Some knowledge of bar code printing
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First, the structure of the barcode and the principle of reading
A bar code is a combination of a wide range of wide and narrow, black and white parallel lines combined in a specific format and spacing. It is usually printed on a commodity or printed matter, can replace various text information, and can be read at any time through an optical reading device. data. The EAN barcode system consists of the barcode symbol itself, the barcode reading device, the interface, and the computer to complete the input and output of the commodity information. There are two kinds of symbols for bar code reading. The bar code symbols are rectangular line patterns. The information reading of the optical scanner is mainly to read and recognize these bar code symbols. The number symbol is a number and letter outside the line, including 0-9 digits, A to Z letters, which can be directly recognized by the naked eye. Generally, it is 8 to 16 digits, the code system is different, and the number of digits is different. The arrangement, width and number of lines of the barcode are determined by each manufacturer and determine the meaning of the code. Generally, there are marks for initial reading and final reading at both ends, and some are also printed with numbers under the line, which can be directly identified or interpreted by an optical text reader, thereby enabling product counting, statistics, and management. 
The reading performance of the bar code system, that is, whether the bar code system can be used normally depends mainly on the reading ability of the system and the printing quality of the bar code. Barcode is a kind of coded information, which is a specific language for people to talk with a computer. Bar code in black and white thick and thin line symbols, thick black lines in the computer as 1, thin black lines represent 0, through the logic conversion, can be expressed as 0 ~ 9 Arabic numerals and arrays, so there must be a reading device Can be used together to read. The reading system mainly includes a scanner and a decoder. The scanner is a component that directly contacts the bar code reading signal. It consists of a light emitter, a photodetector, and an optical lens that reads the information abbreviated by the bar code at an extremely fast rate. When scanning, when the light beam emitted from the light emitter is illuminated on the bar code, the photodetector responds to the intensity of the light reflected from the bar code by the light beam, when the scanning spot is scanned onto the white paper or is in the black line When there is a blank space, the reflected light is strong, the detector outputs a large current; when scanning into the black line, the reflected light is weak, the detector outputs a small current, and the response is different according to the width of the black line, with the barcode The change of light and dark is converted into a current signal of different magnitude, which is amplified and sent to the decoder. The signal is translated into data by a decoder, subjected to local inspection and display, and connected to a keyboard and sent to an electronic computer for data processing. Therefore, the quality of the bar code is related to whether it can be read normally. The ink color is uniform, the layout is not dirty and the paste is clear, and the lines are clear and unbroken, which is the basic quality requirement for bar code printing. 
Second, the printing method of the barcode and the problems that should be noted 
There are two kinds of barcode printing methods. One is to use the printing equipment for batch printing and copying. It is a commercial barcode label production. This method generally prints the barcode together with the pattern. Second, it is controlled by computer to print barcode labels and barcode files in a timely manner. The former applies to a large number of barcodes with fixed specifications and the same content, and is designed and printed at the same time as the outer packaging graphics. The latter can be controlled by computer to achieve instant printing on demand, with strong flexibility. Bar code printing devices include inkjet printers, thermal printers, thermal transfer printers, hit dot matrix printers, and laser printers. In order to ensure that the printed bar code meets the specifications, the plate making process should be considered according to the printing process and the characteristics of the substrate. For the flexographic printing process, the line width can be appropriately reduced during plate making to compensate for the widening deviation in printing. For printing heat shrinkable packaging materials, it is necessary to consider the position of the bar code after shrinking the film, and calculate the vertical and horizontal shrinkage multiples in advance so as to adjust during plate making. 
In order to facilitate normal reading when using, you should pay attention to the color matching of the barcode. For the barcode reading system, the scanner light source is generally a red light source with a wavelength of 630-700 nm, so the red light effect of the ink should be considered. The incident light from the scanner is illuminated on the surface of the bar code of different materials and colors, and the reflection effect is also different. Black ink is completely absorbed by red light, and the reflectance of printed matter to incident light is generally below 3%. Therefore, most barcodes are designed to be printed with black ink. The white ink is completely reflective for red light, and the reflectivity of the printed matter to the incident light is close to 100%, so it is the most ideal blank color. For the above reasons, most of the barcodes on the printed matter are printed on white paper. However, some packaging products, from the decorative effect, also have other color combinations, so you should pay attention to the color according to the nature of the mix. Generally, the color with high reflectance of red light is yellow, orange, red, light brown, and the like. Black, green, purple, cyan, etc., have a low reflectance to red light. Reasonable design of bar code printing color should fully consider the factors such as the reflectivity of color to red light. Transparent film packaging is not suitable for direct printing of barcodes. It should be printed with white ink or yellow or orange red as the substrate, and then printed with dark barcodes, such as black, dark green, dark blue, etc., so that it is easy to read and use. 
Third, some process requirements for bar code printing
1. Ink requirements
The color matching of the ink should fully consider the color cast of the ink, and the color cast of the ink has a great influence on the accuracy of the barcode. In theory, as long as the ink is used according to the color ratio, the bar code printing requirement can be satisfied, but the coloring phase is impure due to the printing ink, and the color cast is caused. Therefore, the color of the ink should be accurately controlled so that the ink density is uniform, the hue is saturated, and the purity is high. It is preferable to determine whether the reflectance of an ink under red light meets the requirements before printing the barcode. The glare and gloss of the gold ink will cause specular reflection effects, affecting the scanner reading, so it cannot be used to print barcodes. In addition, the concentration of the ink and the thickness of the ink layer should also be suitable for bar code printing. Since bar code printing is solid printing, the reflection density that can be achieved by printing is related to the optical characteristics of the ink and the thickness of the ink layer. During the printing process, the reflection density of the printing increases as the thickness of the ink increases. When the thickness of the ink reaches a certain value, the density then reaches a saturated state. General ink saturation density requirements are: black ink 1.8 ~ 2.0; cyan ink 1.45 ~ 1.70; magenta ink 1.25 ~ 1.50; yellow ink 0.90 ~ 1.05; The color ink is 0.8 or more. Due to the difference in printing process, the thickness of the ink layer is also different, generally 2 to 4 μm for offset printing; 8 μm for embossing; 10 μm for flexo printing; 12 μm for gravure; and 30 μm for silk screen.
2. Requirements for substrates. 
Since the scanning light source is incident at a 45° angle when the barcode is read, the reflected light collection angle is 15°. When the reflected light exceeds the 15° range, the reflected light signal cannot be collected, which is equivalent to the black effect. Therefore, in order to meet the characteristics of bar code scanning, the substrate is required to have good light scattering characteristics, and specular reflection cannot occur. Therefore, the whiteness, opacity and gloss of the paper have a certain influence on the reading of the barcode. In addition, consideration should be given to materials with good weatherability, dimensional stability after stress, good colorability, low ink permeability, smoothness and smooth finish. 
3. Printing quality requirements. 
The bar code symbol is the information source for scanning and reading. In order to ensure correct reading, the printed bar code should be neat and clear, the bar code should be no obvious defects, and there is no dirty ink in the blank space. In order to ensure the correct reading of the barcode, the maximum diameter of the defects and stains on the barcode should be less than or equal to 0.4 times the standard width of the narrowest line code. The lines and blanks in the printed bar code should have obvious contrast signals. The reflectivity of the blanks should be as large as possible, and the reflectivity of the lines should be as small as possible. The larger the PCS value (color contrast), the contrast signal of the bar code. The larger, the better the readable performance. 
4. Requirements for printing location. 
The position of the bar code symbol shall be determined in accordance with the principle of non-deformation, easy reading and plate making. That is, it is required to be disposed on the right side of the main display surface of the product package or on the plane connected to the main display surface, and on the back side of the main display surface of the product package. Taking into account the printing process characteristics, the bar code direction should correspond to the printing direction during the imposition, so that the printing deformation can only be expressed in the longitudinal position of the bar code so as not to affect the accurate reading. Due to differences in packaging methods and features, the bar code printing position is also different. The general box packaging barcode is printed on the lower right side of the box; the canned and bottled packaging barcodes are preferably printed on the lower side of the label, but the bar code symbol surface curvature should not exceed 30°; the barrel packaging barcode is preferably printed on the barrel. On the side, if the side cannot be printed, the bar code can be printed on the cover, but the depth of the cover should not exceed 13mm; the bag-shaped package has a bottom and the bottom is large, and the bar code can be printed on the bottom surface or printed on the lower center of the back; The bar code is usually printed on the bottom left corner of the back cover or cover, and the direction of the line is parallel to the spine.综 In summary, bar code, as a data input technology and automatic identification technology, is widely used in commodity packaging, and it plays an important role in the production, sale, storage and inspection of goods, and communication of information. Therefore, correctly understanding and understanding the knowledge of barcodes and printing barcodes is an important part of packaging and printing.