Preface
According to the surface morphology of the printing plate, the printing can be divided into four categories: relief printing, gravure printing, lithographic printing, and stencil printing. In recent years, no printing without printing plates has appeared. The representative of screen printing is screen printing. In addition, there are steel plates and printing plates. Now that printing has been changed to the thermal digital plate making mode, a printing and printing machine has emerged. Recently, an easy-to-read stencil printing press similar to a copier in which a personal computer and a network can be connected has been introduced, as shown in FIG.
The digital stencil printing machine is easy to operate. Its greatest feature is that the more copies are printed, the lower the running cost. For example, if the "Preport" digital stencil printing machine produced by Ricoh in Japan has more than 100 copies, its printing costs are almost only the cost of paper. Because digital stencil printing has the characteristics of energy saving, it is suitable for use in small units such as schools, bookstores, government agencies, hospitals, and retail stores.
The following will introduce the current status of digital stencil printers, the base papers being developed (base papers for printing plates), and the development direction of digital stencil printing.
Change of printing and plate making methods
The printed copy was printed in 1894. The method was to lay wax on a sheet of iron and write it with a stylus to form a fine hole to complete the plate making. The prepared version of the wax paper was put on the frame of the manual printing machine, and the ink was used again. The rollers roll the ink through the wax paper onto the paper. This type of transcription has been adopted by government agencies, universities and news agencies during the war in a simple structure and light printing.
The changes in the platemaking method and the stencil printing method are shown in Figure 2. As with other printing technologies, stencil printing has evolved from the earliest hand stencils to the original version of the typing mechanism and to the discharge stencil. Although the discharge type stencil printing plate produces hydrogen chloride gas, it has an epoch-making progress due to its feature of being able to make plates by photoelectrically reading originals. The flash plate-making method introduced in 1980 shortened the plate-making time, and there was no air pollution. It soon became popular. However, the disadvantage of this method is that it can only be used for ink, photos cannot be made, and the original may be damaged.
In 1986, Ricoh introduced the Preport SS-880 with a digital thermal plate-making method, which made up for the deficiencies of the original method and at the same time absorbed the advantages of digital plate-making technology. This plate-making method has been used up to now. This digital stencil printer has two significant features.
(1) The plate-making machine and the printing press are integrated into one, the operating conditions are improved, and the quality is improved.
(2) Due to the use of thermal digital platemaking technology, the picture quality is improved and digital picture processing technology can be used.
Digital stencil printing machine overview
The digital stencil printer is constructed as shown in FIG. 3 and is generally composed of the following six parts.
1 scanning section
A device for reading the original text information, ie scanning the original, optically collecting the signal, and converting it into an electrical signal.
2 imaging section
According to the signal from the scanning portion, the thermal head is thermally perforated by the thermal head on the reel-shaped stencil sheet, and a graphic edition is produced. After the stencil is cut, the printing cylinder is supplied with a printing plate.
3 collection section
The used stencil sheet is peeled off from the printing cylinder and conveyed to the fork box for compact storage.
4 printing section
The stencil base paper is sleeved on the outside of the printing cylinder and is supplied with ink from the inside, and the printing paper is wrapped thereon, and the ink is continuously transferred onto the paper to form a printing image.
5 Paper feeding section
After the paper stacked on the paper feed table and the paper feed tray is separated one by one, the paper is fed to the print section.
6 Delivery section
The printed sheets are peeled off from the printing cylinder and transferred to the delivery table (or the delivery tray), and then stacked in order.
1. Digital thermal plate base paper and plate making technology
The stencil sheet used in the digital thermal plate is heat-shrinkable. A thermoplastic resin film with a thickness of 1.5 to 2 μm and Japanese rice paper are compounded with a binder, and the surface of the film needs to be coated with an anti-static or anti-adhesion coating. Layer, its structure is shown in Figure 4. The specific process of thermal plate making on the stencil sheet is shown in FIG. 5 .
The heat-sensitive stencil sheet is installed on a plate-making and printing integrated machine through a reel, and one end of the stencil sheet is pressed by a rubber embossing roller onto the heating element of the thermal head. The thermal stencil sheet is suitable for A3 size plate making, width 320mm, and the length of the reel corresponds to the size of 200 to 250 plates. At the time of plate making, the thermal heads arranged in a row selectively generate thermal energy based on the image information, and perforate platemaking in the main scanning direction of the stencil sheet paper film; at the same time, the stencil sheet is driven to perform perforation in the sub-scanning direction by the rotation of the impression roller. This plate-making speed varies depending on the model. If it is 1.5 msec/line, the resolution is 400 dpi for the A4 plate, and the net plate-making time is 5 seconds.
The finished stencil sheet is transported by means of a transfer roller, and the leading end of the stencil sheet is fed to a jig provided on the outer periphery of the printing cylinder, and is cut to a length of one plate and wound around the periphery of the printing roller.
2. Digital thermal plate punching process
The thermal plate making process heats and softens the thermoplastic resin film of the stencil sheet to cause thermal expansion and contraction to form a circular through hole corresponding to the heating element. The through holes are arranged according to the main scanning direction and the sub-scanning direction, and basically form independent holes, which can be formed into a picture. The original idea relies on the size of the aperture of the through hole to control the image level, but the current technology cannot be achieved. Moreover, due to the scattering of the rice paper fiber compounded on the film, the perforation is also in a scattered state, which is a major cause of the lower image quality.
The effect of the stencil made according to this perforated plate making method under the microscope is shown in FIG. 6 .
The plate resolution of the digital thermal plate is determined by the distance between the heating element of the thermal head and the plate material. Generally, the low price model is 300 dpi; the common model is 400 dpi; the high quality model can reach 600 dpi. With a resolution of 600 dpi, the hole spacing on the stencil is approximately 42 μm. The thermal head perforation pattern of the thermal head is shown in FIG.
3. Stencil printing and transfer ink
The printing section of the stencil printing machine is constructed as shown in FIG. When the plate cylinder with the stencil is rotated, the paper contacts with the stencil under the pressure of the impression cylinder, and the ink passing through the stencil at the nip line is transferred to the surface of the paper to form a printed image. If printing with A3 size paper (unlimited paper), the printing speed is 120 sheets/minute and 5 speeds are available.
The plate cylinder consists of a porous body made of a stainless steel sheet that has been corroded to form a large number of micro-circular holes, and a strainer outside it. Inside the plate cylinder, there are a continuous ink supply ink roller and a quantitative ink supply ink knife and a doctor roller. In order to obtain the best embossing area, the outer periphery of the impression cylinder is made of an elastic material and can apply the most suitable embossing force.
The stencil printing is the independent perforation of the ink through the stencil sheet, and is directly transferred to the printing paper. Therefore, in the process of transferring ink to the surface of the paper, the following two points need to be noted:
(1) Since the ink dots transferred to the paper are independent from each other, the ink needs to be penetrated and stretched in order to maintain the density required for the image solid.
(2) Since the surface state of the paper and the viscosity of the ink vary greatly, it is necessary to control the amount of ink transferred to the paper.
The amount of ink transferred to the paper surface has a great influence on the image quality. The amount of ink transferred is large and the image density is high. However, the back side is dirty and dark, so that the image clarity is reduced and the gradation is reduced; the amount of ink transferred is small. in contrast. In fact, when the ambient temperature, type of printing paper, printing speed, and downtime change, the amount of ink transferred will change. Therefore, conditions such as plate-making power, printing speed, and printing pressure should be changed accordingly.
(Ding Yi/Translation)
Source: "Printing Technology - Equipment and Special Printing"