How to control the quality of digital printing? These points must be mastered!
For digital printers, it is an important task for digital printers to discover and correct errors in data files submitted by customers in the shortest possible time, so as to meet the individual needs of customers for products. Here, the author will interpret the key points of digital printing quality control technology.
Pre-press data quality control
01
color mode
When checking the data file initially, check its color mode, which is especially important in color printing.
In digital printing, spot color printing is more commonly used because it can meet the individual needs of customers. Before performing spot color printing, you need to make the necessary settings for the spot color, that is, you need to choose whether to directly use the spot color ink for printing, or to convert the spot color to CMYK color before printing.
02
Layout
The documents received by digital printing should preferably be in common printing file formats such as PDF or PS. The page size is the net size of the printed document. In addition, it is important to ensure that the size of the large version of the document is correct, otherwise it may be reworked and reprinted during post-processing.
03
heart content
The basic elements of version heart include text, images and graphics. In terms of text, the most common problem is font matching. When there is no font used in the customer file, it is necessary to negotiate with the customer to replace the font, or the customer provides the relevant font file, or allows the customer to embed the font in the PDF file when it is generated. In terms of graphics, because it belongs to vector information, it is generally necessary to pay attention to the problems of color mode and file format. In terms of images, the image resolution needs to be controlled first. Generally, the color image resolution should be at least 300dpi, the grayscale image generally requires 600dpi, and the black and white line draft requires 1200dpi; secondly, the color mode of the color image should be CMYK, And it is best to ensure that the output is in TIFF lossless compression format, which is convenient for imaging and printing, and is more conducive to the perfect reproduction of image quality.
Raw material inspection
From the point of view of the digital printing process, the raw materials used in digital printing are mainly paper and ink, and their quality will directly affect the printing output and the color gamut space of the printing output. Only by ensuring the stability of raw materials can the stability of printing output be ensured.
01
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 the paper used in digital printing and traditional printing are different.
(1) Physical and chemical properties
The physical and chemical properties of paper include the physical and chemical properties of paper.
Among them, the physical properties of the paper include basis weight, thickness, tightness, ink absorption and smoothness. In digital printing, the conveying speed of paper greatly affects the quality of printed matter. The basis weight, stiffness, and fiber arrangement direction of the paper will affect the conveyance of the paper. If the basis weight exceeds 100g/m2, it will not Bring problems to transportation. The arrangement direction of the paper fibers is compatible with the basis weight of the paper. When the basis weight of the paper is less than 200g/m2, the arrangement direction of the paper fibers should be consistent with the running direction of the printing press; when the basis weight exceeds 200g/m2, the arrangement of the paper fibers The direction and the running direction of the printing 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 with fewer pinholes.
In addition, digital printing requires the thickness of the paper to be uniform, so as not to affect the uniformity of the printing ink. The tightness of the paper should not be too large, otherwise it will easily cause the paper to be brittle, and the opacity and ink absorption of the paper will also decrease; but the paper must have a certain degree of tightness in order to have tensile strength. The ink absorption speed of the paper cannot be too fast or too slow. If it is too fast, it will cause the ink imprints to be dull, even chalking, through-printing, printing density reduction, and hue changes. If it is too slow, the fixing speed of the ink will decrease and cause Conditions such as adhesion or dirt. Fine prints have high requirements on the smoothness of the paper, so that all the ink can be transferred and high-gloss prints can be obtained.
The chemical properties of paper include paper water content, acidity and alkalinity, etc. In the electrostatic dry powder digital printing, the toner needs to be heated to 120～150℃ to be fixed. The paper will become brittle due to loss of moisture, and will be easily broken during folding. In order to solve this problem, the digital printing system has prepared a relative Paper with a humidity of 30% or running in an air-conditioned environment, and choose paper with a tight fiber interwoven structure and elastic coating. In terms of the acidity and alkalinity of the paper, the ideal paper should be neutral, but due to the complicated pulping and papermaking process, the produced paper may appear acidic or alkaline. When the ink is printed on acidic or alkaline paper , After a period of time, the color of the ink will fade and the printed matter will lose its luster.
(2) Mechanical properties
The mechanical properties of paper include tensile strength and tearing degree. The tensile strength of paper is very important for book paper, especially high-speed digital printing press printing paper. If the tensile strength of the paper is low, the phenomenon of lint and powder falling will occur, and the gloss of the printed matter will also be reduced. Digital printing paper needs to have a high degree of internal tearing and edge tearing. This is because in digital printing, paper is prone to breakage and tear, which reduces printing efficiency and printing quality.
(3) Optical performance
The optical properties of paper include whiteness, gloss and opacity. Digital printing has strict requirements on the gloss of paper. Whether it is using fast-drying ink or slow-drying ink, the gloss of printed matter will increase with the increase of paper gloss. However, high paper gloss does not mean high gloss of printed matter, it is also affected by the smoothness of paper printing. The uniformity of paper gloss is more important than the average level of gloss.
Different types of printed matter have different requirements for paper opacity. In order to prevent people from seeing the graphics on the back of the paper when reading, the higher the opacity of the paper used in magazines and books, the better. Labels, manifests, etc. do not have high requirements on the opacity of the printed paper, as long as no see-through occurs.
02
ink
(1) Glossiness of ink
The gloss of ink is the ability of the ink film to form a smooth mirror surface on the substrate. Digital printing has higher requirements for the gloss of ink.
(2) Water resistance, oil resistance and solvent resistance of ink
Digital printing requires the ink film to remain stable under the erosion of water, oil, solvents and other substances. Prints printed with inks with poor water and oil resistance will change color and fade when they encounter water, oil and other substances, which will affect the effect of printing and copying. Prints printed with inks with poor solvent resistance cannot complete the subsequent glazing and laminating processes.
(3) Light resistance and heat resistance of ink
Digital prints are sometimes exposed to sunlight for a long time, so inks are required to have good light resistance. In addition, some digital printing processes require heating, which requires the ink to withstand high temperatures without discoloration.
(4) Other properties of ink
Digital printing inks are also required to be stable in terms of pH, conductivity, viscosity, permeability, surface tension, density, color difference, etc., can be compatible with printing technology, and must be non-toxic and environmentally friendly. After digital printing ink is printed and imaged, its optical density, drying time, adhesion, non-deviation, water resistance, solvent resistance, stability and other aspects must also meet the printing requirements.
Digital printing quality control method
01
outlet control
Digital printing adopts digital hybrid screening technology. This technology draws on the characteristics of amplitude modulation screening, frequency modulation screening and digital control hybrid screening technology. It can not only reflect the advantages of frequency modulation screening, but also possess the stability and stability of amplitude modulation screening. Operability. Compared with traditional screening technology, this screening technology has greatly improved both in output speed and resolution, and its perfect combination with CTP technology can make the pre-printing or printing present a perfect dot effect.
02
color control
Color is an important measure of the effect of image reproduction. The color control in traditional printing is based on density control. The specific methods include gray balance, color correction, optimal printing density control, dot control, and transfer. The digital printing process is an open system. The input, processing, and output equipment may come from different manufacturers, and the color description and expression of different equipment are also different. With the increase in the number of uses of the same equipment, the difficulty of color reproduction also increases. The color information of a color image must not only be presented on different display devices, but also be transmitted between different media. In this regard, the International Color Organization ICC has developed the ICC Profile profile to describe the color characteristics of the device, through which cross-platform color communication can be realized.
03
Data output and management
With the advancement of digitization, the amount of data is increasing, which requires optimization of the data transmission and management process on the network. For this reason, in the field of prepress, OPI (Open Prepress Interface) specifications and DCS (Desktop Color Separation) specifications have been formulated. The OPI specification contains a series of postscript-compliant annotation sentences, allowing low-resolution images to be used in place of high-resolution images during imposition, and the OPI server will automatically replace high-resolution images during color separation output, reducing the amount of data in network transmission. The DCS specification includes a main EPS file and multiple high-resolution color separation files. The EPS file format is expanded to manage the entire color separation process of the desktop publishing system, which can shorten production time, reduce equipment requirements, and ensure data files Will not be missing in the delivery.
04
Printing registration control
In digital printing, CCD cameras are often used to monitor the registration of the printing press, and the printing press is used for automatic registration control. The advantage of using CCD camera for registration is the use of image processing technology and high registration accuracy; it allows printing other content around the printed detection color standard; it can automatically distinguish between color standard and non-color standard content; improve the utilization rate of the substrate ; Able to adapt to very small color standards; fast registration speed.
Digital printing is the development trend of printing technology. Realizing its digitalization and standardized control is the goal we have been pursuing. We look forward to more quality inspection and control methods in the future to achieve high-quality digital printing.
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