The gravure proofs (hereinafter referred to as concave samples) are important production test basis for gravure platemaking companies and printing companies, and are the link between them. For the plate-making company, the concave shape is the result of the final combination of all their processes and is a reflection of the company’s overall strength. As the final basis for inspection of the final product within the platemaking company, the concave sample will be used to examine the manuscript's color, electrical effect, and some basic parameters of the plate roller. Printing companies tend to use concave samples as the basis for their acceptance of the printing company's version of the rollers, and as proofs of printing color correction in printing, they may even become signature proofs for printing company customers.
The gravure proofer is made by simulating the principle of a gravure printer. The ink is transferred to the substrate by a pressing action between the impression cylinder and the gravure plate roller to form a clear pattern. At present, more domestic use is Japan's GRAVO-PROOF concave prototype (hereinafter referred to as the Japanese machine) and the British JMHEADFORD's concave prototype (hereinafter referred to as the British machine). From the appearance point of view, the latter is simply a monster compared to the former. Japanese machines are very easy to operate and work efficiently. Most proofing jobs can be performed by one operator. British machine production efficiency is worse than that of Japan. Domestic companies have also imitated Japanese machines, but the effect is not satisfactory.
The gravure printing process requires that the proofs produced by the concave prototype be consistent with the results of the printing company's formal printing. Therefore, the construction of the concave prototype is not very different from that of the gravure printer. Some components are simply simplified. The main components include impression cylinders, register devices, plate cylinders, and transmission components. The impression cylinder is the most conspicuous component of the concave prototype. Theoretically, the impression cylinder is larger than the gravure plate cylinder so that the whole layout can be played.
The circumference of the impression cylinder of the Japanese machine is about 1.4M. For the general gravure printing gravure printing drum, the size of its perimeter is sufficient to cope with, but for the printing cylinder of the wood grain type decoration material whose circumference is more than 1.4M, It will look powerless, when the British machine can show its talents. The circumference of the British machine impression cylinder can reach about 4.5M.
The above situation led to the use of Japanese machines for packaging intaglio manufacturing companies, and the use of wood grain decorative materials special manufacturing companies for UK machines.
During proofing, the impression cylinder presses the substrate on the gravure surface by the pressure. When separating, the substrate absorbs the ink in the gravure network and forms an image. The quality of image transfer depends to a large extent on the rubber on the outermost layer of the impression cylinder. The hardness of the outer rubber on the gravure roll like a roller is generally 65 Shore A, which is slightly lower than the hardness of the impression cylinder used by the printing company. The pressure distribution produced by different hardness impression cylinders is different. The deformation of the cylinder with lower hardness under the same pressure is greater than that of the cylinder with large hardness. The elasticity produced by the deformation of the rubber can cause the substrate to deform elastically. A large area of ​​contact between the substrate and the gravure plate cylinder will attract more ink to the surface of the substrate. The hardness of the impression roller produced by the large deformation is small, the contact area with the gravure printing plate cylinder is small, the ink absorption of the substrate is also less. The rubber hardness is not the same, and the difference in the performance of the printed product in the high light part is also obvious, especially when the paper is proofed, it will produce a blank point in the high light part. This phenomenon has a great relationship with the V-shaped structure of the electronic engraving version. The British machine added an auxiliary electrostatic ink-absorbing device during the proofing, which can solve the high-light white point problem.
The register device occupies a very important position in the concave prototype, first of all it requires a very good registration accuracy. Once the registration error is greater than 0.2mm, the effect of the concave sample on the power subgraph will be greatly affected. In general, the first circle of the concave machine must not have registration errors. After each finish, a positioning stop below the impression cylinder will stop the impression cylinder at its last stopped position. When the impression cylinder is again close to the gravure cylinder, their squeezing points are the same as the last time. The position of the pinch points is the same, which ensures the accuracy of proofing. One of the major drawbacks of domestic imitation machines is that they are not accurate in terms of register accuracy.
Second, the registration device is required to have great flexibility. The length of the gravure plate cylinder is quite different, the short is only 400mm, and the platen lines on the plate cylinder do not have a fixed position. Some plate lines are on both sides of the plate cylinder, some are on the pattern, and some are simply There is no blanket line. As long as these plate cylinders can be mounted on the proofing machine, it is required to make the registration accurate and difficult. Flexible packaging materials will cause the printing material to expand and contract during printing. When the printing company is making a version of the drum, there is an increase in the number of colored assembly rollers. This will bring certain difficulties in proofing. Experienced operators can pass through the dots. Pouring out of proofs with high registration accuracy.
The Japanese machine's ink transfer is to transfer the ink to the printing material by pressure driven by the gravure plate cylinder to rotate the platen roller, the speed is generally about 30 meters/MIN, but the platen roller starts from the beginning to the last gravure cylinder The rate of separation is not the same because the substrate is tightly wrapped around the surface of the impression cylinder. Due to the inertia of the drum, the speed increases. The British machine completes the transfer of ink by pressure on the impression cylinder to drive the plate cylinder. The speed of the impression cylinder itself is kept constant. The speed of the substrate that is wrapped in the impression cylinder is the same from beginning to end. This solves the problem of the inconsistent rotation speed in Japan.
In the proofing process, the substrate is firstly wrapped on the impression cylinder, and then the proof is printed according to the printing characteristics of the plate cylinder and the printing sequence. The printing plate in the printing plate is generally transparent. When the sample is proofed, it is first played in a dark color. Then a light color is created on the basis of the coloring. For example, only the four-color electricity and the white version of the printing plate are normally blacked out. The edition will be backed up with a cyan version, a magenta version, then a yellow version, and finally a white version will be based on these four colors. Table printing version of its proofing order is the opposite of the printing plate. Most of the substrates of the plate printing plate are opaque and the proofing is performed from the light-colored version to the dark-colored version.
When proofing must also pay attention to the important factor of proofing ink. The gravure proofer does not have an ink tank. When the impression roller is in close contact with the gravure plate roller, the ink to be dispensed is manually poured onto the gravure plate cylinder. Because the speed of the proofer is not high, it is determined that the viscosity of the ink used for proofing should not be too low, otherwise it will cause a paste plate. The high-speed gravure printing machine can be opened to 160m/min under normal circumstances, the ink viscosity is about 16 seconds, and the viscosity of the ink used for gravure printing should be adjusted to about 20 seconds.
The slow speed of proofing, the large viscosity of the ink, and the low hardness of the rubber on the impression cylinder all contribute to the ink's full transfer to the substrate. The color of the proof is relatively bright.
If the proofing does not add ink to dilute the ink, it will make the printing company suffer too much, because it is very difficult for the printing company to print out a very vivid and bright color effect. It is necessary for the platemaking company to dilute the ink when proofing, so that the printing company will be much easier in color proofing according to the gravure proofs.
In the entire gravure platemaking process in which electric sculpture has achieved no film engraving, the department with the most human influence factors is the gravure proofing workshop. Different proofing ink dilute, different degree of punctuation, different pressure between gravure plate cylinder and impression cylinder, angle of blade, installation method and different pressure, distance between blade and embossing point are different. This kind of effect has an effect.
In the proofing process, according to the different substrates, the relevant parameters of table printing and inward printing are adjusted so that the perfect gravure proofs can be produced and the professional production level of the plate making company can be played.