In the process of visiting customers, we often hear the same questions from customers, that is, what is the difference between the arrival of flexo and relief, and where is the advance of flexo compared with relief. According to our years of experience, especially in recent years, we have been in frequent contact with suppliers and printing factories of narrow width flexographer and satellite UV letterpress, and summed up the differences and characteristics between the two printing methods. The advantages of flexographic printing are introduced many times in various professional magazines in China. This paper simply introduces the differences between flexographic printing and relief printing in several aspects.

Flexographic printing and relief printing belong to relief printing in printing principle. In order to understand and distinguish them easily, people usually call the printing method of using liquid ink and flexographic printing plate as flexographic printing.

The traditional printing method of using paste ink and hard plate is called relief printing.

 ·Printing inking principle

The relief printing device is to transfer the even and thin ink layer to the plate surface as a whole by one or several inking rollers, and then print. The flexographic plate is transferred by the metal anilox roller in contact with the plate. The specifications of the anilox stick are fixed, so the ink transfer amount is also fixed. The ink is stored in the mesh hole before transfer. Because the mesh wall in the mesh hole is very thin, during the transfer process, the ink in each mesh hole is quickly leveled and forms an integrated ink layer on the printing plate when it contacts with the printing plate. Finally, the printing is carried out.

The two are different in inking principle. The relief plate is the contact between the soft rubber stick and the hard plate, and the inking amount depends on the thickness of the ink layer; the flexible plate is the contact between the hard metal anilox roll and the soft plate, and the inking amount depends on the specification of the anilox stick.

·Printing ink

According to the printing inking principle, letterpress printing ink is transferred to the printing plate in the form of a whole film, which requires a certain strength and joint force between the inks, otherwise the ink cannot form an ink film and transfer normally, so letterpress ink is a paste ink with a certain viscosity.

The transfer of flexographic printing ink depends on the rapid outflow and leveling of ink from the mesh, which requires the flexographic printing ink to have low viscosity and fast transfer speed, so flexographic printing ink is a liquid ink with low viscosity and good fluidity.

·Inking system

In order to transfer the paste ink to the plate evenly, there are complex ink conveying systems on the letterpress, including ink bucket, ink bucket roller, ink roller, ink roller, inking roller, etc., which are called long ink path systems. Only through the long ink path system, paste ink can form a uniform ink layer.

Flexible printing uses liquid ink, does not need a complicated ink conveying system, only needs an ink bucket roller to transfer the ink to the anilox stick or contact the anilox stick directly with the ink (gravure ink conveying method). The ink on the surface of the anilox stick is finally removed by the scraper and the ink is stored in the mesh. This kind of ink conveying method is called short ink path system. The short ink path system is characterized by simple mechanism, convenient adjustment and low material consumption.

·Adjustment of ink quantity

There are usually two ways to adjust the ink quantity in letterpress printing.

1. Adjustment of overall ink quantity:

(1) adjust and change the rotation angle or speed of the ink bucket roller, and change the ink transfer amount of the ink bucket roller.

(2) adjust the swing frequency of the ink roller to increase or reduce the contact time with the ink roller, so as to change the ink transfer volume of the system.

2. Adjustment of local ink volume: there are screws for local adjustment of ink volume on the ink bucket of letterpress, and a group of screws are evenly arranged along the axial direction. By adjusting the screw, change the gap between the ink bucket steel plate and the ink bucket roller. By changing the size of the gap, the ink amount of the corresponding position of the screw can be adjusted locally.

The anilox roll on flexographic press is actually an ink metering roll. The technical parameters are fixed, that is to say, the ink transfer is fixed. Only changing different parameters of anilox roller can change the amount of ink, so the amount of ink in flexography is changed by changing the anilox stick. Limited by the printing principle, flexographic printing can only adjust the overall ink, not the local ink.

 ·Printing plate

Although the plates of letterpress and flexography are made of photosensitive resin plates, the thickness, hardness and formula of the two plates are different, so the methods of making film and plate are different.

1. Thickness and hardness of plate

The thickness of the photosensitive resin plate for letterpress printing is generally less than 1mm, and the commonly used specifications are 0.8mm and 0.95mm. The thickness of flexible resin plate is generally between 1-5mm, and the plates with different thickness are selected for different products. In plate hardness, the hardness of the finished relief plate is generally about 90 degrees shore, while the hardness of the finished flexible plate is between 30 and 70 degrees. This is what people usually say, letterpress plate is hard, flexible plate plate is soft. But there is no absolute demarcation line between them, that is, there are soft plates in relief plate for field printing, and there are hard plates in flexible plate for dot printing.

2. Film production

Relief film is normal size, while flexible film needs compression deformation.

Relief plate and flexo plate are different in thickness, so the production method of the plate is also different. Letterpress printing plate can be divided into two types according to the press mode: flat printing plate and round printing plate. The plate material of flat printing is used for flat press printing. The size of plate material and film is the same without any deformation. Circular plate printing refers to the plate making for the plate installed on the plate cylinder. First, the plate and the film are absorbed on the photosensitive drum by vacuum. At this time, the plate is stretched on the upper surface of the photosensitive drum and compressed inside. In the case of plate deformation, print plate with ordinary film, so the plate after being made will not produce dot deformation when installed on the plate cylinder.

In the actual production of plate making, the printing factory will select the corresponding exposure roller according to different plate rollers. For example, Japan's Ko pack label machine is equipped with about ten plate rollers to match the plate rollers of different diameters.

Due to the thickness of the plate, it is inconvenient to use circular exposure, and there will be quality problems due to the lack of vacuum absorption. At present, the plate making is mainly flat printing. In order to ensure that the plate does not deform during printing, the film must be compressed and deformed in advance. The printing plate made of the deformed film can ensure that the pattern dot does not deform and the printing quality. The film deformation is related to plate thickness and roller diameter.

3. Arrangement of film color separation angle

In order to avoid moire, the screen angle of the flexographic plate should be separated from that of the screen stick. At present, the most commonly used angle of the anilox stick is 60 ° or 30 °, while the screen angle of the standard color separation on the printing plate is also 30 °. And 60? , so the angle between them must be staggered. As the angle of the anilox stick is fixed, the plate making company generally adopts the method of changing the same angle of the 4-color plate at the same time (each plate making company has its own standard for the changed angle), so as to avoid the overlap of the angle of any color plate with the anilox stick and achieve the purpose of eliminating the moire.

The relief printing does not use anilox roller, there is no moire problem, so the film color separation angle is arranged according to the standard angle.

4. Plate making

There are two differences between letterpress and flexographic plates in making:

-Plate making step: relief plate making step is main exposure - washing - drying - post exposure. In order to control the height and hardness of the plate, the flexographic plate is first exposed on the back, and then the main exposure is developed. In order to remove the viscosity of the plate surface and transfer the ink better, the process of debonding is added in the plate making process. After debonding, the plate is dried again, and finally the plate surface is hardened by post exposure. Compared with relief plate making, flexo plate making increases back exposure and debonding process.

-In plate making, the washing solution is different; the thickness, resin composition and formula of letterpress plate and flexographic plate are different, so the type of washing solution is different. In recent years, water washable flexographic materials have appeared in the market, but they are not widely used. The drying time of the two plates is different because of the different media. Generally, the drying time of the relief plate is from a few minutes to a dozen minutes, while the drying time of the flexible plate is from an hour to a dozen hours.

·Print pattern

1. Imprinting effect: the printing principle of letterpress and flexographic plate is the same, and there is imprinting effect phenomenon on printing pattern, that is, there are traces of ink being extruded and refluxed on the edge of pattern. This phenomenon appears as white center or white circle on the dot (especially under high pressure); slight ghosting can be seen on the edge of text or line.

The thickness and hardness of relief plate and flexo plate are different, and the deformation of plate is also different when printing, resulting in different impression effect. The pressure of letterpress printing is large, the effect of imprinting is obvious and the contrast is large; the pressure of Flexography Printing is small, the effect of imprinting is slight and there is transition (strictly speaking, the difference between letterpress and flexography pattern needs long-term practice observation and contrast to realize).

2. Field printing; the ink layer of letterpress printing is uniform. In flexographic printing, if water-based ink is used, traces of water marks will be found on the pattern. If UV ink is used, careful observation will also find that there is a uniform pattern on the surface of the field, which is the main difference between the two.

3. Printing color: the pigment solid content of flexographic ink is significantly less than that of letterpress ink, so the color concentration of letterpress ink is significantly higher than that of flexographic ink. Due to the bright colors of letterpress, there is no obvious difference between letterpress and flexography in printing quality, which is also the main reason why letterpress in Europe and Asia has been flourishing for a long time.

4. Ghost phenomenon, also known as glue back shadow, is a common phenomenon in letterpress field printing. The main reason is that the number of ink application is less than the number of ink supply, which mainly occurs in large-scale field printing. Because of the continuous supply of ink for the plate by the anilox stick in flexographic printing, there is no ghost phenomenon in flexographic printing.

We have analyzed the differences between flexo and letterpress from several aspects, some of which may be incorrect or even wrong. The publication of this article hopes to provide some useful help for the readers who are interested in this title.

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