Paper is the most commonly used material for packaging and printing, and its physical properties will have a direct or indirect impact on printing quality. Correctly understand and master the nature of paper, according to the characteristics of the product, the rational use of paper will play a positive role in improving the printing quality of the product.
1. Quantitative
It refers to the weight per unit area of ​​paper, expressed in g / ?, that is, the weight of 1 square meter of paper. The weight of the paper determines the physical properties of the paper, such as tensile strength, tear strength, tightness, stiffness and thickness. This is also the main reason why high-speed printing presses are difficult to print on paper with a basis weight of 35g /? In order to better reduce consumption, improve product quality and equipment printing efficiency.
2. thickness
It is the thickness of the paper. The unit of measurement is usually expressed in μm or mm. Thickness has a close relationship with basis weight and tightness. Generally speaking, if the thickness of the paper is large, the basis weight is correspondingly high. Although some papers are thin, their basis weight is equal to or higher than that of thicker printing technology. This shows that the degree of tightness of the fiber structure of the paper determines the basis weight and thickness of the paper equipment supplies. From the perspective of printing and packaging quality, the uniformity of paper thickness is a very important printing technology. Otherwise, it will affect the supply of materials such as automatic paper feed, printing pressure and ink quality. If the book is printed on paper of different thicknesses, it will cause a significant difference in the thickness of the finished book.
3. Tightness
It refers to the weight of paper per cubic centimeter, expressed in g / C ?. The tightness of the paper is calculated from the basis weight and thickness according to the following formula: D = G / d × 1000, where: G represents the basis weight of the paper; d is the thickness of the paper. Tightness is a measure of the degree of density of the paper structure. If the tightness is too large, the paper is brittle, the opacity and ink absorption will be significantly reduced, the blot is not easy to dry, and it is prone to sticking to the bottom. Therefore, when printing on high-tightness paper, attention should be paid to reasonably control and control the amount of ink applied, and select the ink with corresponding drying property.
4. hardness
It is the performance of the paper to resist the depression of another object, and it is also the rough performance of the paper fiber structure. The paper hardness is low, and a clear imprint can be obtained. The letterpress printing process is generally more suitable for printing with paper with low hardness, so that the printing ink has good color quality and the printing plate has a high printing durability.
5. Smoothness
It refers to the degree of unevenness on the surface of the paper, the unit is expressed in seconds and can be measured. The detection principle is: the time it takes for a certain volume of air to pass through the gap between the glass plate surface and the sample surface under a certain vacuum and pressure. When the paper surface is smoother, the air passing speed is slower, otherwise, the air passing speed is faster. Printing requires that the paper has a moderate smoothness, and the smoothness is high, and the small dots will be faithfully reproduced, but the full version of the print should pay attention to prevent the back side from being dirty. If the smoothness of the paper is low, the required printing pressure is large, and the ink consumption is also large.
6. Dustiness
It refers to the impurity spots on the paper, and there is a clear difference between the color and the paper color. The degree of dust is an index to measure the impurities on the paper surface, which is expressed by the number of dust areas within a certain range per square meter of paper surface. The paper has high dustiness, poor reproduction of the ink color and dots of the printed matter, and the dirty spots often affect the appearance of the product.
7. Sizing
Generally, writing paper, coated paper and packaging paper form a protective layer with water resistance by sizing. What is the degree of sizing? Commonly used duck pen dipped in specially-made standard ink in a few seconds, draw a line on the paper, to see its maximum width of non-diffusion and impermeability, the unit is mm. The paper has a high degree of sizing, the brightness of the ink layer of the printed product is also high, and the ink consumption is small.
8. Ink absorption
It is the ability of paper to absorb ink. Paper with good smoothness and sizing degree has weak ink absorption, and the ink layer dries slowly, and it is easy to stick to dirty prints. On the contrary, the ink absorption is strong, and the printed matter is easy to dry.
9. Aspect
It refers to the direction in which the fiber tissue of the paper is arranged. In the papermaking process, the fiber runs along the paper machine in the longitudinal direction of the paper. It can be identified from the sharp angles presented by the net marks. The one perpendicular to the longitudinal direction is the transverse direction, the longitudinal paper grain printing deformation value is small, the amount of expansion and contraction during the transverse paper grain printing process is large, and the tensile strength and tearing degree are poor.
10. Expansion rate
It refers to the dimensional variation that occurs after the paper has absorbed moisture or lost water. The softer the fiber structure of the paper and the lower the tightness, the higher the expansion rate of the paper; otherwise, the lower the expansion rate of the printing tool. In addition, the paper with good smoothness and sizing degree has a small stretch rate. Such as double-sided coated paper, glass card and A and other offset paper and other equipment supplies.
11. Air permeability
In general, the thinner the paper and the lower the tightness, the greater the permeability. The unit of air permeability is ml / min (ml / min) or s / 100ml (second / 100 ml), which refers to the amount of air that passes through the paper in 1 minute or the time required to pass 100ml of air. The paper with large air permeability is easy to appear in the printing process.
12. BaiDu
It refers to the brightness of the paper. If all the light reflected from the paper is visible to the naked eye, it will be white. To measure the whiteness of paper, the whiteness of magnesium oxide is usually set as 100% as a standard. When the paper sample is irradiated with blue light, the whiteness with a small reflectance is poor. Whiteness can also be measured with a photoelectric whiteness meter instrument. The unit of whiteness is 11% equipment consumables. Paper with high whiteness has a dark printing ink color and is prone to see-through.
13. Front and back
During papermaking, the pulp is attached to the steel mesh and shaped by filtration and dehydration. In this way, if one side of the net has fine fibers and fillers lost with water, leaving a net mark, the paper surface is thicker. The other side is more dense without a net. Smooth, so that the paper has a difference between the front and back sides. Although the production is dried and calendered, there are still differences between the front and back sides. The gloss of the front and back of the paper is different, which directly affects the absorption performance of the ink and the printing quality of the product. If the relief printing process uses thicker paper on the reverse side, the plate wear will be significantly increased. The printing pressure on the front of the paper is light, and the ink consumption is also low. In summary, to correctly understand and identify the main physical properties of paper and its impact on printing, according to the nature and characteristics of paper, take corresponding measures from operation and process technology to avoid blindness in production in order to effectively Reduce or avoid the chance of printing failures, thereby ensuring production efficiency and product printing quality.
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