Experience in the practical operation of flexo printing

Experience in the practical operation of flexo printing

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Flexographic printing, offset printing, gravure printing and screen printing are the four common printing methods, each of which has its own merits. At present, the packaging and printing industry is developing in the direction of high quality, fineness and diversification. However, the proportion of offset printing and gravure printing is relatively large, and flexo printing started late in China, especially in high-quality printing. However, with the continuous application of new technologies, the unique and superior flexographic printing has made great progress and has a very strong market competition potential.

Printing equipment and printing technology are the core of the entire flexographic printing process, which will reflect printing equipment, material selection, job development and operational standards. Proper operation of each process is an essential requirement for the operator.

The usual sequence of operations is as follows:

Feeding roll→feeding paper→adjusting and correcting deviation→reasonable matching anilox roller→upper roll→adjusting pressure→inking→imprinting→registration→tension control→drying system→die cutting→cutting→winding

Feeding roll

After the loading of the roll, the center position of the roll is also the printing position, because the offset correction, die cutting, sectioning, etc. are all based on the central axis, and the printed material should be strictly in accordance with the paper feeding route of the equipment. After each paper-feeding roller and paper-feeding, the machine can be started to make the paper to be printed smoothly, adjust the tension, and the printed material is subjected to a certain smooth tension control to ensure the overprinting precision, adjust the deviation, and make the material in the middle of the printing machine and the printing plate. Position to ensure that the corrective action is sensitive.

Adjust pressure

The adjustment of the flexographic printing pressure is a key step in the quality of the product, which directly affects the fineness of the printed product. The spacing between the printing plate roller and the embossing roller is the same. The 2.08mm (1.7 plate + 0.38 double sided) is optional. The standard feeler gauge of the thickness of the tape makes the resistance of the respective feeler of the two ends the same. The spacing at this time is the most ideal pressure value for printing (but in actual production, the operator needs careful fine-tuning, then the technician needs Practical experience to get the best pressure value).

Under the slow running of the machine, the pressure is started from the first color group. Firstly, the ink transfer of the roller to the plate roller is checked. The pressure system at both ends can be adjusted to the best effect. The pressure between the two rollers is light. Good, it is conducive to the normal ink transfer of the dot network, to ensure the quality of the image and the protection of the plate. Secondly, the rough adjustment pressure of the plate roller and the pressure roller is observed, and the transfer condition is observed, and the degree of clarity of the surface printing of the material is confirmed by the correctness of the transfer pressure (note: this key step is adjusted according to the experience of the technician). It is necessary to overcome the excessive pressure of the flexographic printing, which is the key to the printing plate.

Tension control system

Controlling the constant printing tension by the tension controller during the printing process is the key to accurate overprinting. Therefore, the tension control system is an important mechanism of any web press, which determines the printing accuracy of the printing to a large extent. The tension control mechanism is mainly located in the discharge part, the printing station and the traction and retracting mechanism of the receiving material, the output of the material roll need resistance, the rotation of the material roll has inertia, and the coil material itself also has a certain tension value, plus the printing speed. Pulling traction, the resistance of each color group overprinting to the printed material during the printing process makes the tension control of the unwinding, printing and winding mechanism particularly important. The size of the tension control should be determined according to the thickness of the material and the width of the material. The thicker the material to be printed, the greater the tension value; conversely, the tension value should be small, such as the tension value of the printed material is higher, because the problem of wrinkling, deformation, etc. of the printed material should be taken into account, and the appropriate tension adjustment is generally considered. It is based on the multi-color “cross rule line”, and does not “move” back and forth as the standard. If the “cross line” overprinting is unstable during printing, the tension of the unwinding and winding can be adjusted appropriately so that the color groups “crosshairs” The registration is stable, of course, the tension values are not exactly the same at low speed, medium speed and high speed during printing. It is recommended to fine-tune the tension value at normal speed, and the phenomenon of “walking” often occurs during the printing process. People are used to Look for equipment, materials and other reasons. In fact, it is not the tension that is uncomfortable. If you can carefully adjust the tension, in general, overprinting is not allowed to solve.

Adjustment of pH and viscosity of ink

Effectively controlling the pH and viscosity of the ink during the printing process is the main operational step to ensure the quality of the printed product.

The pH of the water-based ink is about 8.5. At this value, the ink is relatively stable, but in actual production, as the temperature rises and the ammonia in the ink evaporates, the pH value changes, which affects the printing suitability of the ink. A small amount of stabilizer can be added to control the pH value. In normal printing, it is usually required to add 5 ml of stabilizer every half hour and stir it evenly. Basically, the oil black can ensure stable printing suitability. Add stabilizers, otherwise it will be counterproductive and various printing defects will follow.

The viscosity of the ink is the main factor determining the ink transfer performance, print fastness, penetration and gloss. The ink viscosity is too high, the darker the color, the greater the loss of ink, the slower the degree of drying, the lower the viscosity, the more the color changes, and the dot size is enlarged, resulting in a decline in product quality.

Water-based inks have different viscosities when printing layers and full versions. The general grade ink viscosity should be slightly lower. At the time of printing, the speed of the machine also has an effect on the viscosity of the ink. When the printing speed is high, the viscosity of the water-based ink is lower, and the viscosity of the ink is higher when the printing speed is low.

Die cutting station

In order to discuss the flexographic printing technology in a more comprehensive manner, in addition to the above-mentioned operations on the corrugated box flexo printing process, I would like to talk about the die cutting station problem of the narrow flexographic printing press. Narrow-format flexographic printing presses generally have three die-cutting stations. After printing, they are indented and die-cut to obtain the desired printed products. They can be used for the processing of stickers, forms, medicine bags, etc. Round-pressed indentation roller, die-cutting of external cutter roller. If magnetic cutter is used first, magnetic roller of various product specifications must be prepared to form a series of specifications, which can meet the die-cutting of most products of different specifications. Processing requirements.

Another die-cutting roll is called a single roll. It is a knife roll that is inserted into a die-cutting blade. One, two, four or more sipe can be opened as needed. It is mainly used for cutting a single finished product. The cutting single roll can be the same size as the plate roll. Or double the use.

The outer cutter roller must be equipped with a sponge cloth before use to ensure the die cutting precision and avoid damage to the knife edge. The pressure is adjusted to just cut off the printing material for optimum pressure.

In addition, pay attention to post-press processing, glazing, laminating, bronzing, punching, etc.

This article briefly describes some of the theoretical and practical operations of flexo printing, hoping to become a reference for flexographic printing, ever-changing packaging, flexographic printing does not have a fixed model for reference. It is hoped that the printing workers will practice and accumulate experience in production.