The hottest section 4 flexographic printing

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Section IV flexographic printing

I. principle and characteristics of flexographic printing

1 Principle of flexographic printing

flexographic printing refers to a printing method in which the sample preparation and experimental methods are generally strictly stipulated in these standards. The ink is transferred by flexographic plate and ink roller. The layout structure of the printing plate is the same as that of the relief printing plate. The graphics and texts are raised (that is, the graphics and texts are higher than the non graphics and texts) on the same plane, and they are all printed directly

After the plate cylinder is loaded into the machine, the controlled ink of a certain thickness is evenly coated on the image and text part of the plate, and then under the action of printing pressure (given by the embossing cylinder), the ink layer of the image and text part is transferred to the surface of the substrate to form a clear printed copy of the image and text. Therefore, flexographic printing is a direct printing method using rubber plate or photosensitive resin plate and quick drying liquid ink

the transfer process of flexographic printing ink is similar to that of gravure printing. The ink transfer roller is directly placed in the ink tank. When the ink transfer roller rotates, the ink adhered to the surface is transferred to the roller dedicated to transferring ink, and then the roller evenly coats the ink layer on the surface of the image and text part of the printing plate according to the control thickness for printing. Since the ink does not need to be homogenized in the transmission process, the structure of the flexographic printing machine is relatively simple. The principle of flexographic printing is shown in the figure

2. Compared with other printing methods, flexographic printing method has the main characteristics of multi-purpose and economy

① the specifications of the printing machine are complete. Flexographic printing machines have many specifications, which can be selected according to the use of the printed matter, making the use of the substrate more convenient. Generally, it can print narrow strips as small as several hundred meters and substrates about several meters wide

② wide applicability. Flexographic printing has the advantages of convenient plate making, low cost, convenient plate loading, high printing machine speed and simple operation; A wide range of printing materials

③ high production efficiency. According to production needs, it can be multi-color printing on the front, and multi-color printing on the front and back at the same time; The printing plate has high printing resistance. These are creative advantages for improving productivity and reducing production costs

in recent years, the performance of photosensitive resin plates has been continuously improved and stabilized, greatly improving the resolution and point reproducibility of printing plates (up to 1%-95%); The application of modern high technology, such as electronic plate making, laser engraving and digital plate making, makes it possible for flexographic printing to realize multi-level manuscript color overprint; Laser engraving roller technology enables the number of engraving lines to reach a high precision of 1000l/in, so that the ink delivery can be accurately controlled, and 300l/in color layered manuscript can be printed, and its print quality is close to the level of offset lithography; The successful development and wide application of water-based flexo printing ink, UV (ultraviolet) flexo printing ink and uv/eb (ultraviolet/electron beam) flexo printing ink have led to the rapid development of flexo printing. Now, flexographic printing has become one of the main printing methods

3. Types of flexographic plates in the past, the materials for making flexible plates were mainly natural rubber and synthetic rubber. Therefore, the most sensible flexographic plates were hand carved rubber plates and cast rubber plates. Later, flexible photosensitive resin plates were developed, which rapidly improved the printing quality of flexible plates. Here, we mainly introduce the following flexible printing plates:

① hand carved rubber plates. Manual engraving is the simplest flexible plate making method. For simple live parts, you can use manual engraving to make printing plates, and it has the advantages of convenience, quickness and low cost. The state-owned guarantee company has strong manual carving technology, which requires the production personnel to have skilled skills and rich experience, and the printing plate quality is not as good as photographic plate making

② cast rubber plate. Natural rubber or synthetic rubber plate is heated and pressurized in the master mold. The advantages of casting rubber plate are: once the master mold is made, it is easy to produce rubber reproduction plate making, the plate thickness can be controlled freely, and a variety of rubber plates can be produced according to different inks and printing materials, such as isobutylene rubber plate, ethylene rubber plate, etc. Rubber plate is mainly used for packaging and wallpaper printing

③ flexible photosensitive resin plate. The flexible photosensitive resin plate is similar to the photosensitive resin plate used in relief printing in terms of chemical process of image and text formation, plate making process, reproduction performance, etc. the plate is mainly made of rubber resin. The washing method after exposure and the washing solution used are different from the photosensitive resin plate used in 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 of negative film, with good tone reproduction. When the number of lines is 60l/cm, it can reproduce 3% of the points. The plate making process does not need to be heated, so the size accuracy of the printing plate is high, the stability is good, the thickness accuracy is high, and the plate making time is shorter than that of the rubber plate. However, the flexible photosensitive resin version also has some disadvantages. Compared with rubber, it has insufficient elasticity, and the cost of making a replica version is high. It must also check its resistance to ink solvents in advance. Solid resin plates are mainly used for packaging and printing, while liquid resin sheets are mainly used for newspapers and periodicals and packaging and printing

④ seamless version. It is a version made by scanning the original image and using the image signal to control the beam to carve directly on the rubber roller. The production process is simple, but its reproduction performance still has problems. Another kind of seamless version is to cover the surface of the drum with flexible photosensitive resin, and laser engraving is still used when making it. Seamless version can print continuous graphics, such as wallpaper

II. Flexographic printing equipment

1 Classification of flexographic printing machines

flexographic printing machines are mostly multicolor, ranging from two colors to eight colors. According to the arrangement of printing units, it can be divided into three types: CO imprinting type, cascade type and series type

(1) Co imprint printing machine co imprint flexographic printing machine, referred to as Ci printing machine, also known as satellite printing machine. As shown in the figure

the main features of the co imprinting press are that each group of plate cylinders rotate around a large diameter central cylinder, and each takes the central cylinder as the imprinting cylinder to complete the printing process

in the whole printing process, the substrate covers the surface of the embossing cylinder, with good flatness and high chromatic accuracy between color groups. It is suitable for printing color prints of easy to stretch, wrinkled paper, plastic film, metal processing paper and other materials. However, the distance between each printing unit of the co imprint printer is short, the intermediate drying device is simple, and the printing speed is relatively slow

(2) the laminated printer is shown in the figure. Each printing unit of the laminated printer is independent of each other, forming its own printing unit and stacking vertically with each other. The distance between the printing units can be changed, and the intermediate drying device is convenient to place

the substrate creep testing machine of the laminated printing machine is connected in series between the impression cylinder and the plate cylinder of each printing unit, and the transfer of pictures and texts is completed by the printing pressure between the plate cylinder and the impression cylinder

the plate cylinder of each printing unit of the laminated printing machine is easy to exchange and install. When replacing or installing the plate cylinder, it does not affect the combined position of other printing units, which is easy to operate, high mechanical accuracy and strong printing adaptability

the laminated printing machine has a large number of colors, and changes the serial connection mode of the substrate. It can be printed on one side or both sides, which is more convenient and flexible to use

(3) series printing machine series flexographic printing machine is also called combined or seat flexographic printing machine, and its shape is similar to that of combined gravure rotary printing machine, as shown in the figure

each printing unit of the series printing machine undertakes the printing of a color plate, and the embossing cylinder and the printing plate cylinder cooperate with each other to complete the printing process. Each printing unit is arranged in a horizontal straight line. The combination of multiple printing units can complete color printing. The tandem printing machine can not only print with reel materials, but also print sheet (single sheet) materials. It is often used to print non plastic trademarks, newspapers, pieces of corrugated cardboard, commodity packaging materials, etc., and has unique advantages. In addition, it has the characteristics of good additionality and strong variability. For example, it can be matched with the gravure printing machine unit (one or several groups) as needed, and die cutting can be added at the rear of the printing unit: the guiding system of additional drum materials, the ink circulation system, the electrostatic treatment device of plastic film, etc., so as to enhance the surface function and decorative effect of the substrate, as shown in the figure

the wide bookshelf printing machine is a flexographic printing machine with special purposes

the main feature of the wide bookshelf flexographic printing machine is that it can m. Even more than 4m wide paper, plastic film, metal foil substrate for printing, and generally equipped with automatic loading and unloading device of substrate. The wide bookshelf flexographic printing machine can also be linked in series with laminating machine, folding machine, Slitter, rewinder and other required machines to carry out post-processing on the substrate, so that the substrate can meet the needs of commodity packaging

2. The main mechanism of flexographic printing machine

the inking mechanism of flexographic printing machine is relatively simple, but the surface structure of the ink transfer roller is more complex than that of the ink transfer roller (or ink transfer rubber roller) used in the printing machine used in other printing methods

flexographic printing machine is composed of transmission part, paper feeding part, printing part, drying part, guide roller and rewinding part to reduce the unstable friction force and other main devices. The mechanical transmission part is similar to the transmission mechanism of gravure wheel printing machine, which will not be described below

(1) the paper feeding part is a device that places the substrate to be printed and enables the substrate to be printed to be smoothly and accurately fed into the printing part for printing according to the stamping characteristics of the printing machine, which is collectively referred to as the paper feeding part

the paper feeding part of flexographic printing machine has two forms: sheet fed and web fed

① sheet fed device. The printing materials are in the form of single sheets (sheets), such as paper, paperboard, corrugated paperboard, curved paperboard, thin paperboard (E-type corrugated) and other boxes, boxes, packaging and decoration printing, which are generally transported by single sheet automatic paper feeding devices

there are two forms of flexible sheet feeding: jet feeding and vacuum suction conveying

② coil conveyor. The function of the coiled material conveying device is to input the drum shaped substrate to be printed smoothly and properly into the printing part to complete the graphic printing process

in the printing process, the coil diameter gradually decreases from large to small, so that the operating conditions continue to change, and the tension also changes with the coil diameter

the continuous change of coil tension not only makes the elongation of printing film easy to stretch uncontrollable and the transportation can not be adjusted smoothly, but also has a serious impact on the chromaticity of printing, so that the print cannot meet the quality requirements

the continuous change of coil tension not only makes the elongation of printing film easy to stretch uncontrollable and the transportation can not be adjusted smoothly, but also has a serious impact on the chromaticity of printing, so that the print cannot meet the quality requirements

during the transportation of coiled materials, the most important condition is to control the tension of coiled materials so that the coiled materials input to the printing unit can maintain a stable tension

the basic structure of various coil conveying devices is the same, which are composed of conveying shaft, tension control device, coil axial adjustment device and various guide roller systems

a. coil conveying shaft. That is, install the horizontal axis of the coil to be printed. The conveying shaft is divided into cone sleeve type and air cylinder type. Low speed soft printing speed lower than 50m/min

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