Proofing is a very important process in the prepress platemaking process. Proofing can check the production quality of the previous processes. At the same time, proofing proofs are also the standard for color tracking for a large number of subsequent printings. Therefore, proofing plays an important role, which is an important part of the quality management and control of each plate-making company.
There are many factors that affect the quality of proofing, such as: imposition, printing, developing, inking, wiping, solid density, dot increase, maintenance and maintenance of the proofing machine, environmental temperature and humidity, etc. No matter which of these factors has a problem, it will have a great impact on the final proofing results. Therefore, only the standardized and data-based management and operation can make the copy and restoration of the dots meet the requirements during the proofing process, and provide qualified sample proofs for mass printing. Below I will discuss with readers how to effectively control the main factors and related data during the proofing process.
1. Imposition, printing and developing
Proofing imposition is very different from printed imposition. The printed imposition must be assembled in the folding order according to the binding method, and the crosshairs and corners of part of the film should be cut off. Therefore, the imposition personnel can only easily find the edges or lines and color blocks of the image on the film. Align and register the discriminating place. The imposition of proofing does not need to consider the problem of folding hands, and the cross and corner lines on the film will also be saved. As long as the cross lines and corner lines of the films of various colors are aligned during imposition. Generally speaking, the color sequence of the proofing determines the color sequence of the imposition. Because this can improve work efficiency. When fighting the first color, the film should be placed horizontally, not tilted, and the four corners should be glued together with transparent tape. The other three color versions are aligned with them on this basis. The standard of registration is that the cross line and corner line of the upper and lower films are not visible to the eyes.
The control of printing and developing is realized through the UGRA control strip. The UGRA control bar has five functional sections, each with its own different role. as the picture shows:
Section I: Used to control the development of the printing plate, and also to evaluate the exposure of the printing plate. The pass is graded from the third level, and the grade is unclear from the ninth level.
Section II: The micron coil is used to control the exposure time. The pass is to start to clearly distinguish the micrometer coil at 12μm.
Section III: The dot section is divided into 10 blocks from 10% dots to 100% field. It is used to check the exposure and development, and is also used as the standard for measuring the printing characteristics. 2% of the outlets appear qualified, and 3% of the outlets are clear.
Section IV: Fine mesh lines along different angles can be inspected for dot deformation and ghosting.
Section â…¤: Fine dots are divided into positive images 0.5% to 5% and negative images 95% to 99.5%, used to check whether the dots are reduced during the proofing process.
Although the UGRA control strip can be used to control the exposure and development more accurately, in some subtle places, the operator needs to make appropriate adjustments based on experience to optimize the reproduction and restoration of the dots. For example, adjust the suction time for plates of different thicknesses, adjust the temperature and speed of the developing machine, and clean and maintain the glass of the printing machine.
Second, proofing
Proofing machines are very different from printing machines. The printing machine uses the "round pressure round" printing method, while the proofing machine uses the "round pressure flat" printing method. In addition, the degree of automation of the printing machine inking and registration is very high. Proofers are mostly adjusted by the operator according to personal experience, and the registration of each color version is also carried out by hand. Although during the proofing process, the operator must also rely on the density meter to continuously measure the field density and the dot density and dot increase at 80% to adjust the amount of ink supply and the balance of water and ink, but the ink process must still use an ink shovel Shovel the ink onto the proofer roller. Therefore, the quality of the proofing is closely related to the level of the operator.
The field density value is the most important technical parameter for controlling the copying and restoration of network points. Different companies have their own different technical indicators and error ranges. This technical index is determined by each company according to its own specific situation. Generally, the solid density of each color version is Y: 1.30, M: 1.45, C: 1.50, K: 1.80. As for the margin of error, the European standard is ± 0.07. It is said that the error range of a domestic company is controlled within ± 0.05.
As with printing, proofing control of water-ink balance is also very important. The principle of printing is essentially the principle of oil-water incompatibility: the dots on the PS plate exposed to form an image are oleophilic and not hydrophilic, and the unexposed blank parts are hydrophilic and oleophilic. Therefore, maintaining a good water-ink balance is essential. Too little water wiping will result in a dry plate, which is reflected in the proofs as dark levels and gradation; too much water wiping will cause the dots to deform, and the dots on the printed sheets will appear "hollow dots." So, how to control the amount of water? In general, the less water you wipe, the better. The mastered scale is that there is no water on the plate after the water roller passes.
The technical parameters of fountain solution are often overlooked in the quality control of proofing. The technical parameters of fountain solution are mainly conductivity and pH. Their standards are: electrical conductivity 800-1200, pH value 4.8-5.2.
Proofing quality control also involves adjustments to the proofing machine. The contents of adjustment mainly include the following:
①The pressure of the rubber roller is ± 0.03mm (measured once every 120 ° rotation, and the average value is taken), that is, the thickness of the blanket + pad thickness = (2.286 ± 0.03) mm.
②The platen pressure (including PS plate thickness) is + 0.12mm ～ + 0.15mm, that is, the surface of the PS plate is higher than the height of the guide rail.
③Paper table pressure (including paper thickness) is + 0.15mm ～ + 0.18mm, that is, the surface of the paper is higher than the height of the guide rail.
â‘£ There are two rollers on each side of the movable frame. When the frame is moved to the side of the ink leveling system and the machine is not pressurized, the pressure of the roller and the lower rail surface should be rotated just by hand force. Too tight and too loose need to be adjusted.
⑤ Ink leveling system: the width of the ink stick on the platen of the inking roller and the width of the ink stick on the paper table should be uniform.
3. Maintenance and maintenance of the proofing machine
In the process of proofing, the operation of the proofing machine is the most important. Of course, the working state of the proofing machine is also very important. Only by making the proofer in the best working state can we ensure that the outlets maintain good quality during the transfer process, and finally obtain qualified proofs. Because the maintenance and maintenance of the proofing machine is a continuous process, it is necessary to establish a set of practical maintenance procedures, and make it standardized and standardized.
①Maintenance of the ink roller The dot change of the proofing proofs will deteriorate as the ink roller of the proofer hardens. The hardening of the ink roller means that the roller has been used for a long time, and its surface will become light and hard until a "crystalline layer" is formed. When the crystallization of the roller surface is severe, its ink transfer performance will be affected, and the water-ink balance during the proofing process will be difficult to maintain. Especially for magenta ink, due to the decrease in water absorption, it will cause the phenomenon of hollow dots. In this way, the area of ​​the sample dots will be smaller than the dots on the film, and the color will become lighter. That is, the saturation drops. In some examples, when the dots of one color change, it will cause the color tone of the sample to deviate from the expected effect, and the color tone of multi-color overprinting will change more. This shows how important it is to maintain the ink roller. In addition to the regular daily cleaning procedures, there are also specific maintenance measures. That is, every two weeks, the ink roller of the proofer is taken down to "rest", and the spare ink roller is replaced. Only in this way can the ink roller be kept in good working condition, so as to obtain satisfactory dot reduction quality.
â‘¡Maintenance of blankets Blankets are the medium that transfers dots from printing plates to paper. Its performance directly affects the delivery quality of the outlets. After the blanket is used for a long time, chemical residues will adhere to its surface, making the blanket less elastic. This will make the edges of the dots transferred to the paper not sharp enough. Therefore, after working on the blanket for a period of time, let it come down to "rest" to avoid excessive fatigue.
â‘¢ The daily maintenance and maintenance of the proofer itself is generally composed of mechanical parts and electrical parts. For the mechanical part, it is necessary to regularly add lubricating oil to each fixed oil hole and the transmission part (such as a chain) on the surface to avoid excessive wear; while for the electrical part, it is mainly to check and ensure the smoothness of the gas path and maintain Cleaning of the panel surface, etc. During the proofing process, the ink black on the ink roller and blanket should be cleaned before adding other colors of ink. Otherwise, the remaining ink will be mixed with the added ink and affect the quality of proofing. In addition, the dust and paper powder near the plate and the rules should be wiped off on time, because too much accumulation will affect the registration accuracy.
4. Environmental factors
Among the environmental factors, temperature and relative humidity mainly affect the quality of proofing. If the temperature is too high or too low, the water rubbed on the PS plate will evaporate too quickly or too slowly, thereby destroying the water-ink balance. Too large or too small relative humidity in the environment will destroy the water-ink balance, and even produce ink emulsification, resulting in the production of hollow dots. In addition, the more serious is that the temperature and humidity instability will deform the paper, resulting in a decline in registration accuracy. Therefore, it is very important to maintain constant temperature and humidity in the proofing workshop. Generally speaking, the ambient temperature should be 22 ℃ ～ 23 ℃, and the relative humidity is about 50%.
V. Conclusion
There are many factors that need to be controlled to improve the quality of proofing. The above is just a brief analysis and introduction of some main control methods and empirical data. There are many subtle factors that are not listed one by one. In the proofing process, often a small mistake may lead to the generation of waste products. Therefore, a complete and practical management system and operating procedures should be formulated for the specific situation of the company or the department, so that the proofing can be standardized and digitalized. Only in this way can the quality of the proofing be stabilized on a high level.
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