What is the Impact of Mechanical Tension on the Quality of Composite Flexible Packaging Products? A Brief Discussion

In the production process of plastic composite flexible packaging, mechanical tension is present in almost every step of roll-based production. Each tension control system in mechanical equipment can affect the appearance and even the physical properties of plastic composite products. Therefore, let’s explore the manifestations of tension’s influence on plastic composite flexible packaging during production across relevant process equipment, aiming to reduce waste, improve product quality, yield, and production efficiency.

1. What is Mechanical Tension?

First, we need to understand mechanical tension in plastic composite products: What is mechanical tension in the production process?

In this article, mechanical tension refers to a balancing force applied to continuous roll materials in the machine's running direction. Simply put, it straightens and flattens the roll film, keeping the substrate in a relatively unstretched yet taut and smooth state.

Roll film tension is typically measured in pounds per linear inch (PLI) or, in metric units, newtons per centimeter (N/cm). If PLI is known, the total tension applied to the roll can be calculated by multiplying PLI by the material’s width in inches. Conversely, if the total tension (in pounds) is known, PLI can be calculated by dividing the total tension by the roll width in inches.

Relevant formulas:

Pounds per linear inch (PLI) = Total tension (lbs) ÷ Roll width (inches)

Total tension (lbs) = PLI × Roll width (inches)

The tension on the roll substrate can be likened to hanging a weight on the edge of the roll: the tension equals the weight (in pounds), and PLI equals the weight divided by the roll width (in inches). (Example: A 20-inch-wide roll bearing a 50-lb weight has a PLI of 50 ÷ 20 = 2.5 PLI.)

2. The Importance of Mechanical Tension

Why is mechanical tension so critical in plastic composite production?

Without proper mechanical tension, roll materials cannot be controlled. The substrate must maintain "traction" with all machine idler and drive rollers to ensure proper handling and control.

1. Application of Roll Substrate Running Principles

Roll substrate running principles state that the substrate will naturally align perpendicularly to idler or drive rollers. Applications include: guiding rolls through parallel idler and drive rollers, using substrate guides to direct the roll, and applying wrinkle-removal devices.

However, if traction is lost (i.e., the substrate slips on rollers), this principle fails. For example, the substrate may drift sideways, or if it slips on a guide roller, the guiding device won’t direct it as intended. Thus, mechanical tension is essential to maintain traction with rollers.

2. Relationship Between Equipment Tension and Substrate Stretching

All substrates stretch (tighten) in the machine direction under tension. The key is applying enough tension to smoothly feed the substrate while avoiding excessive stretching, which can cause permanent deformation (especially noticeable with printed patterns).

3. Tension Control in Unwinding and Rewinding

Unwinding: Without proper tension control, "roll slippage," wrinkles, or even breakage can occur.

Rewinding: Poor tension control leads to uneven roll edges, inconsistent tightness, or wrinkles.

3. Effects of Poor Tension Control

What issues arise from poor tension control in various processes?

(1) Printing Process

Poor tension in printing affects the substrate and semi-finished products as follows:

Unbalanced unwinding tension or uneven tension gradients cause wrinkles, skewed feeding, or indentations.

Excessive drying oven tension under high heat leads to stretching or excessive shrinkage (e.g., a 200mm print shrinking to 197mm or stretching to 203mm). Stretch-prone materials like MATOPP, NY, or print-grade MDOPE are particularly susceptible.

Excessive rewinding tension causes wrinkles in semi-finished rolls or uneven pattern stretching/shrinking, leading to inconsistent lengths, misaligned edges, and unstable tracking in later processes.

(2) Lamination Process

Lamination tension control is more complex than printing, often involving multiple unwinding stations (e.g., two, three, or even four). Tension matching and substrate flatness requirements are higher.

For a two-layer laminated product:

Unwinding 1: Printed semi-finished roll.

Unwinding 2: Base material (order may vary by company).

Poor tension control causes:

Incorrect rewinding tension: Too high → breaks/stretching; too low → slack/interlayer misalignment → wrinkles/tunneling.

Tension fluctuations: Unstable tension (due to equipment issues or uneven material thickness) causes localized stress variations → tunneling or "bullet" wrinkles.

Mismatched unwinding/rewinding tension: Large differences between pre- and post-lamination tension cause interlayer stretching disparities.

Unmatched unwinding tensions (1 & 2): Warping or severe curling (e.g., NY/PE curls toward the higher-tension side). Stretch-prone structures (MATOPP/PE, BOPP/CPP, etc.) risk pattern misalignment, irregular shrinkage, and inconsistent bag dimensions, complicating downstream processes.

Thus, poor lamination tension control leads to stretching/shrinking, distorted patterns, wrinkles, or tunneling in final products, increasing waste and forcing downstream adjustments.

(3) Slitting Process

Slitting tension control is simpler but critical. Poor tension or misaligned guiding causes cutting inaccuracies (e.g., tracking errors).

High tension: Machine-direction stretching → transverse shrinkage → narrower width → wrinkles.

Low tension: Machine-direction shrinkage → transverse expansion → wrinkles.

Mismatched unwinding/rewinding tension: Rewinding wrinkles.

(4) Bag-Making Process

The main tension in bag-making is unwinding tension. For example, a gusseted stand-up zipper bag involves three tensions:

Bag body unwinding tension.

Gusset unwinding tension.

Zipper feed tension (often passive, driven by the bag material).

Imbalances cause:

Bag body tension issues: Stretching/shrinking, especially in thin materials (MATOPP/PE, etc.), leading to misaligned patterns or localized wrinkles.

Gusset tension issues: Wrinkles during forming or pressing.

Zipper tension issues: Passive tension may cause misalignment, crushing, or derailing if unevenly pulled.

4. Conclusion

In summary, mechanical tension affects the entire plastic composite production process. Poor tension control or mismatches lead to defects, reducing yield and efficiency. In today’s competitive market, batch errors are unacceptable.

Key measures include:

Ensuring equipment operates optimally.

Documenting tension parameters for each process to prevent imbalances or forgotten settings.

Implementing smart tension systems to free operators for other critical tasks.

These steps minimize waste, improve yield, and enhance profitability, making tension control a vital factor in the flexible packaging industry’s success.