Gravure plastic film quality control
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Plastic flexible packaging products are generally printed on various plastic films, and then composite film is formed by laminating a barrier layer and a heat sealing layer, and is cut into a bag to form a product. Printing is the first process of production and the most difficult process to control. Most of the quality problems of products appear in this process. Measuring the grade of a packaged product, the printing quality is the first. Therefore, understanding and controlling the process and quality of printing has become the key to flexible packaging production.
Gravure printing is the main printing method for plastic film printing today. The plastic film uses gravure printing with the advantages of fast speed, rich color, clear and bright, moderate contrast, vivid image and various product specifications. Gravure printing has high printing durability, and large-volume printing of more than one million impressions has its advantages.
The quality of gravure plastic film products is relatively high, and the quality standards for printing appearance are as follows:
1 After the flowering process, the registration error of each color pattern is not more than 0.3 mm. 2 Check the specifications of the film, and the registration error of each color pattern is not more than 0.3mm. 3 Check the ink color of the printed product, and require the hue to be correct, bright, uniform, bright in ink, soft in ink, and smooth in the edges of the print. 4 printing fastness, requires the transparent tape to stick the ink without discoloration. 5 pattern text lines, requiring clear and complete, no defects, no deformation. 6 Reticulation requires a rich hierarchy and clear outlets. 7 The product is neat and tidy, with no obvious dirt or knives. 8 It is required that the deviation of the same color density of the product and the same batch of the same color and color difference meet the requirements of GB7707-87.
In order to better meet the above quality requirements, we can control the gravure film process from the following main aspects:
1. Master the technical points of various plastic packaging film substrates
1.1 BOPP is the most commonly used printing substrate. The production process is a typical plastic gravure production process; it is printed with polyamide or chlorinated polypropylene ink, with appropriate tension and embossing force, and the drying temperature is controlled below 80 °C.
1.2 BOPET printing points: Because PET is usually thin, easy to wrinkle, requires a large tension during printing, has a certain selectivity to the ink, easy to leave the layer with general printing ink, the film is static, easy to block up (not online ), raising hair and scraping marks (knife wire) phenomenon, the humidity of the workshop is good when printing.
1.3 BOPA printing points: BOPA is a film that is easy to absorb moisture. Therefore, the printing machine must ensure that the ambient humidity of the printing machine should not be too high (if the environment with high humidity without dehumidification measures is not suitable for printing nylon film); the unprinted film can not destroy the original aluminum film packaging film, and should be printed immediately after opening. The nylon film needs to be preheated during printing (if the machine does not have a preheating device, the printing plate can be installed in the second color, and the first color is used for preheating), and the preheating temperature is 50 to 80 ° C; The printing tension should be appropriately reduced; the pressure should be moderate; the ink adaptability is strong, depending on the product structure selection, if it is a retort pouch, the cooking ink must be selected; after printing, the product must be sealed and placed in the dry and ripening room for storage, as soon as possible. complex.
1.4 K coating film (KOP, KPA, KPET) printing points: Because the K coating film is either roll-coated or spray-coated, its thickness uniformity is not very good, the surface is not flat, and PVDC is just brittle, during printing The printing tension and the embossing pressure should not be too large, the overprinting is difficult, the ink transferability is poor, and the printing of the shallow screen is easy to take place (some of the dots are lost), and it is necessary to use a higher hardness pressure roller for printing. In addition, it is selective for the solvent. If the solvent is used improperly, the coating may be dissolved. The residual amount of the solvent in the printing film is large and it is easy to stick, so special attention should be paid to drying and cooling. The printing of the matte film can be printed on the smooth surface by the OPP process, but the matte layer on the surface cannot withstand high temperatures, and the drying temperature needs to be controlled.
1.5 pearlescent film is printed with surface-printing ink, and as far as possible, a transparent ink with good transparency is used. Heat shrink film printing should use a very low drying temperature, choose a special heat shrinkable ink (printed ink layer should be able to shrink by heat without falling), if it is a PVC heat shrinkable film, consider the solvent to dissolve the film. Sex.
1.6 unstretched PP, PE film printing tension is small, overprinting is difficult, the design of the pattern should fully consider the amount of deformation of the printing.
2. Preparation of prepress technology
2.1 plate design. At the time of plate making, it is necessary to modify the original designed by the design company according to the characteristics of gravure printing, so that it can be more suitable for printing needs.
For the need of overprinting, such as small lines, text can not be multi-color overprinting, the color pattern can not have small anti-white characters, should not be left blank in the color pattern, put on other small text, require overprinting for portraits, etc. Strict version, black, blue, red, yellow version in the middle as far as possible without other color version. For printing purposes, the bar code should be arranged as if the line direction is consistent with the circumferential direction of the plate. In terms of color assurance, if there are conditions, it is best to separate the large ground color and the layer pattern version, and use the special version as much as possible. The hanging version must fully consider the two color jump zones of 80% and 5%. When the network is tapped, the limit of the dot should be above 10%.
2.2 ink blending. The adjustment of the ink printing adaptability, such as the viscosity and concentration of the ink, is based on the principle that the viscosity of the ink is adjusted with a diluent (solvent), and the viscosity of the ink is determined according to the condition of the plate roll and the machine before printing, according to the solvent in the printing process. Volatile conditions often add solvents, but not excessive, try to ensure that the viscosity of the ink does not change; when the ink is to talk, you must use a thinner (toner oil), should not use solvents to dilute.
The color of the ink is blended. In this regard, you need to pay attention to the following points: 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 should be in the direction of the north window with sunlight. When formulating the spot color, the original color ink should be used for the preparation. The primary color ink mentioned here refers to the ink directly produced by the ink factory, and the brightness and saturation of the ink color can be guaranteed. Color matching with as few ink colors as possible. The more the color number, the greater the color matching error, and the more difficult it is to recolor the color.
3. Proofing
When a set of plates is printed, a standard proof must be confirmed. Some manufacturers take the proofs of the plate making factory as the standard samples, and the final print delivery can not be the same, so the printing machine must be used before the printing, and confirmed by the customer or the relevant personnel as a standard. The standard sample printing should be carried out under normal printing process conditions, otherwise it will be very difficult to trace the sample after printing, so the following points should be noted when proofing:
3.1 Process conditions select the most basic printing process conditions.
3.2 The ink viscosity should be chosen as low as possible.
3.3 Primary color inks should be sampled after adding a certain amount of thinner, and try to use less contaminated inks that have been used repeatedly for proofing.
3.4 It is absolutely forbidden to perform 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 scraping the blade to increase the transfer rate of the ink.
3.5 The ink must be stirred evenly before use, especially for spot color inks.
3.6 The spot color inks that are self-adapted must be accurately recorded and recorded for future reference.
3.7 Try to reach the speed of normal printing when proofing.
3.8 If 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 sample and confirm as a standard sample, sealed as archive data.
After confirming the standard sample, the 2 to 3 m connected to it should be cut as the equivalent standard sample, and it should be properly stored in the place where there is no heat source and no ultraviolet light. After each printing, the cutting part is used as the printing sample.
4. Common troubleshooting
4.1 Dirty version: In the plastic gravure printing, the non-printing surface of the roller is often stuck with a layer of resin (the pigment is entrained in the resin), which is transferred to the substrate and is stained in a sheet form, or appears as a line of dirt. This is the dirty version. In addition to the hardness of the blade, the blade pressure, the blade contact angle, and the quality of the plate cylinder, the dirty plate phenomenon is also related to the viscosity of the ink. When the printing viscosity is too large, the resin, the pigment, and the like have poor fluidity, cannot be uniformly dispersed, and are piled up together, causing the blade and the plate cylinder to be damaged under high-speed operation, causing line-like stains. At the same time, since the resin or the like is also in a super-saturated state, a large amount of resin is piled up together, and under the blade pressure, the resin adheres to the drum due to the affinity of the resin to the drum (the pigment is entrained in the resin), and sheet-like stains appear. In general, the smaller the ink printing viscosity, the less likely the dirty version problem will occur.
4.2 sets of color inaccuracy: mainly the deviation of the diameter of the gravure cylinder; due to the incorrect plate loading, the deformation of the picture; the uneven spacing of the shaft teeth, the gap is too large or too small, or the top tooth phenomenon causes the overprinting is not allowed; The tension of the take-up reel is unbalanced or the thickness of the gravure film is uneven, and the edge of the gravure film is uneven; the pressure between the plate cylinder and the impression cylinder is uneven or the pressure between the plates is inconsistent when the multi-color overprint is applied.
4.3 Dimensional deviation: insufficient or excessive shrinkage allowance. Due to the influence of the tension of the plastic film and the heat drying during printing, the longitudinal dimension of the film changes greatly. Control measures: Adjust the printing tension appropriately to prevent dimensional deviation caused by improper temperature setting.
4.4 reverse printing (ie, bite color): offset printing means that the ink of the previous color in the overprint is reverse printed onto the printing plate of the latter color. Cause: The ink layer was not completely dried after the previous color printing; the second color pressure was too large, or the machine speed was too fast, or the hot and cold air was bad; the substrate printing was poor. Improve the adhesion of the previous color ink, adjust the viscosity of the ink, increase the hot and cold air volume, increase the drying speed of the ink; slow down the vehicle speed; increase the ink supply amount of the latter color; reduce the viscosity of the latter color; correctly arrange the color sequence of the printing plate; Reduce the pressure on the impression cylinder or use a soft rubber roller.
4.5 whitening: the resin does not dissolve or precipitate; drying is too fast. Control measures: Re-ink, properly add slow-drying solvent. There are many faults in gravure printing. In the actual printing operation, the main reason should be carefully analyzed. It can be considered from the aspects of register control, tension control, ink control, temperature control, etc., and quickly eliminated to ensure smooth production. This requires skilled technology, timely grasp the reasons for the above common quality problems, and take decisive measures to solve the problem of gravure quality of plastic film in a timely manner.
4.6 pattern lines are incomplete: the hot air is too large, the ink is easy to dry, the ink is dried too fast in the mesh line, and the ink is dried on the surface of the printing plate; the pressure between the printing plate and the impression cylinder is too small; the angle of the squeegee is not ideal. The plate pressure is not enough; the printing plate has uneven or white spots, the plate is shallow, there are burrs, the ink is less attached; the ink is too thin; the printing speed is too slow. Add butanol in time, clean the area of the ink; adjust the pressure; adjust the angle of the squeegee and the pressure on the plate; deepen the plate, remove the burr; properly add the original ink; adjust the printing speed.
4.7 color difference: color difference refers to the color difference between the roll and the roll during the printing process. Reason: The viscosity of the ink is not well adjusted; the gravure cylinder is blocked; the printing staff is not working properly.
4.8 blocking plate: In printing, with the large amount of solvent volatilization, the viscosity of the ink increases, and it is easy to dry and solidify inside the wire; in the printing process, the position of the blade is far from the printing roller or the angle is unreasonable, the diameter of the plate roller is large or the plate roller enters. When the depth of the ink tank is not enough, the hot air blown from the drying pipe affects the plate surface, which is easy to cause serious blockage; the ink is mixed with impurities; the engraving depth of the plate roller is not suitable; the temperature in the printing is too high, the solvent in the ink evaporates quickly.
The process of gravure printing of plastic film is difficult to control, so it is necessary to summarize some experience in printing to derive your own quality control method.