The tunnel effect refers to the formation of hollow protrusions and wrinkles on one layer of substrate that are flat, and on the other layer of substrate that protrude to form hollow protrusions and wrinkles. It generally runs horizontally and is commonly seen at the two ends of the drum. There are many factors that can cause the tunnel effect. Below, we will provide a detailed introduction.
1.The tension during the composite does not match. After the composite is completed, the previously tensioned membrane will contract, while the other layer with low tension will contract less or no, causing relative displacement and generating raised wrinkles. When coating adhesive on easily stretchable films and compounding with non stretchable films, tunneling effects are particularly prone to occur. For example, there is a composite film with a BOPP/AI/PE three-layer structure.
When the first layer of BOPP is compounded with AI, the BOPP coating enters the drying tunnel for heating and drying. If the unwinding tension is too high, coupled with the heating inside the drying tunnel, the BOPP is stretched, and the elongation of the AI layer is extremely small. After compounding, the BOPP shrinks, causing the AI layer to protrude and form a transverse tunnel. During the second composite, the (BOPP/AI) layer serves as the coating substrate. Due to the AI layer, the film extension is very small. If the tension of the second unwinding PE film is too high, the PE film is easily stretched and deformed.
After the composite is completed, the PE shrinks, causing the (BOPP/AI) layer to bulge and form a tunnel. Therefore, it is necessary to match the tension according to the characteristics of different equipment.
2.The film itself is wrinkled, uneven in thickness, and has loose edges. To composite this type of film, it is necessary to slow down the composite speed and increase the unwinding tension. However, after a period of time, tunnel phenomenon will occur, so the flatness of the film substrate is very important.
3.Improper winding requires adjusting the winding pressure according to the structure of the #composite film. Enlarge the taper of the thick and hard film, and do not cause internal looseness and external tightness, resulting in tunnel phenomenon at the wrinkles. Before coiling, the film should be fully cooled. If the coiling is too loose, there is looseness, and there is too much air between the film layers, which does not fit properly, tunnel phenomenon can also occur.
4.The adhesive has a small molecular weight, low cohesion, and low initial adhesion, which cannot prevent the sliding of the film and cause tunnel phenomenon. Therefore, a suitable adhesive should be selected.
5.Improper amount of glue applied. If the amount of adhesive applied is insufficient or uneven, causing insufficient or uneven bonding force, resulting in tunnel conditions in local areas. If the adhesive is applied too much, the curing is slow, and sliding occurs in the adhesive layer, it can also cause tunnel phenomenon.
6.Improper adhesive ratio, poor solvent quality, and high moisture or alcohol content can cause slow curing and film slip. Therefore, it is necessary to regularly test the solvent and fully mature the composite film.
7. There are too many residual solvents in the composite film, the adhesive is not dry enough, and the bonding force is too small. If the tension is not matched properly, it is easy to cause film slippage.
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Post time: Aug-24-2023
