Talking about inkjet printing and its quality control
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First, Introduction
Inkjet printing is one of digital printing, with the advantages of high speed, small amount of copying, and easy combination. It uses digital data to store images and can be output directly without the need for plates. Therefore, as long as the output can be changed before output, and the process of imposition and printing is saved, a lot of time and consumables are saved. In addition, because its printing method is non-contact, it eliminates the troubles of traditional printing such as printing pressure adjustment, dot enlargement, ink balance (offset) and paper surface smoothness. These features are inkjet printing in today's market. The reason for the ups and downs. At present, inkjet printing has been widely used in lottery printing, large-format inkjet printing, digital proofing, and personalized printing.
Second, the imaging mechanism of inkjet printing
1. Composition of inkjet imaging system
The inkjet imaging system consists primarily of nozzles, ink sticks, charging systems, deflectors, recovery tanks, imaging media, and the necessary transmission control systems. The nozzle and the image receiving medium are in a non-contact state, and the fine ink droplets ejected from the nozzle directly adhere to the substrate to form a visible image. The classification of inkjet printing is shown in Figure 1.
Figure 1 Classification of inkjet printing
2. Inkjet imaging method and its imaging mechanism
There are many kinds of inkjet printing methods. The basic principle is to input the graphic information stored in the computer to the inkjet printer first, and then spray the fog from the nozzle to the surface of the substrate under the control of the electronic computer through a special device. The ink droplets are directly imaged on the surface of the substrate according to the charge effect, becoming the final print. According to the classification of inkjet systems, including continuous inkjet systems and batch inkjet systems.
Continuous inkjet includes two types of binary deflection inkjet systems and multi-value deflection inkjet systems. In the binary deflection process, the ink droplets are in two states of charge, the charged ink droplets are deflected by the electric field, and the uncharged ink droplets are transferred to the substrate. In multi-value deflection, different ink droplets receive different charges, resulting in different deflections that are transferred to different locations of the substrate. As shown in Figure 2.
Figure 2 Continuous inkjet printing system
In batch inkjet printing systems, the most commonly used are mainly thermal bubble inkjet printing, piezoelectric inkjet printing, and electrostatic inkjet printing. The imaging principle is shown in Figure 3.
Figure 3 Intermittent inkjet printing system
In the thermal bubble inkjet printing process, the ink is evaporated by heating, a certain amount of ink is ejected from the nozzle and bubbles are formed under application of pressure, and then transferred to the substrate.
In a piezoelectric inkjet system, a volume change caused by a pressure effect in the ink chamber produces ink droplets, and causes ink droplets to be ejected from the nozzle system and transferred to the substrate.
In an electrostatic inkjet printing process, an electric field is present between the inkjet system and the printing surface. The ink is separated from the nozzle by an electric field and control pulses, and the ink is released to the substrate to obtain a printed graphic.
Third, factors affecting the quality of inkjet prints
In the process of rapid development of inkjet printing, it is inevitably restricted by some factors. At present, the factors affecting the quality of inkjet printing mainly include the following aspects:
1. Color conversion
For prints, the human eye is determined by the reflected light to determine the difference in color. When a red sheet of paper is seen, the red light in the spectrum is reflected because the light source is on the paper, so it is known to be red. However, in print, we must use the subtractive method to define the color, which is the so-called C, M, Y tricolor model. How to accurately convert the color (R, G, B) of the light source system into the color of inkjet or printing system (C, M, Y, K) is the key to affecting the quality of printing.
2. Resolution
The so-called resolution (Dot Per Inch) refers to the number of ink dots printed on the width of one inch. Because inkjet printing, black ink is independent, not through the overprinting of Y, M, C three-color ink, Therefore, the resolution has become the key to determining the quality of black and white printing presses. For a color printer, since it is a multi-color overprint, it does not care whether the dots overlap. Therefore, the so-called true resolution and addressing resolution appear. As shown in Figure 4.
Figure 4 True resolution and address resolution of a color press
3. Levels
Printed images and images are made up of many dots (or pixels Pixel) with different colors and brightness. Each pixel has its own color and brightness. Since the color is analog output, the voltage is used to control the color so that each point can be displayed in continuous color gradation (full color). Therefore, even if the resolution is low, the output image feels very natural and realistic. Therefore, the color scale is very important for the output image and image. The current inkjet printing technology can precisely control the color gradation of ink droplets, and control the color ink at the same position from not spraying to spraying 1 drop until spraying 16 drops, thereby controlling the color gradation of the output image.
4. Print head control
The ink dots are sprayed onto the paper through a nozzle. Generally, the nozzle and the print head are made together. However, the precision of inkjet technology, press cantilever gears and drive belts is a major factor in determining the quality of the print head.
5. Ink
Ink jet printing typically uses non-toxic aqueous inks, typically oil-based or paraffin-based, which do not dry out in the nozzle but are absorbed by the print surface. In addition, the ink must also have a fast drying property to prevent smearing on the paper, so the ink itself is one of the factors that affect the printing quality.
6. Substrate material
After the ink reaches the substrate, the internal structure of the different papers is different, and the surface coating is different, so the penetration and adsorption of the ink on the surface of the paper is different, which causes the color saturation and color space of the printed matter to be different. Generally speaking, in the same printing environment, the inkjet printing effect of the paper with smooth surface coating is better, and many researchers have proved this through experiments.
Fourth, the inspection and control of inkjet printing quality
Inkjet printing uses ink to replicate the color and gradation of the original. It often has high requirements on the viscosity and fluidity of the ink. After the nozzle is used for a period of time, the composition of the ink changes due to the change of temperature. And dust, dust, etc. in the air can cause nozzle clogging. In addition, random inkjet digital printing techniques typically utilize a combination of reciprocating motion of the printhead and linear motion of the paper in the print direction to replicate the object. Therefore, in addition to solving the problems to be solved by other color digital printing, inkjet digital printing also needs to solve the problem of nozzle clogging and correction of the print head, and the nozzle inspection and print head correction can be well solved by the measures proposed by EPSON.
1. Nozzle inspection
Check the various colors by printing the nozzle checker to find out if any of the colors of the print ink are missing. Although the operation method given below is for the EPSON STYLUS PRO7000 color inkjet printer, the processing method for other inkjet printers can be referred to.
The steps for nozzle check are: 1 Load paper. 2 In mode selection mode, press the mode selection button until TEST PRNT MENU appears on the LCD panel. 3 Press the item button and NOZZLE CHECK=PRINT appears. 4 Press the Enter key.
The printed nozzle inspection chart is shown in Figure 5. The series of diagonal lines in the figure are called test lines and are divided into two groups. There is no difference in line thickness, but the lines are different in depth. How many inks are in the inkjet printer, how many lines will be printed. Use these test leads to check which ink nozzles are blocked and decide if you need to clean the printheads.
Figure 5 nozzle check chart
If some line segments are found missing in the test line of the nozzle check chart, the print head needs to be cleaned. Press the wash button on the control panel for more than 3 seconds, or clean it with the printhead cleaning utility in the EPSON Printer Utility in the software. Be sure to confirm that the nozzle check pattern is complete before starting to print the image, otherwise it will affect the quality of the print.
2. Print head calibration
During the printing process, if the print head is out of position, the printed lines, graphic outlines, and text edges will be discontinuous and unsmooth, the image content will be misaligned, and the print head bias error Will accumulate and seriously affect print quality. Therefore, printhead alignment is the basis for ensuring accurate copying of page objects.
Taking the EPSON inkjet printer as an example, the general process of printhead calibration is: loading the paper; setting the thickness of the paper if necessary; printing the calibration pattern (all or single); finding the line with the smallest gap for each calibration pattern And register this number.
To align the printhead, do the following: 1 Load the paper into the printer. 2 Press the mode selection button in the mode selection mode until the Head Alignment Menu is displayed on the LCD panel. 3 Press the item button and PAPER THICK=STD appears on the LCD. If you want to print on the recording medium (paper) provided by EPSON, keep this setting as STD; if you want to print on non-EPSON-provided media (paper), press the "+" or "-" button. This will enter the paper thickness in 0.1 mm increments. 4 Press the Enter key to log in to the user information, and the ADJUST.PATT.=ALL message appears. 5 If you want to print all the adjustment patterns, press the Enter key; if you want to print a single adjustment pattern, press the “+” or “One” button until the pattern number you want to adjust appears (#1, #2, #3) ...), then press the Enter key. The printer starts printing all or selected adjustment patterns. Figure 6 shows the #1~#7 correction pattern. 6 When the printing is completed, the pattern number and the current line group number are displayed, for example, #1=4. If none of the lines in the printed line group are aligned, the print head correction still needs to be performed, but the line group printing with the smallest gap should be selected until the line alignment of a line group is completed, and the print head correction process is completed. Thereafter, press the "+" or "-" key to display the number of the line group. If the gap of the line group 4 in the print result is the smallest, the print head calibration is finished, and the pause mode can be pressed to select the mode. 7 Press the Enter key to log in to the line group number, the next pattern number appears, and then repeat step 6. 8 After setting the pattern, press the Pause button to return to the READY state to determine if you need to continue the printhead alignment until the line gap of a line group is minimized.