Flexographic Printing Actual Operation Analysis

Flexo printing, offset printing, gravure printing and screen printing are common four printing methods, and they have their own merits. At present, the packaging and printing industry is developing towards high quality, precision and diversification. However, the proportion of offset printing and gravure printing is relatively large, while flexo printing has started relatively late in China, especially for high-quality printing. However, with the continuous application of new technologies, unique and superior flexo printing has made great progress and has a very strong potential for market competition.

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

The usual sequence of operations is as follows:

Feeding roll→feeding paper→adjusting and correcting→correctly matching anilox roller→on-print roller→adjusting pressure→inking→imprinting→registration→tension control→drying system→die cutting→slicing→winding

Feeding coil

After loading the roll, the center position of the roll is also the printing position, because the offset correction, die cutting, subsections, etc. are based on the central axis, the printed material should be strictly in accordance with the paper path of the equipment through After each paper guide roller and paper is put through, the machine can be operated to make the material to be printed to feed steadily, and the tension is adjusted so that the printed material is subjected to a certain amount of stable tension control, so as to ensure overprint accuracy, adjustment and correction, and make the material between the printer and the plate. Location to ensure the correctness of corrective action.

Adjust pressure

The adjustment of flexographic printing pressure is a key step in product quality and directly affects the fineness of the printed product. The spacing between the printing plate roller and the embossing roller is the same, and 2.08 mm (1.7 plate + 0.38 side plate) can be selected. The thickness of the standard feeler gauge, so that both ends of their respective feeler pull the same resistance, the spacing at this time is the printing of the most ideal pressure value (but in the actual production, the operator needs to fine-tune, it requires the technical staff Practical experience to get the best pressure value).

When the machine is running at a slow speed, starting from the first color group, the pressure of the roller on the plate roller is checked first, and the pressure can be adjusted to the best effect by the pressure system at both ends. The pressure between the two rollers is light. Well, it is conducive to the normal transmission of ink in the dot network, ensuring image quality and protecting the printing plate. Secondly, the coarse pressure of the plate roller and the pressure roller is observed, the transfer condition is observed, and the clearness 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). We must overcome the pressure of flexographic printing, which is the key to the printing plate.

Tension control system

In the printing process, the constant tension of the printing tension controlled by the tension controller is the key to accurate printing. Therefore, the tension control system is an important mechanism of any web printing press. It largely determines the overprint accuracy of the printing product. The tension control mechanism is mainly set in the discharge part, the printing station and the traction and winding mechanism for placing and receiving materials, the output of the material roll need resistance, the material roll has inertia, and the material itself also has a certain tension value, plus the printing speed. Tension pulling, the resistance of various color printing overprinting materials 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 by the thickness of the material and the width of the material. The thicker the material being printed, the greater the tension value; conversely, the tension value should be small, such as the tension value of the printed thin material is more demanding, because it needs to take into account the printed material wrinkling tension, deformation and other issues, under normal circumstances the appropriate tension adjustment Multi-color "cross-line" is all registered, not "move" back and forth as the standard, if the "crosshair" overprinting is not stable, can be adjusted to adjust the unwinding and winding tension, so that the colored group "crosshair" Until the registration is stable, of course, the tension values ​​at the low, medium, and high speeds are not exactly the same at the time of printing. It is recommended to fine-tune the tension value at normal speed. The phenomenon of “walking” often occurs during the printing process, and people are accustomed to Look for equipment, materials and other reasons. In actual fact, affecting the accuracy of overprinting is often tension discomfort. If you can carefully adjust the tension, under normal circumstances, overprinting can be solved.

Ink pH and viscosity adjustment

The effective control of the ink's pH and viscosity during the printing process is the main operation step to ensure the quality of the print.

Water-based ink PH value of about 8.5, at this value, the ink is relatively stable, but in actual production with the temperature rise and the volatilization of ammonia in the ink, PH value will change, affect the printability of the ink. A small amount of stabilizing agent 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 oil black can ensure more stable printability. 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 transfer performance, printing firmness, penetration and gloss. Ink viscosity is too high, the darker the color, the ink loss, the degree of drying is slow, the viscosity is too low, the color changes, and the dot expansion causes product quality to decline.

Water-based inks have different viscosities when printed at full-scale and full-print. The general level ink viscosity should be slightly lower. When 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 ink viscosity is higher when the printing speed is low.

Die cutting station

In order to discuss the flexo printing technology more comprehensively, besides the above-mentioned operation of the corrugated carton flexo printing process, the problem of a die cutting station for a narrow-width flexographic printing press is also wanted here. The narrow-width flexographic printing presses are generally prepared with three die-cutting stations. After the printing, the wires are indented and die-cut to obtain the desired printed products. The products can be used for the processing of finished products such as stickers, forms, and drug packages. Circular pressure indentation roller, external knife roller die cutting, such as the use of magnetic tools must be prepared with a variety of product specifications of the magnetic roller, composed of serialized specifications, can meet the majority of different specifications size of the product die cutting Processing requirements.

Another type of die-cutting roll is a single-cut roll. It is a knife roller set into a die-cutting blade. It can open one, two, four or more sipes as required. It is mainly used for cutting single-finished products. The cut-off single-roller can be used with the size of the plate roller. Or double use.

The knife roller must be used before the use of the knife roller to ensure the cutting accuracy and avoid the damage of the knife edge. The pressure is adjusted to just cut off the printing material to be the best pressure.

In addition, we must pay attention to post-printing, glazing, lamination, bronzing, and punching.

This article briefly describes some theoretical and practical operations of flexo printing. Hopefully it can become a reference for flexographic printing. The ever-changing packaging and flexo printing operations do not have a fixed model for reference. It is hoped that printing workers will practice more in production and accumulate experience.