Thin paper printing warping and troubleshooting

As used herein, "warp" refers to the warping of the paper in a direction perpendicular to the paper feed direction. When looking at the direction of paper feeding, the paper or the product is curled up on both sides. As the operating speed of the printing press is getting faster and faster, the requirements for product quality are getting higher and higher. Once warpage occurs in the printed matter, it will cause certain troubles for the packaging assembly line operation. Therefore, whether it is a tobacco factory or a brewery, the leveling nature of the printed matter is very strict. The warpage problem is mainly caused by the difference in tension (stress) between the front and back of the thin paper. The humidity of the environment can easily cause warpage problems in the printing of thin paper. Warpage on paper can be analyzed in four ways:

1. Observe the situation in the open part of the paper. That is, without printing, the paper is simply pulled from the roll for comparison. Normal single copper paper warps slightly. This is because the volume of water in the base paper roll is usually only 5% left and right, 'to reach equilibrium in a workshop with a relative humidity of about 50%. If the moisture content increases to about 7%, the amount of paper applied to the front side of the paper during moisture absorption will prevent some of the moisture from entering the paper from the front side, while the back side will be exposed to more moisture, causing the paper to warp slightly. Of course, the positive coating will also have some limitations on the deformation of the paper fibers.

2. Start printing water-based inks and water-based gloss oils, but do not apply UV oil. The boot speed is appropriately higher, the printed semi-finished product is cut into single sheets, and observations are placed. If a large area is printed on the ground and the boot speed is slow, most of the warpage quality accidents occur. On the other hand, if the print area is small, the boot speed is faster and the warpage probability is relatively less. This is because the front of the paper is printed with water-based ink or water-based gloss oil, the surface of the paper is damp and deformed, and there is a brief “arching” phenomenon, contrary to the warpage. However, after drying, the moisture-saturated part of the water after the dehydration, the surface of the paper will have a "shrinkage" phenomenon, that is, paper surface tension caused by warping phenomenon.

3. A layer of UV Varnish was applied to the film to observe if the paper warped after the UV Varnish was applied. If the formula of the varnish is not suitable, it will tend to generate additional shrinkage force during the curing process, causing warpage. In this case, contact the supplier of UV varnish as soon as possible to improve the performance of the UV varnish or increase the number of anilox roller lines of the UV varnish unit.

4. The printed product is flat, but it warps after being placed in the cigarette factory or brewery packaging plant of the printed product for a period of time. The main reason is that due to the high humidity, the product storage conditions are preferably constant temperature and humidity. When the humidity exceeds 70%, quality accidents are likely to occur.

For the warpage problems occurring in the above four stages, we can only adjust the ink and varnish in the second and third stages and increase the startup speed as much as possible. How to improve on the paper and have a double-sided waterproof function? Found in practice can be done by transforming the machine: add two guide rollers above the printing of the first unit, change the paper route, and you can set the flexo printing unit Change to gravure coated unit. The anilox roller is used as a gravure cylinder for printing full-size solids, and at the same time, a seamless rubber dyeing sleeve or a full-flexible flexographic plate cylinder is found as a gravure impression cylinder. Adjust the clutch cylinder pressure and the anilox roller for deinking to ensure that the back of the paper is inked during startup. The back of the paper leaves the anilox roller at a certain distance to prevent the paper from breaking due to wetness. The liquid in the ink tray is used for back coating, and some users simply use a mixture of a diluting agent and water. The thickness of the backside coating can be as needed. Change the number of lines of the screen roller or change the ratio of the back coating liquid. The fundamental purpose of back coating is to increase the “shrinkage” stress on the back of the paper, offsetting the front-side stress due to the ink and varnish, and preventing the back side from humidifying.

Author (dongdong)