Corrugated cardboard direct flexo

Corrugated cardboard boxes are the material with the largest amount of product packaging today. The printing of corrugated paperboard used for making corrugated boxes is still based on flexographic printing. Based on the characteristics of corrugated cardboard and direct flexo printing, the accuracy and quality of the printed products are not ideal.
How to print a good quality product, this article talks about the problems that should be paid attention to in the direct flexo printing of corrugated cardboard and the elimination of common faults.

1. Main features of direct flexo printing on corrugated cardboard

Corrugated cardboard direct flexographic printing refers to a method of using flexographic plate material to transfer ink through an anilox roller to perform direct printing. It has the advantages of low production cost, high efficiency, simple process, high durability, safety and environmental protection, strong applicability, wide range of applications, etc. It is very suitable for printing corrugated cardboard. Corrugated cardboard boxes are generally printed on corrugated cardboard using a flexible version, and then made into boxes, using water-based inks that meet environmental protection and sanitary safety requirements, usually with more monochrome. In recent years, the performance and quality of photosensitive resin plates have been improved, and the application of high-tech such as electronic plate-making, laser engraving plate, digital plate-making, etc. has made the flexible plate direct printing meet the requirements of high-grade color fine products with layers.

2. Direct flexo printing of corrugated cardboard

Issues that should be noted

Factors that affect the quality of corrugated cardboard printing include plate making, equipment, materials, technology, environment and other aspects.

1. Pre-press processing

First, pay attention to the accurate use of color in the design of the original. For the packaging design of corrugated cardboard flexographic printing, in terms of color application, one must create all kinds of elegant, or bold, or warm, or have a strong visual impact; second, reduce the number of printed colors as much as possible, Reduce the number of overprints. The more overprinting times, the more serious the compressive strength of corrugated cardboard decreases.

Second, pay attention to the application of layers. The style of packaging design, some pursue concise and bright, some pursue rich levels. Because the flexible version has high elasticity and is easily deformed, the expansion of the outlet after compression is more serious. In the plate-making process, 1% or 2% of the small dots are easily lost during plate washing. 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 gradation area of ​​the highlight and dark tone that the flexo product can express is relatively less, and the highlight of the better printed product can only reproduce 8% to 10% of the dots.

In image design and production, it is best to use vector software for lines, text, and color blocks to process images or create some special effects. Photoshop and other pixel-based software can be used.

2. Choose the right plate

Currently, photosensitive resin plates are generally used, with thicknesses of 2.84mm, 3.94mm, and 7.00mm. Traditional flexo printing usually uses a 7.00mm thick printing plate. If the pressure is too large during printing, the printing substrate is easily deformed or even damaged. At the same time, the resin plate is deformed and the dots expand. Board failure, affecting the ground and fine lines. Using a 3.94mm thick thin plate and a 3.06mm thick air cushion liner (Shore hardness 20 to 25), not only can reduce the deformation of the printing plate, the printing quality has been greatly improved, but also can reduce the cost of plate material and plate making .

Today, thin-plate technology is becoming more and more popular in high-end corrugated cardboard flexo printing. When using a thin version, pay attention to the selection of pads. The liner includes plastic foam liner, rubber liner (with or without PE base film) and R / bak air cushion liner.

The R / bak lining is a polyurethane type material, which is 2 to 10 times more flexible than resin or rubber, and has the advantages of high elasticity, no deformation, and fast recovery under pressure. It is currently the most widely used liner material in corrugated cardboard printing, which can significantly improve the printing quality and is suitable for fine pattern printing. It is gradually replacing thick plate printing.

3. Pay attention to the selection of anilox roller

Because flexo printing transfers ink through the anilox roller, the quality of the anilox roller and the number of lines determine the amount of ink transfer and the fineness of printing. Generally speaking, high-line-count anilox rollers are used for screen adjustment, so that dot printing is clear; while low-line-number anilox rollers are used for field version to achieve color saturation. If there are both mesh-adjusted images and solid fields, a certain color is generally made into a mesh-adjusted version and a solid version respectively to facilitate printing operations. If a large area is connected to a local gradual change, an anilox roller with a moderate thread count can generally be used. When selecting the number of anilox roller lines, pay attention to match the number of screen lines to avoid and reduce the possibility of moire. In addition, in packaging design, gold ink and silver ink are often used to enrich the effect of packaging design and improve the packaging quality. When using gold ink and silver ink, if the pattern contains mesh tone, it is best to leave some space in the highlight and dark tone, and the dot in the highlight is preferably more than 20%, because this type of ink printing dot is easy to paste.

4. Use high-quality water-based ink

The advantages and disadvantages of water-based ink are the key to ensure the pure and bright color of corrugated cardboard printing. The selected water-based ink requires that the hue, viscosity and other properties are basically the same between batches. The high-quality inks have a delicate touch, viscousness, and suitable concentration. There is no precipitation or agglomeration when diluted with water or alcohol solvents. No foam is generated during stirring. It has good affinity when mixed with other water-based inks and no abnormal reaction. But be careful: Do not mix water-based inks with alcoholic inks, solvent-based inks, or add organic solvents to water-based inks, so as not to cause printing failures and affect printing quality.

Pay attention to measuring the ink viscosity during use, neither too high nor too low. Pay attention to changes in pH. Because in the printing process, due to the volatilization of water, the viscosity of the aqueous ink will increase and the pH value will fall. The pH value should be controlled between 8.5 and 9.5. If it exceeds this range, it can be adjusted with a pH stabilizer. If the viscosity is When it is slightly larger, an appropriate amount of water can be added for adjustment. During use, if there is more foam in the ink, a small amount of defoamer can be added, and the amount added should generally be controlled within 0.2% of the amount of ink. In addition, before use, pay attention to the ink's adhesion, wear resistance, water resistance and dryness. The choice of these properties depends on the use of the corrugated paper. For example, for packaging of frozen products, aquatic products or canned beverages, inks with strong adhesion, quick-drying, and ease of deinking in water should be selected.

5. Ensure printing registration accuracy and control printing pressure

The accuracy of overprinting depends first on the accuracy of the flexo press and whether the paper feeder pushes the cardboard parallel, and then the accuracy of the layout (here, there is no problem of tone compression and shrinkage), plate installation and correction. After the printing plate is finished, you can take a transparent polypropylene film with a thickness of 0.5 mm, draw a straight line at the center of its length, cover the printing plate, draw a line with the edge of the printing plate with a water pen, and accurately draw all the edges (such as overprint three) You can draw another color), and finally press the color of the set, stick the corresponding printing plate in the painted position, and then use double-sided tape to paste the printing plate on the cylinder of the printing machine. The center line of the base is installed with the printing machine. The center alignment of the plate cylinder coincides to ensure accurate overprinting.

In the flexo printing of corrugated cardboard, we must not only pursue the exquisite patterns, but also consider the difficulty of printing. Such as small text and patterns, if you want to overprint, because the overprint accuracy is not ideal, it will inevitably cause the printing pattern to be unattractive and increase the scrap rate. Generally, this problem can be solved by trap printing. The trapping of corrugated cardboard is usually 0.5 ~ 1mm. Overprinting of complementary colors is difficult. There will be obvious black borders at the junction of the two colors, which affects the beauty of the packaging. Relatively speaking, dark and light overprint will be much better.

6. Pay attention to the clarity of printed graphics

The clarity of printed graphics is one of the most intuitive printing qualities. To ensure the clarity of printed graphics, we must first choose a wear-resistant, moderately elastic, high-quality plate. Whether it is a printing plate made with negative film or a hand-carved printing plate, the pattern and line depth on the layout should not be less than 3 mm, the arc should be round, and the straight line should be straight. When printing, the ink should be lightened properly, the ink layer should not be too thick, and the pressure should be moderate and stable (subject to no obvious dents on the facial paper). The speed of the printing machine should generally be controlled to be higher than 800 r / min. If it is too low, the ink on the facial tissue will be uneven and it will show the bottom. At the same time, it is also necessary to prevent paper wool, paper scraps, etc. from entering the printing machine and sticking to the printing plate. If there is, it should be stopped and wiped clean before printing.

7. Pay attention to the effect of full-page printing on the strength of the carton

When corrugated cardboard is printed, the cardboard is transferred to the slotted part through the paper guide roller and printing cylinder on the printing press. The cardboard will be deformed to varying degrees by the pressure of the drum during the transfer process, which will inevitably affect the strength of the carton in the future, especially when the full version is printed. After printing, the strength of the carton of the same material is about 30% lower than before. Therefore, in the production process, three measures can generally be taken to alleviate this problem: First, the gap between the paper guide rollers of the printing machine is adjusted appropriately, which is slightly less than the thickness of the cardboard 0.1-0.15 mm, so that the cardboard to be printed passes smoothly without deformation. should. Secondly, pay attention to adjusting the printing pressure, so that the printing plate pressure can meet the requirements of clear printing, no dents on the facial paper and smoothness. Third, the speed of the printing machine can be properly increased (above 1000 r / min) to make the paperboard pass through the printing machine as soon as possible, reducing the time for the paperboard to be compressed.

8. Pay attention to some problems in printing

In the direct printing of corrugated cardboard flexo, the problems often encountered are: the expansion of the graphic of the printing plate, the roughening of the lines, the background color cannot be covered in the overlay printing, and the printed products show many round holes like pinholes The printing on the facial paper is fuzzy, there are indentations on the surface of the cardboard, the surface of the printed product is powdered and easy to fade, and the surface of the cardboard is fluffed. These problems can be solved either by adjusting the viscosity of the ink, or by controlling the pressure, or adjusting the gap between the feed rollers, or by cleaning, etc., or even by replacing the cardboard.

3. Elimination of common faults in corrugated cardboard direct flexo printing (Table 1).

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