Precautions for packaging screen printing
In packaging and anti-counterfeiting printing, bar code labels are convenient for users to quickly read general product information and certain anti-counterfeiting functions, which has a great role in improving product sales management and anti-counterfeiting security. Although it cannot provide detailed information such as product production, quality assurance, management, etc., the anti-counterfeiting effect is not ideal, and it is gradually being replaced by RFID. However, due to its many advantages such as small equipment investment, convenient printing, and wide range of use, bar codes still have many advantages. Occupying a large area of ​​rivers and mountains, especially in the packaging of ordinary goods with low anti-counterfeiting requirements, it still plays an important role. A variety of printing methods can be used for bar code printing. Let's talk about the technical requirements for bar code printing and some issues to be noted in screen printing.
1. Barcode function and its application in packaging and printing
Barcode is an automatic identification information code. Because of its high data collection rate, strong reliability and low cost, it is widely used in many books, stickers, commodities and packaging. The bar code is composed of lines and number symbols of a certain length, width, and width, which can be read by a computer graphic language through a photoelectric scanning reading device. Bar code is a kind of data input technology and automatic identification technology researched and developed in recent decades, which can reflect many types of information such as the type, manufacturer, weight, amount, ex-factory date and circulation room of the merchant. Communication and other aspects play an important role and are the link between manufacturers, exporters, sellers and customers in the world.
The barcodes currently seen on the market are all early one-dimensional barcodes. With the development of ultra-large-scale integrated circuit technology, the two-dimensional barcode information capacity is small and the barcode area is reduced to improve the ability to resist pollution. The dimension bar code (abbreviated as 2D code) makes the area of ​​the bar code smaller and smaller, the function of the bar code reader is also greatly improved, the reading is more precise and accurate, and the storage space of the bar code is increased. In addition, encryption technology can also be used in the two-dimensional bar code, which greatly improves the anti-counterfeiting performance of the goods. According to the difference in structure, two-dimensional bar codes can be divided into two types: stacked and matrix. In printing, not only black characters can be printed on white paper, but also color printing. It is not limited to printing machines and printing objects. The data can be read from line to surface or even rotated 360 °. The fault correction function can be used to make Recovered and identified barcodes that have been damaged or damaged, with a misread rate of 61 million parts. Two-dimensional barcodes also have large storage capacity, strong damage resistance, high security, fax and photocopying, good data concealment, and information. Features such as product walk.
With the development of technology, a variety of new barcodes have been developed, such as invisible barcodes (including covert invisible barcodes, invisible inks, and photochemically processed invisible barcodes), metal barcodes, and watermark magnetic codes, etc., all of which have improved the application of barcodes. Depth and breadth.
Second, the technical requirements of the bar code
The original negatives printed with bar codes must be registered and issued by the International or National Coding Center. In order for the barcode to be correctly recognized by the barcode recognizer, the barcode must be straight and not broken, the edges of the lines should be smooth and sharp, and there should be no jaggies. The distance between the lines should meet the standard, the blackness of the lines should be sufficient, and the contrast should be large. The technical requirements for bar codes are:
(1) Size zoom. The bar code is usually printed directly from the original, and the enlargement or reduction of printing should be in accordance with the technical requirements of the EAN organization. It cannot be arbitrarily carried out. Generally, the zoom magnification should be controlled at 80% to 200%. The barcode zoom rate has a great influence on the printing pass rate. When the zoom magnification rate is 100%, the printing pass rate is 97.3%; when the zoom magnification rate is 90%, the printing pass rate is 95.7%; when the zoom magnification rate is 85%, the printing pass rate 25%; when the zoom ratio is less than 80%, the printing pass rate is only 10%. Therefore, zoom printing is best done by the provider of the bar code film.
(2) Color matching. Since the barcode reading system specifies that the general scanner light source is a red light source with a wavelength of 630 nm to 700 nm, the red light effect of the ink color should be considered.
The incident light of the scanner illuminates the surface of barcodes of different colors, and different reflections will occur. Black ink can completely absorb red light, and the reflectivity of the printed product to the incident light is below 3%, which is the most safe and ideal bar code color; white ink will completely reflect the red light, and the reflectivity of the printed product to the incident light is close to 100 % Is the safest blank color, so bar codes are generally printed in black and white. However, in packaging and printing, in order to increase its decorativeness, other colors are often used to match the empty space. At this time, it is necessary to pay attention to the appropriate combination of colors according to the red light effect. Yellow, orange, red and other colors have high reflectance to red light, and green and purple have low reflectance to red light. As long as it can meet any color matching of bar code requirements for reflectivity, scanning density and PCS value of printing contrast, it is a reasonable bar code printing color design.
(3) Requirements for substrates. In terms of optical characteristics, in order to ensure that the scanning light source is incident at 45 ° and reflected at 15 °, the substrate needs to have good light dispersion characteristics. In terms of materials, paper substrates generally use the white base of the paper itself as empty white, and have certain requirements for the whiteness, opacity, and gloss of the paper. The whiteness requirement is to make the paper surface have better reflection ability; the opacity is required to prevent the incident light from passing through the back of the paper and the light signal is reduced, resulting in a decrease in reflectivity; the lower gloss is required to reduce the incident light Specular reflection effect. For transparent or translucent print carriers, the same color as their packaging content (especially liquid content) should be prohibited as a bar color to avoid the content color deepening the empty color, making the empty color closer to the bar color, and reducing the PCS value.
In practical applications, this problem is often ignored: for example, when printing a white space, dark blue or dark green barcode on a blue or green liquid transparent packaging, the blue and green contents will make the white space appear light blue or light green; in black Barcodes of white spaces and black bars are printed on the transparent packaging of watermelon seeds. The black content will make the white spaces light gray. At this time, the concentration of the printing ink on the white base should be deepened so that the color of the content will not leak out from the base color, or the color combination should be changed to avoid the above phenomenon. When the packaging decoration color conflicts with the bar code design color, the bar code design shall prevail and the packaging decoration design color shall be modified. When the carrier leaks light, the following measures should be taken: develop a base color with the same color as the empty color, large enough area, and sufficient ink to specifically print the barcode; if the barcode is printed on the plastic film seal and there is a decorative part on the back, it should be sealed An opaque interlayer is sandwiched between the two layers to ensure that the color of the back decoration does not affect the PCS value of the barcode. When using reflective materials such as aluminum foil as a carrier, you can roughen the body color or cover a layer of white, yellow, orange and red as the base color. The barcode is printed with black, dark blue, dark green, and dark brown as the bar code; you can also reflect the body The bar code is printed with white, yellow, orange, and red as the empty color, which is called reverse white printing. The principle of reverse printing is still based on this color design to meet the corresponding values ​​of the reflectivity, reflection density and PCS of the specified bar and space. For the requirements of dimensional stability of printing materials, materials with good weather resistance, stable dimensions, good coloring, moderate ink expansion, small 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 all good printing materials for bar code signs. Corrugated cardboard, which is often used in large packaging, may cause large printing errors due to insufficient surface smoothness and uneven ink permeability. Therefore, except for EAN, UPC and ITF codes with large magnification, they are generally not directly used as substrates Instead, it uses the method of affixing printed labels. Non-polar group polypropylene film with poor coloring power and woven tape with poor dimensional stability cannot be used as the substrate for bar code marking.
(4) Requirements for ink. When matching ink colors, the color shift of the ink should be considered. The color deviation of the ink greatly affects the accuracy of the barcode. In theory, as long as the ink is used according to the color ratio, the barcode requirements can be met, but due to the imperfect hue of the printing ink, the color cast phenomenon will occur, such as the blue ink due to the wrong absorption of red light, will cause the red light The reflectivity of the barcode increases, reducing the PCS value of the barcode. Therefore, the color of the ink should be strictly controlled to make the ink density uniform, hue saturated, and high purity. It is best to determine whether the reflectivity of a certain ink under red light meets the requirements before printing the magnetic barcode.
The reflectiveness and glossiness of metallic inks will cause a specular reflection effect and therefore cannot be used for bar code printing. Since the barcode printing is solid printing, the reflection density that can be achieved 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 printed product increases with the increase of the ink thickness. When the ink thickness reaches a certain value , The density will reach saturation, so pay special attention to the ink concentration and ink layer thickness. Different printing processes have large differences in the thickness of the ink layer, offset printing is 2μm ~ 4μm, relief printing 8μm, flexographic printing 10μm, gravure printing 12μm, screen printing can reach 30μm.
According to the test, it can be calculated that the solid reflection density of the printed products obtained by the above printing types can reach more than 0.3, and the black, cyan, blue, green and other colors can all absorb red light. The rate can meet the requirements. The ink viscosity for bar code printing should not be too large, and pay attention to the ink supply and printing pressure during printing. The ink supply is large, and the substrate cannot be completely absorbed in a short time, and will spread on the surface of the substrate, which reduces the accuracy; the ink supply is small, the lines are not full, and even the phenomenon of broken lines; the printing pressure is too large, the ink shear stress Increased, the fluidity will also increase, on the one hand will cause the spread of ink, on the other hand, the platen cylinder and the platen between the impression area becomes wider, will also cause the bar code bar code wider. These will affect the accuracy of barcode printing, so according to different printing methods, different ink rheology and ink absorption performance of the printing material, adjust and control ink supply, printing pressure, printing speed and other factors. [next]
3. Quality requirements for barcode printing
The bar code symbol is read by the computer, which requires it to have a high printing quality. The main influencing factors are the printing method (divided into contact printing and non-contact printing), printing equipment (including inkjet printing, lithography printing, Thermal induction printing, printing printing, etc.), printing location (on-site printing and off-site printing), etc. There are many devices that can complete the barcode on-site printing, and most of them are controlled by computer, which is easy to operate and reliable. These devices have a certain anti-counterfeiting function for printing bar codes on the spot, but the anti-counterfeiting effect varies greatly between models. Different printing equipment generally has different anti-counterfeiting effects. Barcode printing quality requirements are specified in many national standards. There are:
(1) The printed bar code should be neat and clear with no obvious defects in the bar codes, and no extra black dots in the blank characters. Usually the maximum diameter of defects and stains should be less than or equal to 0.4 times the nominal width of the narrowest bar code; or the area occupied by defects and stains should not exceed 1/4 of the area of ​​the circle whose diameter is 0.8 times the nominal width of the narrow strip.
(2) Control the bar code symbol size error. This is a guarantee to increase the first reading rate of the reading device and reduce the decoding error rate (misrecognition rate). Under normal conditions, the printing tolerance is 45% of the total error, the reading device tolerance is 55% of the total error, and the basic size of the barcode also varies with different magnifications.
(3) Requirements for the optical characteristics of bar code symbols. In order to read reliably, after printing, the lines and spaces in the bar code should have obvious contrast. The reflectivity of the space should be as large as possible, and the reflectivity of the bar should be as small as possible. The better the reading performance.
(4) Requirements for printing tolerances. Due to defects in printing equipment, printing plates, printing inks, etc., there will be deviations in bar code printing. This allowable deviation is called printing tolerance. It must be strictly controlled in printing to ensure that it does not affect the reading effect.
(5) Control the edge roughness. Edge roughness refers to the unevenness of the edge of the barcode, indicating the difference in the vertical size of the element. The requirement for edge roughness is that the element width can meet the dimensional requirements of printing tolerances on all possible scanning trajectories.
(6) Ensure the ink layer thickness and color fastness. In the bar code, the thickness of the ink layer of the bar and the air should be controlled within a certain range (<0.1mm), otherwise the bar code and the air will be on different scanning planes, which will affect the accuracy of the bar code reading. In order to ensure a sufficiently small thickness, ink with good uniformity and diffusibility should be selected, and attention should be paid to the matching performance of the printing ink and the printed substrate.
The barcode printing substrate must have sufficient color fastness so that the barcode can maintain a certain contrast value after multiple scanning and reading, and obtain a reliable input signal. The testing method of the color fastness of the substrate is to rub the cotton bar with cotton wool for 5 times, and then conduct an appearance inspection and use an automatic detector to measure the bar code, symbol size error and contrast value. The requirements for the printing position of the bar code should be in accordance with the guidelines of non-deformation, easy to read and easy to make plate printing. For example, select the right side of the main display surface of the product packaging, the plane connected to the main display surface of the product, the back of the main display surface of the product packaging, etc. For different forms of packaging, there are certain requirements for the location of the barcode printing. For example, box packaging, the barcode is best printed on the bottom of the box, and it is best to avoid printing in the center; rectangular box packaging, the barcode is best printed on the long side of the bottom of the box Of the center. For canned and bottled products, the barcode is best printed on one side of the label paper. The surface curvature of the barcode symbol should not exceed 30 °. If the diameter of the packaging container is too small, the barcode symbol should be turned 90 °. For barrel-type packaging, the barcode is best printed on the side of the barrel. When the side cannot be printed, it can be printed on the packaging lid, but the depth of the lid should not exceed 13mm. If the packaging is a leaky liquid, the bar code must not be printed on the lid. For bag-type packaging, if there is a bottom and the bottom surface is large enough, the bar code should be printed on the bottom surface, otherwise it can be printed in the center under the back; for large packaging bags, the bar code is printed on the bottom right side of the back, but should not be printed on too low Position to prevent the bar code from twisting; the small plastic bag or paper bag without a bottom, the bar code is printed in the center below the back. If there is a seam in the center of the back, it will be printed on the lower right side or printed on the place where it will not be wrinkled or deformed; after the vacuum packaging is evacuated, the packaging bag will cling to the surface of the content, showing an irregular three-dimensional shape, the surface is uneven It will cause the bar code to be deformed and cannot be read. Therefore, it is not suitable to print the bar code directly in the inner bag of the vacuum packaging. An outer bag can be added to print the bar code and other graphics. For blister packaging, the barcode is best printed on the cardboard, and the height of the protruding packaging from the cardboard must not exceed 12mm; when the height of the protruding packaging is greater than 12mm, the barcode should be placed as far as possible from the protruding packaging. Bar codes of other products can be printed on the listing and other locations. During imposition, the direction of the bar code should be consistent with the printing direction, so that the deformation of the bar code occurs at the height of the bar code, without affecting the width and size of the bar code and the accuracy of reading.
4. Issues to be noted in screen printing barcodes
Screen printing is a relatively simple process in all bar code printing methods. In order to ensure the printing quality of bar codes, printing techniques and skills should be improved during the printing process. Generally, the 22.5 ° stretch method is adopted, that is, the angle between the warp and weft direction of the mesh and one side of the frame is 22.5 °. The oblique stretching method can effectively reduce the barrier phenomenon of the mesh to the ink and prevent the occurrence of wire breakage. It is better to choose 300 mesh for the screen. The mesh is too low. The diameter of the mesh is too thick, which will block the ink leakage. The mesh is too high, the amount of ink deposited is too small, and the contrast of the barcode is too small. Thickness should be at least 8μm, which is the key to improving contrast and clarity. When measuring the thickness, use a film thickness meter. When using, press the metal foil with a diameter of 30 mm on the wire mesh. First, let the area not coated with photosensitive adhesive contact the foil, and press the sensor head of the thick film instrument to the metal foil. Read the reading on the LCD screen on the screen (note that it must be cleared before measuring). Then use the same method to measure the thickness of the screen after coating the photosensitive adhesive. The difference between the two is the thickness of the adhesive layer. The thickness gauge can effectively control the randomness of the operation process, which is conducive to the standardization of enterprises and the development of data work.
In the process of screen printing and plate-making, whether the pretreatment of the screen is completely directly related to the fastness of the combination of the photoresist and the screen. Before degreasing, the screen can be immersed in water by grinding with a mesh paste. Use a brush to dip the mesh paste on the printing surface of the screen and back and forth for about 5 minutes. Let the mesh paste fully penetrate into the wire diameter, and then use the degreasing solution for degreasing treatment. Quite a lot of enterprises use household detergent instead of degreasing liquid to remove oil stains on the Internet. This method is not correct. Because there is an ingredient of sheep fat oil in household detergents, it is a softener with relatively mild performance and can protect the skin from damage; but it is also an isolating agent, which will be the effective ingredient of coarse powder and degreasing liquid Isolation will affect the coarsening effect of the grinding net.
When judging the effect of pre-treatment of the net, the screen can be immersed in water and then taken out. When the scraper surface and the printing surface are uniformly left with a layer of water wool, it indicates that the roughening treatment is good, and the place where the water wool surface cannot appear locally indicates that the roughening Incomplete, we have to do it again. The high-quality screen embodies the unification of scientific process steps and perfect operation methods. From the perspective of the development of modern printing technology, either the process steps are completely changed or the operation mode is optimized. As an enterprise employee, it is necessary to gradually regulate and change unreasonable operation methods based on practical experience. Plate-making staff sometimes do not pay attention to the drying of the screen after processing, thinking that it is enough to get rid of the moisture on the screen. Therefore, we must firmly avoid incorrect practices such as throwing the screen by hand, blowing the screen with a hair dryer, or directly putting the wet screen in the oven, and drying it with a towel. These practices cannot be said to have a fatal impact on the production of screens, but they are indeed not conducive to the drying of screens. First, after the screen treatment, the moisture has a strong adhesion to the screen, especially the water in the screen mesh hole can not be removed, the jittering blower will cause an unstable temperature zone on the screen; second, in the air There is dust, and it is inevitable that particles will blow onto the surface of the wire mesh. It is impossible to wipe the moisture in the mesh with a towel. It is even more wrong to put the wet screen directly in the oven. It will make the drying process change. It takes a long time, and more importantly, it will damage the thermal stability of the wire mesh. Use a water-absorbent towel, soak it in water, wring it dry, roll it up into a tube, spread it on the printing surface of the screen, and use the capillary principle to absorb the moisture on the printing plate. The method does not use the towel to wipe dry as the friction movement with the wire mesh, the effect is better. Only after there is no visible moisture on the surface of the screen, it can be dried in the oven. The drying temperature is about 40 ℃. The drying process of the screen should evaporate the water.
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