Analysis of gravure printing technology
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Gravure printing is one of the four major printing methods today. In contrast to letterpress printing, gravure printing has a lower image surface than the printing plane, and the surface of the printing plate is shaded to reveal the gradation of the original. If the image is partially recessed and the amount of ink filled is large, the ink layer left on the substrate after embossing is thick; the image portion is shallow and shallow, and the amount of ink contained is small. The ink layer left on the substrate is thin. The amount of ink in the printing plate corresponds to the light and dark level of the original image. When printing, the plate cylinder is first passed through the ink tank or the ink is used to spread the ink to the entire printing plate, and then the ink adhered to the blank portion is scraped off with a doctor blade to fill the ink in the depressed cavity. Transfer to the surface of the substrate under appropriate printing pressure. Gravure printing has a fast speed (up to 300m/min or more), high printing durability (up to 300-400 million impressions), thick ink color, rich colors, clear and bright, moderate contrast, vivid image and diverse product specifications. Etc. It can be printed on paper, aluminum foil, plastic flexible packaging, thick paper container, liquid paper machine, trademark and other substrates.
With the development of science and technology, the range of gravure printing at home and abroad is getting larger and larger, suitable for printing of various materials, such as pre-printing of color packaging boxes. The paper box produced by gravure pre-printing is better than the traditionally produced carton. High intensity and low cost; and multi-color (up to 12 colors), high speed (above 300m/min), automation (registration automatic control and strong control, etc.), online (continuous process of gravure printing and printing) ) and other directions.
Gravure printing mainly includes: plate making, printing and quality control.
First, gravure plate making
At present, gravure plate making mainly includes: direct etching gravure, electronic engraving gravure, laser engraving gravure, electron beam engraving gravure and the like.
Directly etching the gravure, on the carbon paper sensitized by dichromate, first drying the gravure grid, then drying the continuous tempering image, and then transferring the image on the carbon paper to the drum, through Development, filling, and etching to obtain a printing plate. Since the direct etching gravure is mainly controlled directly by humans, the plate making stability, the uniformity of the solid density, the repeatability of the reprinting, and the sharpness of the characters are poor, and generally only used for ordinary plastic flexible packaging printing.
The electronic engraving gravure is an engraving of the gravure cylinder directly by an electronic engraving machine. It scans the original through the scanning head to obtain a density optical signal, which is photoelectrically converted into an electrical signal. After a series of processing by the electronic computer, it is transmitted to the output end to drive the electronic engraving head and engrave on the copper cylinder. And made a gravure plate.
The laser engraving gravure uses a laser engraving machine to engrave a gravure cylinder; and the electron beam engraving gravure uses an electron beam gravure engraving machine to engrave a gravure cylinder.
The electronic engraving gravure is converted into electric signals of different sizes according to the depth of the original, and the scanning head of the gravure engraving machine converts them into electric signals of different sizes, and the depth of the vibration of the diamond is different, and the different dots of the depth are engraved on the surface of the drum, corresponding to The bright part of the original, the dot is shallow and small, and corresponds to the dark part of the original, the dot is deep and large. Since the hierarchical reproduction of the image is achieved by the size and depth of the dots, the electronic engraving gravure can truly reflect the level of the original. Its plate making stability, field density uniformity, and repeatability are excellent, and the printed images are very clear.
Laser engraving gravure and electron beam engraving gravures increase the cost of plate making due to the high cost of equipment. Although the two methods are engraved, the quality of the plate is very high. However, it is still used in a small amount, and most of the intaglios are still mainly based on electronic engraving.
In order to ensure the quality of the electronic engraving gravure, the size and depth of the engraved cells are specified in the industry standard (see Table 1). However, the size of the cells and the depth and random type (products of different companies), the number of network cables, the degree of old and new knives, the angle of the knives, the thickness of the copper layer (the hardness of the copper layer is 180-210 Vickers) and the quality requirements of the products. Influence, more variable factors, only 70 lines / cm, 130 ° knife angle and new knife in the industry standard as an example.
Second, Gravure printing process
Gravure printing adopts the direct printing method of circular pressing in the park. The utility model has the advantages that the printing machine has a simple structure, the printing plate has high printing durability, the printing speed is fast, the printed ink color is thick, the layer is rich, and the cost in high-volume printing is low. There are many classification methods for gravure printing, which can be divided into three categories according to their use: book gravure printing machine, flexible packaging gravure printing machine, and hard packaging gravure printing machine. The book gravure printing machine is used to print magazines, books, magazines, etc., and is equipped with a folding device at the back; the flexible packaging gravure printing machine is used for printing packaging materials such as cellophane, plastic film, aluminum foil and paper, and can be compounded, glazed and molded later. Cut the device. According to the plastic method, it can be divided into two types: a sheet-fed gravure press and a roll gravure press, and most of them use a web gravure press. Classified according to the printing color number, it can be divided into a monochrome gravure printing machine and a multi-color gravure printing machine. According to the sorting position of the printing color group, it can be divided into a satellite gravure printing machine and a combined gravure printing machine. The satellite gravure printing machine shares a printing cylinder in several color groups, and each color group of the combined gravure printing machine is horizontally arranged into a pipeline.
Gravure printing, according to the work order and proofs to collect ink, substrate; then install the printing cylinder; adjust the lining on the impression cylinder to make the pressure on the parts of the printing plate consistent; and adjust the squeegee to print The angle and distance of the version, boot test. After the test sample is qualified, it can be officially printed after being signed by the workshop supervisor and the customer. The following is a brief introduction to the printing process by using a combined gravure press as an example:
1. Unwinding device and adjustment
1) Web roll paper roll holder The roll paper or plastic film roll is mounted on the printing machine with a tapered tip. When the paper roll is nearly used up, the automatic paper picking device automatically bonds the new paper roll or plastic film roll. On the running paper tape, cut off the old paper tail at the same time, and complete the non-stop change.
2) Tension automatic control system Paper or film is required to maintain a certain tension during the pre-press unwinding process, during multi-color printing or during post-pressing. The tension is too large to cause longitudinal wrinkles, and the tension is too small to cause lateral wrinkles. In short, fluctuations in tension can affect the accuracy of overprinting.
3) Straight gauge adjustment device for paper tape In order to ensure that the transverse rule of the tape of the printing material enters the printing part or rewind, the horizontal adjustment is required. Lateral adjustment is adjusted by gas (liquid) cylinder or by lateral adjustment roller adjustment.
2. Scraper and impression cylinder adjustment
1) Scraper adjustment The sum of the pressure required by the scraper to scrape off the ink on the surface of the plate and the pressure required to resist the impact of the ink. The pressure is proportional to the printing speed. Generally, the speed is fast and the pressure is correspondingly large. In addition, the angle between the blade and the printing plate is generally selected from 30° to 70°. This keeps the knife edge from damaging the layout and makes the quality of the printed product ideal.
2) Adjusting the impression cylinder The impression cylinder can be adjusted after the plate cylinder is in good condition. The printing pressure of the impression cylinder is different for different substrates. Example: cellophane, plastic film, printing pressure is 1 ~ 5Pa. Copper paper, thin cardboard, printing pressure is 8 ~ 15Pa. Printing pressure on white paper, etc. is 20-30Pa.
3. Adjustment of ink drying temperature
Gravure inks are basically solvent-based inks at present. When printing, the operator should adjust the drying temperature of each unit according to the type of printing material, printing speed, graphic area and thickness of the ink layer. The drying temperature is not too high, generally should be controlled below 80 °C, and the maximum should not exceed 100 °C. Otherwise, the printing material will shrink and affect the registration. If it is too low, the ink will dry poorly.
4. Overprinting
After the machine is adjusted, it can be overprinted. First, use the manual adjustment button to register the layout, and then you can adjust the computer. The longitudinal error of the overprint relies on tension adjustment, and the lateral error is adjusted by the tape lateral position control system and the push-pull plate cylinder. Table 3 shows the overprint error requirements for plastic films.
5. Checking the proof in official printing
When printing, the operator should constantly check the proof with the standard, check for errors, and make necessary adjustments.
Third, gravure printing failure and elimination
In gravure printing, in addition to easily causing fire, there are various faults, among which the more common faults and troubleshooting methods are as follows:
1 Ink overflow: The phenomenon is that spots and patterns are generated in the full version of the ink. The main reason is that the ink viscosity is too low, the fluidity of the ink is too large, the printing speed is low, the ink is dried too slowly or has static electricity. The method of elimination is to add the original ink and the adjusting agent to increase the viscosity of the ink, reduce the fluidity, and also increase the printing speed or change the angle of the doctor blade.
2 The ink is dried and embossed in the stencil: the drying speed of the ink is too fast to cause the ink to dry in the stencil. There are many reasons for the burying, such as large ink pigment particles, foreign matter mixing, poor resolvability of the ink fixing agent, high viscosity of the ink, and too shallow ink in the printing plate. The elimination method is to reduce the drying speed of the ink, increase the solvent amount of the ink, improve the resolubility of the ink, remove foreign matter, and reduce the viscosity of the ink. At the same time, the hot air of the drying device should be prevented from blowing to the layout, which causes the ink in the pattern to dry.
3 Printed blind fluff: mainly caused by static electricity, the static electricity of the substrate and ink should be removed, the humidity of the substrate should be improved, or the polar solvent and the adjusting agent should be added to the ink to increase the viscosity of the ink, and the printing speed should be appropriately increased.
The main part refers to the part of the picture that reflects the theme, such as patterns, texts, signs, and so on.
4 printing products are not good gloss: adding appropriate amount of original ink, can also properly improve the drying speed of the ink, replace the high-quality printing materials.
5 Poor stacking: The ink ink film printed first has ink repellency, which is caused by the mismatch of the viscosity of each ink in the stack, and the drying speed of the ink used for the stacking does not match. It can reduce the viscosity of the ink and slow down the drying speed of the overlapping inks.
6 reverse printing and stacking ink: the reverse side of the printed matter, the ink sticks due to stacking, the cause is that the solvent in the ink contains a small amount of high-boiling components, causing the solvent to evaporate too slowly, and the plasticizer and plastic resin in the ink Too much, the drying device is not good, the printing plate is too deep, and the relative humidity in the printing workshop is too large. The method of elimination is to select a resin with good solvent detachment, reduce the content of plasticizer and plastic resin in the ink, and appropriately add a anti-printing preventive agent, and the solvent used in the ink should use a solvent having a narrow boiling point range as much as possible. Improve the drying device to improve drying efficiency. Use a roller with a shallow ink hole and improve the environmental conditions.
7Scratch: The cause of scratches on the printed matter is mainly the damage of the squeegee, the improper pressure of the squeegee, the incorrect angle of the squeegee, the foreign matter mixed in the ink, the viscosity of the ink is too high, and the adhesion of the ink is too strong. The pigment particles are too rough, and the plate cylinder has scratches, irregularities, poor correction, and the like. The method of elimination is to use ink containing no foreign matter, filter before using the remaining ink, adjust the viscosity, dryness and adhesion of the ink, use a high-quality doctor blade, adjust the pressure and angle of the doctor blade, and correct the plate cylinder.
8 Ink hydration: The moisture in the air enters the ink, causing smudging, precipitation of resin, gelation, etc., causing ink gloss reduction, concentration reduction, poor transfer, and easy paste. To avoid hydration, it is best not to use excessively fast drying solvents and to reduce the chance of ink contact with the air.
9 whitening: the ink transferred to the surface of the plastic film, the solvent volatilization is seriously deregulated, and the phenomenon of resin whitening occurs in which the resin is insoluble, white turbid, and partially gelled. In order to prevent whitening, the printing speed can be increased, or the drying box can be brought close to, or the solvent can be dried to slow down the drying speed of the ink.