The water content of the paper changes with the change of the temperature and humidity of the environment. The water absorption and the water loss ability of the paper are related to its own nature, structure and raw materials. In general, the higher the density of the paper, the longer the fibers, the higher the degree of sizing, the lower the water content, otherwise the opposite. The water content of the paper produced by the paper mill is usually 5% to 7%. During the transportation, storage, and printing of the paper, the water content will increase or decrease with different environments. This change in water content will affect the offset printing. It has a certain influence. Suppliers are often implicated in this way. Due to the temperature difference, paper will be distorted during printing, and users will blame the quality of the paper sold by suppliers, leading to a series of claims. In this regard, it is necessary to understand the mechanism of deformation caused by changes in the moisture content of paper, and methods to overcome this effect.
Paper suction
Because the temperature and humidity of the paper itself is lower than the ambient temperature and humidity, the paper must absorb moisture from the air in order to achieve a balance with the environment, resulting in reduced density around the paper, increased fiber gap, paper deformation is "foil", four sides Rippling warping. The difficulty of this type of deformed paper for offset printing is that, for machines with a pendulum-type front gauge, it is difficult to enter the front gauge because the paper is wavy and undulating. The operator must intensify the pressure of the paper in order to achieve its purpose. It is not in the position that the paper is not in place and overprinting is not allowed. Even the paper will go over the front gauge and go directly to the printing unit and it will roll off the plates, blankets and other faults. For the machine with the pendulum front regulator, when the convex part of the paper enters the front gauge, it collides with the front paper gauge and the pad pressure piece, causing the paper to be out of position, which affects the overlay accuracy. At the same time, the unevenness of the paper also brings certain difficulties to the normal separation of paper.
This change in the paper can be solved by the normal paper-hanging method for offset paper, and it is difficult for the coated whiteboard paper of 250 g or more. The solution is: Use paper produced in the same climate area as much as possible, put the cut paper into the offset printing shop a few days in advance, and let it have a process that adapts to the workshop environment. During the printing process, the layout moisture is strictly controlled, and the semi-finished products are stacked neatly. .
2. Paper loss of water
Loss of water on the paper causes shrinkage, which creates a "tight edge," in which the middle is concave (or convex) and the four sides are curled up (or down). When the paper is printed, it is difficult to separate the paper. Second, when it enters the impression cylinder, the stress is not evenly distributed, and wrinkles are easily generated, causing the print to be scrapped. In addition to the measures taken to overcome the above methods, it is also possible to close doors and windows and install curtains to ensure that the temperature and humidity in the workshop are not too low. When loading paper, the paper should be kneaded and pressed.
How long does it take for printing paper and white cardboard to adapt to the room temperature? This problem is mainly determined by the weight of the paper and the original temperature. The time required to adapt to room temperature is related to the weight and thickness of each piece of paper (printed paper and white cardboard), and related to the temperature difference between each piece of paper (printed paper and white cardboard) and the printing shop. Light weight, thin thickness, can adapt to room temperature in a short time. The table below lists the reference time to adjust to room temperature before opening the package.
As mentioned above, changes in temperature and humidity have a major impact on offset printing production. It is recommended that qualified manufacturers should try their best to improve the status of the offset printing plant and provide a relatively stable temperature and humidity environment for offset printing to ensure normal production. If the offset printing shop can not guarantee constant temperature and humidity, it should be based on the form, carefully arrange the parking time, until the printing paper and white cardboard to reach room temperature before opening the moisture-proof packaging.
Source: Henan Graphic Arts