Plastic flexible packaging products are usually printed on various plastic films, and then composite films are formed by compounding the upper barrier layer and the heat sealing layer, and the products are formed by slitting and bag making. Printing is the first process of production, and it is also the most difficult process to control. Most of the quality problems of products appear in this process. To measure the grade of a packaged product, the printing quality is the first. Therefore, understanding and controlling the printing process and quality has become the key to flexible packaging production.
Gravure printing is the main printing method for plastic film printing today. Plastic film using gravure printing has the advantages of fast speed, rich colors, clear and bright, moderate contrast, vivid image, and diverse product specifications. The gravure printing has high printing durability, and the large-volume prints with more than one million prints have their advantages.
The quality requirements of gravure plastic film products are relatively high, and the quality standards of its printing appearance are as follows:
â‘ After the matching process, the registration error of each color pattern is not more than 0.3mm.
â‘¡Check the specifications of the film, the registration error of each color pattern is not greater than 0.3mm.
â‘¢ Check the ink color of the printed product, require correct hue, bright, uniform, bright ink color, soft ink color, and smooth imprinted edges.
â‘£The printing fastness requires the transparent tape to stick and pull the ink without discoloration.
⑤ The pattern text lines should be clear and complete, not incomplete or deformed.
â‘¥ The texture needs rich levels and clear dots.
⑦The product is neat and tidy, without obvious dirt and wire.
⑧ It is required that the same-color density deviation and the same-color color difference of the same batch of products meet the requirements of GB7707-87.
In order to better meet the above quality requirements, we can control the gravure plastic film process from the following main aspects:
1. Master the process essentials of various plastic packaging film substrates
①BOPP is the most commonly used printing substrate, and its production process is a typical plastic gravure production process; polyamide or chlorinated polypropylene ink is used for printing, appropriate tension and embossing force, and the drying temperature is controlled below 80 ℃.
â‘¡BOPET printing points: Because PET is usually thin, it is easy to wrinkle, it needs a lot of tension when printing, it has certain selectivity to the ink, it is easy to delaminate with the general printing ink, the film is static electricity, and it is easy to block the plate ), Fluffing and scratch marks (knife wire) phenomenon, it is beneficial to have high humidity in the workshop when printing.
③BOPA printing points: BOPA is a film that is easy to absorb moisture and deform. Therefore, it must be ensured that the humidity of the printing machine environment is not too high when printing (if there is no dehumidification measures, the environment with high humidity is not suitable for printing nylon films); the unprinted film cannot damage its original aluminum film packaging film, and it must be printed immediately after opening When printing, the nylon film needs to be preheated (if the machine does not have a preheating device, the printing plate can be installed in the second color, and the first color is vacated for preheating), the preheating temperature is 50 ~ 80 ℃; The printing tension should be properly reduced; the pressure should be moderate; the ink has strong adaptability, depending on the choice of product structure. If it is a cooking bag, the cooking ink must be selected; after printing, the product must be tightly packed and stored in a dry and mature room, and as soon as possible. complex.
â‘£K coating film (KOP, KPA, KPET) printing points: because the K coating film is roller-type or spray-type, its thickness uniformity is not very good, the surface is not smooth, and the PVDC is just brittle, when printing The printing tension and imprinting pressure should not be too large, the difficulty of overprinting is difficult, and the ink transferability is poor. It is easy to produce dots (some dots are lost) when printing a light screen, and a high-hardness pressure roller is required for printing. In addition, it is selective for solvents. If the solvent is used improperly, the coating may be dissolved down. The residual amount of the printed film solvent is large and it is easy to block. Therefore, special attention should be paid to drying and cooling. The matting film can be printed with OPP on its glossy surface, but because the matting layer on its surface cannot withstand high temperatures, the drying temperature needs to be controlled.
⑤Pearl film uses surface printing ink for surface printing, and try to choose surface printing ink with good transparency. When the heat shrinkable film is printed, a very low drying temperature should be used, and a special heat shrinkable ink should be selected (the printed ink layer should be able to shrink under heat without falling), and if it is a PVC heat shrinkable film, the dissolution of the solvent to the film must also be considered Sex.
â‘¥ The tension of unstretched PP and PE film printing should be very small, and the difficulty of overprinting is great. When designing the pattern, the printing deformation should be fully considered. [next]
2. Formulation of pre-press process
â‘ Plate design. When making a plate, the manuscript designed by the design company needs to be modified according to the characteristics of gravure printing, so that it can be more suitable for printing needs.
For the purpose of overprinting, such as thin lines and text, multi-color overprinting is not allowed. There should be no small inverse characters on the overprinting pattern, and no blank should be left on the overprinting pattern. Other small text should be overlaid. Strict version, the black, blue, red, yellow version should not be separated by other color versions. For printing needs, the bar code should be arranged so that the line direction is consistent with the circumferential direction of the plate cylinder. In terms of color guarantee, if conditions permit, it is better to make the ground background color and the gradation pattern version separately, and try to use a special edition for the ground background color. The screen hanging version must fully consider the two color jump areas of 80% and 5%, and the dot limit should be more than 10% when the screen is gradually changing.
â‘¡Ink mixing. The adjustment of the ink printing adaptability, such as the viscosity and concentration of the ink, the principle is to adjust the ink viscosity with a thinner (solvent) to adjust, before printing, determine the ink viscosity according to the conditions of the plate roller and the machine, and according to the solvent Solvents are often added in case of volatilization, but not in excess. Try to ensure that the viscosity of the ink remains unchanged. When the ink is to be flushed, a diluent (toner oil) must be used. Never use a solvent to dilute.
Ink color allocation. In this regard, the following points need to be noted: the light source for observing the color should use the standard light source (D65) as much as possible. If there is no standard light source, it is better to be in the direction of the north window with sunlight. When preparing spot colors, primary color inks should be used for blending. The primary color inks referred to here refer to inks directly produced by pigments used in ink factories. The brightness and saturation of such inks can be guaranteed. Use as few ink colors as possible for color matching. The more color numbers, the greater the color matching error, and the more difficult it is to recolor.
3. Proofing
When a set of printing plates is printed, a standard proof must be confirmed first. Some manufacturers use the proofs made by the plate-making factory as standard samples, and the samples cannot be matched at the time of final printing and delivery. Therefore, the printing machine must be used for proofing before printing, which is confirmed by the customer or related personnel as the standard sample. The printing of standard samples should be carried out under the normal printing process conditions, otherwise it will be very difficult to follow the samples when printing, so the following points should be noted when proofing:
â‘ Process conditions Select the most basic printing process conditions.
â‘¡ Choose the ink viscosity as low as possible.
â‘¢ The primary color ink should be sampled after adding a certain amount of diluent. Try to use less frequently used especially contaminated ink for sample.
â‘£It is absolutely forbidden to do all non-standard operations in order to excessively pursue the effect during proofing, such as increasing the density and viscosity to increase the hue, adjusting the pressure, and the scraper to increase the transfer rate of ink.
⑤Ink must be stirred before it can be used, especially spot color ink.
â‘¥ The spot color ink prepared by yourself must be scraped and accurately recorded and kept in the file for reference for future preparation.
⑦Try to achieve the normal printing speed as much as possible during proofing.
⑧If conditions permit, after confirming the basic color of the sample, first print a certain number of products (such as a roll of film), and then take the sample to confirm it as a standard sample, and seal it as a file.
⑨After confirming the standard sample, the connected 2 ~ 3m should be cut as the equivalent standard sample, and kept in a place without heat source and no ultraviolet ray, and the cut part will be used as the print sample for each subsequent printing.
4. Common troubleshooting
â‘ Dirty version: In the plastic gravure printing, the non-printing surface of the cylinder is often stuck with a hazy layer of resin (resin entrained in the resin), which is transferred to the substrate as a sheet stain or a line stain , This is the dirty version. The dirty plate phenomenon is related not only to the hardness of the blade, the pressure of the blade, the contact angle of the blade, and the quality of the plate cylinder, but also to the viscosity of the ink. When the printing viscosity is too high, the fluidity of resins, pigments and other substances is poor, and they cannot be evenly dispersed and piled together, causing damage to the blade and plate cylinder under high-speed operation, resulting in linear stains. At the same time, because the resin is also in a supersaturated state, a large amount of resin accumulates. Under the pressure of the blade, the resin adheres to the roller due to the affinity of the resin to the roller (the resin is entrained with pigment), and flakes appear. In general, the lower the ink printing viscosity, the less likely the dirty version will be.
â‘¡Incorrect color registration: mainly due to the deviation of the diameter of the intaglio cylinder; the graphic is deformed due to the incorrect plate installation; the spacing of the shaft teeth is uneven, the gap is too large or too small, or the phenomenon of top teeth causes inaccurate overprint; the film is opened Uneven tension control of the rewinding shaft or uneven thickness of the gravure film and ruffles; uneven pressure on both ends of the plate cylinder and the platen cylinder or inconsistent pressure between plates during multi-color overprinting.
â‘¢ Dimension deviation: insufficient or excessive shrinkage margin. Due to the influence of plastic film tension and heating and drying during printing, the longitudinal dimension of the film changes greatly. Control countermeasures: Adjust printing tension appropriately to prevent dimensional deviation caused by improper temperature setting.
â‘£Reverse printing (namely bite color): Reverse printing refers to the reverse printing of the ink of the previous color to the printing plate of the latter color in the overprint. Causes: The ink layer failed to dry completely after printing the previous color; the pressure of the second color was too high, or the machine speed was too fast, or the hot and cold air was poor; the substrate printing suitability was poor. Improve the adhesion of the previous color ink, adjust the viscosity of the ink, increase the amount of hot and cold air, increase the drying speed of the ink; slow down the vehicle speed; increase the ink supply of the latter color; reduce the viscosity of the latter color; correctly arrange the color sequence of the printing plate; Reduce the pressure of the impression cylinder, or use a soft rubber roller.
⑤ Whitening: The resin does not dissolve and precipitate; it dries too quickly. Control countermeasures: re-make ink, add slow-drying solvent appropriately. There are many failures in gravure printing. In the actual printing operation, the main reasons should be carefully analyzed. It can be considered to start from the registration control, tension control, ink control, temperature control, etc., and quickly eliminate them to ensure the smooth progress of production. This requires skilled techniques, timely grasp of the causes of the above common quality problems, and decisive measures to solve the plastic film gravure quality problems in a timely manner.
â‘¥ The pattern lines are incomplete: the hot air is too large, the ink is easy to dry, the ink dries too fast in the anilox line, and the accumulated ink is dry and solid on the surface of the printing plate; the pressure between the printing plate and the impression cylinder is too small; the angle of the doctor blade is not ideal, Insufficient pressure on the plate surface; unevenness of the printing plate or white spots, shallow printing plate, burrs, less ink adhesion; ink is too thin; printing speed is too slow. Add butanol in time to clean the plate area ink; adjust the pressure; adjust the angle of the doctor blade and the pressure on the plate surface; deepen the printing plate and remove burrs; add the original ink appropriately; adjust the printing speed.
⑦Color difference: Color difference refers to the phenomenon of color difference between rolls during printing. Reasons: The viscosity of the ink is not adjusted well; the gravure cylinder blocks the plate; the printing staff does not work properly.
⑧ Blocking: During printing, with the large amount of solvent volatilization, the viscosity of the ink increases, and it is easy to dry inside the screen; in the printing process, the position of the scraper is far from the printing roller or the angle is unreasonable, the diameter of the plate roller is large or the plate cylinder enters When the depth of the ink tank is not enough, the hot air blown from the drying pipe affects the surface of the plate cylinder, which is likely to cause serious blockage; impurities are mixed in the ink; the engraving depth of the plate cylinder screen is not suitable; the temperature in printing is too high because the solvent in the ink volatilizes quickly .
The process of gravure printing plastic film is more difficult to control, so we should sum up some experience in printing to get our own quality control method.
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