Countermeasures to prevent paper curling during printing

Paper "curls" refers to the wrinkles that occur in the paper perpendicular to the direction of the paper. Looking in the direction of the paper, both sides of the paper or finished product are curled upward. Since the running speed of modern packaging machinery is getting faster and faster, the requirements for products are bound to be higher and higher. Once the printed products are curled, it will bring some trouble to the packaging line operation. Therefore, whether it is a cigarette factory or a brewery, the flatness requirements for printed products are very strict.

There are two main reasons for analyzing paper curling. One is caused by the difference in the tension between the front and back sides of the thin paper, and the other is caused by the change in the humidity of the paper surface. Tissue paper has a certain thickness no matter how thin it is. If one side (or both sides) of the paper is damp, the tension on the front and back sides is inconsistent, and the stiffness of the printed paper fiber direction is generally better than the horizontal direction. , The paper is bound to curl. Here is an example. In the long-term shutdown state or before the daily start-up, we will find that the paper on the gravure printing machine exhibits a bow-shaped bending state, which is due to the hygroscopicity of the paper.

The printed product is flat, but after the printed product is placed in the tobacco factory packaging workshop for a period of time, the product curls. This is because the relative humidity of the cigarette factory's roll-up packaging workshop is much higher than our printing workshop. Most printed products are covered with ink and varnish on the front, so it is difficult for moisture to enter the paper from the front, so in this environment, there is only one option, that is, the back of the product absorbs moisture, causing curling. If such a problem is encountered, the cigarette factory will always complain that the cigarette wrapper is curled and the cigarette machine cannot be produced normally, which seriously affects the output. At this time, you can try to seal the packaging with plastic bags, and flatten the case back to try to flatten the case skin, but the effect is not obvious, because the most important point is the temperature and humidity conditions of the cigarette factory's roll-up workshop and the printing workshop. The temperature and humidity conditions are too different.

For the anti-curling problem, two methods are recommended. One is to add anti-curling additives to the printing inks (refer to large solid ground inks and varnishes), you can consult the ink manufacturer; the second is to print the back coating on the back of the paper. The fundamental purpose of back coating is to offset the difference in the tension between the front and back sides of the paper and prevent further moisture absorption on the back of the paper, so that the finished product becomes flat. Just to do back coating in the gravure printing machine, it is necessary to simply modify the equipment, that is, add a paper transfer roller in the printing unit, change the paper transfer path, and change the paper to back printing.

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