Solutions to the creasing problem of composite packaging bag edges

In actual production, composite packaging bags often malfunction due to mechanical or other factors, such as bubbles, low composite strength, wrinkles, curling, stretching, or shrinkage. Wrinkling and curling of composite packaging bags can seriously affect customer production and cause significant losses.

Today, Shunxingyuan Packaging, a professional flexible packaging manufacturer, will share its experience in producing composite packaging bags and explain how to avoid wrinkling and curling.

1, Wrinkling and curling are related to material quality

If one end of the composite material or printing substrate is loose and the other is tight, with thickness variations, and the difference in tension between the two ends of the film roll is large, the film will swing significantly after being mounted on the machine, potentially causing wrinkles and curling. When the material enters between the hot steel roller and the hot-press roller, it cannot be horizontal to the hot-press roller, resulting in uneven pressing, causing wrinkles and diagonal lines in the composite product, leading to product scrap.

When the composite material is PE or CPP, if the thickness deviation exceeds 10%, it is also prone to wrinkling. In this case, the tension of the composite material can be appropriately increased to make it as horizontal as possible to the hot-press roller for pressing. However, it should be noted that the tension should be moderate. If the tension is too large, the composite material will be stretched, causing the bag mouth to curl inward. If the deviation of the composite material is too large and cannot be used, it should be handled separately.

Due to the thickness deviation of the material itself, causing product wrinkles and curling, the thickness must be carefully checked before mounting on the machine. If it exceeds 10%, it is best not to use it.

2, Operational errors can also cause wrinkles and curling

For example, uneven pressure of the hot-press roller, or the hot-press roller rotating back and forth during operation, prevents the composite material from feeding smoothly, leading to wrinkles and curling.

The solution is to remove the left and right nuts of the hot-press roller, retighten them evenly, and if a pneumatic cylinder pressure valve locks the hot-press roller, carefully observe and slow down the speed, increase the air valve pressure at the end contacting the hot steel roller to achieve parallel pressing, observe the pressure gauge reading, and feed the material parallel to reduce the probability of wrinkles and curling.

In actual production, curling and wrinkling are also related to the temperature of the hot steel roller. If the temperature of the hot roller is too high, the probability of curling increases, especially in BOPP/CPP and BOPP/PE composite structures. Excessive temperature can even cause the PE film to stick to the steel roller, wasting material and reducing production efficiency.

The solution is to stop the machine, turn off the hot steel roller temperature control, and wait for it to cool down before processing. Generally, the temperature of the hot steel roller is adjusted according to the actual environment and operating effect, and also adjusted according to the width and thickness. Good temperature control will reduce the probability of curling and wrinkling.

3, Curling is closely related to the tension control of the printing substrate and the composite material

Normal production requires the substrate to be coated and glued smoothly and moderately. The tension of the composite material should match the tension of the substrate. If the substrate tension is too high (coating film) and the composite material tension is too low (composite film), the finished bag will curl outwards, resulting in an uneven bag mouth. If the printing substrate tension is too low and the composite material tension is too high, the finished bag will curl inwards, causing poor bag opening, inward curvature of the opening, and difficulty in sealing.

The solution is to adjust the tension of the printing substrate and the composite material through the guide rollers. Adjusting the tension of the printing substrate or composite material is usually done manually with a screw or using a magnetic powder brake.

4, Inappropriate winding tension of the composite product also causes longitudinal and transverse wrinkles

If the winding tension of the composite product is too low, transverse diagonal lines will appear at the bottom as the winding material increases, making the finished bag uneven and affecting its appearance. If the winding tension is too high, due to the rigidity of the paper tube, longitudinal wrinkles will appear in the finished product due to the pulling force, especially at the beginning of the machine operation. Of course, if the paper tube is not smooth, the winding will be uneven, like waves, also affecting and producing wrinkles.

The solution is to start slowly, reducing the pulling force to reduce the probability of wrinkling. The tension should be moderate, aiming for no wrinkles or looseness at the start of winding. After the machine speed is normal, the winding tension should be constant to reduce the probability of wrinkling. Usually, the winding power is achieved by a spring pressure plate or a magnetic powder brake for smooth winding.

5, Curling and wrinkling are also related to uneven gluing

When coating the substrate, if one side has more glue and the other side has less, the side with more glue will bend inward after drying, and may even stick. If the gluing system of the laminator is determined by the gluing roller, the left and right nuts should be adjusted to make the gluing even. If the laminator uses a doctor blade system where the glue amount is determined by the mesh roller depth, the angle of the doctor blade can be adjusted forward or backward.

The best way to check whether the gluing is even is to press the gluing system with the left and right index fingers to observe its thickness, ensuring even coating to effectively reduce curling and wrinkling.

6, Curling and wrinkling are related to the cleanliness of the various guide rollers and traction rollers in the laminator

If the traction rollers are dirty or uneven during substrate operation, diagonal wrinkles are easily caused. The best solution is to use a piece of newspaper, 10cm long and 6cm wide, for padding. Due to inertia, the newspaper will rotate synchronously with the traction roller, solving the diagonal wrinkle problem.

7, Mechanical failures are related to post-lamination wrinkling

The laminator experienced a mechanical malfunction, causing uncontrollable substrate tension. The tension was either too high or too low, preventing the material from feeding smoothly into the hot steel rollers. This resulted in wrinkling and edge curling at the winding station. After careful analysis, two conclusions were reached:

Either the magnetic powder brake malfunctioned, causing inconsistent and uncontrollable tension; or the sensor sensitivity was distorted, leading to inconsistent motor speeds and the aforementioned issues. Analysis ultimately determined that the sensor distortion was the cause. After replacing the sensor with a new one of the same model, the tension was controlled, and the malfunction was resolved.

8, Wrinkling in flexible packaging can be categorized into lamination wrinkling and winding wrinkling.

Lamination wrinkling occurs when one substrate is smooth while the other is raised, forming horizontal or vertical tunnels. The two layers are separated. In winding wrinkling, the two layers remain bonded, but external pressure causes wave-like wrinkles on the surface of the laminated film. Winding wrinkles typically occur near the core.

Common causes of lamination wrinkling include:

Insufficient initial adhesive strength of the adhesive;

High solvent residue;

Excessive or insufficient adhesive application;

Mismatched substrate tension during lamination;

Excessive airflow in the oven;

Too little winding tension;

Poor substrate flatness.

Winding wrinkling is mainly caused by poor tension control during the winding process, resulting in a tighter outer layer and looser inner layer, with large gaps between layers providing space for shrinkage.

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