Influence of Moisture Change of Cardboard in Gift Box Printing

Influence of Moisture Change of Cardboard in Gift Box Printing

At present, white board paper and white cardboard have been widely used in all kinds of packaging, and customers have higher and higher requirements for product quality. New challenges facing the packaging and printing industry.

Moisture content and equilibrium moisture of paper

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, it will be difficult for the ink 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 percent ± 2 percent . White cardboard It is 4 percent -7 percent , and the single-sided coated white cardboard is 8 percent ±2 percent . 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.

In addition, 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 degree , the maximum change in the equilibrium moisture of the paper is about 0.5 percent . However, overprinting during printing requires the control of the moisture content of the paper within ±0.1 percent , otherwise the accuracy of overprinting will be affected. Therefore, the color printing workshop should control the temperature change within ±3 degree 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, it is necessary to take an "overkill" approach. 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 percent equilibrium moisture after 35min treatment; various printing papers need 2-4h to reach 5 percent -8 percent 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 difference between the vertical and horizontal stretch ratios of the paper is not as large as the vertical and horizontal stretch of a single fiber. Taking the relative humidity from 50 percent to 60 percent 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.

The influence of moisture change of cardboard on printing quality

The printability is reflected in the moisture content of the paper, and it is generally required to be as consistent as possible with the relative humidity of the production environment. All kinds of paperboards above 200g/m2 are multi-layer composite papermaking in the papermaking process, and according to their production process requirements, many manufacturers cut, seal, package and pack the paper in time after the paper is rewound from the paper machine. Ships accurately at any time. Due to the multi-layer composite process requirements, the moisture content of cardboard is generally about 5 percent higher than that of ordinary paper. Although the high water content of paper is beneficial for printing, if it is not stored and used properly, it will bring a lot of trouble to printing production. If the package is damaged, the local humidity of the paper in the package changes, causing the paper to be wavy, with tight edges, and one side is high and one side is low; it is difficult to feed paper during printing, and it is difficult to control the balance of ink and water. Inaccurate multi-color overprinting, and other quality drawbacks such as back angle fluttering; especially for products with die-cutting technology in the next process, paper expansion and contraction will also cause problems such as irregular mold making and inaccurate die-cutting dimensions.

Therefore, for the color box wrapping paper above 200g/m2, the following two points should be done in specific production practice:

1. Keep and store paper scientifically and reasonably. In view of the uniqueness of cardboard, in order to prevent the cardboard from dehumidifying or absorbing moisture and causing changes in paper suitability, during paper transportation and storage, great attention should be paid to the integrity of product packaging to prevent packaging damage due to improper transportation and operation. In storage, attention should be paid to both ventilation and light and wind. The temperature should be controlled at 15-25 degree and the relative humidity should be 40 percent -65 percent . The stacking of paper should be classified according to the paper production date, product category, and production address, and the paper should be supplied in sequence according to the production date, so as to provide a relatively stable paper humidity suitability for the next process as much as possible.

2. Effectively control the humidity change of paper. If it is very necessary to keep it properly and arrange the use reasonably before cutting, how to take measures to prevent the change of paper size caused by changes in temperature and humidity after cutting? First of all, the production command must be scientific and reasonable, and paper strives to achieve With printing and cutting, it is best to prepare paper one hour before printing. Large quantities cannot be unpacked and cut all at once. They should be cut in batches according to the printing speed and progress, and the cut paper should be able to Cover with a plastic cover with better sealing performance to minimize the time of direct contact with the air, especially when multi-color products are printed on a single or two-color machine, this method is even more important. Not only white paper but also each color should be treated in the same way after printing, in order to prevent the multi-color overprinting phenomenon caused by the expansion and contraction of the paper caused by the humidity change, and ensure the quality of the printed products.

Influence of moisture change of cardboard after calendering on die-cutting accuracy

In order to increase the beauty of the product and increase the surface finish of the color box, 70 percent -80 percent of the carton packaging needs to be polished and calendered. Glazing means that the printed matter is evenly coated with a layer of varnish on the surface of the printed matter through the varnishing oil roller, and then the solvent is volatilized to form a bright layer by heating and drying. Calendering is the reprocessing on the basis of glazing of the printed matter. The high temperature of 100-130 degree and the pressure of about 138kg/cm2 make the bright layer formed on the surface of the printed matter more compact and brighter.

Due to this special process, the water content of the paper itself is seriously dehumidified in a short period of time, and the paper will inevitably shrink. The shrinkage amount is related to the structure of the paper and the size of the initial humidity. Although its shrinkage has nothing to do with printing overprint, it directly affects the precision of die-cutting. For example, the input size of a product manuscript is X, and the width is Y. If you print 5 pieces horizontally and 3 pieces vertically on a pair of K paper, you will get a product with a pattern size of 3Y5X after printing. After calendering After the paper pattern size shrinks, shrinking △X in the horizontal direction and △Y in the vertical direction, the die-cutting and shrinking products with the same size as the original will definitely have inaccurate die-cutting accuracy, and even embossing, Bleeding, the phenomenon that the pattern is not centered on the left and right. The customer is dissatisfied, and the product will be partially or completely scrapped, causing undue losses to the enterprise.

So how to avoid or reduce such errors in specific production? A relatively simple way is to make molds (die-cutting boards) according to different papers and different shrinkage amounts after calendering, but this often seriously delays the production cycle, and die-cutting The molded size will be smaller than the original size. To this end, we should gradually explore the humidity characteristics of various papers commonly used in this unit and the horizontal and vertical shrinkage after calendering, and add this shrinkage to the input size of the manuscript during printing to make the final shrinkage. 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.

Influence of moisture change of cardboard after glazing and calendering on product folding endurance

Folding resistance refers to the crease properties of the paper when the paper is folded without cracks on the surface of the folded belt, peeling of the fine seam coating, etc. It is usually used to indicate the toughness and surface strength of cardboard and cardboard under folding conditions. . From the paper itself, the folding endurance depends on the length, strength, flexibility and bonding force of fibers. A large part of my country's papermaking raw materials are non-wood raw materials, among which reed, wheat straw, straw, bagasse and other grass raw materials are used more. The paper produced from raw materials such as hemp, hemp, and wood has good toughness, while most grass raw materials have poor toughness after paper formation due to small fiber length and high content of hybrid cells.

The paper we use now. Paper mills generally combine grass fibers with wood fibers to effectively improve the toughness of paper. However, due to the different proportions of wood pulp, the properties of fillers and production processes, the toughness of different papers is quite different, and calendering is a high-temperature and high-pressure dehumidification process for paper. 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 makes the carton produce fluff, peeling, cracks, etc. during the process of die cutting, gluing, and folding. Disadvantages such as broken and rotten, and even cause the whole batch of products to be scrapped. Therefore, in the specific production arrangements and when customers choose paper, special attention should be paid to the performance of the paper and the impact of calendering on the folding endurance of the paper.

In general, 250-380g white-backed whiteboard and white cardboard have good folding resistance, and calendering process can be considered; while gray-backed whiteboard paper is not suitable for calendering. Moreover, due to the inherent characteristics of paper fibers, the folding endurance in the longitudinal direction is obviously better than that in the transverse direction. This factor should be fully considered in the horizontal and vertical arrangement of the carton and the calculation of the format size.

For all calendered products, before die-cutting, the calendered paper should be tested for folding resistance in the horizontal and vertical directions. Water, if the cycle is too late, it can also be placed in a place with high humidity to naturally adjust the humidity, so that after the paper is moderately hygroscopic, the fiber strength and flexibility are enhanced, and the folding resistance is improved, which can reduce waste and loss.

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