Gravure products have always been known for their exquisiteness and fineness. From pictorials, pictures, plastic film packaging to RMB, they are all masterpieces of the gravure process. In addition to single-sheet gravure printing, gravure printing generally uses rotary printing, short ink path, positive single-blade inking, from the erosion of carbon paper, Bumeilan platemaking, electronic engraving platemaking to today's laser engraving platemaking , Gravure has gone through a long road of development.
Since the introduction of gravure printing machines from abroad more than 20 years ago, colleagues in the gravure printing industry have been analyzing and working hard to find solutions. A series of problems, such as inaccurate overprinting, knife silk, color difference, flying ink, etc., are being proposed every year, but the same problem is repeated every year. The loss rate of gravure remains high, generally around 10%. Many years ago, when the profit margin of the gravure flexible packaging industry was still around 30%, the loss of 10% had not attracted the attention of the industry. However, many companies have now reached the edge of low profit. When the Red Sea fighting is extremely fierce, this The loss caused a shock in the industry. Therefore, it is necessary to carefully analyze the quality problems of gravure products, explore the main causes of the quality problems, and then analyze the control points of the gravure yield loss.
Overprint accuracy
The key to the accuracy of rotogravure overprinting is to ensure the stability of the web tension. Almost all register tracking devices, from the early IC470 to DT860, and even the later DT950, the premise of normal operation is that the tension of the system is stable. The photoelectric detection device converts the overprint mark printed on the edge of the film into a pulse signal and inputs it to the computer, compares the pulse with the standard setting in the computer, and calculates the variation, which is proportional to the size of the overprint error. The computer converts it to a voltage value, after processing, drives the motor and mechanical actuator to correct the error, and displays the size and direction of the error on the display screen. Since this automatic adjustment process is a cyclic and uninterrupted dynamic balancing process, the error must be corrected in time and quickly, and the system should not be unstable due to overshoot during correction. Therefore, the photoelectric device that detects the error The conduction angle of is generally 5 ° to 7 ° out of 360 ° rotation of the plate roller, that is, 1/6 time within one rotation of the plate roller to perform the operation. There are two mechanisms for correcting errors. One is the longitudinal roller adjustment mechanism. By adjusting the up and down position of the roller, the front and back of the coil to the imprinting position can be changed. The other is a servo mechanism that changes the phase of the plate roller. Adjust the printing plate. From the point of view of the directness and speed of control, the servo mechanism is obviously superior to the adjustment roller mechanism, but the servo mechanism has certain technical difficulties, and improper design adjustment may make the system unstable. Since the gravure register is based on the precise positioning of the roll feed, the fluctuation of the roll tension will affect the accuracy of the overprint. The factors that cause the tension fluctuation of the traditional gravure printing machine generally have the following six aspects.
(1) Rewind and unwind
The change in diameter causes the change in coil tension.
(2) Changes in tension during acceleration and deceleration.
(3) The change of the tension of the printed substrate itself, such as uneven thickness of the roll material, with ruffles, and a large change in tensile strength in the vertical and horizontal directions, or the roll core is not round, causing the roll to jump.
(4) Changes in the web during the printing process, such as the film is deformed by heat when it is dried, and the traction tension is relatively large, resulting in the film being stretched, or the paper absorbs too much water when inking, and shrinks due to heat during drying, the material Change itself.
(5) There are deviations in the manufacturing accuracy and assembly accuracy of the main components of the unit-type gravure printing machine, such as the base, wallboard, and guide rollers. For example, the flatness and straightness of the base assembly, the verticality of the wall panel and the base assembly, the horizontality of the assembly of the guide rollers and the parallelism between them, their respective runout deviations, mass dynamic and static balance deviations, etc. If the error exceeds the standard, when the coil material runs on many guide rollers, the tension will change slightly. The gears in the transmission should theoretically achieve a gapless precision transmission to ensure that the plate rollers of each color group run synchronously, but because of the gear transmission, especially the normal wear of the equipment after more than 3 years of operation, the gap increases and the printing The transmission synchronization error caused in the process will inevitably change the tension of each printing unit.
(6) The pressure of each color group in the printing process of the unit-type gravure printing machine will affect the running speed of the coil when the material passes through. The theoretical synchronization speed will actually have errors. If the pressure of each color group is different, but the same method of increasing the diameter of the plate roller is used to increase the linear speed of each color group, it will also affect the tension of the entire system.
The unit type considers most of the above factors. However, the compensation for material deformation and the compensation for the loss of mechanical synchronization speed caused by the different pressure of each color group. Due to the difficulty of establishing a mathematical model, the basis of computer automatic adjustment is not solid. Therefore, the control of tension fluctuations is still very limited. The difference between the rotary gravure printing machine and the sheet-fed gravure printing machine in the registration is that the latter ensures the correct positioning of the paper under the action of the front and side rules, and then prints when the paper is at rest, while the former is to roll Tubular paper is dragged into place under constant tension. When printing under dynamic conditions, the accuracy of overprinting is different. The overprint tolerances specified in the industry standard CY / T6-91 "Quality requirements and inspection methods for intaglio printing" are shown in Table 1, and the overprint tolerances in the national standard GB7707-87 "Intaglio printings" are shown in Table 2. The industry standard is for books and periodicals, and the national standard is for film prints. Comparing these two data, the effect of tension on overprint accuracy can be seen at a glance.
So, does the tension of the roll material basically not affect the accuracy of the overprint? Yes, a typical example is a satellite flexo printing machine. In the equipment of this structure, under the action of a rubber pressure roller, the roll material is closely attached to the surface of a metal center impression cylinder with a diameter of 2 to 3 meters, and the wrap angle is more than 85%, and is stamped with the center The drum runs synchronously. There is constant temperature cooling water in the center imprint cylinder to ensure that the temperature on the surface of the cylinder does not exceed 40 ° C, so that the film is not deformed. The distance between adjacent color groups is very short, generally around 0.8 meters. There is no loss of mechanical synchronization speed between the color groups, and no compensation is required. According to the principle that the tension of the web in rotary printing is basically the difference between the front and back linear speeds, the satellite flexo printing machine is actually printed under the condition of zero tension in the printing unit, which avoids the overprinting caused by the change of the tension in the film printing No, as long as the accuracy of the plate roller drive can guarantee the accuracy of overprinting. The registration accuracy of the satellite flexo printing machine that used traditional gear drive 10 years ago can reach about ± 0.1mm. The servo-type satellite flexo printing machine introduced in recent years has an overprint error of ≤0.07mm.
Therefore, the overprinting accuracy of gravure printing is affected by tension fluctuations and material deformation, and has inherent shortcomings. If the printing substrate is pre-shaped, such as BOPET stretched in both directions, the overprinting accuracy is better. If there is no shaped PE, overprinting It is more difficult; even for the same material, such as BOPP, thicker is not easy to deform, and thinner is easy to deform, the accuracy of its overprint is completely different. Judging from the structure of the current gravure printing machine, it will take time to significantly reduce the loss due to overprinting errors.
Table 1 The industry standard CY / T6-91 "Gravure printing quality requirements and inspection methods" on overprint error provisions Click here to view all news pictures
Table 2 Provisions on Overprint Errors in National Standard GB7707-87 "Gravure Decorate Prints" Click here to view all news pictures
Knife wire
The knife wire is a common fault of gravure printing. According to the different parts of the knife wire appearing on the product, it can generally be a bright knife wire and a hidden knife wire. The definition of the knife wire in the national standard is: abnormal stripe ink lines appearing in the direction of the drum rotation caused by the bad scraper. The English expression is Doctor blade strakes. This kind of knife wire appears at the edge of the gravure picture and text. It is the ink line drawn by the ink in the depressed picture and text. We generally call it the knife wire. In gravure printing companies, we often see a bamboo stick clipped on the captain's ears. If a knife wire is found during the inspection, the bamboo stick is used to pick it up on the scraper. There is another kind of knife wire. The definition in the national standard is: comet-shaped stripes caused by dust particles, ink knives or paper scraps on the wall of the net, which is expressed in English as Comet streak. This kind of knife wire appears in the place where there should be no ink on the surface of the chrome plate of the gravure, and it is not limited to a certain point in the axial direction of the plate roller. It appears intermittently, sometimes sometimes. When handling this type of wire, the operator generally retracts the scraper and then presses it back up. Sometimes this treatment will be effective, but most of the time the operator will be at a loss. Some people in the enterprise also call this kind of fault as ink channeling, which means that the squeegee does not wipe the ink, and the ink rushes out under the squeegee. Of course, this kind of ink scraping is different from the ink scraping caused by the blade angle and pressure not being adjusted properly. The latter causes a problem of ink scraping. From the surface of the plate roller, the ink is scraped, but in the product You can clearly see that the film is lightly stained. The hidden knife wire is different. There is no ink on the surface of the plate roller after scraping, and there is no trace of dyeing on both sides of the film, but the ink should not appear on the surface of the plate roller. It becomes a wire on the film.
The reasons for the first type of knife wire are generally: ①unusual particles (precipitation or precipitation) in the ink; ② the presence of inorganic impurities; ③ the impact of poor production environment; ④ the ink viscosity is too high; ⑤ the scraper is defective; ⑥ the scraper The installation angle is incorrect; ⑦ The surface finish of the plate roller and the finish of the mesh hole are not enough; ⑧ The coordination of the ink surface tension with the plate roller is unreasonable.
The main reasons for the second type of knife wire are: ①The data of the CY / T9 industry standard for abnormal radial runout gravure of the plate roller is ≤0.03mm, and the author believes that the standard of single-sheet publishing gravure is used in rotary gravure , The data should be slightly higher; ② the surface finish of the plate roller, the relevant data of CY / T9 is Ra ≤ 0.4; ③ the rigidity of the fixed blade holder and the blade; ④ the ink line leaking from under the blade.
There is also an "active particle hypothesis" in the industry about the generation of knives. It is believed that: ①During the processing and use of ink, resin and pigments break molecular chains under the external force of grinding, impact and stirring, generating polar ions. Or groups, ie active particles; ② the friction between the blade and the plate roller produces more chromium ions on the chrome-plated surface of the plate roller; ③ the active particles are continuously generated and combined during the printing process and adhere to the plate roller, accumulating to a certain extent It will burst out when measured; ④ About 2/3 of the active particles are transferred out, and the remaining part continues to combine and accumulate; ⑤ When foreign particles become adsorbents or nucleating agents, the reaction speeds up, resulting in the appearance of denser filaments.
Therefore, the conclusion of this assumption is that the wire produced in this case is not produced by the scraper, but is the product of the internal chemical reaction in the printing process. The "active particle hypothesis" is actually that the occurrence of such a fault cannot be measured by physical methods, and it is difficult to solve it completely.
The author believes that the cause of the gravure cutter wire can be measured or checked. For the first type of wire, whether it is dust or impurities in the ink, or precipitates, it can be detected by filtering with a certain mesh screen. This detection method is also applicable to the wire formed by the dust that is adsorbed and condensed into excessively large particles due to excessive static electricity and stuck between the scraper and the printing plate. Whether the viscosity of the ink is suitable can be judged by measuring with a viscosity cup. As for the installation of the scraper, it can be solved by visual inspection and manual adjustment, which is not difficult. As long as the quality of the printing plate is retested according to the factory data of the plate-making factory, it can fully grasp the quality. As for the matching of ink tension with the plate roller, it mainly depends on the accumulation and analysis of historical data. The surface tension of the ink is mainly related to the solvent used. Generally speaking, the parameters of the ink and the solvent have not changed, and the tension coordination will not cause problems. , Generally in ink. It should be noted that inks and solvents are chemical products, and printing companies may not be very familiar with them, so they may sometimes make mistakes when purchasing or receiving, so the quality control department of the enterprise should formulate corresponding test methods and test data to Ensure the accuracy of such materials; if problems are found, the simple solution is to switch to them. Of course, you can add some additives to change them. Therefore, the above conclusion of finding the cause of the cutting wire is: as long as abnormal data is found, there will be a corresponding solution.
For the second type of wire, it is even more necessary to investigate the data and the relevant data of the printing press and the plate roller. According to the author's understanding of the front line of gravure for many years, front-line employees generally do not measure these data, so when the relevant data is problematic, they cannot come up with corresponding solutions. In fact, the generation of this kind of knife wire is precisely that the beat of the plate roller is not synchronized with the beat of the blade. If we use the elasticity of the blade or the roughness of the plate roller to make the blade jump with the plate roller, so as not to cause the ink from The purpose of the leakage under the scraper, thus blocking the loophole of the ink line coming out from under the scraper. As for the "active particle hypothesis", the author only wants to ask one question: why flexo printing that also uses solvent-based inks has never encountered the problem of knives?
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