Talking about the practical operation experience of flexographic printing

Flexographic printing, offset printing, gravure printing and screen printing are four common printing methods, each of which has its own merits. At present, the packaging and printing industry is developing towards high quality, precision and diversification. However, offset printing and gravure printing account for a large proportion, while 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 operating the entire flexographic printing, which will reflect the printing equipment, material selection, job formulation and operation level. The correct operation of each process is an essential requirement for operators.

The usual operation sequence is as follows:

Feeding roll → paper feeding → adjustment of deviation correction → reasonable selection of anilox roller → upper plate roller → adjustment of pressure → inking → imprinting → registration → tension control → drying system → die cutting → slitting → winding

1. Loading roll

After the roll is loaded, the center position of the roll is also the printing position, because the correction, die cutting, and sectioning of the plate are based on the central axis, and the printed material should be passed through the paper feeding path of this equipment Each paper guide roller and the machine can start the machine after the paper is passed to make the printed material feed smoothly, adjust the tension, and make the printed material subject to a certain steady tension control, so as to ensure the accuracy of overprinting, adjust the deviation correction, and make the material between the printing machine and the printing plate Position to ensure the correctness of the corrective action.

Second, adjust the pressure

The adjustment of flexographic printing pressure is a key step in product quality, which directly affects the quality of the printed products. The distance between the plate roller and the platen roller is the same, and 2.08mm (1.7 plate + 0.38 double-sided) can be used. Tape) The thickness of the standard feeler gauge, so that the pulling resistance of the feeler gauge at both ends is the same, the interval at this time is the optimal pressure value for printing (but in actual production, it needs careful fine-tuning by the operator, then it needs the technician's Practical experience to obtain the best pressure value).

When the machine is running at a slow speed, the pressure starts from the first color group. First, check the ink transfer from the anilox roller to the plate roller. 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 dots and screens, ensuring the quality of the graphics and images and protecting the printing plate from damage. Secondly, the rough adjustment pressure of the plate roller and the pressure roller, and observe the transfer situation, the clarity of the printing on the surface of the material, is the proof of the correctness of the transfer pressure (Note: This key step is adjusted according to the experience of the technical staff). The pressure of flexographic printing must be overcome, which is the key to the printing plate.

3. Tension control system

In the printing process, the tension controller controls the constant printing tension, which is the key to accurate overprinting. Therefore, the tension control system is an important mechanism for any web printing machine. It determines the overprint accuracy of printed products to a large extent. The tension control mechanism is mainly located in the unwinding part. The printing station and the unwinding and pulling traction and rewinding mechanism and the output of the reel need resistance. The reel rotates with inertia. The reel also has a certain tension value, plus the printing speed. Pulling traction, the resistance of each color group overprint to the printed material in the printing process makes the tension control of the unwinding, printing, and winding mechanisms 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 printed material, the greater the tension value; conversely, the tension value should be smaller, such as the thinner material, the tension value is higher, because the printed material must be wrinkled, stretched, deformed, etc. In general, the appropriate tension adjustment It is based on the multi-color "cross rule line" registration, not "moving" back and forth as the standard. If the "cross line" registration is unstable during printing, the tension of unwinding and rewinding can be adjusted appropriately to make each color group "cross line" Until the registration is stable, of course, the tension value is not exactly the same when printing at low speed, medium speed, and high speed. It is recommended to fine tune the tension value at normal speed. The phenomenon of "walking" often occurs during printing. People are used to Look for reasons such as equipment and materials.

In fact, it is not the tension that affects the accuracy of overprinting. If the tension can be adjusted carefully, under normal circumstances, overprinting can be solved easily. [next]

4. PH value and viscosity adjustment of ink

Effectively controlling the PH value and viscosity of the ink during the printing process is the main operation step to ensure the quality of the printing.

The pH value 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 volatilizes in the water ink, the PH value will change, affecting the printing suitability of the ink. For this, a small amount of stabilizer can be added to control the PH value. In normal printing, it is usually required to add 5ml of stabilizer every half an hour, and stir it evenly. Basically, oil black can ensure more 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 that determines the ink's transfer performance, printing firmness, penetration and gloss. If the viscosity of the ink is too high, the darker the color, the larger the ink loss, the slower the drying degree, the viscosity is too low, the color changes, and the dots expand and the product quality decreases.

When printing a gradation plate and a full plate, the viscosity of the water-based ink is different. The viscosity of the general grade ink should be slightly lower. When printing, the speed of the machine also has an effect on the ink viscosity. When the printing speed is high, the viscosity of the water-based ink is lower, and when the printing speed is low, the viscosity of the ink is higher.

5. Die-cutting station

In order to discuss the flexo printing technology more comprehensively, in addition to the above-mentioned corrugated box flexo printing process operations, I also want to talk about the problem of the die-cutting station of the narrow-width flexo printing machine. Narrow-width flexographic printing presses are generally equipped with three die-cutting stations. After printing, the lines are indented and die-cut to obtain the desired printed products, which can be used for the processing of stickers, forms, medicine packages and other finished products. Round pressure round indentation roll, shape knife roll die-cutting forming, if you use magnetic cutter first, you must prepare magnetic rolls of various product specifications to form a series of specifications, which can meet the die-cutting of most products with different specifications and sizes Processing requirements.

Another kind of die-cutting roll is called cutting single roll. It is a knife roller embedded with a die-cutting blade. One, two, four or more knife slots can be cut out as needed. It is mainly used for cutting single-piece finished products. The cutting single-sheet roller can be the same size as the plate roller. Or double it.

Before using, the knife roll must be covered with sponge cloth to ensure the accuracy of die cutting and avoid damage to the knife edge. The pressure is adjusted to just cut the printing material to the best pressure.

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

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

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