In letterpress printing, the common faults are: dirty back, ink through printing, flying ink, static electricity, etc. These faults affect the printing quality and must be taken to eliminate them.
(1) The back is rubbed dirty
The ink printed on the substrate sticks to the back of another sheet, causing smudging. To prevent the back side from being smudged, a powder spraying device is generally installed in the paper receiving part of the printing press to disperse fine particles of calcium carbonate between the printed sheets. In addition, accelerating the drying of the ink, reducing the height of the stack of printed sheets, and placing paper with good ink absorption between the printed sheets can suppress the smearing phenomenon.
(2) Through-printing of ink
Ink-through printing refers to the phenomenon that the front imprint can be seen on the back of the printed sheet. To prevent the through printing of ink, you can choose paper with high tightness to print, or you can increase the viscosity of the ink and reduce the printing pressure appropriately.
(3) Flying ink
The tiny droplets of ink, which are scattered in the air, are called "flying ink", also known as the mist of ink, which is the most common failure of high-speed rotary letterpress printing machines. To slow down the "flying ink" phenomenon, you can increase the humidity of the printing workshop, using conductive inks such as water-based inks.
(4) Static electricity
During the printing process, the paper is not easy to separate, the paper on the delivery table is not properly received, or the paper on the delivery table is skewed and the overprint is not accurate. It may be caused by static electricity. To eliminate printing static electricity, a static eliminator is generally installed on the printing machine to ionize the air around the printing machine, thereby neutralizing the positive and negative static electricity on the paper. You can also spray an appropriate amount of water mist around the printing machine or the upper back of the impression cylinder to eliminate static electricity on the paper. In addition, increasing the humidity in the printing workshop is conducive to the elimination of static electricity.
(5) Quality requirements of letterpress prints
Product quality refers to the characteristics that a product is suitable for a certain purpose and can meet people's certain needs. Printed products have many uses, different varieties, and different printing methods. According to the technology and equipment of letterpress printing, the quality of printing should meet the following requirements
(1) Size requirements The tolerance of the size of the fine product format is 0.5mm, and the general product is 1.0mm.
(2) Pressure and ink color printing web pressure and ink color are uniform. Text imprints, fine products are clear and complete, and general products have no obvious lack of pen strokes.
(3) Format uniformity of printed book pages requires that the fine product's web uniformity should be greater than 15/16, and the general product's web uniformity should be greater than 14/16.
(4) In the pages of nested printing pages, the positive and negative overprints of each page are accurate. The precision of the overprint error is less than 1.5mm, and the general product is less than 2.5mm.
(5) Fine-looking products, neatly printed pages, no paste, nail shadows, dirty marks, no missing pen breaks. For general products, the printed pages are clean and tidy, without obvious pasting, nail marks, dirty marks, and no obvious missing strokes.
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