The difference between flexo printing and letterpress printing
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Both flexographic printing and letterpress printing are in the form of letterpress printing. In order to facilitate understanding and differentiation, it is customary to use liquid ink and soft printing plates as flexographic printing. Traditionally, paste inks will be used. The printing method of hard plates is called letterpress printing. This article only briefly introduces the "difference between flexo and letterpress printing" in several aspects.
First, the principle of printing ink
The relief printing apparatus transfers the uniform, very thin ink layer in a unitary form to the surface of the printing plate by one or several inking rollers, and then performs printing. The flexographic plate is made by a metal anilox roller that is in contact with the printing plate to transfer ink. The specification of the net stick is fixed, so the amount of ink transfer is also fixed. The ink is stored in the cell space before the transfer. Since the mesh wall in the cell is very thin, during the transfer process, the ink in the cell is quickly leveled and the ink layer on the plate is formed integrally when the ink is in contact with the plate. Finally, printing.
The two are different in the principle of ink. The letterpress is a soft glue stick that is in contact with a hard plate. The thickness of the ink layer changes the amount of ink. The flexographic plate is a hard metal anilox roll that is in contact with a soft plate. The size of the stick changes the amount of ink.
Second, printing ink
According to the principle of printing ink, the letterpress printing ink is transferred to the printing plate in a monolithic film shape, which requires a certain strength and coupling force between the inks, otherwise the ink cannot form an ink film and is normally transferred, so the letterpress ink has a certain viscosity. Paste ink.
The transfer of flexographic printing ink relies on the rapid outflow and leveling of the ink from the cell, requiring the flexographic ink to have a low viscosity and a fast transfer speed, so the flexographic printing ink is a low viscosity, fluid liquid ink.
Third, the inking system
In order to evenly transfer the paste ink onto the printing plate, the letterpress printing machine has a complicated inking system, including; ink fountain, ink fountain roller, ink transfer roller, ink transfer roller, ink roller, ink roller, etc., called long ink. Road system. Only through the long ink system, the paste ink can form a uniform ink layer.
Flexographic printing uses liquid inks, which do not require a complicated inking system. Only one ink fountain roller is used to transfer the ink to the netting stick or the netting stick is directly in contact with the ink (gravity inking method), the surface of the netting stick The ink is finally removed by the squeegee and the ink is stored in the cell. This type of ink transfer is called a short ink path system. The short ink path system is simple, easy to adjust, and low in material consumption, which is the biggest feature of flexographic printing.
Fourth, the adjustment of ink volume
There are usually two ways to adjust the amount of ink in letterpress printing;
1. Adjustment of the overall ink volume:
(1) Adjust the rotation angle or speed of the ink fountain roller to change the ink transfer amount of the ink fountain roller.
(2) Adjust the swing frequency of the transfer roller to increase or decrease the contact time with the ink fountain roller, thereby changing the ink transfer amount of the system.
2. Adjustment of local ink amount: The ink fountain of the letterpress printing machine has screws for partially adjusting the amount of ink, and a set of screws are evenly arranged along the axial direction. The gap between the ink fountain plate and the ink fountain roller is changed by adjusting the screw. By changing the size of the gap, the amount of ink at the position corresponding to the screw is locally adjusted.
The anilox roller on the flexographic printing press is actually an ink metering roller. The technical parameters are fixed, that is, the amount of ink transferred is fixed. Only the anilox roller that changes the different parameters can change the amount of ink infusion, so the change in the amount of flexographic printing ink is achieved by changing the netting stick. Limited by the printing principle, flexographic printing can only perform overall ink adjustment, and local ink adjustment cannot be performed.
Five, printing plate
Although both the relief and flexographic printing plates are made of a photosensitive resin plate, the two plates differ in thickness, hardness and formulation, so they are different in the production of the film and the plate making method.
1, the thickness and hardness of the plate
The thickness of the photosensitive resin plate for letterpress printing is generally less than 1 mm, and the commonly used specifications are 0.8 mm and 0.95 mm. The thickness of the flexographic resin plate is generally between 1 and 5 mm, and different printed materials are selected from different thicknesses. In terms of plate hardness, the hardness of the finished letterpress printing plate is generally about 90 degrees Shore, while the hardness of the finished flexographic printing plate is between 30 and 70 degrees. This is what people usually say, the letterpress plate is hard, and the flexographic plate is soft. However, there is no absolute dividing line between the two, that is, the flexographic printing plate also has a soft printing plate suitable for field printing, and the flexographic printing plate also has a hard printing plate suitable for dot printing.
2, the production of film
The relief film is of normal size, while the flexographic film requires compression deformation.
The relief and flexo plates differ in thickness, so the plates are made differently. The letterpress printing plate is divided into two types: a flat printing plate and a circular printing plate according to the printing method of the printing machine. The plate of the flat plate is used for flat-press printing, and the plate and the film are the same size without any deformation. The circular printing plate refers to the plate making for making a printing plate mounted on a plate cylinder. First, the plate and the film are vacuum-adsorbed on the photosensitive drum, at which time the plate is elongated on the upper surface of the photosensitive drum and compressed inside. In the case where the plate is deformed, the ordinary film is used for printing, so that the finished plate is mounted on the plate cylinder without deformation of the dot.
In the actual production of plate making, the printing factory will select the corresponding exposure roller according to different plate cylinders. For example, the Japanese version of the Ko-pack labeling machine is equipped with more than ten drying cylinders to match different diameters. Plate cylinder.
Flexographic plate making, due to the thickness of the plate, the use of circular exposure is not convenient, the lack of vacuum adsorption will have quality problems, the current plate making is basically based on flat printing. In order to ensure that the printing plate is not deformed during printing, the film must be pre-compressed and deformed. The printing plate made by using the deformed processed film can ensure that the pattern dot is not deformed and the printing quality is ensured. The amount of deformation of the film is related to the thickness of the plate and the diameter of the drum.
3, the arrangement of the color separation angle of the film
In the flexographic color dot printing, in order to avoid the phenomenon of moiré, the screen angle of the printing plate should be separated from the angle of the netting stick. At present, the most common angle of the netting stick is 60° or 30°, and the screen angle of the standard color separation on the printing plate is also 30° and 60°, so the angle between the two must be staggered. Since the angle of the netting stick is fixed, the plate making company generally adopts the method of changing the same angle of the four-color printing plate at the same time (the changing angle has its own standard for each plate making company), avoiding the angle of the same color plate with the same texture. The sticks overlap to achieve the purpose of eliminating the moiré.
However, the relief printing does not use an anilox roller, and there is no problem of moiré, so the separation angle of the film is arranged at a standard angle.
4, the production of printing plates
There are two differences between the letterpress and flexographic plates:
- Plate making step: The steps of the letterpress plate making are the main exposure-flush-dry-post exposure. The flexographic plate is used to control the height and hardness of the plate, first the back exposure, then the main exposure rinse. In order to remove the viscosity of the surface of the printing plate and better transfer the ink, the de-bonding process is added in the plate making step, after de-bonding and then drying, and finally the post-exposure hardening of the printing plate surface. The flexographic plateau adds back-exposure and de-bonding processes compared to the relief plate.
- Different in the plate-making solution: The thickness of the letterpress plate and the flexographic plate are different depending on the resin composition and the formulation, so the type of the plate-washing solution is also different. The letterpress plate washing liquid uses distilled water as the cleaning medium, and the flexographic plate-washing medium is a specially formulated solvent (in recent years, water-washable flexible plate materials have appeared on the market, but it has not been widely used). The two plates use different media and the drying time is different. Usually, the printing time of the letterpress printing plate is from a few minutes to ten minutes, while the drying time of the flexographic printing plate is from one hour to ten hours.
Six, printing patterns
1. Imprinting effect: The printing principle of the relief plate and the flexographic plate is the same, and there is an embossing effect phenomenon on the printed pattern, that is, the edge of the pattern has traces caused by the ink being extruded and reflowed. This phenomenon appears as a white or white circle at the dots (especially when the pressure is high); a slight ghost is visible at the edges of the text or lines.
The thickness and hardness of the relief and flexographic printing plates are different, and the deformation of the printing plates is different when printing, which ultimately results in different embossing effects. The letterpress printing pressure is large, the embossing effect is obvious, and the contrast is large; the flexographic printing pressure has a small embossing effect and a transition (strictly speaking, the difference between the letterpress and the flexographic pattern pattern requires long-term practice observation and contrast to be realized).
2, solid printing: letterpress printing ink layer is uniform. For flexographic printing, if water-based ink is used, traces of water marks will be found on the pattern. If you use UV ink to observe it carefully, you will find a uniform texture on the surface of the ground. This is the main difference between the two.
3. Printing color: The pigment solid content of the flexographic ink is significantly less than that of the letterpress ink, so the color density of the letterpress ink print is significantly larger than that of the flexographic print. Because the letterpress prints are brightly colored, there is no significant difference between the print quality and the flexographic plate. This is also the main reason why the letterpress printing in Europe and Asia is enduring.
4, ghost phenomenon: also known as back to the shadow, is a common phenomenon in the printing of the letterpress, the main reason is that the amount of ink is less than the amount of ink, mainly in large-scale field printing. Since the moiré in flexographic printing continuously supplies ink to the printing plate without interruption, ghosting does not occur in flexographic printing.
Above we have analyzed the difference between flexo and letterpress in several aspects, some of which may be incorrect or even wrong. The publication of this article hopes to provide some useful help to readers interested in this title.