Gravure plate making
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Chapter 4 Plate Making
Section 4 Gravure plate making
Gravure plates, which are distinguished from the production methods, can be divided into two categories, one is engraved gravure, the other is gravure, the engraved gravure has manual or mechanical engraving gravure, and the electronic engraving gravure.
The hand-engraved gravure is carved on the copper plate with various carving knives. It can directly engrave the concave lines. It can also be coated with a resist film on the copper plate to etch the resist film to expose the surface of the copper plate. Chemical corrosion is carried out. The mechanical engraving gravure is made by mechanical engraving with a color engraving machine, a floating engraving machine, a parallel line engraving machine, and a scale engraving machine, or by etching a resist layer on the copper surface to form a gravure.
The gravure lines of hand- or mechanical engraving are exquisite, and the patterns are exquisite. They are mainly used to print banknotes and bonds with anti-counterfeiting value.
The electronic engraving gravure uses an electronic engraving machine to control the engraving knives according to the photoelectric principle, and engraves the cells on the surface of the drum, and its area and depth change simultaneously. (See Figure 4-50b)
Photogravure, also known as photocopying, is made by continuous tempering negatives and gravure screens, through printing, carbon paper transfer, corrosion and other processes. The plate has the same area but different shades from bright to dark, and uses the change in the thickness of the ink to reproduce the light and dark levels of the original. (See Figure 4-20a).
First, the production of plate cylinder
The gravure printed plates are not pre-made on the plate or plate cylinder, but are directly plated on the plate cylinder, and then the plate cylinder is mounted on the printing press for printing.
The plate cylinder has an open hollow cylinder and a closed solid roller. The length and circumference of the cylinder are designed according to the size of the gravure printing machine.
(1) Processing of the drum
The gravure cylinder can be directly processed by a seamless steel pipe according to the diameter of the drum, or it can be rolled into a cylindrical shape for welding, and then processed.
The processing of the drum is divided into roughing, semi-finishing and finishing. Roughing makes the wall thickness of the drum meet the specified requirements, ensuring that the weight of each part is equal when the drum rotates, and the centrifugal force generated is also the same. Semi-finishing is to keep the shaft and the drum concentric. Finishing is the outer circle of the finishing roller, which reaches the specified size.
The processing accuracy of the drum is directly related to the service life of the drum, the plating of the drum, and the quality of the electric carving and printing. The processed drum requires uniform wall thickness, and the center of the center of the shaft and the outer circumference of the drum should not exceed 2 μm. The surface finish of the drum should reach Δ5, and the cylindrical grinding machine can reach Δ7 or more with a diameter accuracy error of ±0.01 μm.
(2) Electroplating of the drum
The drum is pre-plated before copper plating. First, the oil on the surface of the drum is removed by hand or by electrolysis, and then the drum is pickled, and the rusted product oxide film on the surface of the drum is etched away with a chemical. Finally, a nickel layer is plated on the surface of the iron roller, and then copper plating is performed. The copper plating layer is a plate-making layer of the gravure cylinder, and the plate is subjected to corrosion plate making or electronic engraving.
The gravure cylinder is made of acid copper plating, and the copper plating solution is composed of copper sulfate, sulfuric acid, additives, etc., and the temperature of the plating solution is 20 to 35 °C.
The quality standard of the copper layer is: the thickness of the plated copper layer is 100 to 120 μm. The surface is bright and delicate, with burrs, track marks and pitting.
(3) Roller car, grinding
The copper-plated roller needs to be ground on the surface of the copper layer to achieve a surface finish of 8-10.
Second, the gravure plate making process
The gravure is produced by exposing the image of the continuous film to a sensitized and meshed carbon paper, then passing it onto the surface of the drum, developing and etching to form a gravure. The plate making process is:
Carbon paper sensitization → printing → printing → development → filling → corrosion → chrome plating
(1) Carbon paper sensitization
Carbon paper is a photosensitive material for printing, which consists of a paper base and a gelatin emulsion coated with a mixed pigment pigment. Generally, the carbon paper of the factory, the gelatin emulsion layer is not photosensitive. Before printing, the carbon paper should be immersed in a 4% potassium dichromate solution for 3 minutes, taken out and dried, and the adhesive layer has photographic properties.
(2) printing version
The printing plate is carried out in two steps, first drying the net and then drying the film.
Gravure printing, using a squeegee to remove the blank part of the ink, if the area of the inked part is large, the scraper not only scrapes off the blank part of the ink, but also scrapes off part of the ink in the image part, as shown in Figure 4-21·A Shown. Therefore, it is necessary to use a screen to expose the network cable on the carbon paper to divide the graphic into a grid, as shown in Figure 4-21·B. The squeegee is supported by a grid on the surface of the printing plate to prevent the blade from invading the printing portion of the ink.
The screen used for gravure printing is shown in Figure 4-22. The ratio of the width of the transparent line to the opaque line is 1:3 to 1:3.5. The mesh shape has a square shape, a brick shape, a diamond shape, and an irregular shape. Square screens are usually used.
After the carbon paper is exposed to the mesh through the screen, the negative film can be sun-dried to form an image latent image on the surface of the carbon paper.
(3) over the version
The carbon paper that has been exposed to the wire and image is adhered to the surface of the polished copper plate cylinder.
At present, most of the used plates have been dry-processed. The method of passing the plate is as shown in Fig. 4-23. While a small amount of water is added 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
Development is pre-developed and officially developed.
The pre-development is to immerse a part of the plate cylinder in warm water, and rotate the edge to separate the glue layer of the carbon paper from the paper base.
Formal development means that after the paper base is separated from the adhesive layer, the developer is heated to 40 ° C and kept at a constant temperature, and the uncured adhesive layer is completely dissolved. After development, the film is blown dry with a fan.
(5) Fill in the version
On the surface of the plate cylinder, there is no image portion and both sides of the drum, and asphalt paint is applied to prevent corrosion of the portion not to be corroded.
(6) Corrosion
The electrolytic solution of ferric chloride is passed through the hardened film to dissolve the copper layer on the copper surface to form a cell. This process is called corrosion, commonly known as erosive.
The corrosion process is carried out in three steps. First, the gelatin layer expands and absorbs ferric chloride, and then the etching solution penetrates the film layer to reach the copper surface. The third step is to corrode the copper surface, and the etching solution reacts with lead. The chemical reaction is:
Cu+2FeCl3=CuCl2+2FeCl2
In the production, the plate cylinder is etched successively with different concentrations of ferric chloride solution. Figure 4-24 shows the process of etching a plate with a low concentration of corrosive solution after high concentration. Different concentrations of corrosive liquids are etched and etched on the different thickness of the adhesive layer to form a cavity with different depths, and the printed product will reproduce a rich layer.
(7) chrome plating
The gravure is made by etching on the copper layer, and the hardness of copper is generally about 90 to 180 HV. The squeegee is easy to scratch the plate when printing. Since the hardness of metal chromium is very high, it is about 800 to 1000 HV, and the wear resistance is very good. Therefore, when the gravure roll image is finished, a layer of chromium is plated on the copper surface to improve the printing durability of the intaglio plate.
Photogravure, the process of making plates is complicated, the quality is not easy to control, and the range of use is gradually narrowing due to the application of electronic engraving gravure.
Third, the engraving process of electronic engraving gravure
The electronic engraving gravure is a plate making method that appeared in the 1960s. It is characterized by no carbon paper printing and no chemical corrosion. The image-processed swatch is used as the original, and the printing plate is directly engraved on the surface of the copper plate cylinder by using an engraving machine of an electronic circuit.
The electronic engraving gravure has a delicate picture, rich layers, easy to control quality, and is widely used in gravure printing.
(1) The basic principle of the work of electronic engraving machine
The electronic engraving machine is composed of a manuscript roller (or a scanning roller), a plate cylinder, a scanning head, an engraving head, a transmission system, an electronic control system, and the like.
The working principle of the electronic engraving machine is: the scanning head scans the original, and the different light signals reflected from the original are converted into electric signals through the photoelectric converter, and then processed by the amplifier and the data to make the light The strength is converted to the magnitude of the current, and the engraving head is controlled to engrave on the copper cylinder.
When the electronic engraving machine is working, the original cylinder and the engraving cylinder are synchronously operated. At the same time, the engraving system moves along the axial direction of the drum, and the grid is engraved on the engraving cylinder with a sharp diamond knife, as shown in Fig. 4-25. The engraving system is controlled by the scanning system through a computer. The hole network formed on the copper cylinder is generated by an additional signal in the computer. This signal enables the knife to continuously and regularly vibrate. The size and depth of the cell are determined by the density of the original. It is decided that the quantitative relationship between the density of the scanned original and the depth of the engraved cell can be adjusted on the computer.
The functions of the electronic engraving machine are more and more, such as: the change of the magnification of the circular person, the seamless engraving in the circumferential direction, the automatic selection of the level, and the adjustment of the angle of the cell.
(2) Production of intaglio for electronic engraving
The process of making an electronic engraving gravure is:
Scanning film → installation plate cylinder → test → engraving → chrome plating
1. Scanning film
In the past, the scanning negative film was made by continuously adjusting the opaque film, which was expensive and the quality of the film was difficult to control. In the 1980s, the electronic engraving machine was added to the electronic conversion unit, and the concave conversion was carried out according to the designed program, that is, the screen negative film for offset printing was used to engrave the intaglio plate. Therefore, most of the current use of color separation screen printing.
2. Installing the plate cylinder
Install the plate on the electronic engraving machine with a crane and remove the oil, dust and oxide from the plate before engraving. Paste the scanned film on the original cylinder.
3. test
According to the requirements of the original (scanning sheet) and the hue of the ink, the test value is set in combination with the printed product. For example, the decorative printed paper is rough, the ink absorption is strong, and the engraving depth must be 45 μm to 50 μm to meet the printing requirements, and the engraving must be adjusted. Current and voltage on the amplifier.
4. Sculpture
The scanning head scans the original, the engraving head runs synchronously with the scanning head, and the surface of the plate cylinder is engraved into a different depth of the cell.
The new electronic engraving machine has three kinds of dot angles, which can be arbitrarily selected during operation to avoid the occurrence of turtles due to inaccurate overprinting.
When engraving text, the subtle pen can not be lost. You must use fine mesh to engrave. If you use 100 lines/cm, the engraving of the text can achieve very good results.
Nowadays, the electronic engraving gravure has a separate electronic engraving system plate making, that is, the scanner and the electronic engraving machine are separated, and are respectively connected with the input and output interfaces of the image workstation. The scanner can scan the positive and negative negatives, and the Yin can scan the milky white film. The workstation has a variety of image processing functions, which can perform overall and local color correction, cropping, combining and scaling, and color gradation on the image. Make the yellow, magenta, and blue images and line images merge into one. The wire of the electronic engraving machine ranges from 31.5 to 200 lines/cm.