Gravure printing technology
Gravure printing is one of the four major printing methods used today. Gravure printing is the opposite of letterpress printing, where the graphic part of the layout is lower than the printing plane, and the depth of the concave on the surface of the printing plate is used to present the varying levels of intensity of the halo on the original. If the graphic part of the recessed deep, filled with the amount of ink, embossed on the surface of the substrate left after the ink layer is thick; graphic part of the concave shallow, the amount of ink held less, embossed on the surface of the substrate left after the ink layer is thin. The amount of ink on the printing plate and the amount of the original text corresponds to the level of light and dark. Printing, the first plate cylinder through the ink tank or use the ink transfer stick to make the ink coated throughout the printing plate, and then use the ink scraper, scrape off the attached
Gravure printing is one of the four major printing methods used today. Gravure printing is the opposite of letterpress printing, where the graphic part of the layout is lower than the printing plane, and the depth of the concave surface of the printing plate is used to present the varying levels of intensity of the halo on the original manuscript. If the graphic part of the recessed deep, filled with more ink, embossed on the surface of the substrate left after the ink layer on the thick; graphic part of the concave shallow, the amount of ink held less, embossed on the substrate left after the ink layer on the thin. The amount of ink on the printing plate and the amount of the original text corresponds to the level of light and dark. When printing, the first plate cylinder through the ink tank or use the ink transfer stick to make the ink coated with the entire plate, and then use the ink scraper, scrape the ink attached to the blank part, and fill in the recessed cavity of the ink, in the appropriate printing pressure, is transferred to the surface of the substrate. Gravure printing has the advantages of fast speed (up to 300m/min or more), high printing plate resistance (up to 3-4 million prints), thick ink color, rich color, clear and bright, moderate contrast, realistic image, and various product specifications. It can be printed on paper, aluminum foil, plastic soft packaging, thick paper containers, liquid paper ware, trademarks and other substrates.
With the development of science and technology, the scope of gravure printing at home and abroad is growing, suitable for printing on a variety of materials, for example, the pre-printing of color packaging boxes, the use of gravure pre-printing production of cartons than the traditional way to produce good quality, high strength, low cost; and to multi-color (up to 12 colors), high-speed (more than 300m / min), automation (automatic control of registration and strong control, etc.), inline ( The subsequent process of gravure printing and printing continuous development.
Gravure printing mainly includes: plate making, printing and quality control
I. Gravure plate making
At present, gravure plate making mainly includes: direct etching gravure, electronic engraving gravure, laser engraving gravure, electron beam engraving gravure, etc.
The direct etching gravure plate is made on the carbon paper sensitized by dichromate, firstly, the gravure printing grid is tanned, then the continuous sun-toned image is tanned, then the image on the carbon paper is transferred to the cylinder, and the plate is made by developing, filling and etching. The direct etching gravure plate is mainly controlled directly by hand, so the stability of plate making, field density uniformity, repetitiveness and text clarity are poor, and it is generally only used for general plastic flexible packaging printing.
The electronic engraving gravure plate is engraved directly on the gravure cylinder with an electronic engraving machine. It is scanned by the scanning head to obtain the density optical signal, which is converted into electrical signal through photoelectricity and then transmitted to the output after a series of processing by the electronic computer to drive the electronic engraving head to engrave on the copper cylinder to make the gravure printing plate.
Laser engraving gravure plate uses laser engraving machine to engrave the gravure cylinder; while electron beam engraving gravure plate uses electron beam gravure engraving machine to engrave the gravure cylinder.
As the electronic engraving gravure plate, the scanning head of the gravure engraving machine converts it into electrical signals of different sizes one by one according to the shades and levels of the original, and through the different strengths and weaknesses of the **** carving needle vibration, different shades of dots are engraved on the surface of the cylinder, corresponding to the light part of the original, the dots are shallow and small, while corresponding to the dark part of the original, the dots are deep and large. Since the reproduction of the image level is achieved by the change of dot size and depth, the electronic engraving gravure plate can truly reflect the level of the original. Its plate making stability, field density uniformity, and reprint repeatability are excellent, and the printed images are very clear.
Laser engraving gravure and electron beam engraving gravure bring about higher plate making costs due to the high cost of equipment. Although these two methods of engraving out of the plate quality is very high. However, the current use is still relatively small, the majority of gravure plates are still mainly electronic engraving.
In order to ensure the quality of electronic engraving gravure, the industry standard specifies the size and depth of the engraving cavity. However, the size and depth of the cavity random type (different companies' products), the number of lines, the new and old degree of the engraving knife, the angle of the knife, the thickness of the copper layer (the hardness of the copper layer is Vickers 180-210) and the impact of product quality requirements, there are many variable factors, the industry standard is only 70 lines / cm, 130 ° angle of the knife and the new knife for example.
Second, gravure printing process
Gravure printing using the garden pressure garden rotary direct printing method. Its characteristics are: simple printing press structure, high printing plate resistance, fast printing speed, thick ink, rich layers, low cost when printing in large quantities. The classification of gravure printing is more, according to the use can be divided into three categories: books and magazines gravure printing machine, soft packaging gravure printing machine, hard packaging gravure printing machine. Book gravure presses are used for printing magazines, books, pictorials, etc., and are equipped with folding devices at the back; soft packaging gravure presses are used for printing cellophane, plastic film, aluminum foil and paper and other packaging materials, and can be equipped with laminating, varnishing and die-cutting devices at the back. According to the plastic way can be divided into two categories: sheet-fed gravure printing machine and web material gravure printing machine, mostly using web material gravure printing machine. Classified according to the number of printing colors, can be divided into single-color gravure printing machine and multi-color gravure printing machine. According to the classification of printing color group arrangement position, it can be divided into satellite gravure machine and combined gravure machine. Satellite gravure presses share a common impression cylinder for several color groups, while combination gravure presses have each color group arranged horizontally in an assembly line.
Gravure printing, according to the requirements of the work task list and sample sheet to receive ink, substrate; then install the printing cylinder; adjust the lining on the impression cylinder, so that the pressure on each part of the printing plate is consistent; and adjust the angle and distance of the ink scraper to the printing plate, start the machine for test printing. After the test printing sample is qualified, it can be formally printed by the workshop supervisor and the customer's signature. The following combined gravure printing machine as an example of the printing process for a brief introduction: 1:
1. Unwinding device and adjustment
When the paper roll is nearly used up, the automatic paper catching device will automatically bond the new paper roll or plastic film roll to the running paper belt, and cut off the old paper tail to complete the roll change without stopping.
(2) Tension self-control system Paper or film in the pre-press release process, multi-color printing process or post-press collection process need to maintain a certain tension, too much tension easy to produce longitudinal wrinkles, too little tension easy to produce horizontal wrinkles, in short, the fluctuations in tension will affect the accuracy of overprinting.
(3) paper belt transverse rule adjusting device In order to ensure that the substrate into the printing part of the tape or rewinding transverse rule to maintain consistent, the need for transverse adjustment of deflection. The lateral adjustment deviation or through the air (liquid) pressure cylinder adjustment or through the lateral rule adjustment roller adjustment. 2.
2. scraper and impression cylinder adjustment
(1) scraper adjustment scraper to scrape the surface of the printing plate ink required pressure and the pressure required to resist the impact of the ink and the pressure and printing speed is proportional to the general speed, the pressure is correspondingly large. In addition, the choice of scraper and the angle of the printing plate, generally 30 ° ~ 70 °. This can keep the knife does not hurt the page, but also make the printed product quality is more ideal.
(2) adjust the impression cylinder After the plate cylinder can be adjusted after the impression cylinder. For different substrates, the printing pressure of the impression cylinder is different. Example: cellophane, plastic film, printing pressure of 1 ~ 5Pa. copper plate paper, thin cardboard, printing pressure of 8 ~ 15Pa. white paper and other printing pressure of 20 ~ 30Pa.
3. Ink drying temperature adjustment
Gravure ink is basically solvent-based ink, when printing, the operator should adjust the drying temperature of each unit according to the type of substrate, printing speed, graphic area, and ink layer thickness. Drying temperature is not easy to be too high, generally should be controlled at 80 ℃ or less, and the highest should not exceed 100 ℃. Otherwise, it will cause the substrate material shrinkage, affecting the register, too low is bad ink drying.
After the machine is adjusted to register, first use the manual adjustment knob to register the layout, and then you can adjust the computer. The longitudinal error of the register relies on tension adjustment, and the lateral error relies on the paper belt lateral position control system and push and pull the plate cylinder adjustment.
5 formal printing in the verification of the print sample
Formal printing, the operator must constantly check the print sample with the standard sample, check whether there are errors, and make the necessary adjustments.
Third, gravure printing problems and troubleshooting
In gravure printing, in addition to easily cause a fire, there are a variety of faults, the more common faults and troubleshooting methods are as follows:
① ink overflow: the phenomenon of ink full plate part of the production of spots, patterns, mainly due to low ink viscosity, ink mobility is too large, low printing speed, ink drying too slow or static electricity. The method of exclusion is to add the original ink and adjusting agent to improve the ink viscosity, reduce the mobility, you can also improve the printing speed or change the angle of the ink scraper.
② ink drying in the plate pattern and buried plate: the drying speed of the ink is too fast causing the ink to dry in the plate pattern. Caused by the buried version of more reasons, such as ink pigment particles larger, foreign objects mixed in, ink fixing agent re-soluble poor, ink viscosity is too high, the printing plate inking hole is too shallow. Exclusion method is to reduce the drying speed of the ink, increase the amount of ink solvent, improve the re-solubility of the ink, remove foreign matter, reduce the viscosity of the ink. At the same time should also prevent the drying device of the hot air blowing to the layout, causing the dry solid ink within the plate pattern.
③ printed blind paste hair: mainly caused by static electricity, should be removed from the substrate and ink static electricity, improve the humidity of the substrate, or add polar solvents in the ink, adjusters to improve the viscosity of the ink, appropriate to improve the printing speed.
④ poor print gloss: add the right amount of the original ink, but also appropriate to improve the drying speed of the ink, replace the quality of substrate materials.
⑤ stacked color bad: first printed ink film with ink row, caused by the stacked color each ink viscosity mismatch, stacked color with the ink drying speed does not match. Can reduce the viscosity of the ink, slowing down the drying speed of the overlapping ink.
(6) counter-printing and stacking ink: the reverse side of the print, due to stacking and sticky ink, caused by the solvent in the ink, containing a small amount of high boiling point components, resulting in the solvent evaporates too slowly, too much plasticizer and plastic resin in the ink, drying device is not good, the printing plate inking hole is too deep, the relative humidity in the printing shop is too large. The method of elimination is to choose a good solvent detachment resin, reduce the content of plasticizer and plastic resin in the ink, the appropriate addition of anti-printing agents, the solvent used in the ink as far as possible with a narrow range of boiling point solvent. Improve the drying device, improve the drying efficiency. Use rollers with shallow inking holes and improve environmental conditions.
(7) Scratch: the main reasons for scratch marks on the print are scraper damage, scraper pressure is not appropriate, the angle of the scraper is not correct, the ink mixed with foreign matter, ink viscosity is too high, ink adhesion is too strong, the pigment particles are too coarse and hard, the printing plate cylinder has scratches, uneven, poor correction, etc.. The method of elimination is to use the ink that does not contain foreign matter, filter the leftover ink before using it, adjust the viscosity, dryness and adhesion of the ink, and use a high-quality ink scraper.