There are at least two situations where digital proofing technology must be used: first, the final result of the prepress operation is directly recorded on the printing plate, which is known as computer direct plate making (CTP); the second case is that the prepress preparation results are directly in Output on digital printing system. In the above two cases, the step of printing the film after printing is skipped, and there is no reason to use the proofing process of simulated offset printing.
1. Digital Proofing and Digital Printing
There is a principle difference from computer to proofing and digital printing, the only difference is the output speed and output purpose. The digital graphics and text output aimed at proofing uses a technology similar to digital printing. Its focus is to check how the results of the digital job flow will be displayed during actual printing, as a technical basis for mass printing. Because of this feature, the speed requirement for digital proofing equipment is not the main factor, the key is the gap between the results of digital proofing and the actual printed matter. Through years of hard work, with the participation of the color management system, the results of digital proofing are getting closer to traditional offset printing, and are accepted by more and more printing workers.
Digital proofing is the key technology to support the overall digital workflow of CTP workflow printing. It not only involves customers, but also affects the printing shop. Both customers and printer operators need to be trained in the basics of various digital proofing technologies. They should be made aware of the characteristics of the copy technology used in different digital proofing processes, as well as the screening technology determined by the copy characteristics and the corresponding color copy requirements. The degree and limitation of the number of screens can be achieved. Operators of printing presses must abandon the analog half-tone proofing process and tell them that even if the traditional printing process is used to achieve the same effect as the digital proofing, it is actually not difficult.
2. Features of CTP workflow
The CTP workflow omits the process of outputting to film, and the phenomenon of expanding the dots of the color images recorded by the imagesetter will no longer exist. Therefore, an important feature of the CTP workflow is that the network expansion is much smaller than that of film recording and reprinting, so the output curve (transfer function) when the page is recorded on the imagesetter cannot be used; the printer operator must also understand the CTP printing The difference between the version and the PS version requires the use of different water and ink balance adjustment parameters. When planning to use a metal CTP plate in the workflow, the printing conditions also change due to the decrease in the dot expansion value, which will affect the ink density setting and the gradation curve of the image. Compared with the traditional workflow, bookbinding workers need to be guided so that the large format that is discharged digitally can meet all printing and binding requirements.
The key problem to realize the CTP process is to solve the digital proofing. Because once the film is removed from the workflow, digital proofing is imperative. Theoretically, the digital proofing results should reflect the same halftone dot shape, screen line number and layer change rules as the printing process. However, most digital proofing methods usually cannot simulate the halftone dot properties of actual printing, because some digital proofing devices do not use halftone screening technology to simulate continuous adjustment of the original (such as continuous adjustment of digital proofing equipment), even The digital proofing system based on halftone technology may not be able to produce the same dot structure as the CTP printing plate. Proofing and plate generation in the CTP process are two independent processes, so the dots on the proof and the dots on the printed matter It is often different. Therefore, users must note that digital proofing and recording to the printing plate are two RIP processes, which is the main reason that the proofing and the printing plate have different levels of change and resolution. In fact, the printing company should understand that even if the simulation proofing or offset proofing process is adopted, whether the proofing result reflects the actual production effect of the printing process is also uncertain.
3. Key points of digital proofing
Printing companies and their customers have not reached a free position in the process of digital proofing during the transition from analog to digital workflow, because this change is not easy, even difficult, and therefore takes time. Operators of printing presses need time to learn how to match the printing results with the proofing results in color, instead of simply adopting the dot analogy method as before. Note that it is important to match the printed colors with the proofing results, rather than using a magnifying glass to check how close the printed dots are to the proofing dots. This change in concept is the premise of accepting the digital proofing process. In order to check whether the color of the printed matter matches the color of the proofs, you first need to observe it under a standard light source. For the printed products with low quality requirements, visual inspection method can be used; but for the high quality of printed products, instrument inspection is required. The general density meter can only measure the optical density. When the requirements are high, you can consider using a spectrodensitometer or similar equipment to measure the actual color value of proofs and printed products.
4. Digital Proofing Technology
At present, there are two types of digital color proofing methods, one is halftone proofing, and the other is continuous adjustment proofing. Each of these two proofing methods has many technical realizations, such as laser dye removal (Laser Dye Removal) technology, color spray Ink replication technology, dye sublimation technology, laser color printing technology and thermal transfer replication technology, etc. Each digital proofing technology has its advantages and disadvantages, and the prices of equipment and consumable materials may vary greatly. But as long as proper management and calibration are carried out, each digital proofing technology can produce very accurate colors. When discussing digital proofing technology, we cannot only consider the advantages and disadvantages of various color proofing technologies, but also must judge each proofing technology from an economic perspective. For example, inkjet replication technology and dye sublimation technology can produce very accurate color proofing, but the cost of the two is different. Therefore, finding the right digital proofing method for each customer is not an easy task, because the requirements and preferences of customers are varied.
5. Page proofing and layout proofing
Although color proofing is important, page proofing and layout proofing should be equally important. The purpose of page proofing is to check fonts, line drafts, and layouts. Such proofing should begin early in the production cycle. If the user device has the ability to combine page proofing and color proofing, page proofing can be combined with color proofing, but the two should be independent of each other and clearly distinguishable from each other.
The transfer of pre-press preparation results to large format output also requires some type of layout proofing system, as well as a system that performs calibration on color proofing equipment. The operator must confirm that the pages of each publication have been positioned according to the requirements of the book stickers, and that the page numbers have been properly combined according to the folding and saddle stitching requirements. The equipment used to proof the layout results is usually a large-format inkjet printer, such as Atlas produced by Agfa, which is a continuously adjustable drum proofing equipment. If you purchase the Apogee workflow software developed by Agfa, its RIP and output management module Print Drive support special preview function, you can carefully check the job to be output before output.
The layout proofing is to check the correctness of the operation as a whole. If the combination of layouts is not reasonable, the printing and binding workshop will be at a loss. Therefore, the pre-press and plate-making departments should have regular communication with the printing and binding workshops, and mutual understanding is the fundamental method to prevent rework.
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