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In the color printing process, the appearance of dot gain will have a direct impact on the color and level of the printed product. Therefore, how to control the dot gain has been valued by the industry. The dot gain is most likely to occur in flexo printing, and the dot gain is most noticeable. According to the rich experience of plate making, the author now summarizes several methods for controlling the dot gain in the flexographic plate making process as follows for the industry to learn from.
1. Reasonable selection of screen lines
Reasonable selection of the number of screen lines depends mainly on the accuracy of copying the original. In addition, factors such as plate making equipment, printing equipment, and printing consumables should be considered. Under the same conditions, the number of screens for flexo printing is generally lower than the number of screens for offset and gravure.
2. Correct selection of dot shape
At present, the most commonly used dot shapes are round dots and square dots. The correct choice of dot shape, the most important thing is to understand the degree of dot gain of different shapes of dots in different situations. Under normal circumstances, under the condition that the number of screen lines and the percentage of screens are the same, the square dot is the most serious when the square dot is about 50%, and the square dot has the largest circumference; while the circular dot is about 70%, the dot is enlarged. The most serious, at this time the circumference of the circular dot is the largest. In addition, at the 10% to 50% and 80% to 90% tone, the dot gain of the square dot is greater than that of the circular dot; at the 60% to 70% tone, the dot of the circular dot is increased more than the square. Outlets.
3. Using hybrid screening technology
Because flexo printing itself still has certain defects, the range of tones that can be reproduced when using the amplitude modulation dot in the flexo printing process is relatively narrow, and if mixed screening technology is used, it can be well Combine the advantages of AM and FM screening. Generally, FM filtering is adopted in the high-light and dark-adjusted areas, and the level of the screen is changed by the density of the same fine mesh points. The degree of dot gain is not obvious, the dot positions are randomly distributed, and the FM screening is subjected to special calculation processing. It will not cause the overlap between the dots, nor will the spacing between the dots be too large. It is also possible to use a plurality of small dots to form a larger printing dot; in the intermediate tuning region, the amplitude modulation screening is adopted, The dot area is used to represent the image hue, and the dot gain is not too obvious.
Hybrid screening technology enables smooth transitions from random sites to traditional outlets. 5% to 10% and 90% to 95% tonality are digitally processed automatically, rather than simply interpolating the respective density. Compared with the traditional screening technology, this screening technology makes the dot gain in the highlight and shadow areas more stable and easier to predict, and can be effectively compressed during prepress production.
Another advantage of using hybrid screening technology is that it can make the range of printed dots wider, and basically achieve 0-100% tone change. For flexo printing, this problem can be solved very well because the dots in the highlight area are not easy to print and the hybrid screening technology is used.
At present, the hybrid screening technology has been successfully applied to the screening of coarser printed products such as corrugated cardboard printed by flexo printing, which not only can effectively control the dot gain, but also make the brighter and darker regions appear more More details make the printed image clearer and more saturated, which can effectively avoid the appearance of moiré and improve the printing quality.
4. Adjust the color separation level compensation curve
Generally, the color separation level compensation curve is adjusted in the color separation process, and the dot gain portion is compressed to make the final printing level curve close to the ideal level copy curve.
Although the current electronic prepress technology, laser graphic output technology and optical density measurement technology provide some useful means for controlling the dot gain, the complexity of the printing process and the ever-changing printing technology will still bring a lot of dot gain to many printing companies. Big troubles, so really understand the reasons for the increase in outlets, and master effective control measures, will help printing companies to obtain high-quality prints.
The above content is selected from the 12th issue of “ Printing Technology·Packaging and Decoration†of Keyin Media in 2013. For more journal content, please pay attention to the journal channel. The latest journal content clicks into the electronic journal channel of Keyin.com.
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