How to control the quality of digital printing?
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For digital printers, it is an important task to find and correct errors in data files submitted by customers in the shortest possible time to meet the individual needs of customers. Here, the author will interpret the main points of digital printing quality control technology.
Prepress data quality control
1. Color mode
When the data file is initially inspected, its color mode should be checked, which is especially important in color printing.
In digital printing, spot color printing is more commonly used to meet the individual needs of customers. Before performing spot color printing, it is necessary to make necessary settings for the spot color, that is, whether it is to directly use the spot color ink for printing, or to convert the spot color into CMYK color and then print.
2. Layout
The files received by digital printing are preferably common print file formats such as PDF or PS. The layout size is the net size of the printed document. In addition, be sure to ensure that the large version of the file is the correct size, otherwise it may be reworked during post processing.
3. Heart content
The basic elements of the heart include text, images, and graphics. In terms of text, the most common is the font matching problem. When there is no font used in the client file, it is necessary to negotiate with the customer to replace the font, or provide the relevant font file by the customer, or let the customer embed the font when generating the PDF file. In terms of graphics, since it belongs to vector information, it is generally necessary to pay attention to the problems of color mode and file format. In terms of image, firstly, the resolution of the image needs to be controlled. Generally, the resolution of the color image should be at least 300 dpi, the gray image is generally required to be 600 dpi, and the black and white line draft is required to be 1200 dpi. Secondly, the color mode of the color image should be CMYK. And it is best to ensure that the output is TIFF lossless compression format, which is convenient for image printing and is more conducive to the perfect reproduction of image quality.
Raw material testing
From the perspective of the digital printing process, the raw materials used in digital printing are mainly paper and ink, which will directly affect the color output space of the print output work and the print output. Only by ensuring the stability of raw materials can the printing output be stable.
1. Paper
Digital printing has a wide range of substrates, but paper is still the main substrate. With the development of technology, digital printing has higher requirements for paper performance and printability. The physical and chemical properties, mechanical properties and optical properties of digital printing and paper used in traditional printing are different.
(1) Physical and chemical properties
The physical and chemical properties of paper include the physical and chemical properties of the paper.
Among them, the physical properties of paper include quantitative, thickness, tightness, ink absorption and smoothness. In digital printing, the paper transport speed greatly affects the quality of the printed matter. The paper quantitative, stiffness and paper fiber alignment direction will affect the paper transport. If the paper is more than 100g/m2, if the stiffness is sufficient, it will not Bringing problems to transportation. The arrangement direction of the paper fibers is compatible with the basis of the paper. When the paper weight is less than 200g/m2, the arrangement direction of the paper fibers and the running direction of the printing machine should be the same; when the paper weight exceeds 200g/m2, the arrangement of the paper fibers The direction and direction of operation of the press should be perpendicular to each other. The frictionless paper transport system is assisted by a vacuum system, which requires the paper surface to be as smooth as possible and with fewer pinholes.
In addition, the thickness of the paper is required to be uniform in digital printing so as not to affect the uniformity of the printing ink color. The paper should not be too tight, otherwise it will cause the paper to be brittle, and the opacity and ink absorption of the paper will also decrease; but the paper should have a certain tightness so as to have tensile strength. The ink absorption speed of the paper should not be too fast or too slow. If it is too fast, it will cause the ink to be dull, even powdered and printed, the printing density will decrease, and the hue will change. If it is too slow, the fixing speed of the ink will decrease and cause Sticky or sticky. Fine prints require a high degree of smoothness in the paper so that the ink can be completely transferred and a high-gloss print can be obtained.
The chemical properties of paper include paper moisture content, acidity and alkalinity. In digital printing of electrostatic dry powder, the toner needs to be heated to 120-150 ° C to be fixed, the paper will become brittle due to loss of moisture, and it will be easily broken when folding. To solve this problem, the digital printing system is prepared for relative printing. Paper with a humidity of 30% or paper in an air-conditioned environment, and paper with a tight fiber interlaced structure and elastic coating. In terms of acidity and alkalinity of paper, the ideal paper should be neutral, but due to the complexity of the pulp and paper process, the paper produced may be acidic or alkaline, when the ink is printed on acid or alkaline paper. After a period of time, the color of the ink will fade and the print will lose its luster.
(2) Mechanical properties
The mechanical properties of the paper are tensile strength, tearing, and the like. The tensile strength of paper is very important for book papers, especially high-speed digital presses. If the paper has low tensile strength, it will have hair loss and powder drop, and the gloss of the print will also decrease. Digital printing papers need to have high internal tear and edge tearing, because in digital printing, paper is prone to breakage, tearing, etc., reducing printing efficiency and printing quality.
(3) Optical properties
Paper optical properties are whiteness, gloss and opacity. Digital printing has strict requirements on the gloss of paper. Whether it is fast-drying ink or slow-drying ink, the gloss of printed matter will increase with the increase of paper gloss. However, the high gloss of the paper does not mean that the gloss of the printed matter is high, and it is also affected by the smoothness of the printing of the paper. The uniformity of paper gloss is more important than the average level of gloss.
Different types of prints have different requirements for paper opacity. In order to prevent people from seeing the text on the back of the paper when reading, the opacity of the magazine and book paper is higher. Labels, bills of paper, etc. do not require high opacity of the printed paper, as long as the offset does not occur.
2. Ink
(1) The gloss of the ink
The gloss of the ink is the ability of the ink film to form a smooth mirror on the substrate. Digital printing requires a higher gloss on the ink.
(2) Water resistance, oil resistance and solvent resistance of the ink
Digital printing requires the ink film to remain stable under the attack of water, oil, solvents and the like. Prints printed with inks that are resistant to water and oil are in discoloration and fading when exposed to water, oil, etc., affecting the printing and copying effect. A printed matter printed with an ink having poor solvent resistance cannot be subjected to subsequent glazing, filming, and the like.
(3) Light resistance and heat resistance of the ink
Digital prints are sometimes exposed to sunlight for a long period of time, so inks are required to have good lightfastness. In addition, some digital printing processes require heating, which requires the ink to withstand high temperatures without discoloration.
(4) Other properties of the ink
Digital printing inks are also required to be stable in terms of pH, conductivity, viscosity, permeability, surface tension, density, chromatic aberration, etc., and can be adapted to printing technology, and are also non-toxic and environmentally friendly. After printing and imaging, digital printing inks also meet the printing requirements in terms of optical density, drying time, adhesion, non-offset, water resistance, solvent resistance and stability.
Digital print quality control method
1. Network control
Digital printing adopts digital hybrid screening technology. This technology draws on the characteristics of AM screening, FM screening and digital control hybrid screening technology, which not only reflects the advantages of FM screening, but also has the stability of AM screening. Operability. Compared with the traditional screening technology, the screening technology has greatly improved both in output speed and resolution. Its perfect combination with CTP technology can achieve perfect dot effect before printing or printing.
2. Color control
Color is an important measure of image reproduction. The color control in traditional printing is based on density control. The specific methods include gray balance, color correction, optimal print density control, dot control and transfer. The digital printing process is an open system. The input, processing, and output devices may come from different manufacturers. The color description and expression of different devices are also different. The same device increases the number of times of use and the difficulty of color copying. The color information of a color image is not only presented on different display devices, but also transmitted between different media. In this regard, the international color organization ICC developed the ICC Profile feature file to describe the color characteristics of the device, through which cross-platform color communication can be realized.
3. Data output and management
As the digitization process advances, the amount of data is increasing, which requires optimization of the data transmission and management process on the network. To this end, in the prepress field, the OPI (Open Prepress Interface) specification and the DCS (Desktop Color Separation) specification have been developed. The OPI specification includes a series of postscript-compliant comment statements that allow low-resolution images to be used instead of high-resolution images for imposition. The color separation output is automatically replaced by high-resolution images by the OPI server, reducing the amount of data in the network transmission. The DCS specification includes a main EPS file and multiple high-resolution color separation files. The EPS file format has been extended to manage the entire color separation process of the desktop publishing system, which can shorten production time, reduce equipment requirements, and ensure data files. It will not be missing in the delivery.
4. Printing register control
In digital printing, the CCD camera is often used to monitor the registration of the printing machine, and the automatic registration control is performed by the printing machine. The advantage of using CCD camera for registration is the use of image processing technology, high registration accuracy; allowing printing of other content around the printed color code; automatic separation of color and non-color content; improved substrate utilization Can adapt to very small color standards; register speed is fast.
Digital printing is the development trend of printing technology. Realizing its dataization and standardization control is our goal. We look forward to more quality inspection and control methods in the future to achieve high-quality digital printing.