Chapter 4 Principles and Technology of Gravure Platemaking
Section 1 Overview
Gravure printing originated from engraving intaglio printing in the Middle Ages. It is closely related to lithography and letterpress printing. , stencil printing, is an important part of the printing process, and is also a main printing method in modern printing.
1. Gravure printing and its characteristics
Gravure printing is named after its layout characteristics. The graphics and text part of the gravure printing plate is lower than the layout. It is recessed into the printing plate at different depths to express different levels of the original image, and the blank part is on the same layout. When printing, first fill the ink on the printing plate, then use a scraper to scrape off the ink on the surface of the printing plate, and then use the action of pressure to make the ink remaining in the recessed part of the printing plate (i.e. the graphic part) and the paper ( or other printing substrate), and transfer this part of the ink to the paper (or other printing substrate) to obtain the required printed matter.
The difference between gravure printing and photocopying is that it is usually mass-produced at high speed, while photocopying only makes a small number of copies directly from the original. Printing products include books, newspapers, magazines, textbooks, pictures, picture albums, maps, posters, catalogs, forms, securities, packaging materials and various daily printed materials. Compared with flat and letterpress printing, the outstanding advantage of gravure printing is that the printing quality is good. Through ink layers of different shades, the rich layers of the continuous tone original can be more completely expressed. The ink is thick, colorful, and rich. Three-dimensional effect, its ink layer is as much as 5 times thicker than flat printing products and more than 2 times thicker than embossed printing products. Since gravure printing has the advantage of being able to print on various large-format high-grade papers, rough papers, plastic films, metal foils and other substrates with very high quality, it is often used in modern printing to print various exquisite picture albums, albums, etc. Pictorials, especially in the packaging and decoration industry, are widely used, such as various shapes of trademarks, folding cartons, flexible packaging materials (cellophane and various film materials), gift packaging materials, various types of wrapping paper and other commodity packaging, Gravure printing can be used for printing of decorative materials. And because the printing plate of gravure printing has high printing durability, it has the most obvious advantages in mass printing and has good economic benefits.
In short, gravure printing has thick ink layer, bright colors, high printing durability, wide application range, and is suitable for printing continuous and continuous patterns. Now it is widely used in plastic packaging printing, paper packaging printing, decorative printing, transfer printing, publishing printing and other fields.
2. Development of gravure plate making technology
Gravure plate making is an indispensable and important link in gravure printing. With the development of technology, the gravure platemaking process has also changed.
As far as the gravure printing process itself is concerned, with the changes in process technology, the gravure printing process has also experienced its history of rise and fall.
In the mid-15th century, the production of gravure plates was first done by hand. Digging out the copper plate or steel plate by hand with a carving knife.
In the early 17th century, chemical etching was used in the production of gravure printing plates. The specific method is: first apply an acid-resistant anti-corrosion wax layer on the surface of the copper layer, and then use a sharp steel needle to trace on the wax layer. The wax layer of the traced lines is destroyed, causing the copper surface below to be exposed, and In the next step of the etching process, it comes into contact with the acidic solution, thereby forming concave marks.
The invention and application of many technologies during the 18th and 19th centuries laid a solid foundation for the great changes in gravure platemaking technology. These include: the discovery of photosensitivity of potassium dichromate in 1782; the invention of photography in 1839; the discovery of the difference in physical properties of potassium dichromate before and after exposure in 1839; the carbon paper transfer method in 1864; the birth of photogravure technology in 1878 , and was officially put into production in Vienna in 1890. The photogravure method uses photographic technology to make film and uses carbon paper as an intermediate, thus completely replacing manual engraving and greatly improving the quality and speed of plate making. However, due to limitations of the process characteristics, gravure printing at that time could still only print. Lower-grade prints, and the subsequent emergence of the Boomeran platemaking method failed to fundamentally improve the quality of gravure printing.
Until the electronic engraving gravure process appeared, the gravure plate no longer relied solely on one-dimensional changes to reflect the shades of light and shade (the gravure method relies on changes in the depth of the cells, Bumelan The platemaking method relies on changes in the surface area of ??the cells), and electronic engraving gravure relies on the simultaneous changes in the surface area and depth of the cells to reflect the layers of shades and depths. This makes it possible to use the gravure printing process to replicate high-end workpieces that mainly focus on layers.
Especially after computer technology was widely adopted in the field of gravure printing, gravure plate making and printing technology became even more powerful. In terms of gravure printing plate making, it was the first to realize software-free technology. Today, when the offset printing process is still vigorously promoting CTP technology, CTP in the gravure printing field has been successfully operating for nearly 10 years; secondly, it has successfully used digital proofing technology. Today, digital proofing technology has been widely accepted in the field of gravure printing and plays an indispensable role in production. Figure 4-1 is a block diagram of the gravure printing process.
3. Classification of gravure plates
Gravure printing plates can be divided into two categories: engraving intaglio plates and corrosion intaglio plates according to the different ways of forming pictures and text.
(1) Engraving intaglio: It uses manual, mechanical or electronically controlled engraving knives to dig out the graphics and text on the copper plate or steel plate. In order to express the level of the image, the depth and width of the digging are different. . If there is more ink attached to the deep part, the printed color will be thicker; if there is less ink in the shallow part, the printed color will be thin. Engraving gravure plates include: manual engraving, mechanical engraving, and electronic engraving gravure plates.
1． Hand-engraved intaglio: Hand-engraved intaglio uses a method that combines manual engraving and semi-mechanical processing to engrave the original manuscript on the printing plate according to the size requirements.
2. Mechanical engraving gravure: Mechanical engraving gravure is an engraving gravure made by direct engraving or etching with an engraving machine.
3． Electronic engraving intaglio: Electronic engraving machine is used to replace the intaglio made by manual and mechanical engraving. It uses the photoelectric principle on the electronic engraving machine to reflect different amounts of light from the light source according to different levels of graphics and text in the original (if a transmitted original is used, different amounts of light will pass through), and generates corresponding electricity through photoelectric conversion to control the lifting and lowering of the engraving tool. distance, engrave the pre-processed natural metal layout to obtain the required graphics and text. The depth of the printing plate layout changes depending on the level of the original.
Electronic engraving gravure is currently the most widely used printing plate.
(2) Etched gravure plate
Etched gravure plate uses photographic and chemical etching methods to produce gravure plates on the images and texts that need to be copied.
Erosion gravure includes: photogravure and dot gravure
1. Photogravure: Also known as photogravure, this is a commonly used gravure printing plate. Its production method is to make the original into a positive image, then cover it on carbon paper and expose it to make the gelatin on the carbon paper sensitive to light. The part of gelatin that receives sufficient light is fully photosensitive and hardens and becomes transparent. On the contrary, the gelatin in the unexposed part does not harden. The hardened part is equivalent to the bright part on the positive picture, and the gelatin in the image part is not sufficiently photosensitive or not photosensitive. Carefully wrap the exposed carbon paper on the surface of the copper-plated and ground roller to make the hardened photosensitive The film is transferred to form an anti-corrosion layer on the surface of the roller, which is developed and corroded to obtain a gravure printing plate. Since the thickness of the photosensitive film pasted on the surface of the roller is different, the depth of the concave branch after corrosion is also different. These different depths correspond to the levels of the image. In the darkest part of the image, the depression depth of the printing plate is the largest, and the ink layer of the printed matter is the thickest; in the brightest part of the image, the ink layer is the thinnest or has no ink layer, and the thickness of the ink in the middle part also changes accordingly. This will more completely reproduce the subtle levels of the image and enhance the realism of the reproduction. This type of printing plate is most suitable for printing original manuscripts such as photos.
2. Photo dot gravure: This type of gravure is to directly apply photosensitive liquid on the surface of the printing plate cylinder, and then attach a dot photo to print the plate. Under the action of light, the blank parts of the film are photosensitive and hardened. During the corrosion process, these are hardened. The rubber film protects the surface of the roller from being corroded, forming a non-graphic part, while the graphic part is corroded to form dots with the same depth but different areas, forming the required gravure printing plate.
Its characteristic is that it does not use carbon paper to transfer images, but uses positive images of dots for printing.
Section 2: Gravure cylinder structure and processing process
1. Gravure cylinder structure
Since gravure printing machines are all rotary printing machines, Therefore, the gravure printing plate must be cylindrical before it can be printed on the machine. This cylindrical printing plate is called a gravure cylinder. Gravure printing plates are based on the surface of hollow cast iron or rigid cylinders, which are plated with copper as the plate base, and are made through a series of plate-making processes such as printing and corrosion. The printing plate cylinder is divided into two forms: integral structure and combined structure. Figure 4-2 is a schematic diagram of the structure of the integral gravure drum. Its feature is that the plate body and the rotating journal are integrated and cast in one go. The mechanical processing of this structure is relatively simple, and the processing accuracy is easy to ensure, but the casting process is more complicated.
Figure 4-3 is a schematic diagram of the combined structure gravure drum structure. The plate body 4 is generally a seamless steel pipe of corresponding size, and end caps 5 are tightly mounted on both ends. A tapered hole concentric with the outer circle of the plate is processed in the middle of the end cap 5. The taper sleeve 6 fits closely with the taper hole, and the rotating shaft 8 fits with the inner hole of the taper sleeve without any gap and is fastened together by a locking nut to ensure that the rotating shaft 8 is concentric with the outer circle of the plate. The gravure roller of this structure is light in weight, easy to replace, and has good processing performance, but it has many processing parts and requires high precision. It is currently widely used in the packaging industry.
Although the above two types of rollers are different in structure, their compositions are the same. The details are as follows.
(1) Plate body: The plate body is cylindrical, made of cast iron or steel pipe, and is the base body of the drum. Its surface has been finely machined and must be straight and free of defects (such as blisters, pits, etc.).
(2) Nickel plating layer: Since direct copper plating on steel or iron has poor bonding strength and is not strong, nickel plating should generally be performed before copper plating, and should be plated on the surface of the gravure drum first. A thin layer of metallic nickel is used to plate copper on the nickel to ensure the quality of copper plating.
(3) Bottom copper layer: Its thickness is about 2~3mm and can be used multiple times. After the bottom layer is electroplated, it must be turned or ground on a special machine tool or grinder to make the outer diameter and geometric accuracy meet the specified requirements, and to make the surface structure of the copper layer more detailed to ensure the quality of the plated surface layer.
(4) Silver treatment layer: In order to facilitate the separation of the surface copper layer from the bottom copper layer, the surface of the bottom copper is silvered before electroplating the surface copper, that is, it is immersed in a silvering solution or ammonium sulfide. The solution brush evenly brushes a layer of silvering solution or ammonium sulfide solution on the bottom copper surface, and then the surface copper is electroplated.
(5) Surface copper layer: The thickness of the surface copper layer is about 0.13~0.15mm, and is for one-time use only. Every time a new printing plate is made, the old copper surface needs to be removed and a new layer of copper needs to be replated. After the surface copper is electroplated, fine and ultra-fine processing such as grinding, grinding and polishing must be carried out to ensure the surface quality (geometric accuracy and smoothness) of the surface copper.
(6) Chrome plating layer: In order to improve the hardness and wear resistance of the printing plate surface, a layer of chromium is plated on the surface after the printing plate is made.
In order to ensure the quality of printed products, the surface quality of the gravure printing plate cylinder is required to be very high. The surface of the processed printing plate should be mirror-like without pits or processing marks. There are very strict requirements on geometric accuracy and smoothness:
① The outer circumferential surface of the rotating shaft and the outer circumferential surface of the plate cylinder should be strictly Concentric, tolerance ≤0.03mm.
②The out-of-roundness tolerance of the outer surface of the plate drum is 0.01mm.
③The cylindrical tolerance of the outer surface of the plate drum is 0.02mm.
④The roughness of the outer surface of the plate cylinder is 0.02▽ (smoothness is ▽ll) or more.
2. Processing process of gravure cylinder
Due to the needs of gravure technology, strict accuracy requirements are put forward for the production of gravure cylinder. The processing process is complex and difficult, and many processes are still complicated. Special methods are used for processing on specialized processing equipment, which are introduced below.
(1) Processing process of new gravure cylinder
Figure 4-4 is a schematic diagram of the processing process of new gravure cylinder.
1. Roller material.
Nowadays, the general body material of gravure printing is seamless steel pipe, because this material is available in the market and does not need to be specially made. It is generally cheap, has sufficient supply, and the processing performance of steel is also relatively good.
2. Metalworking. The processing of rollers is generally carried out on ordinary lathes and grinders. Rough, semi-finish and finish machining are required to ensure that the dimensional accuracy meets the requirements and the surface roughness is about 0.8.
3. Skim. Use weak alkali or organic solvent to remove grease and dirt on the surface of the processed roller to make the surface of the roller clean and facilitate nickel plating.
4． Nickel plated. Nickel plating is an important work that must be performed before copper plating on the gravure cylinder. Due to the poor bonding strength of direct copper plating on steel pipes, nickel plating is generally performed first, and a thin layer of metallic nickel is plated on the gravure drum to plate copper on the nickel layer.
Gravure nickel plating is carried out in a solution based on nickel sulfate. Under the action of pulsating direct current, the anode nickel plate emits electrons, and Ni-2e→Ni is deposited on the cathode gravure drum surface, that is, Ni 2e→Ni, in this way, the gravure steel is rolled on the metallic nickel.
Recipe: Nickel sulfate 250g/1
Boric acid 40g/1
Sodium chloride 25g/1
Current 2A/dm2
Temperature 50℃
5. Bottom copper plating. Copper plating is a key process in gravure processing, and its quality directly affects the processing quality of the printing plate. The copper plating of the gravure roller adopts the copper plating process of copper sulfate electroplating solution. Under the action of direct current, electrode reactions occur at both the cathode and the anode. The pure copper block of the anode releases electrons and generates divalent copper ions, Cu-2e→Cu2. ??The cathode gravure roller The electrons are obtained and a deposition reaction occurs, which reduces the copper ions to copper atoms on the surface of the drum, Cu2 2e → Cu. After countless electrode reactions, a layer of copper is plated on the surface of the gravure drum.
Formula: 220g copper sulfate/1
50g sulfuric acid/1
Appropriate amount of hardness additive
Current density 15～20A/dm2
Temperature 25℃
6. Turning and grinding. After the bottom copper is plated, the surface accuracy and dimensional accuracy cannot meet the specified requirements, and must be turned and ground. Turning and grinding can be performed on general lathes and grinders, or on dedicated turning and grinding machines. Special attention should be paid to: During turning and grinding, the same processing positioning reference must be used to ensure the mutual position accuracy of each processing surface. The thickness of the processed bottom copper layer is about 2~3mm.
7. Skim. The principle is the same as 3.
8. Silver treatment. Also known as "silvering" treatment, or pouring isolation solution. The main components of commonly used treatment solutions are: ammonium sulfide [(NH4)2S] and sodium hydroxide. There is also a "silvering solution" prepared with silver nitrate and potassium cyanide, but because potassium cyanide is toxic, it is rarely used. During treatment, rotate the drum slowly and pour the treatment liquid evenly on the surface of the drum to form a thin isolation layer. It is required that the copper sheet can be firmly adhered to, cannot be detached during the printing process, and the copper sheet (surface copper) can be peeled off from the drum smoothly after printing.
9． Plated copper. Copper plating is a very important process. The surface copper layer is required to have fine crystallization, bright and smooth surface, and uniform thickness. Therefore, when plating surface copper, the content of copper sulfate in the electroplating solution, the concentration of sulfuric acid, the temperature of the electroplating solution, the current density, and the PH value of the electrolyte should be strictly controlled. controlled within allowable values. Careful operation ensures the quality of surface lead. The surface copper thickness is about 0.13~0.15mm.
10. Turning and grinding. After copper plating, the surface of the drum needs to be finely processed. It is generally carried out on a combined turning and grinding machine tool, using a diamond tip and a diamond grinding wheel for turning and grinding, and cutting the copper layer on the surface of the drum to the required diameter size and surface accuracy requirements. Turning and grinding must be completed in one installation.
11. polishing.
After the roller is ground, use a feather cloth polishing wheel, or use special soft charcoal to super-finish the surface of the roller to make the surface smoothness of the roller reach a mirror-like surface with a surface roughness of above 0.05▽.
(2) The processing process of the old gravure cylinder
The processing process of the old gravure cylinder is shown in Figure 4-5. The requirements for each process are the same as before and will not be repeated. If the size of the drum remains unchanged, just peel off the copper surface and repeat the silver treatment, copper plating, grinding and polishing. There is no need to perform the various processes before silver treatment.
Section 3 Gravure Platemaking Method
Gravure is the basis of gravure printing. In modern gravure printing, the production methods of gravure include traditional photogravure platemaking method and photodot gravure platemaking method. and electronic engraving method. They have their own characteristics, which are introduced below.
1. Electronic engraving plate making method
The electronic engraving plate making method is a modern plate making method that integrates modern machines, optics, electricity and electronic computers. It can be quickly and accurately , to produce the required gravure with high quality. According to the different engraving tools, it can be divided into: electromechanical engraving method, laser engraving method and electron beam engraving method.
1． Electromechanical engraving method
Electromechanical engraving method is a rapidly developing method of high-speed and fully automatic gravure mesh formation. It changes the concave holes formed by corrosion in the past, and instead uses a diamond engraving knife to directly engrave the intaglio copper surface. The process is to first electronically divide the original into dot pieces or continuous tone images, and then scan the dot pieces or continuous tone images through the objective lens on the scanning head. The size or depth of the dots is converted from the size of the optical signal of the scanning density into an electrical signal. The size is then input into an electronic computer. After a series of computer processing, changing currents and digital signals are transmitted to control and drive the electric engraving diamond carving knife, and the gravure mesh with different sizes and depths is carved on the surface of the mirror copper roller, which forms The process is shown in Figure 4-6.
Figure 4-6 Engraving mesh step process diagram
Now take the working process of K304 electronic engraving machine as an example to illustrate the process flow of mechanical and electronic engraving. Figure 4-7 shows its process flow.
Figure 4-7 Electronic engraving gravure process flow
(1) Film production is based on the different conversion heads of electronic engraving machines (divided into O/C conversion heads and O/T conversion heads Two types), can use continuous harmonization and scanning dot film respectively. If an O/C converter is used, the film should be a continuous image-adjusted film shot with a small stretchable white opaque polyester photosensitive film; if an O/T converter is used, the film should be a scanning screen film. The film can be either positive or negative, depending on the complexity and requirements of the workpiece.
(2) Roller installation The rollers of gravure printing machines can be divided into two types: shafted rollers and shaftless rollers. When installing the shaftless roller, two tips must be used to hold against the tapered holes at both ends of the gravure roller; the shafted roller must be installed on the sleeves at both ends and then connected to the electric engraving machine with a coupling. After the drum is installed, use a 1:500 gasoline and engine oil mixture to remove dust, oil, and oxides from the drum surface to make the drum surface clean and free of dirt.
(3) Film pasting Before pasting, use clean gauze and an appropriate amount of absolute alcohol to wipe the surface of the film and scanning roller clean. When pasting, the center line of the film should be perpendicular to the direction of scanning movement and completely in contact with the surface of the scanning roller. Otherwise, due to inaccurate scanning focus, the image will be blurred, affecting the tonal level and clarity of the engraving.
(4) Programming refers to the corresponding data and work instructions input to the electronic computer to control the work of the electric engraving machine. The programmer must be familiar with product specifications and dimensions, customer requirements, layout and layout, and according to the pattern content , Specifications and sizes are selected using network cables, network angles and layered curves. When the layout size is large and the layering pattern is large, it is appropriate to use a thicker line number such as 60 lines/cm, and use corresponding screen angles and layering curves according to yellow, magenta, cyan, and black. If you want to copy a pattern with smaller size and rich layers, it is better to use a thinner line count, such as 70 lines/cm, to reflect the subtle layers. Text line patterns should use harder layered curves.
(5) Trial engraving: By adjusting the current value of the control box, the appropriate dark tone (full tone), high light (5%) dot and channel sizes can be obtained.
Electronic engraving dots can be divided into four shapes, represented by 0#, 2#, 3#, and 1#, which are called mesh angle shapes, as shown in Figure 4-8. Different mesh angle shapes are obtained by changing the rotational speed, feed speed and engraving frequency during electric engraving.
Figure 4-8 Schematic diagram of the mesh angle shape
For example, at a higher speed, the point shape is elongated into a ◇ shape; at a lower speed, the point shape is flattened into a mouth shape. Trial engraving is a very important task, which is directly related to the tone level of the printed matter. Therefore, during trial engraving, corresponding dark tone and high-gloss dots should be selected according to different screen line numbers, dot shapes, and printing materials. The diagonal and channel dimensions of the dots can be determined with a dot tester.
(6) When scanning and calibrating, use the white surface of the scanning roller as the base surface, so that there is an obvious contrast between the black density graphics and text on the film and the white base surface. In order to ensure the first quality during gravure printing For tonal printing, the scan head setting should have an appropriate density difference, which can be achieved by moving the optical head to the 5% screen density area. The digital input value for this density is calibrated at 768, the diagonal of the first ink hole ( The test engraving highlight dots is at this value, and the remaining numbers (768~1023) are input to make enough difference between the white base surface and the blank part of the film, which ensures that the edges formed by the engraving paste and the background part are all There will be no impact on the engraving or printing process.
(7) Engraving After the above work is completed, the electric engraving machine officially starts engraving. When the scanning head scans the film, the engraving head synchronized with the scanning performs engraving based on the scanning signal. The movement of the engraving head is driven by a quartz oscillator, and the maximum engraving speed of the engraving head can reach 4000 grains/s dots.
2. Laser engraving method
The laser engraving method was successfully trial-produced by the British Crossfield Company more than 20 years ago, and it first exhibited a laser gravure engraving machine in 1977. The company started by filling the mesh of an etched copper roller with epoxy resin, hardening it, and then grinding it on a grinder to give it a smooth surface. During engraving, a carbon dioxide laser beam with variable energy is used to irradiate the drum. Because the copper surface can reflect the laser beam, the original copper mesh wall is retained, and the epoxy resin is removed from the mesh. The amount of removal depends on the size of the laser beam. related. The main disadvantage of the laser engraving method is that the material properties of copper and epoxy resin are very different, and it is technically difficult to achieve a smooth non-printing surface, leaving stains on the non-printing area. In addition Plate making also requires electroplating and corrosion equipment. Since then, the company has switched to spraying epoxy resin on the surface, and then treating the surface of the drum before laser engraving. The printing area is composed of fine grooves of different depths arranged in a spiral. Instead of the grid, the ink-filled area appears
"continuous grooves". In 1982, the company modified the laser engraving control system to break the previously continuous grooves, thus forming pits similar to a grid. Figure 4-9 shows the working principle of the laser engraving method. The working process is that the laser beam emitted from the carbon dioxide laser, according to the information requirements of the gravure original, is turned into a beam through the electronic computer control modulator and light energy regulator. The required laser light shines on the surface of the gravure copper cylinder through the reflector and condenser (lens), melting and evaporating the epoxy resin to form a required pit. These pits form a printing plate corresponding to the original.
Figure 4-10 shows the laser engraving process. A brief description is as follows.
(1) Erosion roller According to the traditional corrosion method, the surface of the finely processed gravure roller is etched into the required grid shape for spraying.
(2) Spray epoxy resin. Use the electrostatic spraying method to spray specially prepared epoxy resin powder to coat the surface of the drum with epoxy resin. Then move the drum to an infrared furnace and start at 180°C. Start melting and slowly rotate the drum, the whole process is controlled by a microcomputer. To achieve sufficient coating thickness on the roller, a second spray can be applied. At the end of the hardening process the temperature reaches 200°C. The whole process takes about 1.5 hours. Finally, the drum is cooled. In order to make the surface finish of the drum meet certain requirements, it must be polished and polished to make the drum easy for laser engraving.
(3) Laser engraving uses CO2 gas laser (power 150W), and the epoxy resin surface where the laser beam touches is evaporated. The drum speed is 1000r/min. The engraving speed depends on the drum circumference, generally 75mm per minute. The maximum size of the laser engraving drum is 2600mm in length and 1600mm in circumference.
According to relevant information, the laser engraving machine can use two input methods: online operation and non-online operation.
Online operation means that the laser engraving machine is connected to an electric extension. The originals loaded on the electronic extension machine can be color films, black-and-white manuscripts or reflective manuscripts. The maximum size is 580mm~610mm. When online, the scanning signal is directly input into the laser engraving machine that operates synchronously with the electric extension machine, so that the depth of the resin filled in the grid carved by the laser beam adapts to the modulation value of the corresponding part of the image and text.
When operating offline, the information from the electric extension is stored on the disk and then input into the laser engraving machine for engraving when needed.
(4) Chrome plating: After the engraved gravure roller is cleaned and inspected, a layer of chromium is plated on the traditional chrome plating machine to improve wear resistance and ensure durability.
After printing is completed, the coating on the roller can be peeled off and the grid can be filled with epoxy resin for the next engraving. A roller can be reused more than ten times.
Advantages of laser engraving: good quality, clear image, suitable for 20-70 lines/cm; accurate reproduction, no need to correct the roller, high production capacity (engraving length 2600mm, circumference 1200mm roller, only 35 minutes ); can automatically repeat continuous engraving, especially suitable for packaging printing.
3. Electron beam engraving method
The only company engaged in the production of electron beam engraving gravure rollers is the German Haier Company. The first electronic engraving machine was made in early 1980.
(1) The basic structure of the electron beam engraving machine. The frame of the electron beam engraving machine is a cast iron bed base, as shown in Figure 4-11.
It forms a drum processing chamber with a vacuum box of the same length. There are two bearing seats in this chamber, which are driven by stepper motors through lead screws and move independently along the length of the machine. The bearing seat has two auxiliary devices, which are automatically clamped after the roller is installed on the machine. The bearing seat has a high-power motor to drive the drum. The speed of the drum depends on the diameter of the drum, generally 1200-1800r/min.
When the drum is engraving, the electron beam gun is stationary, and the drum moves left and right in front of the electron beam gun. The electron beam gun is installed at the rear of the processing chamber in the middle of the machine, passing through the vacuum box hood. The distance between the electron beam gun and the drum is driven by a stepper motor. The electron beam engraving machine also has electronic cabinets, high-voltage generators and vacuum pumps that control the electron beam gun and machine.
(2) Engraving process: Install the engraved gravure roller on the centering device with the cover opened, and center the roller shaft to the axis of the bearing seat. Other steps are automatically controlled by multiple microcomputers, and the sequence of operations is as follows.
①Close the lid of the vacuum chamber.
②Clamp the roller.
③The vacuum pump evacuates air to create a vacuum in the electron beam gun processing chamber.
④ Rotate the drum and perform the initial positioning of engraving at the starting end of the drum.
⑤ Turn on and adjust the electron beam.
⑥Start engraving.
Most of the time the control and adjustment process is parallel, and it takes 6 minutes to start engraving. For a roller with a length of 2400mm and a circumference of 780mm, the engraving time is 15 minutes. Another drum with the same length and a circumference of 1540mm takes 22 minutes to engrave. During engraving, only the engraving drum rotates and moves laterally, and the electron beam gun is fixed. After engraving is completed, the electron beam is automatically cut off, the drum is braked, the processing chamber is inflated, and finally the drum is released. The entire preparation time for engraving a roller is less than 15 minutes. It can be seen that 2 rollers can be engraved per hour.
When engraving with electron beam, the high-energy electron beam penetrates about 5μm into the copper layer and is stopped in the atomic field. It transfers all the kinetic energy to the copper, thus producing superheated copper molten liquid.
The plasma pressure generated in the electron beam squeezes the molten copper out of the molten zone from the side. The peripheral speed of the drum of 20 to 30 meters per second will throw the molten liquid in the form of drops along the tangent line of the rotating drum. Tiny copper droplets flew in the vacuum, cooled down slightly, and then hit the replaced reflective plate in a molten state, turning into copper slag. After electron beam impact, the molten zone in the cells recrystallizes under the action of the surface tension of the molten copper to form a smooth surface, and the cell shape is hemispherical. As shown in Figure 4-12.
(3) Advantages of electron beam engraving technology
① Electron beam engraving has high speed (100,000-150,000 grids/s) and is easy to modulate and deflect.
②The electron beam moves synchronously with the rapidly rotating drum between shots, that is, the electron beam is always at the same position of the drum when shooting the cells.
③The electron beam can engrave any number of lines and angle of the network lines.
④ The use of a special electron beam engraving dot device significantly improves the reproduction of outlines, which is very important for the reproduction of text and lines.
⑤The production efficiency is 1 to 2 times higher than that of electronic engraving.
⑥The grid shape produced by electron beam engraving is hemispherical. When the high-heat copper melts, the vaporization cylinder is squeezed out of the grid by plasma pressure, and the thickness of the recrystallized surface is less than 5/μm. Therefore, the inner wall of the grid is smooth and the grid wall has no defects, which is conducive to achieving very good ink transfer under high-speed printing conditions.
2. Photogravure plate making method
Photogravure plate making method, also known as carbon paper printing method or carbon paper etching method, is a traditional gravure plate making method. ,
The process is to use photography or electrolysis to make a continuous sun-adjusted pattern from the original, and then first dry a concave mesh (also called a grid) on sensitized carbon paper, and then dry it. Continuously adjust the image. The exposed carbon paper already has concave mesh and positive pattern text. It is passed on a degreased roller and developed to form grids with different densities on the passed carbon paper. After filling the plate, it is etched with ferric chloride etching solutions of different concentrations to form gravure meshes of the same size and different depths. The formation process is shown in Figure 4-13.
The photogravure plate making process flow is shown in Figure 4-14. The main process is described below.
(1) Photography. For original manuscripts of any nature, such as drawings, color photos, black and white photos, etc., they must be photographed first to create continuous sun-adjusted pictures in the opposite direction to the original. If the original is a color image, it should first be photographed into four color positive pictures of yellow, crystal red, cyan, and black. After correction, it should be tanned into a four-color positive picture. The text manuscript is photographed into the required format using phototypesetting methods.
(2) Imposition If the positive pictures or text made by photography need to be combined for printing, they should be assembled into a printing layout according to the layout requirements and technical parameters.
(3) Carbon paper sensitization Carbon paper is composed of paper base, white gelatin and carbon pigment. It itself has no photosensitive properties. Only after sensitization treatment to make the gelatin layer absorb potassium dichromate (K2Cr2O7) can it have photosensitive properties. The sensitization treatment is generally carried out 24 hours before drying the carbon paper. The sensitizing solution consists of potassium dichromate and water. At room temperature of about 22°C and relative air humidity of 65%, put the carbon paper into a potassium dichromate solution with a concentration of 2.5% to 5% (temperature of about 15 to 18°C) and soak it for 3 ~4min, take out and dry in the sun, put into a sealed iron cylinder, protect from light and moisture, and set aside for later use.
(4) Carbon paper screen printing The screen used for photogravure screen printing has two mesh shapes, one is a square mesh and the other is an irregular mesh. Its characteristic is that when exposed, the small black squares are opaque and only the white lines are transparent. Through exposure, the carbon paper adhesive layer has a mesh latent image, and after corrosion, a mesh wall of different shades is formed. During printing, the inked part of the gravure plate has a large area. When scraping the ink with a scraper, the scraper not only scrapes off the ink in the blank part, but also scrapes off part of the ink in the graphics and text, which affects the quality of the graphics and text. In order to prevent this situation from happening, without affecting the image layer transfer, the screen wall is dried so that it can be used as a support for the squeegee during printing to ensure that the ink in the graphic part is not invaded by the squeegee. The quality of the print.
(5) Dry the carbon paper and place the assembled sun pictures on the drying plate