Printing Enterprise Standardization Plan—Flexible Packaging

Plastic flexible packaging products are generally printed on various plastic films, and then combined with a barrier layer and a heat sealing layer to form a composite film, which is then cut and bagged into products. Printing is the first process of production and the most difficult to control. Most product quality problems occur in this process. To measure the grade of a packaging product, printing quality is the first priority. 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 crisp, moderate contrast, lifelike image, and diverse product specifications. Gravure printing has high printing durability, and its advantages are even more obvious when printing large quantities of more than one million copies.

Gravure printing plastic film products have higher quality requirements, and the printing appearance quality standards are as follows:

① After the registration process, the registration error of each color pattern shall not be greater than 0.3mm.

② Check the specifications of the film. The registration error of each color pattern is not greater than 0.3mm.

③ Check the ink color of the printed product, which requires correct, bright, uniform hue, bright ink color, soft ink color, and smooth imprint edge.

④Printing fastness requires that the transparent tape sticks to the ink and does not discolor.

⑤The lines of pattern and text must be clear and complete, not incomplete or deformed.

⑥The mesh pattern requires rich layers and clear dots.

⑦The product is clean and tidy, with no obvious dirt or slivers.

⑧ It is required that the density deviation of the same color of the product and the color difference of the same color in the same batch should 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 various plastic packaging films Key points of the base material process

①BOPP is the most commonly used printing base material, and the key points of its production process are a typical plastic gravure production process; use polyamide or chlorinated polypropylene ink for printing, appropriate tension and The imprinting force and drying temperature are controlled below 80°C.

②BOPET printing key points: Because PET is usually thin and prone to wrinkles, it requires greater tension during printing and has a certain selectivity for ink. It is easy to separate layers with general printing ink, and the film has high static electricity. It is prone to plate blocking (not surfing the Internet), fluffing and scraper marks (knife wire). It is beneficial to have high humidity in the workshop during printing.

③Key points of BOPA printing: BOPA is a film that easily absorbs moisture and deforms. Therefore, when printing, it is necessary to ensure that the environmental humidity of the printing machine is not too high (if there is no dehumidification measure, the environment with high humidity is not suitable for printing nylon film); the unprinted film cannot damage its original aluminum film packaging film, and it must be printed immediately after opening , the nylon film needs to be preheated when printing (if the machine does not have a preheating device, the printing plate can be installed after the second color, leaving the first color free for preheating), and the preheating temperature is 50~80°C; The printing tension should be appropriately reduced; the pressure should be moderate; the ink has strong adaptability, depending on the product structure selection. If it is a retort bag, retort ink must be selected; the printed product must be sealed and stored in a drying and curing room as soon as possible. complex.

④ K-coated film (KOP, KPA, KPET) printing points: Because K-coated film, whether it is roller coating or spray coating, its thickness uniformity is not very good, the surface is uneven, and PVDC It is rigid and brittle. The printing tension and imprinting pressure should not be too high during printing. Overprinting is difficult and the ink transferability is poor. When printing shallow screens, it is easy to have spots (some dots are lost). A higher hardness pressure roller is required. For printing.

In addition, it is selective for solvents. Improper use of solvents may dissolve the coating. The printed film has a large solvent residue and is easy to stick, so special attention must be paid to drying and cooling. The matting film can be printed on the glossy surface using the OPP process, but the matting layer on the surface cannot withstand high temperatures and the drying temperature needs to be controlled.

⑤The pearlescent film uses surface printing ink for surface printing, and try to use surface printing ink with good transparency.

When printing heat shrinkable film, a very low drying temperature should be used, and special heat shrinkable ink should be selected (the printed ink layer must be able to shrink by heat without falling off). If it is a PVC heat shrinkable film, the dissolution of the film by the solvent must also be considered. sex.

⑥The tension of unstretched PP and PE films should be very small when printing, and overprinting is difficult. The deformation of printing must be fully considered when designing patterns.

2. Pre-press process development

① Plate design

During plate making, the original manuscript designed by the design company needs to be modified according to the characteristics of gravure printing. Make it more suitable for printing needs. In order to meet the needs of overprinting, multi-color overprinting cannot be used for small lines and text. There cannot be small reversed white characters on the overprinted pattern. It is also not suitable to leave spaces on the overprinted pattern and cover other small text. For portraits, etc., more overprinting is required. For the strict edition, try not to separate other color editions between the black, blue, red, and yellow editions. For printing needs, the barcode should be arranged so that the line direction is consistent with the circumferential direction of the cylinder. In terms of color guarantee, if possible, it is best to make separate plates for the solid background color and layered pattern plates, and try to use special editions for the solid background color. The two color jump areas of 80% and 5% must be fully considered when hanging the screen. When hanging the screen in a gradient, the dot limit should be above 10%.

②Ink preparation

The principle of adjusting the adaptability of ink printing, such as the viscosity and concentration of the ink, is to adjust the viscosity of the ink with a diluent (solvent) before printing. Determine the viscosity of the ink based on the condition of the plate roller and the machine. During the printing process, add solvent frequently according to the evaporation of the solvent, but not excessively. Try to ensure that the viscosity of the ink remains unchanged. When ink is to be washed, diluent (ink-adjusting oil) must be used, and avoid Dilute with solvent. Ink color mixing. In this regard, you need to pay attention to the following points: try to use a standard light source (D65) to observe the color. If there is no standard light source, it is best to use a sunny north window. When preparing spot colors, primary color inks should be used for preparation. The primary color inks mentioned here refer to the inks produced directly from pigments by the ink factory. The brightness and saturation of the ink color can be guaranteed. Use as few ink colors as possible for color matching. The more colors there are, the greater the color matching error will be, and the more difficult it will be to re-match the color.

3. Proof a set of printing plates

When printing, you must first confirm a standard proof. Some manufacturers use the plate-making factory's proofing draft as a standard sample, which cannot be compared when the final printing is delivered. Therefore, a printing machine must be used to proof before printing, and it will be used as a standard sample after confirmation by the customer or relevant personnel. The printing of standard samples should be carried out under normal printing process conditions, otherwise it will be very difficult to follow the samples during subsequent printing, so the following points should be paid attention to when proofing:

① Select the most basic printing process for process conditions condition.

②The viscosity of the ink should be as low as possible.

③ A certain amount of diluent should be added to primary color ink before proofing. Try to use as little ink as possible that has been used many times, especially contaminated ink.

④ It is absolutely prohibited to do any non-standard operations in order to excessively pursue the effect during proofing, such as increasing the concentration and viscosity to increase the hue, adjusting the pressure and scraper to increase the ink transfer rate, etc.

⑤The ink must be stirred evenly before use, especially spot color ink.

⑥The spot color ink prepared by yourself must be accurately recorded and kept as a file for future reference.

⑦ Try to achieve normal printing speed when proofing.

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), then take the sample and confirm it as a standard sample, which will be sealed and stored as archival data.

⑨After the standard sample is confirmed, the connected 2 to 3m should be cut out as the equivalent standard sample, and stored properly in a place without heat sources and ultraviolet rays. The cut portion should be used as a printing reference for each subsequent printing. Use it like this.

4. Common troubleshooting

①Dirty plate

In plastic gravure printing, a hazy layer is often stuck to the non-printing surface of the roller. The resin (pigment entrained in the resin) is transferred to the substrate and appears as flake-like stains or linear stains, which is a dirty plate.

The phenomenon of dirty plates is not only related to the hardness of the scraper, the pressure of the scraper, the contact angle of the scraper, and the quality of the plate roller, it is also related to the viscosity of the ink. When the printing viscosity is too high, resins, pigments and other substances have poor fluidity and cannot be dispersed evenly and accumulate together, causing damage to the scraper and plate cylinder under high-speed operation, resulting in linear dirt. At the same time, because the resin is also in a supersaturated state, a large amount of resin accumulates together. Under the pressure of the scraper, the resin adheres to the roller due to its affinity for the roller (pigment is entrained in the resin), and flake dirt appears. Generally speaking, the smaller the printing viscosity of the ink, the less likely it is for dirty plates to occur.

②Inaccurate color registration

The main reason is that the diameter of the gravure roller is deviated; due to incorrect plate mounting, the graphics and text are deformed; the spacing between the shaft teeth is uneven, and the gap is too large or too small. There may be top teeth causing inaccurate registration; the tension control of the film unwinding reel is uneven or the thickness of the gravure film is uneven or ruffled; the pressure at both ends of the plate cylinder and the impression cylinder is uneven or the printing is uneven during multi-color overprinting. Pressure is inconsistent between versions.

③Dimensional deviation

Insufficient or excessive shrinkage allowance. Due to the influence of plastic film tension and heating and drying during printing, the longitudinal dimensions of the film change greatly. Control countermeasures: Properly adjust the printing tension to prevent dimensional deviations caused by improper temperature settings.

④Reverse printing (i.e. color bite)

Reverse printing means that the ink of the previous color is reversely printed on the printing plate of the latter color during overprinting. Reasons: The ink layer after printing of the previous color was not completely dry; 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 had poor printability. Improve the adhesion of the previous color ink, adjust the ink viscosity, increase the hot and cold air volume, and increase the drying speed of the ink; slow down the vehicle speed; increase the ink supply of the next color; reduce the viscosity of the latter color; correctly arrange the printing plate color sequence; Reduce the impression roller pressure or use a softer rubber roller.

⑤Whitening

The resin does not dissolve or precipitate; it dries too quickly. Control measures: Make ink again and add slow-drying solvent appropriately. There are many faults in gravure printing. In actual printing operations, the main reasons should be carefully analyzed. You can consider register control, tension control, ink control, temperature control, etc. to quickly eliminate them to ensure the smooth progress of production. This requires skilled technology, timely grasp of the causes of the above common quality problems, and decisive measures to solve the quality problems of plastic film gravure printing in a timely manner.

⑥The pattern lines are incomplete

The hot air is too strong and the ink dries easily. The ink dries too quickly in the anilox lines and the ink dries and solidifies on the surface of the printing plate; printing plate and imprinting The pressure between the rollers is too small; the angle of the scraper is not ideal enough, and the pressure on the layout is not enough; the printing plate has uneven texture or white spots, the printing plate is shallow, has burrs, and the ink adheres poorly; the ink is too thin; the printing speed is too slow.

Add butanol in time to clean the ink on the layout; adjust the pressure; adjust the angle of the scraper and the pressure on the layout; deepen the printing plate and remove burrs; add original ink appropriately; adjust the printing speed.

⑦Color difference

Color difference refers to the color difference between rolls during the printing process. Reasons: The viscosity of the ink is not well adjusted; the gravure roller is blocked; the printing staff does not work properly.

⑧Blocking

During printing, as the solvent volatilizes in large quantities, the viscosity of the ink increases, and it is easy to dry and solidify inside the screen line; in the printing process, the position of the scraper is far away from the printing roller. Or the angle is unreasonable, the diameter of the plate roller is large or the depth of the plate roller entering the ink tank is not enough, the hot air blown from the drying pipe affects the surface of the plate cylinder, which can easily cause severe plate blockage; impurities are mixed in the ink; the depth of the plate roller screen line engraving is not suitable ; If the temperature is too high during printing, the solvent in the ink will evaporate quickly. The process of gravure printing plastic film is difficult to control, so you need to sum up more experience during printing to come up with your own quality control methods.