Seminar on

Impact of Varying Plate Screening 

Techniques on Print Quality

 in Flexography

By

Ms. Gargi Sachin Shah

Under the Guidance of

Prof. Dr. Akshay V. Joshi

 

Pune Vidyarthi Griha’s College of Engineering and Technology & G. K. Pate (Wani) Institute of Management, Pune, India 

Academic Year 2020-21

In flexographic printing flexible printing plate is used. The image on the inked plate is in raised form and  non-image area is in recess form. They   rotate at high speeds to transfer the ink from the anilox roll to the substrate (paper). Each color requires a different printing plate (Fig.1). Now-  a-days continuous-tone images are reproduced in two stages. Firstly, the image must be digitized using a scanner or other digitizing device and care must be taken to capture enough data to provide sufficient resolution for the size of image to be printed, and also its halftone screen resolution. Then the scanned image is placed onto a page in a page-layout program. When the completed result is output to a printer, the page-layout program converts the digitized image to halftone dots. The process of screening is also referred as halftoning, and the resulting image is a halftone. After printing, the image will totally resemble the original image, with the smaller dots producing light shades or highlight areas and the larger dots making the dark shades or shadow areas [1].  An important factor in the improvement of flexographic print quality is advances in screening technologies. These screening technologies when combined with plate technologies and cure technologies, are of great advantage in the screening world: highlight dots and surface screening. Screening is converting the continuous tone image to the printable image comprised of dots of various shapes and sizes. The faithfully image reproduced and how much of the original contrast, saturation, and detail is preserved depends on how we screen an image. The different screening technologies are at the heart of flexo platemaking which are AM (Amplitude Modulated) Screening, FM (Frequency Modulated) Screening and Hybrid screening. Surface screening has brought better results to the issue related to the ink transfer and solid ink coverage helping those vibrant and saturated brand colors to become more consistent. Printer choosing and curing a plate is important, similarly screening selection is a vital element in implementing high-quality flexo. Screening selection will depend on your process and must be optimized for each process [2].

Fig 1: Flexographic Process

AM Screening

The basic working is that in an AM screening half tone dots are equally spaced and arranged on a grid and the number of dots on the orthogonal grid is fixed (fig. 2). Amplitude modulated (also known as conventional screening) is dependent on the size (“amplitude”) of the dots. In AM screening, the size of the dots is modulated or altered on a grid to create the illusion of different tones. Issues that come along with conventional screening, including loss of detail, subject moiré, screening moiré, rosette visibility and the optical bump [3]. The dots are equally spaced in AM screening, so this tends to create rosettes patterns along with moirés is clearly visible to the naked eye. To minimize the moirés effect, dots are designed to be placed at certain angles for a CMYK 4 color process that are C (75°), M (15°), Y (0°), K (45°) [4]. 

Fig 2: AM Screening Dot Placement

Image copyright ©2021 Phillips Printing. All Rights Reserved, Phillips Printing Co.

FM Screening

In frequency modulated (FM) screening, the size of the dot is constant, but the number (“frequency”) varies. In FM screening the dots are pseudo randomly placed  and not aligned along fixed screen angles to avoid causing artifacts (fig. 3). FM has  pseudo-random arrangement of the dots that is no direction to the screen, and therefore no screen angle. The result is that FM screening can overcome many of the reproduction problems associated with AM screening. With second order FM screens (the latest advancement in FM), FM-spaced dots are designed to grow in size through the tone scale in a fashion similar to AM dot structure. Flat tint reproduction can have a grainy appearance. Smooth transitions from mid-tone to shadow can be difficult to maintain. One advantage of stochastic screening is its ability to remain stable during the run [3].

Fig 3: FM Screening Dot Placement

Image copyright ©2021 Phillips Printing. All Rights Reserved, Phillips Printing Co.

XM (Cross Modulated) / Hybrid Screening

Screenings that don’t resemble AM or FM methodology are generally referred to as a “hybrid” technology. Hybrids are of two types - second order FM screens and hybrid AM screens. These technologies take advantage of both FM and AM screening, combining them together to produce the desired result [5]. XM makes use of FM screen in the highlighted and shadow areas and AM screen in the mid-tones (fig. 4). XM allows the smooth transition from one screen to the other. XM screens combine the best characteristic features of both AM and FM [4]. Hybrid AM/XM screening  helps to recover the lost part of the tone range by varying the size of highlight and shadow dots so they never get smaller than a size that can be considered for the printing process. Hybrid AM/XM screens are so called because they leverage a technique borrowed from FM screening [5].

 

Fig 4: XM Screening 

Image copyright ©2007 Rujna, XM SCREENING TECHNOLOGY. All Rights Reserved.

 

AM screening technique proves itself to be a method with great print stability and less dot gain in mid tone areas due to the factors (fixed screen angle, screen ruling and screen shape) that affects the representation of darker and lighter parts to be inadequate. AM-FM screening is also referred to as transitional screening and it produces best output and grants a better dot gain curve. The advantages of this method is that rich details are achieved in highlight and shadow area, the graininess of the mid tones decreases, in flexography printing highlight break is avoided and tone break effect is reduced, it helps in achieving the optimum dot size and, also as there is no requirement of applying bump curves during the plate making process, so time required is less in prepress. AM/FM hybrid screening helped to improve the image details and smoother gradients also the highlight areas are produced more faithfully. Hybrid screening had random distribution of dots and AM had regular distribution. Hybrid screening gives more dot gain as compared to AM screening due to no use of bump up curve in hybrid screening. For effective results the prepress operator should make combination with FM and AM dots and to avoid problems during printing, a minimum dot-size should be previously specified [6]

Advantages of stochastic screening over conventional screening are there is no chance of apparent moire patterns and there are no screen angles involved in overlapping halftone screens with stochastic screening which makes it easy to print four color over each other with forming any moire patterns. There are no patterns formed by the randomly placed dots. Secondly slight misregistration errors on press can cause color shift issues, but with stochastic screening misregistration did not cause any color deviation. Stochastic screening also helps to improve image quality as it has ability to increase ink densities, which in turn helps tonal range and contrast to get better. Lastly, the overall sharpness and detail of images are the result of the small dots used in stochastic screens. The print contrast was result of stochastic screenings which enhanced image quality by improving overall tonal range and contrast. The graininess issue and benefit of reproducing sharp images is solved by small minimum dot-size held. Stochastic screening creates a larger gamut as a result of its application to the expanded gamut process. Tonal Value Increase is the issue when using stochastic screening with the flexographic printing process. It is possible the single stochastic screen would be sufficient to avoid the moire and perhaps reduce the TVI issues [7].

XM screening technology obtains unchanged high-screen frequencies where there is no loss of detail and with no extra problems on press because in most hybrid techniques, the intersection between AM and FM is clearly visible to the naked eye, and the complicated screening algorithms can slow the pre-press process. The true benefits and potential of computer-to-plate workflow is understood by using XM screening. In XM screening to capture fine details FM screening is applied in highlights and shadows, and to achieve smooth gradations AM screening in mid-tones. The main principle of XM methodology states that the press is part of the imaging system. XM more over is same as AM screening. The stability of AM is decreased in highlights and shadows (the line ruling increases) and FM behaves opposite to AM.  Hence XM screens combine the best performance characteristics of both, enabling a wide range of stability on press. For a smooth transition from one screen to the other without visibility XM uses FM screens in the highlight and shadow areas and AM in the mid-tones. AM dots become smaller when rendering highlights until the minimum reproducible dot size on press is reached but from that point on, dots are withdrawn from the grid – providing the desired tone. Likewise, in the shadows, the screen smoothly evolves from one screening type to the other without visibility to naked eye. The shadows and highlights may look stochastic, but this is not true stochastic screening. even if FM  uses smaller dots controlled in FM mode, they are aligned as a continuation of the AM screen angles established in the mid-tones. The result is an entirely new order of screening, called XM or cross-modulated screening. All the three methods AM, FM and XM screening are good, especially at rendering details. Screening is somewhere a determining factor in print quality [8].

Solid screening with Microcell technology can solve the density deficiency as well as the evenness of the color block is improved in the flexographic printing. The density value of the solid screening is not constant when PE is adopted as the carrier for printing. In addition, with the same exposure parameters and screening modes, the effect of the solid screening and value of solid density for coated paper were better than that of PE film. The Microcell technology, under different laser booster values and substrates, effectively improved the solid density and uniformity in the solid area and the solid screening with PE as the substrate effectively improved the uniformity and value of solid density for the flexographic printing. The Microcell technology will not only increase the investment of technology, but also the cost of production [9]. 

References

1. What is Flexographic Printing? Retrieved from https://www.printmatics.com/flexographic-printing
2. Flexo Plates: It’s all about the dots. Retrieved from https://www.flexpackmag.com/articles/90160-flexo-plates-its-all-about-the-dot
3. AM, FM & In Between Tuning Your Screening Dial Retrieved from https://idealliance.org/files/2006-3tech2.pdf
4. AM Screening v/s FM Screening. Retrieved from https://www.flexoglobal.com/blog-articles/2019/kymc-01-am-screening-vs-fm-screening.html
5. Hybrid AM Screening/XM Screening. Retrieved from https://www.flexoglobal.com/flexomag/09-May/flexomag-Pritchard.htm
6. Youssef K. (2015). The Impact of FM-AM Hybrid Screening and Am Screening on Flexographic Printing Quality. International Design Journal, Volume 5, Issue 4, pp 1471-1476. Retrieved from https://journals.ekb.eg/article_93687.htm
7. Trey R.  (2013). Stochastic Screening for Flexography and Its Application to Expanded Gamut Printing. TAGA Proceedings. 142-154.  Retrieved from https://www.printing.org/taga-abstracts/t130142
8.  Valdec D., Vusic D. and Tomisa M. (2007). XM Screening Technology. rujna, Zadar, Hrvatska, 26-29.  Retrieved from https://bib.irb.hr/datoteka/317114.XM_SCREENING_TECHNOLOGY.pdf
9. Enyin F. and Jinghuan G.  (2017). Research in the Solid Microcell Technology in the Flexographic Printing. Advances in Graphic Communication. Retrieved from https://doi.org/10.1007/978-981-13-3663-8_74