Problems with Screen Printing Bar Codes

In packaging and anti-counterfeit printing, bar code labels are convenient for users to quickly read commodity general information and certain anti-counterfeiting functions, and have a great effect on improving product sales management and product anti-counterfeit security. Although it cannot provide detailed information such as product production, shelf life, and management, the anti-counterfeit effect is not ideal, and it has gradually been replaced by RFID (radio frequency identification tag), but the barcode has the advantages of small investment in equipment, convenient production of printing, and application range. Many advantages, such as wide use, still occupy a large area of ​​rivers and mountains, and it still plays an important role in the packaging of ordinary goods with low anti-counterfeiting requirements. Bar code printing can use a variety of printing methods, the following will talk about bar code printing technology requirements and some issues that need to pay attention to in screen printing.

First, the function of bar code and its application in packaging and printing

Bar code is an automatic identification information code. Due to its high data collection rate, high reliability and low cost, it is widely used in many books, stickers, goods and packaging. The bar code is composed of a certain length, width and width of lines and numbers, and can be read by the photoelectric scanning reading device graphics language. Barcodes are kinds of data input technologies and automatic identification technologies that have been researched and developed in recent decades. They can reflect many types of information such as the type, manufacturer, weight, amount, date of manufacture, and circulation room of suppliers, and are used in the production, sales, storage, and inspection of commodities. Exchanges and other aspects play a major role in linking manufacturers, exporters, sellers, and customers around the world.

The bar codes currently seen on the market are all early one-dimensional bar codes. With the development of VLSI technology, the capacity of one-dimensional bar code information is small, the bar code area is reduced, and the ability to resist pollution is improved. Dimensional barcode (2 D code for short) makes the area of ​​the barcode smaller and smaller, and the function of the barcode reader is greatly improved. The reading is more precise and accurate, and the storage space of the barcode is increased. In addition, encryption technology can also be used in two-dimensional bar codes to greatly improve the anti-counterfeiting performance of products. According to the structural differences, two-dimensional barcodes can be divided into two types: stacked and matrix. In printing, not only black characters can be printed on white paper, but also can be printed in color, regardless of the printing machine or the printing object. The reading of data can be rotated 360° from line to surface, and the malfunction correction function can be used. Damaged or damaged bar code recovery recognition, misread rate of 6.1 million, two-dimensional bar code also has a large amount of storage, damage resistance, high security, fax and photocopying, data hiding, Information follows the characteristics of the product.

With the development of science and technology, many new types of barcodes have been developed today, such as invisible barcodes (including covert invisible barcodes, invisible inks, and photochemically processed invisible barcodes), metal barcodes, and watermarked magnetic codes, all of which enhance the use of barcodes. Depth and breadth.

Second, the technical requirements of the bar code

Original negatives printed with barcodes must be registered and issued by an international or national coding center. To make the bar code be correctly identified by the bar code recognizer, it is required that the bar code be straight, not broken, the edges of the lines should be smooth and sharp, no jaggies should appear, the distance between the lines should be in line with the standard, the line blackness should be sufficient, and the contrast should be large. The technical requirements for barcodes are: (1) Size scaling. Barcodes are usually printed directly from the original, and the technical requirements of the EAN organization must be magnified or reduced. Printing is not arbitrarily carried out. Generally speaking, the zoom magnification should be controlled at 80 to 200 percent. The bar code zoom rate has a great influence on the printing pass rate. When the zoom magnification is 100%, the pass rate is 97.3%; the zoom ratio is 90%, the pass rate is 95.7%; the zoom ratio is 85%, the pass rate is It is 25%; while the zoom ratio is less than 80%, the print pass rate is only 10%. Therefore, zoom printing is preferably performed by the provision mechanism of the barcode film.

(2) Color matching. Since the bar code reading system provides that the general scanner light source is a red light source with a wavelength of 630 to 700 nm, the red light effect of ink should be considered. The incident light of the scanner is irradiated on the surface of different color barcodes, and different effects of reflection may occur. Black ink can completely absorb red light, the reflectivity of the incident light on the print is below 3%, is the most safe and ideal bar code color; white ink is completely reflective of red light, the reflectivity of the print on the incident light is close to 1 00% is the safest blank color, so bar codes are generally printed in monochrome. However, in the packaging and printing, in order to increase its decorativeness, it is often the case that other colors are used in conjunction with empty spaces. At this time, it is necessary to pay attention to selecting the right mix according to the red effect of the color. There are yellow, orange, red, etc. colors with high red reflectivity, and green, purple, and other colors with low red reflectivity. Any color combination required by bar codes for reflectivity, scan density, and print contrast PCS values ​​is a reasonable bar code print color design.

(3) Requirements for substrates. In terms of optical characteristics, in order to ensure that the scanning light source is 45° angle incident and 15° reflective, the substrate is required to have good light dispersion characteristics. In respect of materials, paper substrates generally have white blanks on the paper itself, and have certain requirements for the whiteness, opacity and gloss of the paper. The whiteness requirement is to make the paper surface have a better reflective ability; the requirement of opacity is to prevent the incident light from passing through the back of the paper and reduce the light signal, resulting in a decrease in reflectivity; a lower gloss is required to reduce the incident light. Specular reflection effect. For transparent or translucent printing carriers, the same color as the contents (especially the liquid contents) of the package should be disabled as the stripe color to prevent the color of the contents from deepening the space color, making the empty color closer to the stripe, and lowering the PCS value.

This problem is often overlooked in practical applications. For example, a white open space, dark blue or dark green bar code is printed on a transparent package of blue or green liquid, and the blue and green contents make the white space pale blue or light green; on black The barcode of the white open space and the black bar is printed on the transparent packaging of the watermelon seeds, and the black content will make the white open space grayish. At this time, the concentration of the white-base printing ink should be deepened so that the color of the contents will not be revealed from the base color, or the color matching may be changed to avoid the occurrence of the above phenomenon. When there is a conflict between the color of the packaging and decorating design and the color of the bar code design, the bar code design shall prevail and the color of the packaging and decorating design shall be modified. When the carrier leaks light through the color, the following measures should be taken: open up a color with the same space color, large enough, the ink is strong enough to print the bar code; if the bar code is printed on the plastic film seal and the back part of the decoration, should be sealed An opaque interlayer is sandwiched between the two layers to ensure that the color of the backside decoration does not affect the bar code PCS value. When aluminum foil or other reflective material is used as a carrier, the color of the body can be roughened or the base color of white, yellow and orange red can be blank, and the bar code can be printed in black, dark blue, dark green, or dark brown; the body of the reflective material can also be used. For bar color, printing bar codes with white, yellow, orange, and red blank colors is called reverse printing. The principle of reverse printing is still based on this color design that satisfies the required reflectivity, reflection density, and PCS corresponding values ​​of bar and space. For the dimensional stability requirements of the printing materials, materials with good weather resistance, dimensional stability after stress, good colorability, moderate ink expansion, low permeability, moderate smoothness and smoothness should be selected. Coated paper, offset paper, whiteboard paper in paper, biaxially oriented polypropylene film in plastic, aluminum foil in metal, and tinplate are good substrates for bar code marking. The corrugated cardboard often used in large packages may not be directly used as a substrate, except for large-scale EAN, UPC, and ITF codes because the surface is not sufficiently smooth and the ink permeability is not uniform, which may cause large printing errors. Instead, it uses adhesive printing labels. Non-polar group polypropylene films with poor tinting strength and braids with poor dimensional stability cannot be used as substrates for bar code marks.

(4) The requirements for the ink. In ink color matching, ink color shift should be considered. The color shift of the ink has a great influence on the accuracy of the bar code. In theory, as long as the use of ink in accordance with the color matching can meet the requirements of the bar code, but due to defects in the printing ink impure defects, color cast phenomenon occurs, such as the blue ink due to the wrong absorption of red light, will cause red light The reflectance rises, reducing the PCS value of the bar code. Therefore, the color of the ink should be strictly controlled so that the density of the ink is uniform, the saturation of the hue is high, and the purity is high. It is preferable to determine whether the reflectance of a particular ink under red light meets the requirements before printing a magnetic barcode.

Reflectiveness and gloss of metallic inks (such as gold) can cause specular reflections and cannot be used for bar code printing. Since bar code printing is field printing, the reflection density that can be achieved by printing is related to the optical characteristics of the ink and the ink layer thickness. In the printing process, the reflection density of the printing ink increases with the increase of the ink thickness, and when the ink thickness reaches a certain value The density is saturated, so pay special attention to the ink concentration and ink layer thickness. Different printing processes, the ink layer thickness is quite different, offset printing is 2 ~ 4 μm, embossing 8 μm, flexo printing 1 0 μm, gravure printing 1 2 μm, screen printing can reach 30μm.

According to the test, the reflective density of printed matter obtained from the above printing types can reach 0.3 or more, and black, cyan, blue, and green colors can absorb all the red light. Therefore, the bar code stripe colors and reflections are printed using the above-mentioned printing methods. The rate can meet the requirements. Barcode printing ink viscosity should not be too large, and in the printing should pay attention to the amount of ink and printing pressure. The amount of ink supply is large, the substrate cannot be completely absorbed in a short time, and it will spread on the surface of the substrate to reduce the precision; the ink supply is small, the lines are not full or even broken, etc.; the printing pressure is too high, the ink shear stress When it is enlarged, the fluidity also becomes larger. On the other hand, the ink spreads. On the other hand, the printing area between the plate cylinder and the impression cylinder becomes wider, and the bar code symbol width becomes wider. These will affect the accuracy of bar code printing, so according to different printing methods, different ink rheology and ink absorption properties of substrates, adjust the control of ink supply, printing pressure, printing speed and other factors.

Third, the bar code printing quality requirements

The bar code symbol is read by the computer, which requires its high printing quality. Its main influencing factors are printing methods (contact printing and non-contact printing), printing equipment (inkjet printing, photolithographic printing, Thermal-induction printing, print-printing, etc.), printing sites (on-site printing and off-site printing), etc. There are many devices that can complete the on-site printing of barcodes, and most of them use computer control, which is easy to operate and reliable. The use of these devices to print bar codes on the spot has certain anti-counterfeiting capabilities, but there are major differences in anti-counterfeiting effects among various models. Different printing devices generally have different anti-counterfeiting effects. The quality requirements for bar code printing are specified in many national standards. There are:

(1) The printed bar code should be neat and tidy, and there should be no incomplete defects. There should be no residual black spots in the blank. It is generally stipulated that the maximum diameter of the defect and stain should be less than or equal to 0.4 times the nominal width of the barcode of the narrowest line; or the area occupied by the flaw and stain should not exceed 1/4 of the circular area with a diameter of 0.8 times the nominal width of the narrow strip.

(2) Control the bar code symbol size error. This is to improve the readability of the reading device and reduce the decoding error rate (error rate). Under normal conditions, the print tolerance is taken as 45% of the total error, and the reading device tolerance is 55% of the total error. At the same time, the bar code basic size tolerance also varies with different magnifications.

(3) Optical property requirements for bar code symbols. For reliable reading, the line and space in the bar code should be obvious after printing

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