Issues and Solutions for Heat Sealing in Bag Making Machines
11/05
2025
Bag manufacturing is a critical process in the production of flexible plastic packaging. If bag quality fails to meet standards or consumption rates are excessively high, the consequences range from rendering previous production stages futile to triggering claims from downstream customers that severely impact the company's operations.
Heat sealing is achieved through the combined action of heat and pressure. On one hand, at the molten sealing surface, macromolecules undergo diffusion, penetration, and entanglement to achieve a sealed joint. On the other hand, under the pressure of the heat sealing tool, molten polymer molecules fuse the upper and lower film layers into a single entity. After cooling and setting, this joint maintains a certain strength.
The quality of composite packaging bag production and heat sealing significantly impacts every stage of the packaging process, transportation, content storage, and product distribution. Below are common issues encountered during bag production and their solutions.
1. Incomplete heat sealing leading to content leakage or spoilage.
Cause: Poor contamination-resistant heat sealability of the film, primarily due to unsuitable resin used in the inner sealing layer.
Solution: Use films with superior contamination-resistant heat sealability. Generally, LDPE offers moderate contamination resistance, while EVA exhibits excellent resistance when VA content is high. LDPE also provides good contamination resistance, and ionic resins or metallocene polymers deliver outstanding contamination-resistant heat sealability.
2. Brittle seals or seal fractures.
Causes:
(1) Excessive heat sealing temperature.
(2) Excessive pressure.
(3) Prolonged heat sealing time.
(4) Excessively sharp edges on the upper sealer or damaged PTFE coating.
(5) Bottom seal silicone rubber is too hard.
(6) During lamination and curing, adhesive penetration into the film reduces substrate toughness (impact resistance) and increases brittleness.
(7) After cooling and storage, plastic composite packaging bags exhibit increased seal strength but also a tendency toward brittleness.
Countermeasures:
(1) Select appropriate processing temperature, pressure, and heat sealing time based on the heat sealing characteristics of the inner sealing layer material.
(2) Improve the surface condition of the upper heat sealing blade to ensure a smooth sealing surface.
(3) Ensure complete coverage with PTFE cloth.
(4) Select silicone rubber pads of suitable hardness.
3. Indistinct outer contour on the heat-sealed surface (blurred knife marks)
Causes:
(1) Insufficient cooling time.
(2) Poor contact with the cooling plate.
(3) Melting of the silicone pad edges due to heating.
(4) Uneven longitudinal heat sealing pressure.
(5) Irregular or dull edges on the heat sealing blade.
Countermeasures:
(1) Adjust cooling time.
(2) Adjust cooling plate.
(3) Replace or adjust heat sealing blade.
(4) Prevent edge melting caused by silicone pad overheating.
4. Bag warping after heat sealing.
Causes:
(1) Inconsistent thickness of the laminated film.
(2) Excessive heat sealing temperature or prolonged sealing time.
(3) Non-straight running path of the laminated film at the longitudinal sealing blade section.
(4) Insufficient cooling.
(5) Insufficient cure time.
(6) Poor heat resistance of the surface base film.
(7) Improper tension matching control during lamination, resulting in residual stress after curing and setting—particularly prone when using thin laminate films.
Countermeasures:
(1) Adjust floating roller tension.
(2) Select inner sealing substrates with low-temperature heat sealability.
(3) Adjust heat seal temperature to an appropriate level.
(4) Ensure thorough cooling.
(5) Conduct sufficient curing.
(6) Re-evaluate the surface substrate film selection.
(7) Adjust tension across all sections of the laminating equipment to minimize shrinkage rate differences between the two substrates.
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