Causes and Control Measures of Coated Paper Warping Problems

1. Reasons for paper warping:

Paper warping is a phenomenon in which the paper surface is uneven due to moisture absorption and dispersion in the air. The main reason is that during the production process of the paper, due to the imbalance of moisture on the front and back of the paper and the shrinkage of the front and back of the paper, Caused by the difference, the side with less moisture in the paper will absorb moisture faster or emit moisture slower than the side with more moisture. This causes the paper to warp in the opposite direction. In layman's terms, the paper will warp in which direction where is more moisture. It is mainly caused by factors such as unreasonable paper machine configuration or unreasonable production technology. Paper warping is very easy to occur, especially during low-quantity coating processes, whether it is external coating or in-machine coating. After the base paper is sized, coated and dried, every time the paper goes through a wet and dry cycle, problems such as paper deformation and warping will occur due to the different moisture content on the front and back of the paper. 

2. Measures to control paper warping:

Methods to control paper warpage during the production process. Generally, the moisture difference between the two sides of a single-sided coated paper web after drying is about 2% to 3%. Therefore, during the production process, it is necessary to increase the moisture on the back side to balance the moisture difference between the two sides. Control the occurrence of paper warping. Common methods to control warpage during production include the following:

2.1 The warpage problem can be adjusted from the aspect of glue application. The warpage problem can be controlled by using metering rods of different sizes to adjust the amount of glue applied on the front and back sides. During the production process, in order to increase the moisture content of the back surface by adjusting the amount of glue applied on the back surface, the amount of glue applied on the back surface is generally slightly higher than the amount of glue applied on the front surface by about 0.3 g/m2. In terms of solid content, in order to facilitate quick adjustment during the production process, the prepared rubber can be diluted with online dilution water and then fed into the working tank of the sizing machine for use. Generally, it is recommended to set the solid content to be controlled at 9%~ Between 13% and 13%, if the solid content is too high, it is very easy for the rubber paste to appear on the scraper rod, which will affect the service life of the scraper rod. If the solid content is too low, it will lead to poor coating of the scraper, uneven sizing, and in severe cases, the scraper will return material, resulting in sizing streaks.

2.2 Control the moisture difference between the front and back sides of the paper web by adjusting the drying intensity of the front and back sides. The specific operation is achieved by adjusting the steam inlet pressure ratio of the upper and lower row drying cylinders in the drying section, increasing the temperature of the lower row of drying cylinders, and reducing the temperature of the upper row of drying cylinders. According to actual production proof, the last group of upper and lower row drying cylinders in the high-temperature section of the drying section of the paper machine The cylinder steam supply method and the post-drying upper and lower row dryer steam supply method adopt the steam injection heat pump system steam supply structure. The dryer tail steam is reused after being pressurized by the heat pump. Compared with the ordinary three-stage steam supply structure drying cylinder The exhaust steam goes to the previous group of drying cylinders and is reused after supplementing with fresh steam, which significantly improves the drainage effect of the drying cylinders.

2.3 Use back coating adjustment. Use the paper back-coated device to increase the moisture on the back by coating or gluing it on the back, so that the moisture on both sides of the paper is close to or the same, thereby achieving the purpose of controlling warpage. Commonly used methods in production include using paint with a solid content of 30%-35% for light coating on the back. Or use starch glue with a solid content of 7%-9% for back sizing. There are two coating modes for back coating: scraper coating and scraper coating. Personally, it is recommended to use scraper coating. Most of the coating scrapers currently used are alloy materials, so the wear resistance of the scraper is better than that of the scraper rod. A lot, and the scraper has good flexibility. If the paper breaks and wraps around the back roller, it is not easy to damage the back roller when using a scraper coating.

2.4 The steam humidification device humidifies the paper web by spraying steam onto the surface of the paper web. Through the steam humidification effect, the moisture on the surface of the paper web is increased and the moisture deviation between the front and back is reduced to prevent the occurrence of warpage. . The steam humidification device can also adjust the moisture deviation of the banner by using a proportional valve to control the steam flow of the banner in sections, so that the moisture content of the entire paper web can be as uniform as possible. In addition, because the steam re-humidification device has limited ability to control warpage, if there is no back coating device in production, it is generally necessary to add 2-4 humidification devices to achieve the effect of controlling warpage. 

2.5 Conditioning of the cold cylinder. After the normal paper web comes out of the dryer section, the surface temperature of the paper is generally around 60-70°C. Because the temperature is too high, if it directly enters the paper reel, the temperature of the paper profile cannot be uniform after being rolled. The ground is reduced. Affected by the ambient temperature and humidity, after the paper is rewinded and slit, due to the large difference between the paper temperature and the ambient temperature, it is easy to deform due to the expansion and contraction of the paper during use. In the current production process, the commonly used paper cooling The only equipment is the cooling cylinder. Its appearance is the same as that of the dryer. The main difference is that the inside of the cold cylinder is filled with constant-temperature water. After the paper web is cooled by the cooling cylinder, the surface temperature of the paper web will drop from 60~70 ℃ to 40~50 ℃. In addition, after the paper web passes through the cold cylinder, the water vapor in the surrounding air will condense on the surface of the cold cylinder, which can relatively increase the humidity of the paper web surface by 1% to 2%, so that the paper can obtain better softness and elasticity. , at this time, the paper enters the paper winding machine, and after rewinding and slitting, the temperature and humidity of the paper reaches a relative balance with the temperature and humidity of the surrounding environment, which plays a role in reducing the deformation and warping of the paper during use.

3. Summary

The above is in terms of preventing paper warping. Our company, Qingdao JIERUIXIN Machinery & Technology Co., Ltd., has many years of practical production experience. In order to ensure the long-term stability of paper quality, it needs to be controlled and adjusted through all aspects of the production process. It not only requires equipment and technology In combination, it is also necessary to adopt different control strategies according to the ration of different paper types to effectively control the occurrence of paper deformation and warping problems.