Introduction
In the final stage of flexible packaging production, the appearance and physical properties of the finished roll are the ultimate criteria determining whether it can be successfully delivered to the customer. For operators and equipment purchasers, understanding how tension control affects stretch film rewinding quality is crucial. If mistakes are made at this stage, even the most perfectly extruded film can be instantly turned into scrap.
The core challenge of a high-precision stretch film rewinder lies not in how fast it spins, but in how precisely it can “control” the film. Stretch film (LLDPE) possesses extremely high elasticity and self-adhesiveness, making it exceptionally sensitive to changes in tension. Excessive or insufficient tension can directly lead to core collapse, end-face misalignment, or film stretching and deformation. This article will combine principles of rheology with modern electrical control technology to provide a comprehensive analysis of this core process, revealing the mechanical secrets behind a perfect film roll.

The Fatal Damage of Excessive Tension: Accumulation of Internal Stress

To understand how tension control affects the quality of stretch film rewinding, we must first examine what happens when the film is “pulled too tight.”

Paper Core Collapse and Starring Defects

Stretch film behaves much like an elongated rubber band.

• If high tension is maintained throughout the rewinding process, each layer of film will attempt to contract inward.
• As the number of layers increases, this inward compressive force (radial pressure) accumulates exponentially.
• Ultimately, the immense internal stress will crush the inner paper core. If the core is rigid enough to resist crushing, the inner layers of the film will undergo irreversible wrinkling and deformation, resulting in star-like radial cracks on the end face—a phenomenon known as “starring.”

Necking and Width Loss

Another obvious sign of excessive tension is a narrowing of the film.
If the tension exceeds the film’s yield point during slitting and rewinding of a parent roll from an upstream wide-width stretch film making machine, the film will stretch and elongate while simultaneously shrinking in width. This “necking” not only results in the customer receiving rolls with insufficient width but also compromises the film’s original puncture resistance.

The Dangers of Insufficient Tension: Looseness and Slippage

When discussing how tension control affects the quality of stretch film rewinding, we must avoid the other extreme: applying too little tension.

Layer Misalignment and Telescoping

When rewinding tension is insufficient, the friction between layers weakens.

• This makes it extremely easy for air to become trapped between the film layers, forming bubbles.
• During vibrations from handling or transportation, the loosely packed inner layers will slip relative to one another.
• The end face of the entire roll will bulge outward like a telescope, a phenomenon known in the industry as the “telescoping effect” or “layer misalignment.” Such products are completely unusable on automatic stretch wrappers and must be returned as scrap.

Remedies Using Surface Winding Technology

For adhesive films, surface winding technology is employed to expel air without excessive stretching.

• Modern stretch film slitter rewinder machines typically utilize surface winding technology.
• By utilizing direct contact between the lay-on roller and the film roll’s surface, air is forcibly squeezed out. The perfect coordination of tension control and lay-on roller pressure is the key to solving the problem of loose rolls.

Taper Tension: The Soul of Perfect Rewinding

Having understood the dangers of excessive and insufficient tension, what is the correct tension?

Control Logic for Linear Decay

The ultimate answer to how tension control affects stretch film rewinding quality lies in “variation.”

• To prevent internal stress from crushing the paper core, the rewinder cannot use constant tension; it must use “taper tension.”
• Principle: During the initial winding stage (near the core), higher tension is applied to ensure the film is tightly wound without slippage. As the roll diameter increases, the PLC control system automatically and linearly reduces the tension (for example, reducing tension to 60% of the initial value when the roll is full).
• This “tight inside, loose outside” structure is the only physical principle for producing high-quality industrial film rolls.

Servo Drive and Dynamic Compensation

During rapid acceleration or deceleration (such as the moment of flying knife cutting), inertia causes severe fluctuations in tension.

• Traditional magnetic powder brakes have slow response times, which can easily lead to film slack during the startup phase.
• High-speed stretch film rewinding machines, equipped with fully servo-driven motors and low-inertia dancer rolls, can detect and compensate for tension fluctuations within milliseconds, ensuring completely consistent tension from the first meter to the 1,000th meter.

The Interdependence of Waste Control and Tension

Poor tension control not only ruins finished products but also increases additional costs.

Downtime Losses Caused by Film Breaks

Unstable tension is the primary cause of film breaks.

• Each film break requires manual rethreading, which generates a significant amount of waste and severely impacts OEE (Overall Equipment Effectiveness).
• Closed-loop Solution: Directly feed the trimmed uneven edges or waste film generated during tension adjustment through a pipeline into the plastic granulating machine in the workshop for in-line recycling. While this mitigates losses, reducing waste at the source through precise tension control is the true path to lean production.

Comparison Table of Common Tension Defects and Solutions

To visually demonstrate how tension control affects the quality of stretch film rewinding, we have compiled the following troubleshooting matrix.

Conclusion

In summary, exploring how tension control affects the quality of stretch film rewinding essentially involves finding the optimal balance within the physical limits of the film through mechanical and electrical means.

• Too tight leads to brittleness: causing core deformation and film tearing.
• Too loose leads to looseness: resulting in air bubbles and delamination, leading to returns.
• Taper is key: Tight on the inside and loose on the outside is the golden rule of stretch film rewinding.

For packaging companies, investing in a rewinding machine equipped with full-servo closed-loop tension control is a crucial step toward eliminating quality complaints and enhancing product value. When commissioning the equipment, be sure to use the defect checklist above to fine-tune your tension curve, ensuring that every roll of film becomes a perfect work of art.