At present, whiteboard paper and white cardboard have been widely used in various types of packaging. The requirements of customers for product quality are getting higher and higher. In addition to the requirements for printing fine, carton molding also requires more precise and more perfect, which is undoubtedly New challenges faced by the packaging and printing industry.
Paper moisture content and balance moisture
Paper moisture content has a great influence on printing. Water is too low, paper is brittle, and static electricity is easily generated during printing. If the content is too high, the ink will be difficult to dry. Moisture changes have a great influence on various properties of paper. With the change of moisture, the quantitative, tensile strength, flexibility, and folding resistance will change, and the paper size will also expand and contract, even curling and warping. Edge, wrinkle, lotus leaf and other phenomena. All kinds of paper have an optimum moisture content. Under this value, the various properties of the paper are in good condition. The optimum moisture content of cast coated paper and cast coated white paperboard is 7%±2%. White cardboard For 4%-7%, single-coated white cardboard is 8% ± 2%. This is the moisture content of the paper when shipped.
During storage in a printing plant, paper is used as a hygroscopic material. It absorbs moisture from the air. The rate of absorption depends on the relative humidity and temperature of the air. It also transfers water to the air and loses moisture. The rate depends on the moisture content of the paper. And the environment temperature and humidity. When the moisture absorption rate and the water loss rate are the same, the paper and the air are in balance and the moisture content in the paper is constant. In this case, the moisture contained in the paper is called balanced moisture.
The relative humidity refers to the ratio of the moisture content in air at a certain temperature to the moisture content that the air can hold to saturation at that temperature. The same paper equilibrates moisture at different relative humidity.
In addition, the balance of moisture changes also has the following characteristics:
1 affected by the nature of the paper. At the same relative humidity, the more hydrophilic the paper, the higher the equilibrium moisture; otherwise, the lower. Papers without excipients have a higher moisture balance, and fillers, sizing, coated paper, and low moisture balance. The same species and its thick products have higher moisture balance than thin products because of the high ratio of raw paper rich in water absorbency in thick products.
2 affected by temperature. At the same relative humidity, the temperature change is about 15°C, and the equilibrium moisture change of the paper is about 0.5% at maximum. However, the overprint of the paper on the moisture content of the paper needs to be controlled within ±0.1%. Otherwise, the accuracy of overprinting will be affected. Therefore, the color printing shop must control the relative humidity at the same time, and control the temperature change to ±3°C.
3 is reached by the balance of the trip - hygroscopic or dehumidification effect. Under certain relative humidity, the moisture content of the paper when it is in equilibrium with low moisture absorption is lower than the moisture content when it is dehydrated with high moisture in the same relative humidity. This is the hysteresis hysteresis effect of the paper. To make the water content of the paper consistent with the original, it is necessary to adopt the "overkill" method. That is, paper that reaches equilibrium at a certain relative humidity must be placed in an environment where the relative humidity is lower than the original relative humidity if it is to be dehumidified after moisture absorption in a high-humidity environment, and vice versa. In addition, the speed of moisture absorption and desorption is different, and the speed of moisture removal is much slower. However, regardless of moisture absorption and desorption, the starting speed is relatively fast, and the closer it is to equilibrium, the slower it is. This process is also related to paper and air mobility. In the standard wet, temperature, the single sheet of cigarette paper is treated for 35 minutes, and it can reach 5.8% balanced moisture; all kinds of printing paper need 2-4h to reach 5%-8% of balanced moisture; the time for packaging cardboard is longer. . This hysteresis between the change in relative humidity and the change in the moisture content of the paper causes a hysteresis between the deformation of the paper and the change in relative humidity.
4 Affected by the directionality of the paper. The so-called directionality means that the balance of paper moisture is inconsistent in the longitudinal and transverse directions, and the transverse stretch rate is far greater than the longitudinal stretch rate. Therefore, the transverse stretch deformation of the paper is larger than that of the longitudinal direction. Tests have shown that the stretch of a single fiber in the transverse direction corresponds to about 20 times the longitudinal dimension. However, the orientation of the fibers inside the paper is very diverse. It is only that the number of fibers arranged in the vertical direction is usually more.
Therefore, the gap in the vertical and horizontal telescopic ratios of the paper is not as great as that of the single fiber. Taking the relative humidity from 50% to 60% as an example, the vertical and horizontal telescopic ratio is about 3:7, ie about 2.3 times. This ratio differs depending on the degree of orientation of the fibers, and the stronger the directionality, the greater the ratio. The papermaking of paper with scattered and disorderly arrangement of fibers is one of the key issues for reducing the horizontal stretching rate and avoiding misregistration.
5 Affected by both sides of the paper. A very flat paper, when its environment changes, the paper will absorb or dehumidify in the air. If it is hygroscopic, the opposite side of the stretch is larger than the front, the paper will curl to the front; if it is dehumidified, the reduction of the opposite side is greater than the front, the paper will curl to the opposite side. Regardless of the direction of curling, it is always oriented along the longitudinal direction of the paper. This is caused by the fact that the positive and negative sides of the paper are oriented differently. In particular, cardboard shows even more performance in this area.
Effect of moisture change on printing quality of board paper
The printability is reflected in the moisture content of the paper and is generally required to be consistent with the relative humidity of the production environment. Paperboards of 200g/m2 or more are multi-layer composite papermaking in the papermaking process. According to the production process requirements, many manufacturers cut and seal plastic packaging after the paper is rewinded from the paper machine. Accurate delivery at any time. Due to the multi-layer compounding process requirements, the water content of the board is generally about 5% higher than the water content of the plain paper. Although the high water content of the paper is advantageous for printing, it will bring a lot of trouble to the printing production if it is not well preserved and used. If the package is damaged, the local humidity of the paper in the package changes, making the paper wavy, tight, and appearing high on one side and low on one side; the paper feeding is difficult during printing, the ink and ink balance is difficult to grasp, and changes in humidity can cause the paper to expand and contract. Multi-color overprinting is not allowed, such as the quality of defects in the back corners; in particular, there are still die-cutting processes in the next process. Paper retracting can also cause irregularities in die making and inaccurate die-cutting dimensions. Therefore, in the case of packaging box paper of 200g/m2 or more, the following two points should be made in the specific production practice: