How much do you know about the impact of paper moisture changes on printing High Quality Paper Box?

How much do you know about the impact of paper moisture changes on printing High Quality Paper Box?

The moisture content of paper has a great influence on printing. If the moisture content is too low, the paper will become brittle, and static electricity is easily generated during printing; if the content is too high, the ink will be difficult to dry. The change of moisture also has a great influence on various properties of paper. With the change of moisture, its quantitative, tensile strength, flexibility, and folding endurance will change, and the size of the paper will also expand and contract, and even curl and warp. Edges, wrinkling, ruffles, etc.

All kinds of paper have an optimum moisture content value. Under this value, the various properties of the paper are in a good state. The optimum moisture content of cast-coated paper and cast-coated white board is 7% ± 2%. White cardboard It is 4%-7%, and the single-sided coated white cardboard is 8%±2%. This is the moisture content of the paper when it leaves the factory.

During storage in the printing plant, paper, as a hygroscopic material, can absorb moisture from the air at a rate that depends on the relative humidity and temperature of the air; it can also transfer moisture to the air and lose moisture at a rate that depends on the moisture content of the paper and ambient temperature and humidity. When the moisture absorption rate and the water loss rate are the same, the paper and the air are in equilibrium, and the water content in the paper remains unchanged. In this case, the moisture content of the paper is called equilibrium moisture.

The so-called relative humidity refers to the ratio of the moisture content in the air at a certain temperature to the moisture content that the air can hold when it is saturated at that temperature. The same paper has different moisture balance under different relative humidity.

How to balance moisture

The changes in equilibrium moisture have the following characteristics:

① Affected by the nature of the paper. Under the same relative humidity, the more hydrophilic the paper, the higher the equilibrium moisture; otherwise, the lower. The paper without excipients has a higher equilibrium moisture, while the paper with fillers, sizing and coating has a lower equilibrium moisture. For the same variety, the balance moisture of thick products is higher than that of thin products, because the ratio of base paper rich in water absorption is high in thick products.

② Affected by temperature. Under the same relative humidity, when the temperature changes by about 15°C, the maximum change in the equilibrium moisture of the paper is about 0.5%. However, overprinting during printing requires the control of the moisture content of the paper within ±0.1%, otherwise the accuracy of overprinting will be affected. Therefore, the color printing workshop should control the temperature change within ±3°C while controlling the relative humidity.

③ It is affected by the itinerary to achieve equilibrium - moisture absorption or dehumidification. Under a certain relative humidity, the moisture content of paper when it reaches equilibrium by low moisture absorption is lower than that when it reaches equilibrium by high moisture dehumidification under the same relative humidity. This is the moisture absorption hysteresis effect of paper. In order to make the moisture content of the paper consistent with the original, an "overkill" approach must be taken. That is to say, the paper that reaches equilibrium under a certain relative humidity must be placed in an environment with a lower relative humidity than the original relative humidity if it is to be dehumidified to achieve the original equilibrium moisture after absorbing moisture in a high-humidity environment, and vice versa.

In addition, the speed of moisture absorption and dehumidification is also different, and the dehumidification speed is much slower. But regardless of moisture absorption or dehumidification, the starting speed is relatively fast, and the closer it is to equilibrium, the slower it is. This process is also related to paper quality and air mobility. Under standard humidity and temperature, a single cigarette paper can reach 5.8% equilibrium moisture after 35min treatment; various printing papers need 2-4h to reach 5%-8% equilibrium moisture; packaging cardboard takes longer . The hysteresis between the relative humidity change and the paper moisture content change will bring about a hysteresis effect between the paper deformation and the relative humidity change.

④ It is affected by the orientation of the paper. The so-called directionality means that the equilibrium moisture of the paper is inconsistent in the longitudinal and transverse directions, and the transverse expansion rate is much larger than the longitudinal expansion rate. Therefore, the transverse expansion and contraction of the paper is larger than that in the longitudinal direction. Tests show that the expansion and contraction of a single fiber in the transverse direction is equivalent to about 20 times that in the longitudinal direction. However, the arrangement directions of the fibers inside the paper are various, but the number of fibers arranged in the longitudinal direction is usually more.

Therefore, the gap between the vertical and horizontal expansion ratios of paper is not as large as the vertical and horizontal expansion of a single fiber. Taking the relative humidity from 50% to 60% as an example, the ratio of vertical and horizontal expansion and contraction is about 3:7, or about 2.3 times. This ratio varies according to the degree of orientation of the fibers, and the stronger the orientation, the larger the ratio. One of the key problems to reduce the horizontal stretch ratio and avoid the occurrence of inaccurate overprinting is to make paper with scattered and irregular fiber arrangement direction.

⑤ Affected by the two-sidedness of paper. A perfectly flat sheet of paper absorbs or dehumidifies in the air when its environment changes. If it is moisture absorption, the elongation of the reverse side is greater than that of the front side, and the paper will curl to the front side; if it is dehumidified, the shortening of the reverse side is greater than that of the front side, and the paper will curl to the reverse side. No matter which direction it is curled, it always takes the longitudinal direction of the paper as the axis, which is caused by the different degrees of orientation of the front and back sides of the paper. Cardboard, in particular, shows even more in this regard.

Causes of paper deformation and its influence on printing quality

In the process of manufacturing and use, there are many factors that affect the deformation of paper. The physical and chemical properties of pulp, beating conditions, selection of chemicals and fillers, papermaking performance, paper machine equipment, and moisture gain and loss during use will all affect The size of the paper varies. Here we only discuss the dimensional changes caused by moisture gain and loss in the finished paper during transportation and printing.

Paper is mainly composed of cellulose, and cellulose is a very hydrophilic substance, which has strong water absorption and dehydration. During storage, transportation, and printing, the moisture content of the paper changes due to changes in temperature and humidity, resulting in expansion and contraction. The main reasons for the change of paper size caused by the change of paper moisture are: when the moisture of the paper changes, the single fiber that constitutes the paper can absorb moisture to expand or lose moisture to shrink, so that the shape of the entire fiber changes, which causes the size of the paper to change; there is hydrogen inside the paper. Bonding, when the paper absorbs or loses moisture, the effect of hydrogen bonds is strengthened or weakened, causing the fibers to tighten or push away from each other.

The shape change of the fiber inside the paper due to the gain and loss of moisture will eventually lead to a change in the size of the paper, which will adversely affect printing. When the water content of the paper is too large, the bonding force between the fibers, fillers and rubber materials in the paper is easily reduced, which in turn affects the surface strength of the paper, resulting in the occurrence of undesirable phenomena such as powder drop and hair loss after printing. The occurrence of powder and hair loss reduces the hardness and stiffness of the edge of the paper, and it is easy to cause unstable positioning during paper feeding, resulting in misregistration. If the water content of the paper is too low, the elasticity of the paper is poor, and the dot imprinting is easy to expand during printing, resulting in a large dot expansion rate.

Therefore, the paper should not be printed under the condition of high humidity that is not suitable for its moisture content, but should be printed under the condition of medium humidity, which is conducive to the guarantee of overprinting accuracy. In order to keep the moisture content of the paper uniform on the entire paper surface, and to adapt to the temperature and humidity of the printing workshop, at the same time, in order to reduce the sensitivity of the paper to the environmental humidity and improve the stability of the paper size, generally before printing, take certain measures.

Discuss about the inaccuracy of overprinting caused by the expansion and contraction of paper

Paper is a material containing variable plant fibrous tissue. It has the function of absorbing moisture and releasing moisture. When the moisture in the air is constantly changing, it will adjust and balance with the air humidity. When the air humidity is higher than the moisture content of the paper, it will absorb the moisture in the air and expand and elongate; otherwise, it will release moisture and shrink and shorten to achieve a relative balance with the environmental humidity.

Lithographic offset printing is based on the principle that oil and water are immiscible with each other. Therefore, the paper will generally be elongated after printing, but it is rare to shrink and shorten. In order to eliminate such failures, the relevant printing processes are discussed:

1. When printing, try to use horizontal silk tufts. Paper is divided into horizontal silk tresses and longitudinal silk tresses. In the papermaking part of the papermaking process, since the pulp (plant fibers) flows vertically to the moving wire mesh, the fibers are suddenly accelerated, and most of them will be connected along the moving direction of the wire mesh, forming what we call Vertical and horizontal strands. Plant fibers in paper are hollow and translucent tubular objects. We can vividly say that the length direction of the fiber itself is what we call the longitudinal strands, and the direction between the fibers and the fibers is the transverse strands.

The experimental results show that the cohesion force of the fiber itself is greater than the bonding force between fibers (mainly hydrogen bonding force). After the paper is damp, the expansion and contraction in the direction of the longitudinal thread is smaller than that in the direction of the transverse thread, and the two are roughly 1:2. Therefore, the product with more accurate overprinting must consider the deformation relationship of the longitudinal and transverse silk strands of its paper. Therefore, when printing, the direction of the longitudinal strands of the paper should be parallel to the cylinder axis, and the strands of the same batch of printing paper should be kept in the same direction.

2. Overprint clear water (commonly known as pressurized water). In the case that the printing paper is not well mastered, in order to prevent the paper from being stretched during printing, the method of overprinting the clean water can be used to make the paper adapt to the printing requirements, and then start printing the color. have a certain effect.

3. Control the moisture of the printing layout. The moisture of the printing layout is the key factor to control the expansion and contraction of the paper. The moisture of the layout is transferred to the paper through the blanket. The greater the moisture of the layout, the greater the expansion and contraction. Therefore, the moisture control of the layout should be as small as possible, especially when printing 80 grams of double-adhesive paper.

4. Use a plate drawing machine to elongate the plate. Generally, the single-color machine is in the third and fourth-color printing process, and the two-color machine is in the second printing process. The image on the paper is stretched and enlarged, resulting in inaccurate overprinting. This is because the PS plate has good ductility and can be stretched by a plate drawing machine, so that the printing plate image and the image on the paper are evenly elongated and the overprinting is accurate.

At present, most small and medium-sized enterprises in China generally do not have air-conditioning equipment. Most of the methods are to sprinkle some water around the machine to increase the air humidity. For products with low requirements for non-dot printing, you can use the nozzle to spray water on the edge of the paper and then print on the machine; when the air humidity in the workshop is high, it is best to cover the cut paper with a plastic cover to prevent it from becoming longer due to moisture. ; If it is a secondary printing, you can use the method of borrowing the line to make up for the shrinkage of the image.

Generally speaking, there are many processing methods to solve the inaccurate overprint caused by paper expansion and contraction. As long as the discussion and management of the printing process are strengthened, it can generally be solved.

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