This article will continue to explore the printing method of woven bags. In the previous article “Can PP Woven Bags Be Printed?”, we have introduced the characteristics and working principle of flexographic printing. In this article, we will introduce another relatively high-end and high-cost printing method – gravure printing in the production of woven bags.
Woven bags laminated with gravure printed film
1. Working principle
Gravure printing is a direct printing method, which directly imprints the ink contained in the concave pit on the substrate. The thick and light level of the printed picture is determined by the size and depth of the pit. If the pit is deep, there are more ink, and the ink layer left on the substrate after the imprinting is thicker; On the contrary, if the pit is shallow, the ink content is less, and the ink layer left on the substrate after imprinting is thinner. The intaglio printing plate is composed of a pit corresponding to the original text and the surface of the plate. When printing, the ink is filled into the pit. The ink on the surface of the plate is scraped off with a scraper. The plate has certain pressure contact with the substrate, and the ink in the pit is transferred to the substrate to complete the printing.
Gravure printing, as a printing technology, has played an important role in the field of printing packaging and graphic publishing, because of its thick ink layer, bright color, high saturation, high printing resistance, stable quality of printing products and fast printing speed. From the application situation, gravure printing is mainly used in fine publications such as magazines, product catalogue, packaging printing, banknotes, stamps and other securities, and also in special fields such as decorative materials; At present, gravure printing is mainly used in soft packaging printing in the international market. With the development of gravure printing technology, gravure printing has been widely used in paper packaging, wood grain decoration, leather materials and drug packaging. Of course, gravure printing also has limitations, its main disadvantages are: the pre printing technology is complex, the cycle is long, the plate making cost is high; Because of the use of volatile solvent, the harmful gas content in the workshop is higher, which is harmful to the health of workers; Gravure printing practitioners require relatively high treatment.
The types of gravure printing are divided into two types according to the method of plate making: engraving gravure and etching gravure.
Engraving gravure is to directly carve the concave corresponding to the original text and text directly on the surface of the plate cylinder with a cutter. According to the control method of the cutter, it can be divided into manual engraving gravure, mechanical engraving gravure and electronic engraving gravure.
(1) Manual engraving gravure
Manual engraving gravure is made by technicians on the surface of plate drum by hand according to the original text and text. The plate material can be made of copper plate or steel plate. The manual work is heavy, the plate making cost is large and the cycle is long. But the gravure lines of manual carving are clear, and the printing products are of strong sense of hierarchy, which is difficult to forge. Most of them are used to make the printed gravure of the securities and high quality art.
(2) Mechanical engraving gravure
Mechanical engraving gravure is made on the surface of plate roller by mechanical control cutter. It reduces the heavy work of manual engraving, which has the advantages of fast plate making speed, short cycle and low plate making cost. Mainly used for the production of securities of printing gravure.
Because the above two kinds of engraving plate making costs are high and the plate making period is long, the scope of use is limited. But things are divided into two, as far as anti-counterfeiting packaging is concerned, manual engraving gravure may be the best way to make plate. Because secret anti-counterfeiting is a main method of anti-counterfeiting packaging, technicians can use various methods to carve out some secret records in the process of manual engraving gravure. The secret records on the manual engraving intaglio are not easy to be found, and it is more difficult to distinguish on the printed materials, which forms the secret anti-counterfeiting.
(3) Electronic engraving gravure
This type of plate is controlled by electronic control device to carve the engraving tool on the surface of the plate drum. The area and depth of the engraving tool are changed simultaneously by using the electronic engraving machine and the photoelectric principle to control the carving knife and carve the net holes on the surface of the drum.
The etched gravure is made by etching one ink pit on the surface of the plate drum by chemical etching according to the original text and text. According to the different methods of graphic and graphic conversion of the original, the etched gravure can be divided into etched gravure, photographic gravure and dot gravure.
Etch gravure is made by combining engraving and etching. That is, the shape of the original text is carved by hand, and then the concave is made by the method of corrosion.
Photographic gravure is widely used, which is the most widely used plate in gravure printing, mainly used for printing and painting.
The intaglio is mainly used for packaging and decoration printing and building materials printing.
As like as two peas in the gravure printing, the concave body is exactly the same as the original manuscript, and the ink on the surface of the plate is not printed. When the plate is in contact with the substrate, the ink in the pit is transferred to the substrate surface to complete the printing process. Therefore, gravure printing has some unique characteristics different from other printing methods. The following is discussed separately.
The gravure printing is based on the ink in the pit carved by the original text and text. The thickness of the lines and the thickness of the ink can be controlled arbitrarily during the engraving, and it is not easy to be imitated and forged, especially the depth of the ink pit. The possibility of realistic carving according to the printed image and text is very small. Therefore, the current paper money, stamps, stocks and other securities, are generally printed in gravure, with a good anti-counterfeiting effect. At present, some enterprises have already adopted gravure printing consciously in trademark and even packaging decoration, which indicates that gravure printing is a more vigorous anti-counterfeiting printing method.
General soft materials can be used as the substrate of gravure printing. For example, plastic, paper, aluminum foil, etc., especially for some materials easy to extend and deform, such as textile materials, have better adaptability, which is incomparable between the convex printing and the print printing. For those printing materials with uneven and smooth plane, such as woven bags, we use gravure printing on BOPP film to print patterns, and then combine BOPP film with plastic woven bags to achieve printing effect.
High quality printing
Gravure printing has a large amount of ink, graphics and graphics with convex, rich levels, clear lines, high quality. The printing of books, periodicals, pictorial, packaging and decoration is mostly gravure printing.
Gravure printing has a long period of plate making, high efficiency and high cost. But the plate is durable, so it is suitable for large-scale printing. The larger the batch, the higher the benefit, the lower the efficiency for the smaller batch printing. Therefore, gravure method is not suitable for printing small batch trademarks.
One of the basic printing methods. The picture and text of the printing plate is lower than the blank part. When printing, the whole plate is inked, and then the layout is scraped to make the ink left only in the graphic part and transferred to the substrate to become a printed product. It can be divided into three categories: engraving gravure, photo etching gravure and electronic engraving gravure.
Engraving gravure is developed from the art of engraving. M. finigura, Italian goldsmith, was invented in 1452. At first, it was all hand carved, and later there was chemical etching. In modern times, machinery was used as much as possible. The layout consists of points and lines of different depth and thickness. The lines of the printed matter are slightly convex, clean and clear, and can prevent forgery. Therefore, they are mostly used for printing banknotes, stamps and other securities.
In the late 19th century, K. klick, born in Bohemia, invented the gravure printing method and the gravure printing method with scraper by using the previous photographic techniques and carbon paper passing the plate. The ink part of the layout text is a regular arrangement of small holes (net holes), generally in square, the size is the same, but the depth is different, ink content is different. The printing is thick and solid, and can achieve the printing effect consistent with the original image color level. It is the most common one used in gravure printing, so it is also called traditional photographic gravure. For more than half a century, several new photographic etching gravure have been developed, such as the size of ink holes, and the depth and depth of the holes are consistent or inconsistent, and the printing quality is good.
In the middle of the 20th century, electronic engraving machine was used to carve the concave plate. It used the scanning head and the diamond cutter controlled by computer to carve the ink holes of the picture and text on the cylinder in the inverted pyramid shape, and the size and depth changed. The printing quality is no less than that of photographic etching gravure, and has the advantages of simple operation, short time of plate making and no waste liquid treatment.
Most of the printing plates of modern gravure printing are made on the surface of the cylinder, and the printing method of circle pressing is adopted. Usually the press drum is on the top and the plate roller is under. The lower part of the plate drum is immersed in the ink tank, and the layout obtains ink from the groove (ink pump is also used to jet ink or transferred to the layout by the ink roller immersed in the ink tank). A thin steel scraper is arranged on the surface of the plate drum above the ink groove to wipe off the ink without pictures and pictures on the plate (also used for wiping with a reverse rotating ink roller). Ink retained in the ink hole (or line) of the layout graphic and graphic shall be transferred to the paper (plastic film, aluminum foil, etc.) passing through the two rollers when it is transferred to the tangent of two rollers to print the text and text.
Compared with offset printing or copper plate embossing, copying monochromatic or color photographic manuscript has the advantages of thick ink layer, rich layers, good printing quality, low waste paper rate, printing plate resistance and long-term storage, especially in the printing effect of coarse paper, glass paper, plastic film and metal foil. However, the gravure plate is long and expensive, so only color pictures, picture albums, pictorial periodicals, book illustrations, postcards, trademarks, packaging decoration, securities, architectural decoration materials with high quality requirements are suitable for intaglio printing. And the printing products are large, gravure printing is more reasonable. Because of the poor quality of small size characters, the books and periodicals mainly based on the characters do not need gravure printing.
Gravure printing ink must be easy to enter the ink hole of the plate and transfer to the paper, so the flow is good, and the volatile benzene is commonly used as solvent. In order to prevent the pollution of air and fire prevention by solvent, we should pay attention to airtight pollution sources, recovery solvents and ventilation and air exchange in the workplace. The research and application of water-based gravure ink which requires no reduction in printing quality are actively explored at home and abroad.
Gravure printing is easier to operate than flat printing because of its high automation and good plate making quality. The process flow is as follows:
Preparation before printing → upper version → adjustment rules → formal printing → post printing processing
Gravure printing machine
The preparation of gravure printing includes: according to the requirements of the construction order, prepare the substrate, ink, scraper, etc., and lubricate the printing machine.
Plastic film is the main substrate of gravure printing. The commonly used plastic films are polyethylene, polypropylene, PVC, etc. Because the surface of plastic film is smooth and the performance of adhesive ink is poor, the film surface should be treated before printing. Generally, corona treatment is adopted. The method is to pass through the plastic film in two electrodes, and force the air to ionize and generate discharge phenomenon by high frequency oscillation pulse to form corona. The free oxygen atom and oxygen molecules combine to generate ozone, which makes the film surface form some strong groups and invisible “burr”, thus improving the surface tension and roughness of the film, It is beneficial to the adhesion of ink and adhesive.
Gravure printing, solvent volatile ink, low viscosity, good liquidity, strong adhesion. The commonly used solvents are toluene, xylene, gasoline, alcohol, etc. before printing, add appropriate amount of solvent to the ink for release, and it is better to filter before use.
The main feature of gravure printing machine is to use a scraper to wipe off the blank part of the printing plate. The blade is a special steel sheet with width of 60mm to 80mm and length of 1000mm to 1500mm (depending on the size of plate drum). The blade of the blade must be straight. If there is a “Crescent” type of scar, a wide diagonal bar will appear on the print, that is, the empty bar phenomenon will occur.
Printing plate is the basis of printing, which is directly related to the quality of printing. The printing plate needs to be reviewed before the last edition. Check whether the dot is neat and complete, whether the chrome plated plate has the phenomenon of dechromation. The text plate requires that the lines are complete and free from defects, and the pen cannot be broken and the pen is not broken. The plate can only be installed on the printer after detailed inspection.
During the operation of the last plate, special attention should be paid to protect the layout from being damaged. The rules at the mouth of the mouth and the pushing and pulling rules should be aligned. The plate drum should be fastened on the printer to prevent the plate drum from loosening during formal printing.
Adjust the rules
After the preparation before printing is completed, carefully calibrate the plate, check the conditions of feeding, feeding, receiving and pushing and pulling, and make proper adjustment, correct the pressure, adjust the ink supply and adjust the ink scraper.
The adjustment of the blade is mainly to adjust the distance between the blade and the plate and the angle of the blade, so that the pressure of the blade on the layout is even and does not damage the plate.
In the process of formal printing, we should often check grapefruit sample, whether the dot is complete, whether the overprint is accurate, whether the ink color is bright, whether the viscosity and drying of the ink match the printing speed, whether the scraping knife is uneven, and there are traces, knife lines, broken knife edges, etc. on the printing sheet.
In the working area of gravure printing, good ventilation equipment shall be provided to eliminate harmful gases. The solvent shall be recycled. The electrical appliances on the printing machine shall be equipped with explosion-proof devices, and the maintenance shall be checked frequently to avoid fire.
6 Digital plate making
According to the actual situation in China, there are three ways to make digital plate of gravure printing: electronic engraving machine, laser film cutting and post-corrosion treatment, and direct laser ablation of electroplating alloy.
Gravure printing cylinders
1. The anti-counterfeiting effect of manual engraving gravure printing is better.
Secret anti-counterfeiting is a main aspect of commodity anti-counterfeiting, which is implemented by trademark. Manual engraving gravure printing provides necessary means for it. Manual engraving gravure printing is easy to implement secret and anti-counterfeiting when making plate. The depth of the pit determines the thick and thin level of printed text. The imitator can easily copy the size of the picture and text and the thickness of the lines, but it is difficult to copy the printed products with the same thick and light level (i.e. the same deep ink pit). This is the main reason why the gravure is necessary for Securities for a long time.
The less the printing, the better the anti-counterfeiting effect. Because of the long cycle of manual engraving gravure, the cost is high, so the price of the printed products is higher when the printing quantity is low. The purpose of the counterfeiters is to pursue economic benefits. When they are unprofitable or not, they are unwilling to take any risks. Therefore, it can be said that the effect of the low-volume trademark printing with gravure printing may be better than that of large batch gravure printing.
2. Plate making and printing are carried out in two places
The printing and printing in different places have good anti-counterfeiting effect. Printing plants generally have the ability to make plates, but the plate making work of trademark users should be selected separately. One key to secret anti-counterfeiting is to know that the fewer people are better, even if they know, they should only know a part, but not let them know the whole process of trademark design and printing. Even if there is a loophole in some link, Other links are enough to meet the needs of anti-counterfeiting. When printing trademarks in printing factories, enterprises should send responsible personnel to the site to supervise and print, in order to prevent the plate theft and trademark theft. After printing, the printing plate shall be taken back and saved.
3. Combination with anti-counterfeiting ink technology
Ink anti-counterfeiting technology is also a secret anti-counterfeiting aspect, the gravure printing technology and ink anti-counterfeiting technology are combined to achieve double secret anti-counterfeiting, anti-counterfeiting effect will be better. Some printing anti-counterfeiting ink has larger pigment particles, which requires a thick ink layer. While the ink layer of gravure printing is thick, the quality of the printed products is strong, and the combination of the two will get double anti-counterfeiting and decoration effect. If the fluorescent ink is used in gravure printing, it not only ensures the micro convexity of gravure printing, but also ensures the use requirements of fluorescent ink. Both secret anti-counterfeiting functions are guaranteed. Ink should be allocated by itself, and then handed over to printing enterprises for use. After printing, ink should be recovered and preserved, so as to achieve better anti-counterfeiting effect.
8 advantages and disadvantages
① Advantages: ink expressiveness about 90%, rich color. Strong color reproduction. Good layout tolerance. There is a huge amount of printing. A wide range of paper applications, other than paper materials can also be printed.
② Disadvantages: plate making is expensive, printing is also expensive, plate making is more complex, a small number of quantity printing is not suitable.
Application scope of gravure printing
Engraved gravure printing, because of its exquisite lines and not easy to counterfeit, is used in the printing of securities, such as banknotes, stocks, gift certificates, stamps and commercial credibility of the certificate or stationery and so on. Because of its high cost of plate making and printing, the general printed matter is seldom used. As for photogravure, although its plate making process is more complex and its cost is also more expensive, it is not suitable for printing a small number of prints. Generally, it is used in a large number of prints, such as color magazines and the popular building materials printing, etc. Gravure printing because of the use of high-speed rotary type, not only the speed is fast, but also the ink film is far thicker than relief or lithography.
In addition to books and newspapers, gravure printing is mainly used to print packaging and decoration materials. According to the characteristics of gravure printing, the printed products should meet the following quality requirements.
Monochrome intaglio printing, bright, medium, dark, clear, coordinated, delicate. The network is clear and complete. The layout is even and tidy.
Color intaglio printing: the image is bright, medium and dark, clear, harmonious and delicate. The color is natural and harmonious. The dot is clear, complete and the angle is accurate. The outline of the image is clear and the error of overprint is allowed.
Appearance of printed matter: the layout is clean, uniform and without obvious dirty mark. The position of image and text is accurate. The color of printing plate is basically the same, the size error of fine print is not more than 0.5mm, the general print is not more than 1.0mm, and the front and back overprint error is not more than 1.0mm.
Printing quality control
1. Automatic overprint device
An automatic overprint device is installed on the web gravure press. The device consists of a scanning head, a pulse generator, an electronic controller, an adjusting motor and a register adjusting roller.
When the overprint mark on the printed sheet passes through the scanning head, the pulse signal will be sent to the electronic controller. If the overprint mark of the second color is wrong, the time of pulse occurrence will be uneven. Therefore, the electronic controller starts the adjusting motor to make the overprint adjusting roller between the first color and the second color move slightly to eliminate the overprint error.
2. Electrostatic ink absorption device
Gravure printing press is generally equipped with electrostatic ink absorption device. The device consists of a high voltage adjustable generator, an aluminum guide roller, a special embossing roller with conductive surface and insulating bottom layer, an automatic cut-off mechanism for static electricity and an over-current short-circuit protection mechanism. Using static electricity to adsorb the ink in the printing plate hole on the substrate surface can improve the ink transfer rate by about 20%, especially the brightness of the printed matter.
The faults in gravure printing are mainly caused by printing plate, ink, substrate, ink scraper, etc.
(1) Uneven ink color
The phenomenon of periodic ink color change on printed matter. The elimination methods are: correcting the roundness of the plate cylinder, adjusting the angle and pressure of the doctor blade, or replacing the doctor blade with a new one.
(2) The imprint is thick and fluffy
The image of the printed matter is layered and pasted, and there is burr on the edge of the image. The elimination methods include: removing the static electricity on the substrate surface, adding polar solvent into the ink, increasing the printing pressure appropriately, adjusting the position of the scraper, etc.
The phenomenon that ink dries up in the hole of the printing plate, or the hole of the printing plate is filled with paper wool or paper powder, is called blocking. The elimination methods are: increase the content of solvent in the ink, reduce the speed of ink drying, and use the paper with high surface strength for printing.
(4) Ink overflow
The phenomenon of spot appearing in the field part of printed matter. The elimination methods are: add hard inking oil, improve the viscosity of the ink. Adjust the angle of the doctor blade, improve the printing speed, change the deep hole printing plate into the shallow hole printing plate.
There are traces of a scraper on the printed matter. The elimination methods are: using no foreign matter mixed in clean ink printing. Adjust the viscosity, dryness and adhesion of ink. Adjust the angle between the doctor blade and the printing plate by using high quality doctor blade.
(6) Pigment precipitation
The phenomenon of lightening the color of printed matter. The elimination methods are: printing with ink with good dispersion and stable performance. In the ink to add anti condensation, anti precipitation additives. Fully rolling, often stirring ink tank.
(7) Sticky and dirty
Ink stains on printed matter. The elimination methods are: choose the ink with fast volatilization speed for printing, increase the drying temperature or reduce the printing speed appropriately.
(8) Ink shedding
Printing ink on the plastic film adhesion, hand or mechanical friction off phenomenon. The elimination methods are: to prevent the plastic film from getting wet, select the ink with good affinity to the plastic film for printing, re surface treatment of the plastic film to improve the surface tension.
1. Large and medium-sized packaging gravure printing enterprises will occupy the mainstream, especially cigarette package gravure printing enterprises are more in pursuit of scale efficiency, high quality and high-tech content.
2. Due to environmental protection and health reasons, food, medicine, tobacco, wine and other industries pay more and more attention to the environmental protection of packaging materials and printing process, gravure printing enterprises pay more attention to the environment of printing workshop. Environmental friendly ink and varnish will be more and more popular. The closed scraper system and quick change device will be popularized and applied. Gravure printing machine suitable for water-based ink will be widely used.
3. Gravure printing machine independent drive technology and electronic shaft drive as the technical platform, will realize the comprehensive upgrading of gravure printing machine, various uses of gravure printing machine will use electronic shaft drive. Electronic shaft drive and registration systems are mainly from Japan and Europe, but they can only be used in a small number of domestic gravure presses. The development of domestic overprint system will be the key to upgrade the domestic gravure press.
4. The proportion of wide gravure press and paper gravure press will continue to increase. On the one hand, decorative printing (mainly wood grain paper printing) and carton gravure printing market will grow rapidly. On the other hand, the range of flexible packaging gravure printing machine will continue to increase, and the number of large format gravure printing machine will increase rapidly. Gravure printing machines for paper and paper-based composite materials (wood paper cigarette label, aseptic packaging, gift wrapping paper, etc.) will grow rapidly.
5. Gravure production line processing will be more diversified, more and more powerful function configuration, such as glazing, die cutting, cross cutting, soft label cutting, etc. Accordingly, gravure press management system, remote technical support system will be more and more used. At the same time, in order to meet the personalized needs, from unwinding, printing to line processing, winding and other parts will be modular, the most important of which will be gravure printing trolley or gravure printing and flexo printing interchange trolley.