In filament winding, precise control over fiber tension is not just a good practice—it is essential for producing composite parts with predictable performance, structural integrity, and repeatability. Whether you're manufacturing pressure vessels, pipes, or custom composite components, inconsistent or uncontrolled fiber tension can compromise dimensional accuracy, induce residual stresses, and result in defects such as fiber waviness, bridging, or dry spots.

This article provides a technical deep dive into the significance of fiber tension, the factors influencing it, and the methods available for measuring and controlling it effectively.

Understanding the Role of Fiber Tension

During filament winding, continuous fiber tows are pulled from creels, impregnated with resin (wet winding) or pre-impregnated (dry winding), and then laid onto a rotating mandrel under controlled tension. The fiber tension applied during this process has multiple functions:

Maintains Fiber Alignment: Tension ensures that fibers are laid down smoothly and follow the intended winding pattern without buckling or misalignment.
Controls Fiber Volume Ratio (FVR): The tighter the fibers are wound, the higher the packing density—resulting in a higher FVR, which directly influences mechanical properties such as stiffness and tensile strength.
Minimizes Voids and Resin Pooling: Adequate tension helps press the fiber bundle against the mandrel, reducing the likelihood of trapped air and ensuring consistent resin distribution.
Affects Curing-Induced Stresses: Pre-tensioned fibers can influence the internal stress state of the composite during post-curing and cooling, impacting dimensional stability and performance under load.

Consequences of Improper Fiber Tension

Tension Too Low Tension Too High

Fiber slippage Fiber stretching of breakage

Loose or uneven winding Deformation of mandrel or layup

Waviness and bridging Increased residual stresses

Resin-rich zones Fiber misalignment

Poor dimensional control Risk of damage during winding

Maintaining optimal fiber tension is a balancing act that requires understanding material behavior and machine dynamics.

Influencing Factors and Their Effects

1. Fiber Type and Structure

Glass fibers are relatively tolerant to tension changes but require higher tension to maintain alignment.
Carbon fibers are brittle and prone to damage under excessive tension.
Aramid fibers (e.g., Kevlar®) are more elastic and can accumulate internal stress under high tension if not controlled properly.

2. Winding Speed and Acceleration

Rapid changes in winding speed can cause tension spikes or drops if not synchronized with feed systems. Acceleration/deceleration curves should be smooth to avoid dynamic tension swings.

3. Path Complexity and Winding Angles

Parts with complex geometries (e.g., domes, transitions) introduce changes in tow path length and surface curvature. High winding angles or crossovers increase resistance and friction, influencing the effective tension seen by the fiber.

4. Creel and Feed Path Configuration

The distance between the creel and mandrel, the number of guide rollers, and the friction at each contact point all contribute to the final tension at the point of laydown. Longer or poorly aligned feed paths lead to increased friction losses and tension variability.

Methods of Tension Control

There are three primary categories of tensioning systems in filament winding:

1. Passive Tensioners

Rely on mechanical resistance (e.g., friction brakes or weighted pulleys)
Simple and low-cost, but sensitive to wear and ambient conditions
Tension is not adjustable in real time

2. Open-Loop Active Tensioners

Use motorized rollers to pull fiber under predefined torque
Speed and torque can be programmed but are not adjusted based on feedback
Suitable for applications where part geometry and speed are constant

3. Closed-Loop Active Tensioners

Incorporate sensors (e.g., load cells, tension arms) to measure real-time tension
Feedback is used to adjust motor torque dynamically
Provides the highest precision and adaptability, especially useful for variable geometries or high-speed production

Tension Monitoring and Calibration

Regardless of the system used, regular tension calibration is essential. Common practices include:

Dry Runs: Running fiber through the system without resin to observe baseline tension and behavior
Sample Mandrel Tests: Winding test patterns on smaller mandrels to verify laydown quality
Real-Time Tension Logging: Recording tension data throughout the winding cycle to identify fluctuations or inconsistencies

In advanced setups, tension profiles can be pre-programmed to vary across different winding layers, compensating for curvature or changing load paths in structural parts.

Best Practices for Fiber Tension Management

Always match the tension level to the fiber type and part geometry.
Use tension compensation strategies for transition zones and dome ends.
Periodically inspect tensioning hardware for wear, contamination, or misalignment.
Validate tension settings by examining part quality and comparing against simulation or mechanical test results.
Train operators to recognize symptoms of under- or over-tensioning through visual inspection of fiber laydown and surface consistency.

Conclusion

Precise fiber tension control is fundamental to the success of filament winding, directly influencing part performance, consistency, and structural reliability. With the right understanding of materials, process dynamics, and equipment capabilities, manufacturers can ensure consistent quality and optimize production processes.

As the demand for lightweight, high-performance composites continues to grow, mastering the nuances of fiber tension management will remain a key differentiator in advanced manufacturing environments.

Understanding Fiber Tension in Filament Winding: Why It Matters and How to Control It