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. Floor.
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.
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 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.
Stencil printing and transfer ink
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.
Source: "Printing Technology - Equipment and Special Printing" Ding Yi / Translation