What are the reasons for the poor transparency of composite packaging bags? See the answer!

Some composite packaging bags have very high transparency requirements, but during the composite processing of plastic films, packaging bags often encounter issues such as insufficient transparency. So, what are the reasons affecting the transparency of composite packaging bags?

Today, Shunxingyuan Packaging, a professional flexible packaging manufacturer, will combine its own experience to explain the reasons for poor transparency in composite packaging bags and their solutions.

I. Improper adhesive selection

For example, if the adhesive itself has a darker color or insufficient fluidity, poor leveling performance, it cannot spread fully on the composite base film. Generally, the higher the solid content of the main adhesive, the better its fluidity, which facilitates spreading on the film. Overall, adhesive with 75% solid content has better transparency than 50%, and 50% is better than 40% or 35%. For films requiring very high transparency, using 50% and 40% adhesives for lamination is difficult to meet the requirements.

II. Process issues

Firstly, if the drying tunnel temperature of the laminating machine is too high, and drying is too fast, the solvent on the surface of the adhesive rapidly volatilizes (evaporates), causing the adhesive surface to form a skin too early. Then, when heat penetrates into the inner layer of the adhesive, the solvent beneath the adhesive film will vaporize, and when the gas breaks through the surface film, it forms ring-like structures similar to volcanic vents, also making the adhesive layer less transparent. Secondly, if the laminating roller or doctor blade has defects, and a certain point is not pressed firmly, forming a gap, it will also cause the laminated film to be opaque.

Additionally, if there is too much dust in the air of the working environment, and the hot air drawn into the drying tunnel after adhesive application also contains dust, which sticks to the surface of the adhesive layer or gets trapped between the two base films during lamination, a large amount of dust can cause opaqueness or poor transparency.

Solution: Enclose the adhesive application area of the laminating machine, and use a high-mesh filter screen at the air inlet of the drying tunnel to prevent dust from being drawn in (i.e., remove dust from the hot air in the drying tunnel).

Furthermore, if there is no spreading roller or the spreading roller is not clean, it will make the laminated film insufficiently transparent, or if the adhesive application amount is insufficient, the coating is uneven with blank areas, and small air bubbles are trapped, causing mottling or opaqueness.

Solution: Check and adjust the adhesive application amount to ensure it is sufficient and evenly coated, which otherwise causes the so-called "pockmarked film".

III. Other issues

It should be noted that insufficient hot drum temperature during lamination, failure of the hot-melt part of the adhesive to melt, excessively high cooling roller temperature, and failure to cool abruptly can all lead to poor transparency of the laminated film.

Solution: The temperature of the hot drum should not be lower than 70 degrees Celsius. The hot-melt part within the adhesive only begins to melt when the temperature reaches 65 degrees Celsius. After melting, not only does transparency improve, but lamination fastness also increases. The cooling roller should use circulating cooling water or chilled water for cooling. The faster the cooling speed, the better the transparency, the better the flatness of the laminated film, and the better the fastness.

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