Discussion on curling of single-sided coated paper

Due to the difference in the production process of single-sided coated paper, there is a serious difference in the properties of the two sides of the paper, which makes it very sensitive to the relative humidity of the outside world and is prone to curl problems, which has a great impact on printing and subsequent use. This article aims at this issue, through a technical communication with a number of printing factory technicians in the country, made a more detailed summary.

1. The reason for the single copper curl problem:

Due to the special requirements of single copper paper products, the coating amount on both sides is different: the front side is coated with 20 grams (abbreviated as the coated side), and the reverse side is coated with only 1 gram (commonly known as uncoated side). It can be said that the single copper paper On the one hand, the coating is on the other, and on the other is paper fiber. The expansion coefficients of water and fiber are different: the expansion rate of the fiber is large, and the expansion rate of the coating is small. At the same time, there is also a big difference in the water absorption of the two. The fiber absorbs water quickly and the coating absorbs water slowly.

When the paper is produced, because its moisture content is stable, the state of the fibers and the paint is relatively balanced, and the surface of the paper web is very smooth. When the relative humidity of the external environment changes (that is, the humidity of the environment does not match the moisture content of the paper), curling of the paper occurs: When the humidity of the environment is higher than the relative humidity of the paper (if the weather changes: it rains or snows, the humidity of the environment changes. Increased), the paper will absorb moisture and curl on the coated surface; when the environmental humidity is lower than the relative humidity of the paper, the paper loses moisture and curls toward the uncoated surface. However, if the deviation of the relative humidity of the environment from the relative humidity of the paper is between 5% and 5%, there is generally no problem with curling that is too obvious. At the same time, if the paper's moisture content changes during the printing process (such as the overdrying of paper in gravure printing and flexo printing), the balance of fibers and paint is destroyed, and the corresponding change will occur: if water is lost, the paper will lose moisture. It curls toward the uncoated surface; when it absorbs moisture, it curls toward the coating surface.
2. The main influencing factors of single copper curl in printing:

Due to the different printing methods, the influence of single copper on curling after printing is not the same. In offset printing, the main factor for curling is that the ambient temperature and humidity do not match the relative humidity of the paper. For gravure and flexo printing, in addition to the mismatch between ambient temperature and humidity and the relative humidity of the paper, excessive drying is one of the main reasons.

Offset printing is achieved through the oil-water immiscible principle. Although there is a water process in the printing process, the water absorption capacity of the coated surface is very low and basically does not affect the moisture content of the paper, so curling does not generally occur. problem. If the ambient temperature and humidity match the relative humidity of the paper, the curling problem will not be substantially caused.

For gravure and flexo printing, besides being affected by the environment's temperature and humidity, as well as offset printing, drying equipment is required to dry the ink during the printing process. While drying the ink, some moisture in the paper will be evaporated. The balance of the state of the fibers and the paint is destroyed, and it is also very likely to cause curl problems (curl to the uncoated surface).

3, how to avoid the occurrence of curl problems:

1) Strictly control the ambient temperature and humidity. Because the ambient temperature and humidity have a great influence on the curl of single copper, the printing environment must be strictly controlled and always be stable. It cannot change with changes in the outside weather. The production workshop must be equipped with equipment for adjusting temperature and humidity. The moisture of single copper produced by our company is generally controlled between 4.5% and 5.5%. The temperature and humidity of the production workshop are also strictly controlled: temperature 20±5°C, humidity 50±5%RH. Therefore, if the temperature and humidity of the printing environment are controlled according to our control standards, curling does not occur under normal conditions.

2) The printing conditions are optimized. For gravure and flexo printing, if the printed material is curled, the drying temperature of 1-2 degrees or the speed of 3 to 5 meters per minute can be reduced as long as the ink can be dried. This can basically reduce the degree of curling.

3) For gravure and flexo printing, if there is a problem with curling, it can also be improved by back-processing the paper. The specific approach is: on the back with a free color group or flip frame coated with 0.5-1 g / M2 of water, or coated with 2-3% of gum arabic 0.5-1 g / M2, this will have a good curl after printing improve. The back coating is to compensate for the loss of moisture during drying, while the back coating of gum arabic can also reduce the loss of water.

4) For the cutting workshop of the printed matter, strict temperature and humidity control must also be adopted. At the same time, the finished small-scale mark of the finished product should be sealed with a plastic film as far as possible to avoid being affected by the external temperature and humidity during transportation or storage. Curling deformation, resulting in the finished product can not be used and scrapped. Especially for wine labels, because the humidity of automatic labeling environment is relatively large, generally it can reach more than 70%, so the sealed packaging is very important, and the winery should not try to dismantle too much at one time when labeling, and the guarantee does not affect the production. Under the circumstances, try to unpack the wine label as little as possible.

4. How to determine whether the paper matches the environment or printing conditions:

Take 50 x 50cm square original paper samples, cut it in a printing environment along the diagonal line with a paper knife about 50cm long cross-shaped hole, flat in this environment for about 10 minutes, observe the intersection rolled up . If the height of the roll-up site does not exceed 1 cm, it is proved that the paper matches the ambient temperature and humidity and does not cause curling. Otherwise, the two mismatches may cause curling problems. In this case, the ambient temperature and humidity must be adjusted accordingly, or the printing speed and drying temperature must be adjusted accordingly during printing.

If the paper is tested in accordance with the above method, the paper and the environment are matched. Under normal printing conditions, a small amount of paper is printed first, a few samples are taken, and the paper is inspected according to the method of detecting the base paper. If the height of the rolled-up part does not exceed 1 cm, the printing is confirmed. The drying temperature and speed are suitable, and the paper is not excessively dried, otherwise it proves that the paper has been excessively dried, the printed product is likely to be curled, so the drying temperature, printing speed need to be adjusted, or the paper needs to be back-coated with water or gum arabic.

Using this method, you can easily determine whether the printing environment, printing conditions and paper are matched, whether curling occurs after the paper is printed, it is possible to prevent curling problems in the finished product after a large amount of printing, and adjust the printing conditions and printing process in time to avoid loss. expand.

In addition, if you want to accurately determine whether the paper is compatible with the environment, whether the printed paper product is over dried, and whether the printed product will be curled, you need a sword-type temperature and humidity meter to use it to determine the relative humidity of paper products and the environment. The temperature does an accurate test.

Source: Paper Shopping Guide