From traditional printing to desktop publishing (DTP) to today's CTP and digital printing, the printing industry has undergone a process of constant change. However, no matter how the printing industry changes, people are concerned only with the transmission and reproduction of graphic information. Therefore, it is always extremely important to evaluate the quality of the reproduction of graphic information and use effective means to detect and control the quality. So, when the printing industry moves from DTP and CTP to full digital network production, how should we conduct quality inspection and control? This article will discuss this issue.
Main printing production process
The DTP technology that emerged in the 1970s and 1980s has caused great changes in the printing platemaking process. At the beginning, DTP was mainly based on word processing systems, and it solved part of the pre-press process. By the early 1990s, the printing industry had entered a more mature stage of color desktop publishing. During this period, desktop publishing has gradually developed into a set of processes Process. CTP is an extended form of pre-press digitization at the output. With the continuous decline in the price of CTP plate materials and the continuous maturity of related technologies, the CTP system has entered a practical stage by the mid-1990s. "Digital printing" is essentially an extension of pre-press digitization to printing and post-press. For the definition of digital printing, there is no unified statement. Broadly speaking, the so-called digital printing is a printing method in which data such as characters and images stored in a computer are recorded on a certain medium in a digital form, and then the information is transferred to the substrate by this medium. At present, digital printing can be divided into two ways. One is to completely eliminate the printing plate and use electronic imaging or inkjet technology to directly image the digital printing on the medium, such as the digital printing machine launched by HPIndigo and Xeikon; the second is the semi-digital printing machine with printing plate. Such as the Heidelberg DI (Digital Direct Imaging System) printing machine. The latter can also be summarized as on-machine CTP.
According to the different levels of digitization, the existing printing technology can be divided into the following four workflows.
1. CTF process: DTP → CTF → traditional printing machine → printed matter
2.CTP process: DTP → CTP (offline computer direct plate making) → traditional printing machine → printed matter
3. Semi-digital printing process: DTP → semi-digital printing machine (computer-to-machine direct plate making) → printed matter
4. Full digital printing process: DTP → digital printing machine → printed matter We call the first CTF process as the traditional printing process, and the latter three as the digital process.
Print quality control elements and control tools
Print quality refers to the comprehensive effect of various appearance characteristics of printed matter. From the perspective of copying technology, print quality should be based on "faithful reproduction of the original". Whether in the traditional printing process or in the digital process, the printed matter must be faithfully copied to the original. There are 4 control elements for printed matter: color, gradation, clarity, consistency.
1. Color is the basis of product quality, which directly determines the quality of the product. Color control or management has always been a hotspot and technical key for printing professionals' research and analysis.
2. The gradation is the gradation, which refers to the change of the gradation of the color recognizable by the image. It is the basis for accurate color reproduction.
3. Clarity refers to the clarity of image details, including three aspects, the clarity of the subtle levels of the image, the clarity of the edges of the image outline, and the clarity of the image details.
4. Consistency is uniformity, which includes 2 aspects. On the one hand, it refers to the consistency of the ink volume of different parts of the same batch of printed matter, that is, different ink areas. It is generally measured by the consistency of the vertical and horizontal solid density of the printed matter. It reflects the stability of different parts of the printed matter printed at the same time. On the other hand, it refers to the consistency of the density of different batches of printed products in the same location, which reflects the stability of the printing press.
For printed matter, as long as these four aspects are well controlled, that is, the color, gradation, clarity, and consistency of the printed matter can be well controlled, you can get high-quality printed matter.
One of the methods of printing quality control is to use conventional signal strips, test strips, control strips, ladders, and test instruments and charts to scientifically and quantitatively control printing quality (including printing quality and printing engineering quality). At present, there are many types of signal strips and test strips used in various countries, such as the GATF system in the United States, the Brunner system in Switzerland, the Fogra system in Germany, and the Glinda system. In China, the signals and test strips of GATF and Brunner of Switzerland are mostly used.
Common testing tools for printing include signal strips, test strips, control strips, and ladders. The signal bar is mainly used for visual evaluation, the function is relatively single, and it can only express the appearance quality information of the printed matter. Such as the sun ladder, GATF code signal bar, color signal bar, etc. The test strip is a multi-functional marking element mainly based on density meter detection and evaluation. It is a test strip combining visual identification and density meter test, and numerically calculating with the help of charts and curves. The control bar is a multifunctional control tool that combines the visual evaluation and test evaluation of the signal bar and the test bar. Such as Brunel's third generation control strip. The ladders have unequal density or dots, which are used to control the printing and printing quality.
The printing process mentioned above needs to use signal bars and control bars to record data in the operation of its main links to lay the foundation for standardized production. Quality inspection and control generally find problems through proofing. In the traditional printing process, traditional proofing is used, that is, proofing by round flattening. In the digitalization process, digital proofing has gradually replaced traditional proofing. The digital files to be printed can be directly output to the proofing machine to see whether there are problems with the text, specifications, and images. Since film is no longer used in the digitalization process, and even printing plates are no longer used, quality evaluation and control largely depend on the digital proofing. So, in today's digital and networked environment, what factors are restricting its printing quality in the digital process? What are the difficulties and advantages of quality inspection and control?
Method for testing and controlling printing quality in digital environment
1. Transmission of data flow in digital process
The digital production process makes the data flow and control flow in actual production not as intuitive as the traditional process. What we see is only the input layout elements, the layout information expressed by the display device, and the final output color digital proofs and CTP printing plate. The operation information exists in the form of data. Any small mistakes or errors will cause the failure of data transmission or abnormal output results, and will also exceed the control capabilities of the operator. It is difficult or impossible to check the problem, so these variables become Uncontrollable factors in the prepress operation. In order to ensure the correct and smooth flow of data, we must find a set of practical data flow control methods.
In the production of the printing industry, there are two types of information flow for printing jobs, namely "graphic and text information flow" and "production control information flow". The graphic information flow solves the problem of "what to do"; the control information flow solves the problem of "how to do" and "what to do". In the digitization process, both types of information are digitized, and both can be recorded, processed, and transmitted by the computer. The control nodes of the data flow in the digitalization process are file pre-check, printout, PDF normalized digital layout, and output after RIP.
2. Color management under open system
Color reproduction is an important measure of image reproduction quality. The color control in the traditional process is based on the definition and description of the color attributes with density control as the core, and the color recognition and correction are performed through a closed closed-loop system. Specific control methods include gray balance, color correction, optimal printing density control, dot transfer and its control.
The digital printing process is an open system, and the input, processing, and output devices may all come from different manufacturers. Various devices have different descriptions and expressions of colors, and the same device will also wear out as the number of uses increases, and the expression of colors will also vary, thereby increasing the difficulty of color reproduction. And because of the emergence of the network and the need for information exchange, the designed color images or color image files are no longer limited to local publication and printing; they often need to be viewed or copied in other places. The color information of a color image must not only be transmitted between different devices, such as a computer monitor (even if it is the same device, devices produced by different manufacturers have different colors), but it also needs to be transferred between different media.
This requires a system whose function is to make the color transfer results between all media are basically the same, without aliasing. Also known as "what you see is what you get", this system is called a color management system. The International Color Organization ICC has developed a standard for describing the color performance of devices-ICCProfile. Using the ICCProfile profile, cross-platform color communication can be achieved, so that when other computers add color management functions, these files can be converted at will. Color management is carried out through the characteristic file. A color management system is an application system that includes computer hardware, computer software, and color measurement equipment. Its goal is to form an environment in which various devices and materials that support this environment match each other in the transmission of color information and achieve undistorted transmission.
The new color control method with color space transformation as the core-color management has made the technical process in the field of printing and communication begin to enter a new digital production stage, which solves many problems of color reference and conversion in the production of print media. Digital proofing, CTP, digital printing and digital production processes are constantly emerging, and are moving towards the goal of "what you see is what you get".
3. Data transmission and management
As the degree of digitization deepens in the digitization process, the amount of data increases geometrically. Although there is a fast network, it is still necessary to optimize the transmission and management of data on the Internet. To this end, the pre-press field has formulated two related specifications: OPI (OpenPrepressInterface) specification and DCS (DesktopColorSeparation) specification. The OPI specification allows low-resolution images to be used in imposition, and the OPI server automatically replaces the corresponding high-resolution images during color separation output, thereby reducing the amount of file transmission on the network. The DCS specification is an extension of the EPS file format, which can manage the entire color separation process of the desktop publishing system, which is beneficial to shorten production time and reduce the requirements for equipment. In the printing process, data flows in all links. Therefore, it is extremely important to ensure that the data files are not missing during the transmission process, and that the interpretation of the files by different platforms is consistent. Otherwise, the files transferred to the back end will get incorrect output results due to data loss. In order to carry out unified file control in the integrated production of printing, the CIP3 standard has been developed to CIP4, and its members have also developed a new file format JDF.
4. Digital proofing
Proofing is divided into traditional proofing and digital proofing. In the traditional production process, the traditional proofing is basically used, and the round flattening proofing method is generally used. Of course, digital proofing is also used within a certain range, but there are problems in proofing quality and accuracy.
With the development of digital proofing, digital proofing is very useful in digital workflow. Digital proofing is a "no film", digital proofing technology that is produced with the development of network technology, CTP technology and pre-press digitalization. It is an important part and development direction of the modern printing process digital process. Digital proofing means that the data of pages (print layouts) produced by the color desktop publishing system, without any analogue processing, is directly output by color printing equipment (jet printing, laser, static electricity, etc.) in a digital manner, which can meet the design, Checking the quality and errors of the pre-press process such as production can provide the basis and standards for the subsequent printing, and can also be used as the proofing technology for the proof of the user's signature. The digital proofing system is generally composed of a color inkjet printer or a color laser printer. The color of the printing proofing is simulated by the color printer, which replaces the traditional long process processes such as output film, printing, mechanical proofing, etc., using the original page (printing layout) Data to obtain color proofs.
The development of a new generation of RIP has effectively promoted digital proofing. "RIP once, output multiple times" ensures that the files used for proofing and printing are consistent. In this process, the file is interpreted only once by RIP and can be output to the printer and plate-making machine, thereby eliminating many repetitive tasks, saving time, and ensuring the consistency of the proofs. In the digital printing production process, digital proofing is essential. Digital proofing uses ink and paper, which are different from the inks and substrates used for printing, which determines the difference in color reproduction and printing effect of digital proofing. At present, this difference is narrowing. Some new printers have 7 or 8 colors, so that the effect of proofing is closer to the actual effect of printing.
Digital proofing is not only different from the traditional flattening printing method of the proofing machine, but also different from the rounding printing method of the printing machine. It is based on the color range of the printed product color and the same RIP data as the printed content. The color printing with larger space matches the printing method with smaller color space to reproduce the printing colors, which can meet the requirements of various printing methods such as flat, concave, convex, soft, and net, and can produce proofs according to the actual printing conditions of users , Solve the problem that the proofing and subsequent actual printing process can not match, which brings difficulties to printing.
Compared with traditional proofing, digital proofing has the following advantages: â‘ Less equipment investment, small footprint, and low environmental requirements. â‘¡ Save human resources, reduce costs, and rely less on operator experience. â‘¢Fast speed, stable quality, strong repeatability and low cost. â‘£Wide adaptability, especially suitable for direct plate making, gravure printing and flexo printing, which can not be proofed or difficult to proof. It can not only simulate the effects of various printing methods, but also can be combined with CTP and digital equipment of digital printing presses to truly realize automated workflow.
5. Online detection technology
At present, many countries in the world have begun to adopt advanced online measurement and control printing technology. The conventional test usually uses printed measurement and control strips. The test results cannot be fed back online to the printing process to control the printing process in time and improve the printing quality. One printing process is controlled; while advanced online measurement and control printing technology is tested, direct feedback control is provided. The test can not only target the measurement and control strip, but also directly test specific printed products. The basis of this development is the development of computer technology and calculation. Speed ​​has allowed analysis and testing of full images in a very short period of time, such as Heidelberg's online quality control scanning system and online control system for printing ink volume.
Online detection is usually through the CCD camera lens, continuous printing of the printed matter on the printing production line, and the captured image is timely transmitted to the computer for analysis and processing. The detection content can include: color detection, layout detection, handwriting detection, position fit detection and Missing prints, paste editions, dirty editions, etc.
in conclusion
At present, most domestic printing companies are in the transition period from the traditional closed product production system to the expansion of the open product production system, that is, the transition period from the traditional printing process to the digital printing process. Most of their production, process and quality control systems follow backward Empirical production control methods have the problems of separation before and after printing production, random technical parameters, post-recovery of quality control, poor production stability, large fluctuations in quality, and excessive material waste, etc., which cannot meet user needs and have weak market competitiveness. Therefore, in the digital environment, the implementation of quality control requires the implementation of new technical means and the establishment of technical objectives.
(1) Establish a scientific and reasonable technology, process and quality control system, and gradually adapt to the use of digital technology to achieve the production and development requirements of color graphic products. That is how to use digital technology to integrate the digital color control and management platform of printing production process, quality and management according to the digital production environment that has been established or to be established, and rationalize the production technology process and quality management method.
(2) Solve the need for the transformation and extension of the printing industry's production model from manufacturing to product (information) services. In the whole process of printing production, how to reduce time redundancy, cost redundancy, personnel redundancy, expand new application areas and realize products and personalized re-value-added services, form a new technology concept and core competitiveness suitable for the domestic production environment .
(3) Multi-party cooperation, comprehensively improve the printing industry equipment, technology and management level, improve the quality of personnel through technical training, and standardize, standardize and digitize the printing experience.
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