Stamp printing process and anti-counterfeiting technology

Stamps are special prints that most of us will contact and use. In addition to their function as securities, many stamps (especially rare stamps) are also valuable works of art and collectibles. The stamps are full of ornamental and scientific features. They have the infinite charm of the world, the ancient times and the modern world. They have a high collection value, and they have been continuously appreciated as the time goes by and the quantity of collections has continued to decrease. Therefore, the printing of stamps not only requires exquisite pictures, but also a considerable amount of anti-counterfeiting technology. Today, with the continuous development of anti-counterfeiting technology, the technical content of stamp printing has increased. Therefore, the printing of stamps is different from the printing of general books and advertisements.

First, the stamp printing technology

1. Platemaking technology

At present, the printing of stamps is mainly based on gravure printing (some of which are printed with relief printing and offset printing). This is mainly to ensure its seriousness and falsity. China's stamps are generally printed in gravure. Gravure printing is a printing method that transfers graphics below the plate to the substrate. The prints printed by the intaglio printing method have clear, well-defined lines, thick ink layers, good hand feeling, and strong anti-counterfeiting. Therefore, they are used for the printing of many banknotes, stamps, and many other documents, and their quality is recognized.

There are many kinds of intaglio production techniques. In addition to hand-carving techniques, there are many kinds of engraving methods such as machine engraving, electronic engraving, and laser engraving. These engraving methods each have their own characteristics and uses.

(1) Photopolymer gravure. This plate is composed of a base layer, a photosensitive layer, and a protective layer. The first is to make the color separation film on the plate for exposure, known as separation exposure. The image and text are transferred to the plate material, and the polymerization reaction in the exposed area becomes an insoluble area. Then do not add film to perform a deep exposure to control the depth and rinse with water. The part where the polymerization did not occur was washed out with water. After rinsing, intermediate exposure is performed to increase the hardness and the resistance of the plate. At the end, no film was used for exposure, which increased stability. This printing plate is used on a gravure printer consisting of a plate cylinder, a doctor blade assembly and an impression cylinder. It is suitable for the printing of gold, silver, Xubu pearl powder or aluminum foil and plastic film.

(2) Hand-engraved gravure. First of all, the pattern to be printed is drawn as a sketch draft. According to the sketch draft, a point-and-tone pattern or text is formed on the steel plate with a knife. The hand-carved steel plate is called a small original plate. Small originals require heat treatment to increase hardness. The plate was pressed on the steel shaft surface by a plate press. After the steel shaft was heat-treated to increase the hardness, the pattern on the shaft surface was pressed by the plate press. The graphic and then turned on the plate, this process is called "version." After that, the image on the steel plate was pressed with a hot press on a 1 mm thick plastic plate. The single-plate plastic printing plate is manually modified to make a large version of a large sheet of plastic version of the copper plate, nickel plated copper plate, nickel plate and then turned into a machine printed on the nickel plate or iron plate for machine use . Hand-engraved gravure is mainly used for the printing of securities (such as banknotes, stamps, etc.). However, the engraving and intaglio making process in stamp printing has now adopted the electronic engraving method.

(3) Photoetching gravure. First, color manuscripts are separated to make a monochrome negative image. After the revised version of the negative image is placed on the continuous shooting machine, it is continuously shot into a large Zhangyang figure color separation film, and the large Zhangyang picture is printed on the printing plate. The plate is then etched with a solution of ferric chloride and finally plated on the plate to increase the resistance to printing.

Gravure-printed stamps have used two kinds of plate-making techniques: traditional gravure platemaking and modern gravure platemaking.

(1) Photo etching gravure stamp plate making process. Traditional gravure stamp plate making, the original manuscript photograph separation, decomposition of the required several colors of continuous tone chart, and then artificial correction, the corrected Yin picture through the connected camera, the various color version to reduce to The stamps have the same size and size as the positive ones, and they are continuously photographed into positive editions of tens or hundreds of full-page stamps, followed by printing.

When making a plate, the gravure grid (also known as the cable) is sunrayed onto carbon paper, and the stamp pattern (positive image) is also sunrayed onto the existing grid of carbon paper, followed by carbon. The film with a grid pattern on the paper was transferred to a previously prepared printing cylinder, and the surface of the cylinder was subjected to deep and shallow corrosion with ferric chloride solution. In order to increase the printing capacity of the printing cylinder, a layer of chromium is finally plated on the surface of the printing cylinder for printing on the machine. The traditional gravure plate making process is cumbersome, slow, long production cycle, poor product quality stability, and is technically difficult to handle, difficult to grasp, and is currently not used in the printing of stamps. With the printing industry stepping into the era of electronic printing, the plate-making process of gravure stamps has also been improved from traditional hand-made plates to modern electronic plates. After the transition of electronic subcontracting, copying, copying and whitening - electronic engraving, it entered the stage of the optical film making combined with the electronic color separation high-end networking system and electronic engraving workstation.

The stamp manuscript is first separated by an electronic color separation machine, and the separated color image information is transmitted to the computer for correction and editing. The printing of stamps is different from that of ordinary prints. In addition to the use of three primary colors, it often requires the addition of various special colors. Therefore, it is necessary to make corrections to the layout after separation. At the same time, due to the relatively small size of the stamps, the exquisite designs and the high quality required, this poses great difficulties for overprinting during printing, especially for high-speed printing. For this reason, special technology is also required for overprinting of various colors during stamping. Processing to ensure print quality. In addition, the arrangement and placement of text and borders need to be performed in the computer. When the layout of a stamp was completed, the operator transmitted the electronic data to the electronic engraving workstation. The electronic engraving workstation edits and typesets according to the layout specifications of the data and the full-page stamp and the layout requirements of the printing machine, and also needs to perform smooth processing on the edges of the characters and lines, and perform final fine corrections on the patterns. Afterwards, through the electronic engraving machine, dozens or even hundreds of stamps with the same level of color can be engraved on the electrolytic copper on the surface of the printing roller. The printing roller can be printed on the machine after chrome plating.

The modern photogravure stamp platemaking process, which uses optical film operations, goes from an electronic color separation high-end networking system to an electronic engraving workstation, improving the speed of plate making, shortening the production cycle, the accuracy of plate printing, and the good repeatability of platemaking quality. Stability is conducive to the digitization, standardization, and even standardization of the process.

(2) The engraving process of engraving gravure stamps. There are many kinds of engraving and gravure platemaking methods, such as: mechanical plate printing method, electroplating plate method, etching method and so on. Our country adopts mechanical printing method. The law first reduced the stamp manuscript pattern to the size of a flat steel plate according to the size of the stamp. The engraver used hand knives to engrave the pattern on the copper plate with different shapes, widths, and depths. The line shows the different levels in the picture. This carved small steel plate is used as the original engraving engraved plate (called the original steel plate), use it to reproduce the printing plate. Gravure stamps are printed on a rotary machine. Therefore, when making a printing plate, the original steel plate must be heat-treated to strengthen its hardness, and then put on a special plate press machine to mechanically press the small steel plates. The point of inconsistency, the line was transferred to another small steel shaft, and the convex lines and points were obtained; the small steel shaft was also heat-treated and placed on the plate press, and the economical pressure applied the pattern to the specification of the full-page stamp. The pieces were transferred to a printing roller coated with an electrolytic copper layer on the surface to obtain concave graphic points and lines. After chrome plating, the printing rollers could be printed on the machine and the engraving of the engraving and gravure stamps was completed. The engraving gravure stamp has its unique effect. The pattern lines are clear, the ink layer is thick, and it is slightly raised on the paper. The fine points and lines are clearly distinguishable. The constant color will help prevent the stamp from being forged. Although the art of engraving and gravure plate making and printing is very difficult, its anti-counterfeiting performance is good, so most of the world's stamps are printed using the intaglio carving process.

2. Printing techniques for stamps The printing of gravure stamps uses a special gravure transfer machine, four-color, five-color, six-color stamp printing presses, and a gravure stamp printing machine with one-color engraving gravure plus five-color or four-color gravure overprint. These special stamp presses are equipped with counting numbers, perforations, and cutting devices. Stamps can be counted, punched, and full-size stamps cut to size. The ink used for gravure printing is a solvent-based liquid ink. When the printing plate is immersed in the ink fountain, the ink on the non-graphic part of the plate is scraped off by the squeegee, and the graphic part is the concave part of the printing plate. The ink is just stored. After being imprinted on the impression cylinder, this part of the ink is transferred to the paper. After multi-color overprinting, an exquisite stamp is transferred onto the paper. The printing ink used for the engraved stamps is a viscous resin ink. Its ink transfer form is similar to that of the emboss, but since the graphic part of the engraving intaglio is concave, the non-graphic part (flat) ink on the plate is printed. It depends on a special wiping device to wipe off. At present, there are roughly four types of ink wipes for engraving and printing: cloth wiper, trichloroethylene wiper, water wiper, and paper eraser. Rubbing the ink with a cloth, the three-dimensional impression and gloss of the printed ink layer are better, but the printing speed is slower; Trichloroethylene wiper printing speed is faster, but toxic, harmful to the human body; water wipe ink is a More advanced methods of wiping ink, but there are also problems of waste water treatment and pollution; paper wiping ink is suitable for small paper printing, and is not suitable for such large-volume stamp printing in China. China's engraving stamp printers are equipped with cloth rubbing devices.

In addition to the single photo ticket or the single carved ticket mentioned above, the intaglio stamp also includes a postage stamp that has been overprinted and embossed, known as the shadow carving stamp. It is a favorite of philatelists. The printing of shadow carving stamps overprints the combination of two different printing methods: gravure and engraving gravure. Although engraving and gravure is an ideal process for printing valuable securities, it is difficult to express rich colors and delicate levels. Photogravure also has its disadvantages. Because some stamps have a darker color and require more ink for printing, gravure ink is a fluid with very low viscosity. Therefore, irregular watermarks often appear in deep-adjusted areas and the stamps are damaged. In addition, since the gravure printing patterns are all composed of dots of different depths and different sizes, zigzag slivers will appear at the edges of the thin lines or characters, which will affect the smoothness of printing. In order to make stamps more perfect, the method of engraved intaglio and photogravure mixed overprinting is usually used to exert the respective advantages of the two printing methods so as to obtain more ideal printing effects, such as the famous “Landscape of Huangshan Mountain” and “Landscape of Mount Lushan”. The "Zodiac" stamps and "China Ancient Pagoda", "Lushan" and "Rare Bird" stamps issued in recent years have all been printed in this way.

Combining two different printing methods with different properties, the process is particularly difficult to overprint accuracy, their respective needs of the printing pressure is completely different, the ink properties are not the same, and there are different requirements for the printability of the paper Therefore, the mechanical error in printing is large, and the rejection rate is also higher than other printing methods. When designing the printing process of such stamps, it is necessary to fully understand and utilize the characteristics of these two printing methods, and to avoid weaknesses in order to obtain better printing results. This type of overprinted version of the stamp is generally based on the engraving version, to make the screen pattern of the sketch effect, and then lined with the color of the gravure smudge. Therefore, the main lines and ideas of the engravings should be emphasized and real, and the contours and outlines of the motifs should be made. The inset color of the photogravure can be shallow and imaginary, supplemented by the detailed changes of colors and levels, so that even mechanical errors exist. It will not affect the clear and complete contours of the subject landscape. The printed stamp lines are solemn and straight, the theme scene is prominent, and the three-dimensional effect is strong.