The paper or cardboard used for humidity control should be reasonably controlled in paper storage, which often affects its performance due to improper storage. Paper is a very hydrophilic material. Changes in the paper's moisture content will cause the paper to deform and become brittle, causing inaccurate overprinting of the printed product, and problems such as breakage during the forming process of the paper product. Changes in ambient temperature and humidity are the main factors affecting the moisture content of paper.
The paper not only absorbs water during contact with the paper, but also has the ability to absorb moisture from the humid air and drain it into the dry air. The paper swells after absorbing water, the linear size and area in all directions will increase, and the mechanical strength will decrease. The paper shrinks after dehydration, and the size and area of ​​straight lines in all directions will shrink, becoming stiff and brittle. Therefore, to control the paper performance changes, it is necessary to control the paper moisture content and the temperature and humidity of the paper environment.
In the entire printing production process, the paper is taken out from the warehouse, cut, processed after printing, and formed, and transferred from one workshop to another workshop. At this time, it is necessary to consider the temperature and humidity changes that may exist due to the space change, resulting in Changes in paper performance.
(1) The warehouse should pay attention to the conditions of paper storage, and control the temperature and humidity of the warehouse environment. If the paper is under high humidity, it will absorb the surrounding water and curl and deform, showing ruffles; if the paper is stored in a very dry environment, it will cause the water in the paper to be dispersed and make the paper brittle and fragile , There is a tight edge phenomenon. Therefore, the paper should be stored in a place close to the use environment.
(2) Due to the changes in the surrounding environment, there are differences in temperature and humidity in the printing workshop. The paper should be adjusted for humidity before being printed, especially single sheets. When a single sheet of paper is unpacked by hand, the moisture content cannot be the same as the temperature and humidity in the printing workshop. Humidity adjustment should be carried out to make the moisture content of the paper uniform and consistent with the temperature and humidity in the printing workshop.
There are usually three methods for adjusting the humidity of offset sheet paper:
(A) Hanging and adjusting humidity in the printing workshop or in a drying room with similar temperature and humidity to the printing workshop.
(B) Humidification treatment is carried out in a drying room that is 6% -8% higher than the relative humidity in the printing workshop.
(C) Put the paper in a humid place to humidify, and then go to the printing workshop or the drying room with the temperature and humidity similar to the printing workshop to balance the moisture, so that the moisture content of the paper is uniform.
Comparing these three methods, the latter two methods are better, but some equipment and space are needed. Due to the limitation of conditions, most of the printing plants in China currently use the first method, which is usually called the natural humidity control method. If the paper is stored in an environment without temperature and humidity control, it is necessary to send the paper to the printing workshop for humidity adjustment 24 hours before printing, so that the moisture contained in the paper is compatible with the temperature and humidity of the printing workshop.
In this way, the accuracy of overprinting can be ensured and the quality of the printing can be improved. In addition, the humidity control measures for web and sheet paper are different. The roll paper is packed in paper rolls (such as cattle paper), and its outer packaging can protect the moisture content of the inner roll paper from the external environment and humidity. Only open the outer packaging before putting it on the machine.
(3) The temperature and humidity control of the printing and processing workshop is the control of the entire process of printing and assembly. Some printing plants only pay attention to the temperature and humidity control of the workshop, and ignore the temperature and humidity control of the post-press processing workshop. Corresponding air conditioners, humidifiers and other devices are installed in the printing workshop to ensure the stability of the printing workshop environment.
The same method should be adopted in the post-press processing workshop to ensure a stable processing environment so that paper products can be processed smoothly. Otherwise, when the printed products enter the dry and cold post-press processing workshop from the temperature and humidity controlled printing workshop, the front printing is good. The paper, cardboard, and paperboard are processed into a box-type process, and the edges of the paper will be broken and the whiteness will be exposed, causing great waste in production.
Printing companies must fully consider the impact of small links on overall production and product quality when formulating a series of plans for individual business procurement, warehouse management and production environment control. They must have a holistic and comprehensive management, create a good production environment and improve production. quality.
The impact of environmental temperature and humidity on offset printing is in accordance with the requirements of standardized standards. The environment of the offset printing workshop should be constant temperature and constant humidity, which not only helps prevent paper deformation, improves the accuracy of product overprinting, but also ensures the printing suitability of the ink. But in fact, due to the different climatic conditions in different regions, companies invest in the environment of offset printing workshops differently, which makes it difficult for most manufacturers to guarantee the constant temperature and constant humidity of the offset printing workshop environment, which brings some practical difficulties and process failures to offset printing production.
Effect of temperature and humidity changes on paper The water content of paper changes with the change of environmental temperature and humidity. The paper's ability to absorb water and lose water is related to its own properties, structure and raw materials. Generally speaking, the higher the density of the paper, the longer the fiber and the higher the sizing degree, the lower the water content, otherwise the opposite. The water content of paper produced by a paper mill is usually 5% to 7%. During the transportation, storage and printing of paper, the water content will increase or decrease with different environments. This change in water content will affect offset printing. It is often harmful and useless.
Paper water absorption: Since the temperature and humidity of the paper itself is lower than the environmental temperature and humidity, in order to achieve the balance with the environment, the paper must absorb moisture from the air, resulting in reduced density around the paper, increased fiber gaps, and paper deformation showing "lotus edge" ", Warped on all sides.
The difficulty of this deformed paper for offset printing is: For the machine with a hem type front gauge, it is difficult to enter the front gauge due to the undulation of the paper gripper. In order to achieve the purpose, it is necessary to increase the pressure of the paper, and the consequences are The side gauge pulls the paper in place and the overprint is not accurate. Even the paper will go over the front gauge and enter the printing unit directly, causing the printing plate and blanket to be damaged.
As for the front swing-type front gauge, when the convex part of the paper enters the front gauge, it collides with the paper tongue of the front gauge and the pressure pad, which causes the paper to not be in place and affects the accuracy of overprinting. At the same time, the paper is slightly uneven, which also makes Feida difficult to separate the paper normally. This change of paper can be solved by the normal paper drying method for offset paper, but for coated whiteboard paper above 250g / m2, the paper drying is difficult to work.
Our solution is: try to use paper produced in the same area as the factory's climate, stack the cut paper into the offset printing workshop a few days in advance, let it have a process to adapt to the workshop environment, and strictly control the moisture of the plate during the printing process , Neatly stack semi-finished products.
Paper loss of water: Paper loss of water will cause shrinkage, forming "tight edges", that is, concave (or convex) in the middle, and four sides raised (or down). When printing on this kind of paper, one is difficult to separate, the other is When it enters the impression cylinder, the stress is unevenly released, and it is easy to wrinkle, which makes the product scrapped. In addition to the above methods, we also closed doors and windows and installed curtains to try to ensure that the temperature and humidity of the workshop are not too low. When loading paper, feed and rub the board.
Because the temperature is too low, the internal cohesion of the ink increases, the activity of the oil molecules weakens, and the dispersion force decreases, which makes the ink not easy to break the wire. During printing, the imprint gripping force is not enough to overcome the combined force of the adhesion and peeling force of the blanket to the paper. , So the paper is stuck on the blanket. Despite measures such as reducing the printing pressure, replacing the blanket, and reducing the printing speed, the failure could not be eliminated, and production had to be stopped. The next day, after the workshop temperature rose, there was no more paper sticking to the blanket, and production returned to normal.
In summary, changes in temperature and humidity have a significant impact on offset printing production. It is recommended that qualified manufacturers try to improve the current status of the offset printing workshop, and provide a relatively stable temperature and humidity environment for offset printing to ensure normal production.
The DOING Inflatable Couch uses environmentally friendly PVC as a raw material, and gas enters through pressurization. Compared with traditional furniture, the Air Couch can be placed indoors and outdoors, so it is welcomed by many young people who pursue quality of life.
â— The inflatable sofa is small after deflation, and it is very convenient to store and carry, trendy and comfortable.
â— Blow Up Couch air bag with three-dimensional liner. Based on the pressure of different parts of the body, it can actively adjust the supporting force.
â— We support customization to meet your specific needs.
Inflatable Couch,Inflatable Sofa,Blow Up Couch,Inflatable Sofa Bed,Air Couch,Inflatable Chair
Danyang Doing Articles Co.,Ltd , https://www.doingarticles.com