Several problems should be paid attention to in direct flexo printing of corrugated cardboard
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Factors affecting the quality of corrugated board printing are plate making, equipment, materials, technology, and environment.
1. Prepress processing
First of all, the use of color should be paid attention to in the design of the original. For the packaging design of corrugated board flexo printing, in the use of color, one must create a variety of works that are elegant or unconstrained, or warm, or have a strong visual impact; second, to reduce the number of printing colors as much as possible, Reduce the number of overprints. The more the number of overprinting, the more severe the compressive strength of corrugated cardboard is reduced.
Second, pay attention to the application of the hierarchy. The style of packaging design, some pursuit of simplicity and clarity, and some pursuit of rich layers. Since the flexible version has high elasticity and is easily deformed, the dot enlargement is more serious after being pressed. In the plate making process, 1% or 2% of the small dots are easily lost during the washing process. During the printing process, 3% of the dots tend to expand to about 10%, and it is impossible to achieve a gradient from 0 to 100% or a very soft layer. Effect. Therefore, the flexographic products can display relatively high levels of highlights and shadows, and the better-printed products can only reproduce 8% to 10% of the dots.
In image design, lines, texts, and color blocks are best used with vector software to process images or make some special effects. You can use software such as Photoshop.
2. Choose the right plate
At present, a photosensitive resin plate is generally used, and the thickness is 2.84 mm, 3.94 mm, 7.00 mm, and the like. The traditional flexo printing is usually 7.00mm thick printing plate. If the pressure is too large during printing, the printing substrate is easily deformed or even damaged, and the resin printing plate is deformed and the dot is expanded. When the printing pressure is too small, the printing substrate is easy to produce the clothing. Board failure, affecting the field and fine lines. Using a 3.94mm thick thin plate and a 3.06mm thick air cushion lining (Shore hardness 20~25), not only can the plate deformation be reduced, the printing quality can be greatly improved, and the plate and plate making cost can be reduced. .
Today, thin plate technology is increasingly popular in high-end corrugated cardboard flexo printing. When using a thin plate, pay attention to the choice of the pad. The gasket is covered with a plastic foam pad, rubber lining (with or without PE base film) and R/bak air cushion lining.
R/bak lining is a polyurethane type material, which is 2 to 10 times more flexible than resin or rubber. It has the advantages of high elasticity, no deformation, fast compression and so on. It is currently the most widely used cushioning material in corrugated board printing, which can significantly improve the printing quality, is suitable for fine pattern printing, and is gradually replacing thick printing.
3. Pay attention to the matching of the anilox roller
Since the flexo printing is carried out by the anilox roller, the quality of the anilox roller and the number of lines determine the amount of ink transferred and the fineness of printing. In general, the mesh adjustment uses a high-line anilox roller to make the dot print clear; while the solid version uses a low-line anilox roller to achieve color saturation. If there are both the mesh image and the field, a certain color is generally made into a mesh version and a solid version to facilitate the printing operation. If a large area is connected to a local gradient, an anilox roller with a moderate number of lines is generally available. Select the number of anilox lines and pay attention to matching the number of screen lines to avoid and reduce the possibility of generating moiré. In addition, in the packaging design, gold ink, silver ink, etc. are often used to enrich the packaging design effect and improve the packaging grade. When using gold ink or silver ink, if the pattern contains mesh, it is better to leave more space in the highlights and darks, and the dots in the highlights are better than 20%, because such ink printing dots are easy to paste.
4. Use high quality water-based ink
The advantages and disadvantages of water-based inks are the key to ensuring that the color of corrugated board printing is pure and bright. The selected water-based inks require that the hue and viscosity of the batches are basically the same. The high-quality ink has a delicate hand feeling, is viscous and suitable in concentration, and has no precipitation or agglomeration when diluted with water or alcohol solvent. No foam is generated during stirring, and the affinity with other aqueous inks is good, and there is no abnormal reaction. However, it should be noted that water-based inks should not be mixed with alcohol-based inks or solvent-based inks, or organic solvents should be added to aqueous inks to avoid printing failures and affect print quality.
Pay attention to the measurement of ink viscosity during use, neither too high nor too low. Pay attention to the change in pH. Because in the printing process, due to the volatilization of water, the viscosity of the aqueous ink will increase and the pH will decrease. The pH should be controlled between 8.5 and 9.5. If it exceeds this range, it can be adjusted with a pH stabilizer. When it is slightly larger, add an appropriate amount of water for adjustment. In the process of use, if there is more foam in the ink, a small amount of defoaming agent can be added, and the amount of addition is generally controlled within 0.2% of the amount of ink. In addition, before use, pay attention to the adhesion, abrasion resistance, water resistance and dryness of the ink. The choice of these properties depends on the use of the corrugated paper. For example, for the packaging of frozen products, aquatic products or canned beverages, inks with strong adhesion, quick drying, and water-repellent deinking should be selected.
5. Ensure print registration accuracy and control printing pressure
The accuracy of overprinting depends first on the accuracy of the flexo press and whether the paperboard is parallel to the paper feed, and the accuracy of the layout (the compression and reduction of the tone is not involved here), the loading and correction. After the plate is finished, you can take a transparent polypropylene base with a thickness of 0.5mm, draw a straight line at the center of its length, and cover it on the plate. Use a pen to draw lines along the edge of the plate to accurately trace all the edges (such as overprinting three). The color can be painted again), and finally, according to the overprint color, the corresponding printing plate is stuck in the painted position, and then the printing plate is attached to the cylinder of the printing machine with double-sided tape, and the base line and the printing machine are installed. The center of the plate roller is aligned to ensure accurate overprinting.
In corrugated cardboard flexo printing, it is necessary to pursue the exquisite pattern and the printing difficulty. Such as small texts and patterns, if you want to overprint, because the overprinting accuracy is not ideal, it will inevitably lead to the print pattern is not beautiful and increase the scrap rate, generally can solve this problem through trapping. The trapping of corrugated cardboard is usually 0.5~1mm, the overprinting of complementary colors is difficult, and the black borders appear at the junction of the two colors, which affects the aesthetics of the packaging. Relatively speaking, dark and light overprints will be much better.
6. Pay attention to the clarity of printed graphics
The clarity of printed graphics is one of the most intuitive print quality. To ensure the clarity of the printed graphics, we must first select a plate that is wear-resistant, moderately elastic, and of good quality. Whether it is a printing plate made with a negative film or a hand-cut plate, the pattern and line depth should not be lower than 3mm, the arc should be round, and the straight line should be straight. When printing, the ink should be properly lightened, the ink layer should not be too thick, and the pressure should be moderate and stable (there is no obvious dent in the paper imprint). The speed of the printing press should generally be controlled above 800r/min, too low, the unevenness of the ink on the face paper will reveal the bottom; too fast, the ink will be too thick, and the layout will be blurred. At the same time, it is necessary to prevent paper wool, paper scraps, etc. from entering the printing press and sticking to the printing plate. If it is, stop it, wipe it clean and reprint it.
7. Pay attention to the effect of full-page printing on the strength of the carton
When the corrugated board is printed, the cardboard is formed by passing the paper guide roller and the printing cylinder on the printing machine to the grooved portion. During the transfer process, the paperboard will be deformed to varying degrees by the pressure of the drum, which will inevitably affect the strength of the carton in the future. Carton of the same material, after printing, its strength is reduced by about 30% compared with before. Therefore, in the production process, generally three measures can be taken to alleviate this problem: First, the gap between the paper guide rollers of the printing press is appropriately adjusted, which is slightly smaller than the thickness of the cardboard of 0.1 to 0.15 mm, so that the to-be-printed cardboard passes smoothly without being deformed into should. Secondly, pay attention to adjust the printing pressure, the printing plate pressure can meet the printing clear, the surface paper feels no dent, and can be flat. Thirdly, the speed of the printing press (above 1000r/min) can be appropriately increased, so that the cardboard can pass through the printing machine as soon as possible, thereby reducing the pressing time of the cardboard. 8. Pay attention to the handling of some problems in printing
In the flexographic printing of corrugated board, the problems that are often encountered are: the expansion of the printing plate and the thickening of the lines, the inability to cover the background color in the overlay printing, and the printed matter exhibits many round holes such as pinholes. The paper printing is blurred, the surface of the cardboard is indented, the surface of the printed product is powdery and easy to fade, and the surface of the cardboard is raised. The solution of these problems, or by adjusting the viscosity of the ink, or by controlling the pressure, or adjusting the gap of the paper feed roller, or cleaning, etc., can even be replaced by the cardboard.