When performing multi-color printing on offset paper, in order to ensure the printing quality and meet customer requirements, all links must be well controlled. I think the focus should be on the following aspects.
1. Control of printing plate quality
(1) When printing, increase the actual exposure slightly so that the dots on the printing plate are slightly smaller than those on the film to compensate for the increase in dots caused by pressure during printing.
(2) During the baking process, when the printing plate undergoes the process of changing from cold to heat and then cooling, the phenomenon of expansion and contraction will occur accordingly. Therefore, in order to ensure the accuracy of overprinting, it is generally not recommended to bake plates when performing multi-color printing.
(3) Proofreading should be proficient and accurate. Try to reduce the number of proofreading to avoid overprinting caused by artificial deformation.
2. Pressure control
(1) Due to the rough surface of the offset paper and good ink absorption performance, in order to make the printing ink thick and the dots clear, the printing pressure should be appropriately increased during printing.
(2) The pressure between the water roller and the ink roller should be appropriate while maintaining good water transfer and ink transfer performance.
3. Selection of printing consumables
(1) Paper. In the process of multi-color printing, paper with good sizing degree and strong printability should be selected to avoid the phenomenon of fluffing and dusting. At the same time, we must also pay attention to the effect of paper pH on printing quality. Before printing, the paper should be suspended.
(2) Ink. Quick-drying gloss ink is permeation-drying ink, which can avoid the phenomenon of powdering and matt, sticky dirt, and roughening caused by resin ink. It is more suitable for printing on offset paper, so it is recommended to use quick-drying gloss ink.
(3) Blanket. In order to ensure that the dots can be better restored and the imprint can be transferred well, an air cushion rubber cloth should be used.
4. Control of printing conditions
(1) The pH value of fountain solution is controlled at 5.5 ～ 6.5. If the pH value of fountain solution is not properly controlled, it will affect the drying of the blot and the printing quality.
(2) Strictly control the moisture of the layout and maintain the balance of water and ink during printing to achieve the best printing status of "small ink dry water".
(3) Control of workshop temperature and humidity. In the offset paper printing process, when the humidity in the workshop is high, it is easy to produce wrinkled paper, brushing, powder falling, etc., and when the humidity is low, it is easy to generate static electricity. In order to achieve the best results in printing, it is more appropriate to control the workshop temperature at 22-25 ° C and humidity at 55% -65%.
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