At present, white board paper and white paper board have been widely applied to various types of packaging. The customer's requirements for product quality are getting higher and higher. In addition to the requirements for beautiful printing, carton forming is also required to be more accurate and perfect. This undoubtedly gives The packaging and printing industry presents new challenges.
The paper moisture content and the equilibrium moisture content of the paper have great influence on the printing. The water content 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 have a great influence on various properties of paper. With the change of moisture, the quantitative, tensile strength, flexibility, and folding resistance will change, and the paper size will also expand and contract, even curling and warping. Edge, wrinkle, lotus leaf phenomenon. All kinds of paper have an optimum moisture content value. At this value, the various properties of the paper are in good condition. The optimum moisture content of cast coated paper and cast coated white paperboard is 7%±2%, white cardboard is 4%-7%, and single-coated white cardboard is 8%±2%. This is the moisture content of the paper when shipped.
During storage in a printing plant, paper is used as a hygroscopic material. It absorbs moisture from the air. The rate of absorption depends on the relative humidity and temperature of the air. It also transfers water to the air and loses moisture. The rate depends on the moisture content of the paper. And the environment temperature and humidity. When the moisture absorption rate and the water loss rate are the same, the paper and the air are in balance and the moisture content in the paper is constant. In this case, the moisture contained in the paper is called balanced moisture.
The relative humidity refers to the ratio of the moisture content in air at a certain temperature to the moisture content that the air can hold to saturation at that temperature. The same paper equilibrates moisture at different relative humidity.
In addition, the balance of moisture changes also has the following characteristics:
1 affected by the nature of the paper. At the same relative humidity, the more hydrophilic the paper, the higher the equilibrium moisture, and vice versa. Papers without excipients have a higher moisture balance, and fillers, sizing, coated paper, and low moisture balance. In the same variety, thick products have higher moisture balance than thin products because of the high ratio of raw paper rich in water absorbency in thick products.
2 affected by temperature. At the same relative humidity, the temperature change is about 15°C, and the equilibrium moisture change of the paper is about 0.5% at maximum. However, the overprint of the printing needs to control the moisture content of the paper at ±3°C.
3 is reached by the balance of the trip - the impact of moisture or moisture. Under certain relative humidity, the moisture content of the paper when it is in equilibrium with low moisture absorption is lower than the moisture content when it is dehydrated with high moisture in the same relative humidity. This is the hysteresis hysteresis effect of the paper. To make the paper water content consistent with the original, we must take the "overkill" approach. That is, paper that reaches equilibrium at a certain relative humidity must be placed in an environment where the relative humidity is lower than the original relative humidity if it is to be dehumidified after moisture absorption in a high-humidity environment, and vice versa. In addition, the speed of moisture absorption and desorption is different, and the speed of moisture removal is much slower. However, regardless of moisture absorption and desorption, the starting speed is relatively fast, and the closer it is to equilibrium, the slower it is. This process is also related to paper and air mobility. At standard wet and temperature conditions, a single sheet of cigarette paper is treated for 35 minutes to achieve 5.8% of equilibrium moisture; various types of printing paper require 2-4 hours to reach 5%-8% of equilibrium moisture. It takes longer for packaging cardboard. This hysteresis between the change in relative humidity and the change in the moisture content of the paper causes a hysteresis between the deformation of the paper and the change in relative humidity.
4 Affected by the directionality of the paper. The so-called directionality means that the balance of paper moisture is inconsistent in the longitudinal and transverse directions, and the transverse stretch rate is far greater than the longitudinal stretch rate. Therefore, the transverse stretch deformation of the paper is larger than that of the longitudinal direction. Tests have shown that the stretch of a single fiber in the transverse direction corresponds to about 20 times the longitudinal dimension. However, the orientation of the fibers inside the paper is very diverse. It is only that the number of fibers arranged in the vertical direction is usually more. Therefore, the gap in the vertical and horizontal telescopic ratios of the paper is not as great as that of the single fiber. Taking the relative humidity from 50% to 60% as an example, the vertical and horizontal telescopic ratio is about 3:7, ie about 2.3 times. This ratio differs depending on the degree of orientation of the fibers, and the stronger the directionality, the greater the ratio. The papermaking of paper with scattered and disorderly arrangement of fibers is one of the key issues for reducing the horizontal stretching rate and avoiding misregistration. In actual operation, it is only necessary to control the lateral direction with a large expansion ratio in an allowable range.
5 Affected by both sides of the paper. A very flat paper, when its environment changes, the paper will absorb or dehumidify in the air. If it is hygroscopic, the opposite side will have a greater elongation than the front side, and the paper will curl downwards; if it is dehumidified, the reverse side will be shorter than the front side, and the paper will curl toward the reverse side. Regardless of the direction of curling, it is always oriented along the longitudinal direction of the paper. This is caused by the fact that the positive and negative sides of the paper are oriented differently. In particular, cardboard shows even more performance in this area.
The influence of the change of moisture on the printing quality of the paperboard The printability is reflected on the water content of the paper. Generally, it is required that the relative humidity of the production environment is as consistent as possible. Paperboards of 200g/m2 or more are multi-layer composite papermaking in the papermaking process. According to the production process requirements, many manufacturers cut and seal plastic packaging after the paper is rewinded from the paper machine. Accurate delivery at any time. Due to the multi-layer compounding process requirements, the water content of the board is generally about 5% higher than the water content of the plain paper. Although the high water content of the paper is advantageous for printing, it will bring a lot of trouble to the printing production if it is not well preserved and used. If the package is damaged, the local humidity of the paper inside the package changes, causing the paper to become wavy, tightly edged, and appearing high on one side and low on one side; the paper feeding is difficult during printing, the ink and ink balance is difficult to grasp, and changes in humidity can cause the paper to expand and contract. Multi-color overprinting is not allowed, the quality of defects such as the back corner rejection; especially in the next process there are die-cutting products, paper stretching can also cause irregular mold, mold cutting size is not accurate and other issues. Therefore, in the case of packaging box paper of 200g/m2 or more, the following two points should be made in the specific production practice:
1 Keep and store paper scientifically and reasonably. In view of the uniqueness of cardboard, in order to prevent the paperboard from dewetting or absorbing moisture and causing the paper to adapt to changes, in the transportation and storage of paper, the integrity of the product packaging should be paid attention to so as to prevent the packaging from being damaged due to improper transportation and operation. Storage should not only pay attention to ventilation, but also avoid light and wind, temperature control at 15-25 °C, relative humidity 40% -65%. The stacking of paper should be stacked according to the paper production date, product category, and production address. Paper use should be based on the order of production date in order to provide paper as much as possible to provide the next process with a more stable paper humidity fitness.
2 Effectively control the humidity of the paper. If it is necessary to properly store and rationally use the product before cutting, how should measures be taken to prevent changes in paper size caused by changes in temperature and humidity after cutting? First of all, it is necessary to be scientific and reasonable in the production and command. The paper strives to be printed along with printing, and it is best to be able to prepare the paper in one hour before printing. The large quantities cannot be opened and cut all at once, and must be batched according to the printing speed and progress. Cutting, and cutting paper is best to use a plastic cover with good sealing performance to cover the product, so as to minimize the time of direct contact with the air, especially when multi-color products are printed on single and double color machines. The method becomes even more important. Not only white paper but also every color must be processed in the same way to prevent the multicolor overprinting caused by the humidity change, and to ensure the quality of the printed product.
The effect of moisture change on the die-cutting precision after calendering In order to increase the beauty of the product and increase the smoothness of the color box surface, there is now 70%-80% of the carton packaging needs to be coated and calendered. Polishing is that the printed product is evenly coated with a coating of varnish on the printing surface by a glazing roller, and then the solvent is volatilized to form a bright layer by heat drying, and the calendering is a reprocessing on the basis of the calendering of the printing product, and 100 is used. - The high temperature of 130°C and the pressure of about 138kg/cm2 make the bright layer formed on the surface of the print more dense, and the brightness is higher.
Due to this special process, the water content of the paper itself is severely dehydrated in a short time, and the paper inevitably shrinks. The amount of shrinkage is related to the size of the paper structure and the initial humidity. Although its shrinkage has nothing to do with printing overprinting, it directly affects the precision of die cutting. For example, if the length of the original input of a product is X, and the width is Y, if you traverse 5 pairs of K-sheets and 3 bails, you will get a product with a pattern size of 3Yx5X after printing. After calendering, the paper pattern shrinks in size, shrinks in the transverse direction by ΔX, and shrinks in the longitudinal direction by ΔY. After die-cutting the die with the same size as the original, die-cutting accuracy will certainly occur. Embossing, bleeding, and patterning do not center around. If the customer is not satisfied, it will lead to partial or total scrapping of the product, causing undue losses to the company.
How to avoid or reduce such errors in specific production? Of course, the simpler method is to make the mold (dieboard) according to different paper and different shrinkage after finishing the calendering. However, this often seriously delays the production cycle, and the size of die-cutting will be smaller than the original size. For this purpose, we should gradually explore the humidity characteristics of various types of paper used in this unit and the horizontal and vertical shrinkage after calendering. Add this amount of shrinkage to the input size of the manuscript when printing, so that the final distance after contraction, The entire graphics size meets the standard requirements, which also allows the die-cut version to be produced in advance to ensure product quality and cycle time.
The effect of moisture on the folding endurance of the product after veneering and calendering. The folding endurance is the property of the paper crease, which does not cause cracks, peeling of the slit coating, etc., when the paper is folded. Indicates the toughness and surface strength of cardboard and board in the folded condition. From the paper itself, the good or bad folding resistance depends on the binding force between the fibers. A large part of China's papermaking raw materials are non-wood raw materials, among which more reeds, wheat straw, straw, bagasse and other grassy materials are used. Due to the influence of the properties of these raw materials, different toughness and brittleness will appear after paper forming, and cotton, The toughness of the paper produced from raw materials such as hemp and wood is better, and most grass materials have poor fiber length and have a high content of heterocells.
The paper we are using now, paper mills generally use grass fibers and wood fibers to properly match the toughness of the paper. However, due to the different proportions of wood pulp, filler properties, and different production processes, there is a big difference in the toughness of different papers. Calendering is a high-temperature and high-pressure dehumidification process for paper. By calendering, the individual fibers of the paper itself become brittle due to dehydration, the ability of fibers to slip and disperse stress is greatly reduced, and the folding endurance performance is weakened. In the process of folding the cardboard box in the mold-folding box, it causes fluffing, peeling, cracks, crumple and other ills, and even causes the entire batch of products to be scrapped. Therefore, in the specific production arrangements, customers choose paper, pay special attention to the performance of the paper and the effect of calendering on the folding resistance of the paper.
Under normal circumstances, 250-380G white board white board, white cardboard, better folding resistance, you can consider the calendering process; and gray-white paper, not suitable for calendering. Moreover, because of the inherent characteristics of paper fibers, the folding endurance in the longitudinal direction is significantly better than the folding endurance in the transverse direction. It is necessary to fully consider this factor in calculation of the opening size in the paper box.
Before calendering the product before die-cutting, it is first necessary to perform folding-resistance tests on the paper after calendering in the transverse and longitudinal directions. If it is found that the folding resistance is poor, the calendered product can be re-watered on the machine from the reverse side of the paper, and if the cycle is available, humidity can be naturally adjusted in places with high humidity. After such a proper moisture absorption of the paper, the fiber strength and flexibility are enhanced, the folding endurance is improved, and waste and loss can be reduced. (Shen Hong) ("China Packaging News")