Causes of single-sided coated paper curling problems and avoiding measures

Due to the difference in the production process of single-sided coated paper, there is a serious difference in the performance of the two sides of the paper, which makes it very sensitive to the relative humidity of the outside world, prone to curling problems, and has a relatively large impact on printing and subsequent use. Aiming at this problem, this article made a more detailed summary through technical exchanges with technicians from many printing plants across the country.
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1. Reasons for curling problem of single copper:

Due to the special requirements of single-copper paper products, the coating amount on both sides is different: about 20 grams on the front side (coated side for short), and only 1 gram on the reverse side (commonly called uncoated side), it can be said that single copper paper One side is the coating, and the other side is the paper fiber, and the fiber and the coating have different expansion coefficients for water: 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 paint absorbs water slowly.

When the paper is produced, the surface of the paper web is very smooth because of its stable moisture content, the relatively balanced state of fibers and paint. When the relative humidity of the external environment changes (that is, the environmental humidity does not match the moisture content of the paper), the paper will curl. When the environmental humidity is higher than the relative humidity of the paper (such as weather changes: rain or snow, the environmental humidity will Increase), the paper will absorb moisture and curl to the coated surface; the ambient humidity is lower than the relative humidity of the paper, the paper will lose moisture and curl to the uncoated surface. However, if the deviation between the relative humidity of the environment and the relative humidity of the paper is within ± 5%, there is generally no obvious curling problem. At the same time, during the printing process, if the moisture content of the paper changes (such as the paper is excessively dried in gravure and flexo printing), the balance between the fiber and the coating is destroyed, and the corresponding change will occur: if the moisture is lost, the paper will Curl toward the uncoated side; absorb water and curl toward the coated side.

2. The main factors affecting the curling of single copper in printing:

Due to different printing methods, the influencing factors of single copper after printing are different. In offset printing, the main cause of 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 the ambient temperature and humidity and the relative humidity of the paper, excessive drying is also one of the main reasons.

Offset printing is based on the principle of incompatibility between oil and water. Although there is a water supply process during printing, the water absorption capacity of the coated surface is very low, which basically does not affect the moisture content of the paper, so curling generally does not occur. problem. If the temperature and humidity of the environment match the relative humidity of the paper, the curl problem will not be caused.

For gravure and flexo printing, in addition to being affected by ambient temperature and humidity like offset printing, in the printing process, drying equipment is required to dry the ink, and at the same time drying the ink, part of the water in the paper will be evaporated. The state balance between the fiber and the coating is broken, so it is also easy to cause curling problems (curling toward the uncoated surface).

3. How to avoid the curling problem:

1), strictly control the environmental temperature and humidity. Because the environmental temperature and humidity have a great influence on the single copper curl, the printing environment must be strictly controlled and always kept stable, and cannot change with the change of outside weather. The production workshop must be equipped with equipment to adjust the temperature and humidity. The moisture of the single copper produced by our company is generally controlled between 4.5-5.5%, and the environmental temperature and humidity of the production workshop are also strictly controlled: temperature 20 ± 5 ℃, humidity 50 ± 5% RH. Therefore, if the temperature and humidity of the printing environment are controlled according to our control standards, curling problems will not occur under normal circumstances.

2) The printing conditions are optimized. For gravure printing and flexo printing, if the printed matter curls, in the case that the ink can be dried, try to reduce the drying temperature by 1-2 degrees or increase the vehicle speed by 3-5 meters per minute, which can basically reduce the degree of curl.

3) For gravure and flexo printing, if curling occurs, you can also use paper back processing method to improve. The specific method is: use 0.5-1 g / M2 of clear water on the back of the color set or flip frame, or 0.5-3 g / M2 of 2-3% gum arabic, which will have a good curl after printing improve. Back coating is to compensate for the moisture lost during drying, and back coating of gum arabic can also reduce the loss of water.

4) For the printing workshop, there must be strict temperature and humidity control. At the same time, the finished small standard should be sealed with plastic film as far as possible to avoid being affected by external temperature and humidity during transportation or storage. Curled and deformed, making the finished product unusable and scrapped. Especially for wine labels, because the environmental humidity of automatic labeling is relatively large, it can generally reach more than 70%, so sealed packaging is very important, and the winery should try not to open too much at one time when labeling, to ensure that it does not affect production Under the circumstances, try to unpack the wine label as little as possible.

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

Take a 50 × 50cm square of the original paper sample, and use a paper knife to cut the cross-shaped openings about 50cm long diagonally in a printing environment, and lay it flat in the environment for about 10 minutes to observe the degree of curling of the intersection . If the height of the rolled part does not exceed 1cm, it proves that the paper matches the temperature and humidity of the environment and will not cause curling. Otherwise, the two do not match, which may cause curling problems. At this time, the temperature and humidity of the environment must be adjusted accordingly, or the printing speed and drying temperature must be adjusted during printing.

If the paper is matched with the environment through the above method, under normal printing conditions, first print a small amount of paper, take a few samples, and test according to the method of detecting the base paper. If the height of the rolled part does not exceed 1cm, it means that the printing is in progress. The drying temperature and speed are suitable. The paper is not over-dried. Otherwise, the paper has been over-dried, and the printed product is likely to curl. Therefore, the drying temperature and printing speed need to be adjusted, or the paper needs to be coated with water or gum arabic.

Using this method, you can easily determine whether the printing environment, printing conditions and paper match, and whether curling will occur after paper printing, which can prevent curling problems in the finished product after a large amount of printing. Adjust the printing conditions and printing process in time to avoid loss. expand.

In addition, if you want to accurately determine whether the paper matches the environment, whether the paper product is excessively dried during printing, and whether the printed product will curl, you need a sword temperature and humidity meter, which can be used to measure the relative humidity of the paper product and the environment. Do accurate temperature detection.

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