How to Prevent Wrinkling During EVOH High-Barrier Film Production？

Evoh (ethylene vinyl alcohol copolymer) is renowned for its outstanding Gas Barrier Properties and mechanical strength. It has become a critical material across premium packaging and industrial applications, particularly in food packaging, fresh produce, medical devices, automotive fuel tanks, and underfloor heating systems. However, during the blown film extrusion process, EVOH-based films are highly prone to various types of wrinkling. This not only compromises the visual quality of the film but can also significantly degrade its barrier performance and functional reliability.

 

1.Why Wrinkling Happens in EVOH Film Production

 

EVOH resin has unique processing characteristics due to its high moisture sensitivity, high melting point, and considerable shrinkage rate. These factors make the film particularly susceptible to wrinkling if not properly controlled during production. Common causes include:

 

Temperature Sensitivity: EVOH material reacts sharply to temperature fluctuations. Localized overheating or uneven cooling can cause variations in molecular orientation, leading to wrinkles.

 

High Shrinkage During Cooling: EVOH contracts significantly as it cools. If cooling is uneven, vertical wrinkles or “ribbing” may form.

 

Moisture-Induced Degradation: Inadequate drying of raw material results in high moisture content, which destabilizes the melt and causes irregular extrusion, leading to random creases.

 

2.Key Process Controls to Minimize Wrinkling in EVOH Film

2.1 Material Preparation and Melt Stability

 

Thorough Drying: EVOH resin must be dried to a moisture content of ≤0.05%, with a recommended dew point of ≤ -40°C.

 

Melt Pressure Stabilization: Use of melt gear pumps is highly recommended to eliminate flow instability caused by pressure or temperature fluctuations.

 

Precision Filtration: Employ fine filtration (≥120 mesh) along with continuous automatic screen changers, such as the Gneuss SF system, to ensure melt purity and flow consistency.

 

2.2 Cooling System Configuration

 

Dual Air Ring Cooling with Negative Pressure: Especially suitable for EVOH and PA-based barrier films, this system provides independent upper and lower air streams for uniform, stable cooling while minimizing bubble instability.

 

Air Ring and Die Concentricity: Proper alignment between the die and air ring is essential to prevent vertical wrinkling. Use segmented air rings with infrared sensors or temperature guns to adjust airflow evenly around the bubble circumference.

 

IBC (Internal Bubble Cooling) System: Critical for stabilizing internal pressure and reducing lateral movement that causes transverse wrinkles.

 

2.3 Process Parameter Optimization

 

Blow-Up and Draw Ratios: A balanced approach—lowering the blow-up ratio while increasing the draw ratio—helps lower the frost line and stabilize the film bubble. Care must be taken to maintain strength in both directions.

 

Temperature Management: Keep die temperature variations within ±2°C to prevent localized overheating or poor plastication.

 

Controlled Cooling Rate: Stage cooling using upper low-flow pre-cooling and lower high-flow final shaping can improve overall uniformity.

 

2.4 High-Precision Haul-Off, Flattening, and Winding Systems

 

Flattening Boards and Heat Dissipation: Longer flattening boards improve performance. Install heat-dissipation brushes on plastic rolls to prevent thermal collapse and wrinkles caused by residual heat.

 

Effective Flattening Rollers: Essential to eliminate edge waves or inconsistent web tension, thereby preventing spiral or diagonal wrinkles.

 

Low-Tension Winding: Due to EVOH polymer's high shrinkage, adopt taper tension control with low initial tension. Ensure film is cooled to near room temperature (ideally below 40°C) before winding.

 

Winding Method Selection: Choose between contact, center, or gap winding based on surface sensitivity, smoothness, and shrinkage characteristics of the film.

 

3.Common Wrinkle Types and Their Solutions

 

4.FAQs

 

Q1: What problems arise from insufficient drying of EVOH resin, beyond wrinkling?

Insufficient drying leads to hydrolysis, causing melt instability and degraded film properties. It weakens mechanical strength (e.g., tensile strength, elongation) and increases brittleness. Critically, it reduces crystallinity, which directly impairs EVOH plastic's oxygen barrier function—undermining its core purpose in high-barrier packaging.

 

Q2: How should the EVOH layer thickness be optimized in co-extruded structures to reduce wrinkling?

Symmetrical structures are preferred to minimize internal stress. The EVOH layer typically makes up 5–10% of the film structure. Overly thick EVOH layers can lead to stress concentration and buckling. Proper balance of tie layers and rigid support layers (like PA or PP) is essential for maintaining film stability.

 

Q3: How can you distinguish whether vertical wrinkles are caused by cooling issues or die output irregularities?

Use a rotation test: if the wrinkle position moves when rotating the die or air ring, the issue is likely cooling-related (uneven airflow). If wrinkles remain fixed regardless of rotation, the cause is likely uneven die output—check die gap, temperature, or contamination in the flow channels.

 

Producing EVOH high-barrier films without wrinkles requires a deep understanding of the material’s behavior and a systems-level approach to process control. From precise drying and melt stabilization to advanced cooling and tension management, every stage of production plays a role. By mastering the unique challenges of EVOH processing, manufacturers can ensure flawless, high-performance films that meet the rigorous demands of global packaging markets.