Photogravure, also known as photocopying, is made by continuous negative adjustment film and gravure screen, after printing, carbon paper transfer, corrosion and other processes. The plate area from bright to dark has the same cell area but different shades. The thickness of the ink layer is used to reproduce the light and dark levels of the original.
The production of gravure is to expose the image of the continuous negative film to the sensitized and gridded carbon paper, and then pass the plate to the surface of the drum, develop and etch it to make the gravure plate, the plate making process is:
Carbon paper sensitization → printing → overprinting → development → filling → corrosion → chrome plating
(1) Carbon paper sensitization
Carbon paper is a photosensitive material for printing. It consists of a gelatin emulsion coated with mixed pigments on the paper base and surface. Generally, the carbon paper that is shipped from the factory has no sensitivity to the gelatin emulsion layer. Before printing, the carbon paper needs to be immersed in a 4% potassium dichromate solution for 3 minutes, taken out to dry, and the adhesive layer has photosensitivity.
(2) Printing
Exposure is carried out in two steps, first exposure of the network cable and then exposure of the negative film.
For gravure printing, use a doctor blade to remove the ink in the blank part. If the area of ​​the inked part is large, the blade will not only scrape off the ink in the blank part, but also scrape off part of the graphic part of the ink, as shown in Figure 4-21 · A As shown. Therefore, it is necessary to use a screen to expose the network line on the carbon paper to divide the graphics into grids, as shown in Figure 4-21 · B. Support the squeegee with a grid on the surface of the printing plate and text to prevent the squeegee from invading the printed part of the ink.
The screen used for gravure printing is shown in Figure 4-22. The ratio of the width of the transparent line and the opaque line is 1: 3 to 1: 3.5. The mesh shapes are square, brick, diamond and irregular. A square screen is usually used.
After the carbon paper is exposed to the network line through the screen, the negative film can be exposed to form an image latent image on the surface of the carbon paper adhesive layer.
(3) Version
The carbon paper that has been exposed to the wire and the image is attached to the surface of the polished copper plate cylinder.
At present, most of the used edition machine adopts the dry method for edition. The method of printing is shown in Figure 4-23. While adding a small amount of water between the surface of the plate cylinder and the carbon paper gelatin layer, the carbon paper is adhered to the surface of the copper roller by the pressure of the pressure roller.
(4) Development
The development is divided into pre-development and formal development.
The pre-development is to immerse a part of the plate cylinder in warm water, and to separate the glue layer of the carbon paper from the paper base while rotating.
Formal development means that when the paper base is removed from the glue layer, the developer is heated to 40 ° C and maintained at a constant temperature to dissolve all the unhardened glue layer. After developing, use a fan to dry the film.
(5) Fill in the version
On the surface of the printing plate cylinder, the parts without pictures and text and the two sides of the cylinder are coated with asphalt paint to prevent corrosion of parts that should not be corroded.
(6) Corrosion
The ferric chloride etching solution is used to penetrate the hardened glue film to dissolve the copper layer on the copper surface to form a net hole. This process is called corrosion and is commonly known as rotten version.
The corrosion process is carried out in three steps. First, the gelatin layer expands and absorbs ferric chloride, and then the etching solution penetrates through the film layer to reach the copper surface. The third step is to corrode the copper surface. The etching solution reacts with lead.
Cu + 2FeCl3 = CuCl2 + 2FeCl2
In production, the plate cylinder is corroded successively with different concentrations of ferric chloride solution.
The process of corroding the printing plate with high concentration first and then with low concentration corrosive liquid. Corrosion liquids of different concentrations will be permeated and etched on the plates with different thicknesses of adhesive layers to form different depths of holes, and the printed products will reproduce a rich layer.
(7) Chrome plating
The gravure is made by etching on the copper layer, the hardness of copper is generally about 90 ~ 180HV. When printing, the doctor blade can easily scratch the printing plate. Due to the high hardness of metal chromium, about 800-1000HV, the wear resistance is very good, so when the gravure roll graphics are completed, then a layer of chromium is plated on the copper surface to improve the durability of the gravure.
For gravure, the process of making plates is complicated, and the quality is not easy to control. The range of use is gradually narrowing due to the application of electronic engraving gravure.
Plate-making process of electronic engraving gravure
Electronic engraving gravure is a plate-making method that appeared in the 1960s. It features no carbon paper for printing and no chemical corrosion. Taking the image-processed free film as the original, the engraving machine of the electronic circuit is used to directly engrave the mesh holes on the surface of the copper plate cylinder to make a printing plate.
Electronic engraved gravure has a delicate picture, rich layers and easy quality control. It is widely used in gravure printing.
(1) The basic principle of the work of electronic engraving machine
The electronic engraving machine is composed of the original drum (or scanning drum), plate cylinder, scanning head, engraving head, transmission system, electronic control system, etc.
The working principle of the electronic engraving machine is: the scanning head scans the original, and the light signals of different strengths reflected from the original are converted into electrical signals by the photoelectric converter, and then through the amplifier and data processing, the optical The strength is converted into the size of the current, and the engraving head is controlled to engrave on the copper drum.
When the electronic engraving machine works, the original cylinder and the engraving cylinder run synchronously. At the same time, the engraving system moves along the axial direction of the cylinder, and a sharp diamond knife is used to engrave the holes on the engraving cylinder according to the signal, as shown in Figure 4-25. The engraving system is controlled by the scanning system through a computer. The hole network formed on the copper drum is generated by an additional signal in the computer. This signal can make the cutter continuously vibrate regularly. The size and depth of the network hole are derived from the density of the original It was decided that the quantitative relationship between the density of the scanned original and the depth of the carved holes can be adjusted on the computer.
The functions of the electronic engraving machine are more and more, such as: can change the magnification of the square person in the circle, seamlessly engrave in the circumferential direction, automatically select the level, adjust the angle of the net hole, etc.
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