At present, whiteboard paper and white cardboard have been widely used in various types of packaging. Customers have increasingly higher requirements for product quality. In addition to exquisite printing, carton molding also requires more precise and perfect. This is undoubtedly New challenges facing the packaging and printing industry.
Paper moisture content and equilibrium moisture
The moisture content of the paper has a great influence on the printing. The moisture is too low, the 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 also have a great influence on the various characteristics of paper. With the change of moisture, its basis weight, tensile strength, flexibility, folding resistance, etc. will change, and the paper size will also expand and contract, even curling, warping Edge, wrinkle, lotus leaf and other phenomena. All kinds of paper have an optimal moisture content value. At this value, the various properties of the paper are in a good state. The optimal moisture content of cast coated paper and cast coated white cardboard is 7% ± 2%. White cardboard 4% -7%, single-sided coated white paperboard is 8% ± 2%. This is the moisture content of the paper when it leaves the factory.
During storage in the printing house, paper acts as a hygroscopic material, it can absorb moisture from the air, the rate of absorption depends on the relative humidity and temperature of the air; it can also pass moisture into the air and lose moisture, the rate depends on the moisture content of the paper And ambient temperature and humidity. When the rate of moisture absorption and the rate of water loss are the same, the paper and the air are in equilibrium, and the moisture content in the paper is unchanged. In this case, the moisture content of the paper is called equilibrium moisture.
The 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 saturate at that temperature. The same kind of paper has different equilibrium moisture under different relative humidity.
In addition, the changes in equilibrium moisture have the following characteristics:
â‘ Affected by the nature of the paper. At the same relative humidity, the more hydrophilic the paper, the higher its equilibrium moisture; otherwise, the lower it is. The paper with no auxiliary materials applied has a higher equilibrium moisture, while the paper with filler, sizing and coating has a lower equilibrium moisture. Thick products of the same variety have a higher equilibrium moisture than thin products because of the high ratio of water-absorbent base paper in thick products.
② Influenced by temperature. At the same relative humidity, when the temperature changes by about 15 ° C, the maximum change in paper equilibrium moisture is about 0.5%. However, the change of paper moisture content in overprinting during printing needs to be controlled within ± 0.1%, otherwise it will affect the accuracy of overprinting. Therefore, while controlling the relative humidity, the color printing workshop should control the temperature change to ± 3 ℃.
â‘¢ It is affected by the journey to equilibrium-moisture absorption or dehumidification. The moisture content of the paper at a certain relative humidity when absorbing moisture from low moisture to reach equilibrium is lower than that at the same relative humidity when it is dehumidified by high moisture to achieve equilibrium. This is the hysteresis effect of paper moisture absorption. In order to make the paper moisture content consistent with the original, we must adopt the "overcorrection" method. That is, a paper that has reached equilibrium at a certain relative humidity. If it needs to be dehumidified to reach the original equilibrium moisture after absorbing moisture in a high humidity environment, it must be placed in an environment with a lower relative humidity, and vice versa. In addition, the speed of moisture absorption and dehumidification is also different, the dehumidification speed is much slower. But whether it is absorbing or dehumidifying, 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 sheet of cigarette paper can be treated for 35 minutes to achieve a 5.8% equilibrium moisture; various printing papers require 2-4 hours to reach a 5% -8% equilibrium moisture; packaging cardboard takes longer . This lag between the change in relative humidity and the change in paper moisture content will bring about the lag effect between the deformation of the paper and the change in relative humidity.
â‘£ Affected by the directionality of the paper. The so-called directivity refers to the paper's balanced moisture is not consistent in the longitudinal and transverse directions, the lateral expansion and contraction rate is much greater than the longitudinal expansion and contraction rate, therefore, the paper's lateral expansion and contraction deformation is greater than the longitudinal direction. Tests have shown that the expansion and contraction of a single fiber in the transverse direction is equivalent to about 20 times the longitudinal direction. However, the direction of the fibers in the paper is varied, but it is usually the number of fibers arranged in the longitudinal direction. Therefore, the gap between the stretch ratio of the paper in the vertical and horizontal directions is not as large as that of a single fiber. Taking the relative humidity from 50% to 60% as an example, the ratio of vertical and horizontal expansion is about 3: 7, which is about 2.3 times. This ratio varies according to the degree of fiber alignment. The stronger the directivity, the greater the ratio. Making paper with random and irregular fiber arrangement directions is one of the key problems to reduce the lateral expansion and contraction rate and avoid misregistration.
⑤ Influenced by both sides of paper. A very flat sheet of paper will absorb or dehumidify in the air when its environment changes. If it is hygroscopic, the elongation of the reverse side is larger than the front side, and the paper will curl to the front side; if it is dehumidification, the shortening of the reverse side is larger than the front side, the paper will curl to the reverse side. Regardless of the direction of curling, the longitudinal direction of the paper is always taken as the axis, which is caused by the different degrees of orientation of the front and back sides of the paper. Especially cardboard, the performance is even greater in this regard.
The Influence of Variation of Paperboard Moisture on Printing Quality
The printability is reflected in the moisture of the paper, and it is generally required to be as consistent as possible with the relative humidity of the production environment. In the papermaking process, various types of paperboards of 200g / m2 or more are multi-layer composite papermaking, and according to their production process requirements, many manufacturers cut, seal and package in time after the paper is rewinded from the paper machine. Ship accurately at any time. Due to the multi-layer composite process requirements, the moisture content of paperboard is generally about 5% higher than that of ordinary paper. Although the high moisture content of paper is more advantageous for printing, if it is not preserved and used badly, it will bring a lot of trouble to printing production. If the packaging is damaged, the local humidity of the paper in the package changes, making the paper wavy, tight edges, one side is high and one side is low; it is difficult to convey paper during printing, the water and ink balance is difficult to grasp, and changes in humidity will also cause the paper to expand and contract, causing Multi-color overprinting is not accurate, the back mouth flicks and other quality defects; especially the products in the next process also have die-cutting technology, and paper expansion and contraction will also cause irregular mold production and inaccurate die-cutting dimensions.
Therefore, the following two points should be achieved in the specific production practice for the color box packaging paper above 200g / m2:
1. Scientific and reasonable storage and storage of paper. In view of the uniqueness of paperboard, in order to prevent the paperboard from dehumidifying or absorbing moisture and causing changes in the suitability of the paper, during the paper transportation and storage, great attention should be paid to the integrity of the product packaging to prevent packaging damage due to improper transportation and operation. In the storage, attention should be paid to both ventilation and protection from light and wind. The temperature is controlled at 15-25 ° C and the relative humidity is 40% -65%. Paper stacking should be classified according to paper production date, product category, production address, paper use should be in accordance with the order of production date, in order to provide a more stable paper humidity suitability for the next process.
2. Effectively control the humidity change of the paper. If it is necessary to properly preserve and arrange the use before cutting, then how to take measures to prevent the paper size change caused by temperature and humidity changes after cutting? First of all, the production command should be scientific and reasonable, and the paper should strive to achieve With printing and cutting, it is best to be able to prepare the paper one hour before printing. Large quantities cannot be unpacked and cut at once. It should be cut in batches according to the printing speed and progress, and the cut paper is best. Use a plastic cover with good sealing performance to minimize the time of direct contact with air, especially when multi-color products are printed on single and double-color machines. This method is even more important. Not only the white paper but also each color should be processed in the same way to prevent the inaccuracy of multi-color overprinting caused by paper expansion and contraction caused by changes in humidity to ensure the quality of printed products.
Influence of Variation of Paperboard Moisture on the Precision of Die Cutting after Calendering
In order to increase the beauty of the product and the surface finish of the color box, 70% -80% of the carton packaging needs to be polished and calendered. Varnishing is to apply a layer of varnish uniformly on the surface of the printed matter through the varnish roller, and then heat and dry to evaporate the solvent to form a bright layer. Calendering is reprocessing on the basis of glazing of printed products. With a high temperature of 100-130 ℃ and a pressure of about 138kg / cm2, the bright layer formed on the surface of the printed product is more dense and the brightness is higher.
Due to this special process, the moisture content of the paper itself is severely dehumidified in a short time, and the paper will inevitably shrink. The amount of shrinkage is related to the paper structure and the initial humidity. Although its shrinkage has nothing to do with printing overprinting, it directly affects the accuracy of die cutting. For example, the input size of a product original is X and width Y. If you print 5 horizontally and 3 vertically on a pair of K paper, you will get a product with a pattern size of 3Y × 5X after printing. After the size of the paper pattern shrinks, it shrinks △ X in the horizontal direction and △ Y in the longitudinal direction. The die-cutting products with the same size as the original will definitely show inaccurate die-cutting accuracy, and even indentation, Bleeding and the phenomenon that the pattern is not centered. If the customer is dissatisfied, the product will be scrapped partially or completely, causing undue losses to the enterprise.
So how to avoid or reduce such errors in specific production? The simpler method is to make molds (die cutting boards) according to different papers and different shrinkages after calendering, but this often results in serious delays in the production cycle and die cutting The size of the molding will be smaller than the original size. To this end, we should gradually explore the humidity characteristics of the various papers commonly used in this unit and the amount of horizontal and vertical shrinkage after calendering, and add this amount of shrinkage to the input size of the original manuscript during printing, so that the final shrinkage of the image distance, The entire graphic size meets the standard requirements, so that the die-cutting plate can be made in advance to ensure the quality and cycle of the product.
Effect of Variation of Board Paper Moisture on Product Folding Resistance after Glazing
Folding resistance refers to the nature of paper creases that do not cause cracks and peeling of the coating on the surface of the folding belt when the paper is folded. It is usually used to indicate the toughness and surface strength of cardboard and cardboard when folded. . From the paper itself, the folding resistance depends on the fiber length, strength, flexibility and the bonding force between the fibers. A large part of China's papermaking raw materials are non-wood raw materials. Among them, reeds, wheat straw, straw, bagasse and other grass materials are mostly used. Due to the performance of these raw materials, different toughness and brittleness will appear after paper formation. Paper made from hemp, hemp, wood and other raw materials has good toughness, but most grass raw materials have poor toughness after paper formation due to the small fiber length and high content of miscellaneous cells.
The paper we use now. Paper mills generally mix grass fibers with wood fibers to effectively improve the toughness of paper. However, due to different preparation ratios of wood pulp, different filler properties and different production processes, there is a big difference in the toughness of different papers, and calendering is a high temperature and high pressure dehumidification process for paper. Through calendering, the paper itself is single The root fiber becomes brittle due to dehumidification, the ability of the fiber to slip and disperse stress is greatly reduced, and the folding resistance is weakened, which causes the carton to produce fluff, peeling, cracks, etc. during the die cutting, gluing and folding process. Broken and other ills, even causing the entire batch of products to be scrapped. Therefore, in the specific production arrangements and when customers choose paper, they must pay special attention to the performance of the paper and the impact of calendering on the folding resistance of the paper.
In general, 250-380g whiteboard and white cardboard with good folding resistance can be considered calendering process; grayboard whiteboard paper is not suitable for calendering treatment. And because of the inherent characteristics of paper fibers, the folding resistance in the longitudinal direction is significantly better than the folding resistance in the horizontal direction. This factor should be fully considered in the calculation of the horizontal and vertical arrangement of the carton and the size of the format.
Before the die-cut products are die-cut, the fold resistance test of the calendered paper is required in the horizontal and vertical directions. If the fold resistance is found to be poor, the calendered product can be passed from the reverse side of the paper on the machine. Water can also be placed in a place with high humidity to naturally adjust the humidity if the cycle is too late. In this way, after the paper is properly absorbed, the fiber strength and flexibility are enhanced, and the folding endurance is improved, which can reduce waste and loss.
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