Section IV Flexo Printing

First, the principle and characteristics of flexographic printing 1. Flexographic printing principle Flexographic printing refers to the use of a flexible version of the printing ink transfer ink through the anilox roller. The layout structure of the printing plate is the same as that of the relief plate, and both the graphic part and the raised part (that is, the graphic text is higher than the non-graphic part) are in the same plane, and all of them are directly printed.
After the plate cylinder is loaded into the machine, a controlled thickness of ink is evenly coated on the plate graphic portion, and then the ink layer of the graphic portion is transferred under the effect of the printing pressure (given by the impression cylinder). The surface of the substrate, forming a clear printed copy of the text. Therefore, flexographic printing is a direct printing method using a rubber plate or photosensitive resin plate and a fast-drying liquid ink.
The transfer process of flexographic printing ink is similar to that of gravure printing. The transfer roller is directly placed in the ink tank. When the transfer roller rotates, the ink adhering to the surface is transferred to an anilox roller dedicated to transfer the ink, and then the screen. The roller coats the ink layer evenly over the surface of the printing plate image portion for printing at a controlled thickness. Since the ink does not need a uniform ink transfer process, the structure of the flexographic printing press is relatively simple. The principle of flexographic printing is shown in Figure 1-65.
2. Flexographic printing features Compared to other printing methods, the flexographic printing method has the main features of versatility and economy.
1 The specifications of the printing press are complete. Flexographic presses have many specifications and can be selected based on the intended use of the printed matter to make the use of the substrate a more method. It is generally possible to print strips as small as a few hundred meters and substrates that are approximately several meters wide.
2 wide applicability. Flexible printing plate making is convenient, the cost is low, the plate loading is convenient, the printing press has high speed, and the operation is simple; the printing materials are widely selected.
3 high production efficiency. According to production needs, it can be printed in front of multiple colors, and it can print in front of and behind the same time in multi-color printing; the printing plate has a high printing rate. All of these are favorable conditions for improving productivity and reducing production costs.
In recent years, the performance of the photosensitive resin plate has been continuously improved and stabilized, greatly improving the resolution and dot reproducibility of the printing plate (up to 1%-95%); modern high-tech such as electronic plate making, laser engraving, and digital plate making The application makes it possible for flexo printing to achieve level overlay color overlay; the laser-engraving anilox roll technology enables the engraving line number to reach a high precision of 1000 L/in, which can already make the ink volume accurately controlled and can print 300 L/in. The color level draft, its print quality is close to offset offset printing level; water-based flexo printing ink, UV (ultraviolet) flexo printing ink and UV/EB (ultraviolet/electron beam) flexo printing ink successfully developed and widely used, making flexo It has developed rapidly. Now, flexo printing has become one of the major printing methods.
3. Types of Flexible Plates In the past, the materials for making soft plates were mainly natural rubber and synthetic rubber. Therefore, the most flexible plates were hand-engraved rubber plates and cast rubber plates. Later, a flexible photosensitive resin plate was developed to rapidly increase the quality of flexographic printing. Here, we mainly introduce the following types of flexo plates:
1 hand-engraved rubber plate. Hand-sculpting is the simplest way to make a flexographic plate. For a simple job, you can use hand-engraving to make a plate, which has the advantages of convenience, speed, and low cost. The hand-engraving technology is strong, requiring the production staff to have skilled skills and rich experience, and the quality of the plate is not as good as photo-engraving.
2 casting rubber plate. The natural rubber or synthetic rubber plate is heated and pressurized in a master form. The advantages of the casting rubber plate is that once it is made into a mother shape, it is easy to produce rubber reproduction plates, the plate thickness can be freely controlled, and various rubber plates such as isobutylene rubber plate and vinyl rubber plate can also be produced according to different inks and printing materials. Rubber plate is mainly used for packaging and wallpaper printing.
3 flexible photosensitive resin plate. The photosensitive photosensitive resin plate is similar to the photosensitive resin plate used in letterpress printing in terms of chemical process, plate making process, and reproduction performance. The plate is mainly made of rubber-based resin, and the rinsed ear is exposed after exposure. The rinsing liquid is different from the photosensitive resin plate used for relief printing.
Flexible photosensitive resin plate is divided into solid resin plate and liquid resin plate, and its plate making process is different. The flexible photosensitive resin plate is made by direct exposure with a negative film. The reproducibility of the attached film is good. When the number of screen lines is 60L/cm, 3% dot can be reproduced. The plate making process does not require heating, so the plate has high dimensional accuracy and good stability. , Thickness and precision, plate-making time is shorter than the rubber plate. However, the flexible photosensitive resin plate also has disadvantages. Compared with rubber, its elasticity is insufficient, and the cost for making a replica plate is high. It is also necessary to check in advance its resistance to the ink solvent. The solid resin plate is mainly used for packaging printing, and the liquid resin plate is mainly used for newspapers and packaging printing.
4 seamless edition. By scanning the original and using an image signal to control the beam directly on the squeegee, the production process is simple, but its reproduction performance is still problematic. Another kind of seamless plate is coated with a flexible photosensitive resin on the surface of the roller, and laser engraving is still used in the production. Seamless printable continuous graphics, such as wallpaper.

Second, the flexographic printing equipment 1. Flexographic printing press classification Flexographic printing presses are mostly multi-color, ranging from two-color to eight colors. According to the arrangement of printing units, it can be divided into three types: co-embossing, cascaded and tandem.
(1) Co-imprinting Press Co-imprinting flexographic printing press, abbreviated as CI printing press, also known as satellite printing press. As shown in Figure 1-66.
The main features of a co-imprinting printing press are that each group of printing cylinders rotates together around a large-diameter central drum, and each center drum serves as an impression cylinder to complete the printing process.
Co-imprinting printing machine In the entire printing process, the substrate covers the surface of the impression cylinder with good flatness and high color registration accuracy between various color groups. Suitable for printing easily stretched, wrinkled paper, plastic film, metal Color prints of materials such as paper. However, the distance between each printing unit of the co-imprinting printing press is short, the intermediate drying apparatus is simple, and the printing speed is relatively slow.
(2) Laminated Printer As shown in Figure 1-67, each printing unit of the laminating printing press is independent from each other, self-forming printing units, and vertically stacked on each other. The distance between the printing units can be changed and the intermediate drying device can be conveniently placed.
The substrates of the laminating press are serially interposed between the impression cylinder and the plate cylinder of each printing unit, and the transfer of the graphic is completed by the printing pressure between the plate cylinder and the impression cylinder.
It is more convenient to exchange and install the plate cylinders of each printing unit of the stacking printing machine. When changing or installing plate cylinders, it does not affect the combined position of other printing units, and it is easy to operate, with high mechanical precision and strong print adaptability.
Laminated printing machine overprint color number, and change the series of substrates, can be printed on one side, can also be double-sided printing, use more convenient and flexible.
(3) Tandem printing press A tandem type flexographic printing press, also known as a modular or seat type flexographic printing press, has the appearance similar to a combined gravure web press, as shown in Figure 1-68.
Each printing unit of the tandem printing press bears one color version of the printing, and the printing cylinder and the printing drum cooperate with each other to complete the printing process. Each printing unit is arranged in a horizontal line. Combination of multiple printing units can complete color printing. The tandem printer can not only print web material, but also sheet (single) material. Commonly used for printing not in plastic labels and news newspapers, sheet corrugated cardboard packaging materials, etc., have unique advantages. In addition, there are features such as good attachability and strong variability. For example, according to the needs of the gravure printing machine unit (a group or several groups), add die cutting at the rear of the printing unit: additional web guide system, ink circulation system, plastic film electrostatic processing device, etc. Substrate function and decorative effect, as shown in Figure 1-69.
A wide-format bookshelf press is a special type of flexographic printing press.
The main feature of the wide-format bookshelf flexo printing machine is that it can print papers, plastic films, and foil substrates that are 1.6-4m wide and even more than 4m wide. Moreover, it is generally equipped with a device for automatically loading and unloading substrates. Broad shelf type flexo printing machine can also be combined with complex machine, folding machine, slitting machine, rewinding machine and other required mechanical linkages in series to process the substrates for post processing so that the substrates can be adapted to the needs of commodity packaging.
2. The main mechanism of the flexographic printing press The ink transfer mechanism of the flexographic printing press is relatively simple, but the surface structure of the transfer roller is more complicated than that of the transfer roller (or transfer roller) used in other printing methods. .
The flexographic printing press consists of a transmission part, a paper feeding part, a printing part, a drying part, and a main device such as a guide roll and a rewinding part. The mechanical transmission section is similar to the transmission mechanism of the intaglio printing press and will not be described below.
(1) The paper feeding section places the printing materials to be printed, and the printing materials to be printed are smoothly and accurately fed to the printing section for printing according to the embossing characteristics of the printing machine, collectively referred to as the feeding section.
The paper feeding section of the flexographic printing press has two forms of sheet feeding and web feeding.
1 sheet feeder. Printing materials are single-sheet (sheet) forms, such as paper, cardboard, corrugated cardboard, curved cardboard, thin cardboard (E-type corrugated) and other boxes, box packaging and decorating printing, generally using a single automatic feeding device .
Flexible sheet feeding methods include jet feeding and vacuum suction feeding.
2 coil conveying device. The function of the coil conveying device is to steadily and properly input the roll-shaped printing materials to be printed into the printing portion to complete the graphic printing process.
In the printing process, the diameter of the coil gradually becomes smaller, so that the operating conditions change continuously, and the tension also changes continuously as the diameter of the coil changes.
The continuous changes in the tension of the web not only make it impossible to control the elongation of the easily stretched printing film, the conveying cannot be smoothly adjusted, but also has a serious impact on the printing color printing, so that the printing cannot meet the quality requirements.
The continuous changes in the tension of the web not only make it impossible to control the elongation of the easily stretched printing film, the conveying cannot be smoothly adjusted, but also has a serious impact on the printing color printing, so that the printing cannot meet the quality requirements.
The most important condition in the coil conveying process is to control the tension of the coil, so that the coil of the input printing unit maintains a stable degree of tension.
The basic structure of all kinds of different types of coil conveying devices is the same. They are composed of a conveying shaft, a tension control device, a coil axial adjustment device and various guide roller systems.
a. Roll feed shaft. That is, install the horizontal axis of the web to be printed. Conveyor shaft cone sleeve type and cylinder type two. The web conveying shaft of the low-speed flexographic printing press with a printing speed lower than 50m/min adopts a conical sleeve structure. When it is used, the steel mandrel is inserted into the paper tube in the middle of the coil, and both ends are fixed with a tapered sleeve, and then used. Nuts locked. The high-speed flexographic printing press uses a cylinder-type conveying shaft. When the cartridge shaft is used, the mandrel of the gas cylinder device is inserted into the middle of the coil and then the high-pressure air flow of 4 kg or more is input, so that the civil axis expands and expands, and the fixed grid is opened. The cylinder type conveyor shaft is easy to operate, saves time and effort.
b. tension control device. The role of the tension control device is to control the roll substrate without changing the diameter, maintain a certain degree of tension, and prevent the substrate from wrinkling, breaking and skewing.
Tension control devices are divided into manual controllers and automatic controllers. Low-speed flexographic printing presses commonly use manual controls to adjust the web tension.