insulating pieces

Key Engineering Notes

• Local thickness buildup can appear at the overlap seam, folded edges and supplementary patch boundaries
• Insulation reliability depends heavily on patch or folded-edge adhesion; long-term bonding performance requires dedicated validation
• Process performance must be compared using identical material, equipment and acceptance criteria during trial runs

4. U-Type vs. Ring-Type Wrapping: Structural & Performance Comparison

Comparison Point	U-Type Wrapping	Ring-Type Wrapping
Main Film Path	Across the bottom and two broad faces	Around the four vertical faces
Bottom Coverage	Incorporated into the main film path	Depends on edge-folding design and any supplementary insulation
Local Thickness Buildup	Mainly associated with side folds and corner overlaps	Associated with the overlap seam, folded edges and supplementary pieces
Main Process Controls	Film alignment, fold geometry, tension and corner formation	Film alignment, seam position, overlap width and supplementary-piece placement
Main Inspection Focus	Fold integrity, corner lifting, coverage and local dimensions	Seam integrity, edge lifting, supplementary insulation and local dimensions
Environmental Performance	Requires validation of the complete design	Requires validation of the complete design
Yield and Cost	Must be established under defined production conditions	Must be established under defined production conditions

Neither wrapping method has universal yield or cost advantage. Total cost evaluation should include film consumption, supplementary parts, cycle time, scrap, rework, changeover and inspection workload. Continuous bottom coverage is an insulation structural feature and cannot be interpreted as validated waterproof capability.

5. PET Insulation Film Selection for Prismatic Cells

PET describes only the substrate. Validate these key parameters before selecting film grades:

1. Electrical performance: Define insulation resistance and dielectric withstand requirements with clear test conditions and acceptance criteria. Flat film sample testing cannot capture defects introduced during cutting, folding and handling of finished cells.
2. Local thickness buildup: Evaluate finished thickness after folding and overlapping and feed the data into module tolerance assessment. Take measurements at defined locations on wrapped cells instead of relying solely on nominal raw film thickness.
3. Adhesion & aging: Test peel strength on the actual casing finish, and validate performance after relevant thermal, humidity and mechanical exposure cycles.
4. Foldability: Verify the film can conform to cell corner geometry without tearing, whitening, excessive stretching or film lifting under production tension. Confirm stable release and tension behavior for roll-fed processes.

CANRD’s PET protective film product is designed for lithium-ion cell surface protection. Its qualification cannot be directly reused for prismatic power-cell blue-film applications. Verify electrical, adhesive and aging requirements for your target use case before grade selection.

6. Common Wrapping Defects & Root Cause Investigation

Defect	Possible Contributors	Investigation Focus
Air bubbles	Surface contamination, uneven application pressure, trapped air	Surface preparation, roller contact and film application sequence
Wrinkles	Film skew, unstable film tension	Film tracking accuracy, cell positioning and tension control
Lifted corners / edges	Poor wetting, residual film stress, unsuitable fold geometry	Adhesive compatibility, fold design and thermal aging behavior
Film tearing	Sharp casing features, excessive tension, unsuitable tooling contact	Casing edge quality, cutting quality, film tension and tooling specification. Any casing rework must follow the cell manufacturer’s approved process.
Exposed metal	Wrong blank size, registration error, misplaced supplementary pieces	Coverage drawing, cutting dimension and placement accuracy
Excessive local thickness	Uncontrolled overlap or accumulated folds	Blank geometry and folding sequence

Build baseline process recipes and adjust parameters through controlled trials. Record film lot, casing condition, machine settings and defect location for repeatable quality improvements.

7. Dual-Mode Wrapping Equipment: Can One Machine Handle Both Processes?

Yes. Published patent CN112208830A describes an equipment architecture compatible with both wrapping methods. It demonstrates a feasible dual-process concept, but does not guarantee production yield, changeover speed or suitability for all cell sizes.

When evaluating equipment, run trials using your actual cells and target film. Review these items:

• Supported cell dimension and tolerance range
• Required tooling changes between wrapping patterns
• Film feeding, cutting and tension stability
• Automatic handling of supplementary insulating patches
• Inline inspection function and rejected part management
• Demonstrated cycle time and changeover repeatability

CANRD’s aluminum-plastic film forming machine is built for pouch cavity forming. It serves a separate manufacturing stage from external prismatic-cell wrapping.

8. Step-by-Step Process Selection Workflow

1. Define required insulation coverage and reserved clearances for terminals and safety vents
2. Analyze mechanical and thermal interfaces with cooling pads, compression components and structural parts. Include folded regions and seams in tolerance assessment
3. Compare complete process routes. Evaluate material consumption alongside handling steps, cycle time, rework and inspection. Lower raw film usage does not automatically deliver lower total cost
4. Validate finished cells through project-specific tests: dimensional measurement, adhesion and edge-lift evaluation, insulation resistance, dielectric withstand, environmental aging and module assembly trials
5. Set test methods and acceptance limits according to cell, module and customer specifications

9. Process Development Priorities

Three core priorities for wrapping process improvement:

1. Inspection integration: inline visual inspection to detect seam displacement, wrinkles, exposed casing and missing supplementary insulation. Where thickness data is required, deploy dedicated dimensional measurement systems separate from visual defect inspection.
2. Film-layout optimization: reduce unnecessary local thickness buildup while preserving full insulation coverage. Validate dielectric performance, mechanical durability and aging behavior after layout modification.
3. Material–equipment matching: qualify film and adhesive performance under the intended operating conditions. New cell chemistries such as solid-state designs still require external insulation assessment; external insulation requirements are determined by cell architecture and materials rather than cell chemistry alone.

Conclusion

U-type wrapping creates a continuous main film path spanning the cell bottom and two broad faces. Ring-type wrapping wraps film around the four vertical cell surfaces.

The suitability of each configuration depends on insulation coverage requirements, seam and folded edge layout, module mechanical interfaces, material aging performance and validated process capability. No single wrapping type is universally preferred for EV or 3C applications. Selection must be completed on a project-by-project engineering basis.

For R&D project discussions, prepare cell dimensions, casing material, wrapping drawings, operating environment and validation criteria. Visit https://www.canrud.com/ to explore relevant material, equipment and validation support matching your development stage.

FAQs

Q1: What is the core difference between U-type and ring-type battery wrapping?

A1: U-type wrapping carries the main film across the bottom and two broad faces. Ring-type wrapping carries it around the four vertical faces. Remaining surfaces are finished following the insulation design.

Q2: Is U-type wrapping always better for EV prismatic cells?

A2: No. Selection depends on required coverage, module interfaces, dimensional tolerances, material behavior and demonstrated process capability. The wrapping naming convention alone is not sufficient for decision-making.

Q3: Does blue PET film make a lithium battery waterproof?

A3: Its primary role is electrical insulation, and it may also provide surface protection. It does not, by itself, establish a waterproof rating. Waterproof performance must be validated on the full cell and module assembly.

Q4: Can aluminum-laminated pouch film replace PET blue film for prismatic cells?

A4: No. Pouch laminate provides primary pouch-cell enclosure with moisture-barrier and heat-sealing functions, while PET blue film delivers external casing insulation. Direct substitution requires independent engineering assessment.

Q5: How should wrapping yield be compared?

A5: Use identical cell design, inspection criteria and yield definition. Compare first-pass acceptance, rework, scrap and sustained process performance under representative operating conditions.