Zipper Bag Production Process

Heat-Sealing Bag Production Process Control

The primary function of packaging bags is to protect product quality throughout storage, transportation, and sales. This means ensuring that products remain undamaged, intact, and free from leakage or deterioration despite varying environmental conditions. Therefore, the sealing process must provide adequate strength and airtightness, capable of withstanding pressure from the contents, ensuring the product remains secure. As bag-making is the final production step before delivery, quality control is crucial.

1. Material Selection

1.1 Outer Layer Material

Zipper bags typically use non-heat-sealable materials. The outer layer should have high heat resistance and minimal thermal deformation, such as BOPA, BOPET, BOPP, or paper.

1.2 Inner Layer Material

The inner layer is the heat-sealable material. Its heat deformation temperature should be at least 30°C lower than the outer layer to ensure proper sealing. If the temperature difference is too small, controlling the sealing temperature becomes difficult, affecting seal strength.

The heat-sealing strength of zipper bags increases with the thickness of the heat-seal layer. For back-seal bags and stand-up pouches, the inner layer should be ≥30μm thick to ensure a strong seal. A thinner inner layer may compromise sealing performance.

1.3 Zipper Material

The zipper strip should have a lower heat-sealing temperature than the inner layer, typically 5–10°C lower, for optimal sealing. For example:

- PE L-101 has a sealing temperature 10°C lower than PE LL-201, making it suitable for various PE inner layers to achieve optimal sealing.

2. Pre-Production Preparation

1. Ensure the laminate layer is fully cured to prevent deformation of the heat-seal edge after bag-making.

2. Select appropriate heat-sealing jaws, clean them thoroughly, and inspect the thermal insulation cloth for any damage.

3. Check all parts of the bag-making machine, pre-set the sealing temperature based on past experience, and preheat for 20 minutes until the temperature stabilizes.

4. Adjust temperature, pressure, and sealing time (or speed) to ensure optimal coordination for the best sealing effect.

3. Heat-Sealing Temperature

Heat-sealing temperature directly affects sealing strength. Factors to consider when setting the temperature include material type, thickness, and processing speed.

- Too low: The seal won’t fully bond, even if pressure and time are increased.

- Too high: The sealing material may get damaged, reducing seal strength.

4. Sealing Pressure

Sealing pressure is the first parameter to be adjusted. Standard settings include:

- Heat-sealing jaw pressure: ≈3mm thickness for balanced pressure (left-right/front-back).

- Too low: Leads to weak seals, air bubbles, or incomplete sealing.

- Too high: Causes misalignment between sealing and traction, leading to thinner, brittle seals with reduced strength.

5. Sealing Time (or Speed)

Sealing time affects both sealing strength and bag appearance:

- Longer time: Results in stronger seals but increases shrinkage and wrinkling.

- Shorter time: May lead to weak seals and poor airtightness.

6. Cooling Process

The cooling process stabilizes the heat-seal weld:

- Use the lowest temperature that prevents condensation for cooling.

- Ensure smooth circulation of cooling water. If cooling water is unavailable, increase flow rate to improve cooling efficiency.

- Insufficient cooling can lead to residual internal stress, affecting seal strength and appearance.

7. Number of Heat-Sealing Passes

More heat-sealing passes improve seal strength. General requirements:

- At least two heat-sealing passes.

- Longitudinal sealing passes depend on the heat-sealing jaw length-to-bag length ratio.

- Lateral sealing passes depend on the number of horizontal heat-sealing units in the bag-making machine.

8. Process Control

Continuous quality checks during production include:

1. Sealing Strength

2. Bag Dimensions, Flatness, and Appearance

- Ensure alignment and meet customer specifications (e.g., tear-notch positioning).

3. Zipper Opening Force

- PE zipper internal opening force: ≥50N/50mm

- PP zipper internal opening force: ≥60N/50mm

- PE LL-201: ≥70N/50mm

- PE Q501: ≥80N/50mm

- External opening force: Adjusted per customer requirements.

4. Seal Integrity Testing

- Leakage test: Apply dye solution to detect leaks.

- Water inversion test: Fill the bag with water, invert it for 24 hours, and check for leaks.

5. Flatness of the Zipper Bag

Conclusion

Heat-sealing bag production is a heat transfer process with numerous influencing factors, including:

- Lamination peel strength

- Heat resistance of printing inks

- Additive migration in the heat-seal layer

- Heat-seal width

- Uniform thickness of laminated films

During production, all these factors should be considered and continuously monitored to ensure consistent bag quality that meets circulation and usage requirements.