Types and Causes of Curling and Warping After Bag Making

Types and Causes of Curling and Warping After Bag Making

Types and Causes of Inward Curling After Bag Making

a) Low heat resistance of the heat-sealing layer material, excessive shrinkage, causing inward curling.

b) Inconsistent tension between two substrates, with the inner layer having higher tension than the outer layer, resulting in inward curling.

c) Heat-sealing material has a lower heat resistance temperature than the bag-making heat-sealing temperature.

d) The base film has a lower heat shrinkage rate than the inner film, causing inward curling after bag making.

Types and Causes of Outward Curling After Bag Making

a) The heat shrinkage rate of the outer layer material is higher than that of the inner layer material.

b) Residual solvent inside the film, especially if it contains a high amount of OH radicals, is prone to moisture absorption, leading to the formation of bubbles during heat sealing.

c) Excessive glue coating, not completely dried, with a high residue of solvent causing tunneling at the heat-sealing site.

d) The heat resistance temperature of the outer layer material is lower than that of the heat-sealing material, resulting in outward curling.

e) After maturation and shaping, residual stress remains, especially when the substrate tension is too high, and the composite material tension is too low, making the finished bag more likely to curl outward.

S-Shaped Curling

a) Excessive moisture absorption by BOPA film leading to S-shaped curling after bag making.

b) Significant difference in the longitudinal and transverse heat shrinkage ratios of BOPA film, ideally controlled within a 0.5% difference.

c) Excessive heat shrinkage of BOPA film causing curling, with longitudinal and transverse shrinkage rates exceeding 3.0% (for general composite packaging).

In summary, the curling and warping after bag making can be attributed to various factors such as film thickness variation, improper heat-sealing conditions, uneven tension, and differences in heat shrinkage rates. Understanding the specific type and direction of curling helps identify the root causes and adjust corresponding process parameters for a resolution.

II. Non-Synchronous Shrinkage-Induced Curling

Apart from the mentioned factors, the non-synchronous shrinkage of the composite film can result in material curling and warping. This manifests as either "inward curling" or "outward curling" of finished bags, indicating the presence of non-synchronous shrinkage (thermal stress or shrinkage rate differences and directions) within the composite film.

Generally, CPP has a heat shrinkage rate not exceeding 1%, PE film has longitudinal shrinkage rates between 1.4% and 7.2% and transverse rates between -0.4% and -1%, while BOPA film should have shrinkage rates controlled within 2.5%. Any deviation beyond these values, particularly exceeding 0.5% difference between different films, can lead to non-synchronous shrinkage and subsequent curling. Resolving this issue may require changing the film, as adjusting process parameters may not be sufficient.

III. Curling After Cooking/Steaming

For BOPA//CPP structures, bags are likely to curl outward after hot treatment. BOPA//PE structures may exhibit either outward or inward curling after heat treatment, depending on factors like bag-making orientation. Bags made with a "horizontal out" approach are more likely to curl outward after heat treatment.

Controlling the shrinkage rate of BOPA cooking films (resistance to 121°C for 40 minutes) to around 5% and keeping the difference between longitudinal and transverse shrinkage rates below 0.5% can reduce the probability of curling. When receiving film shipments, it's advisable to conduct shrinkage rate tests under similar processing conditions and make adjustments if significant deviations are observed.

IV. Summary

The reasons for curling and warping can be summarized as follows:

Inconsistent film thickness in the composite film, leading to varied heat-sealing temperatures and shrinkage rates.

Excessive heat-sealing temperature or duration causing wrinkling at the heat-sealed area.

Uneven running trajectory of the composite film in the longitudinal heat-sealing knife section, resulting in wrinkling.

Insufficient cooling during bag making, especially when the heat-sealed area is not adequately cooled, leading to shrinkage wrinkling.

Inadequate maturation time, excessive residual solvent, causing bubbles, delamination, or corner curling.

Poor heat resistance of the surface layer film leading to outward curling.

Improper tension matching during the composite process, resulting in shrinkage curling, with the side of higher tension curling more.

Excessive heat-sealing layer, especially PE layer, shrinkage causing inward curling.

Excessive glue coating causing outward curling.

Inconsistent substrate tension during the composite process, resulting in curling, with the side of higher tension curling more.

Excessive shrinkage of nylon film causing S-shaped shrinkage curling.

The smaller the difference in the 45-degree diagonal heat shrinkage rate of the nylon film, the better the product balance, with differences below 1.5% having a lower probability of bag corner curling.

Too much residual solvent inside the film causing outward curling.

High moisture absorption of the base film causing outward curling after bag making.

Significant difference in the shrinkage ratio between the outer and inner films causing outward curling. Conversely, inward curling occurs with a difference in the opposite direction.

Lower heat resistance temperature of the outer layer material than the heat-sealing temperature causing outward curling.