Seminar on 

Impact of Screen Ruling on Flexo Dot Fidelity

By

Mr. Pramit Patekar

Under the Guidance of

Prof. Dr. Akshay V. Joshi

 

Pune Vidyarthi Griha’s College of Engineering and Technology,Pune 

Academic Year 2020-21

Flexography printing process is a modern version of letterpress printing process. This process of printing can be used on mostly all types of substrates. The substrates include plastics, metallic film, label stock, corrugated cardboard, fabric, etc. (Fig. 1). Flexography holds its own in the areas of large orders and long print runs, particularly of packaging products and labelling. This process uses quick-drying, semi-liquid inks. Food packaging is an important market because of the ability of flexography to print on non-porous substrates. Flexographic printing is popular due to its speed, cost-effective, and environmentally friendly nature. Water-based, UV curable, and solvent-based are the main variety of flexographic inks.

Fig. 1 – Flexographic Printing Process.

Image copyright © ServoArtpack. All Rights Reserved.

The flexographic printing process uses flexible photopolymer plates and they are wrapped around the rotating cylinder on a web press. These inked plates have a slightly raised image and rotate at high speeds to transfer the image to the substrate (Fig. 2).

Fig. 2 – Flexographic Printing Plate.

Image copywrite © FlintGroup. All Rights Reserved.

These inks can print on many types of absorbent and non-absorbent materials and is capable of achieving high quality printed images. These factors make it very popular in food packaging, textiles and label printing industries. The deformations of the printing plate in this printing process caused by the pressure between the printing plate and the substrate is a major limitation of flexography [1]. The number of screen lines per unit length is called as screen ruling. It is also called screen frequency or half-tone resolution and is measured in lines per inch (lpi). The screen ruling will determine the amount of detail in the final printed result, a low lpi means fewer details (less bigger dots) and high lpi means a lot of details (more small dots) [2] (Fig. 3).

Fig. 3 – Image with Different Screen Rulings.

Image copyright © Esko. All Rights Reserved.

The use of different screen ruling has a considerable influence upon relief depth and a cross-section of the printing elements which again have a significant impact on the quality of the final product [3]. The dot gain increases with the increase in screen ruling. It may be attributed to more number of dots at higher screen ruling that cause a merging effect [4]. The resolution of the image for halftones is determined by screen ruling of that image. The screen ruling is used to determine the dot frequency in a given area of an image (Fig. 4).

Fig. 4 – Screen Ruling.

Image copywrite © Sappi. All Rights Reserved.

The screen ruling of the anilox roller must be higher than that of the plate inorder to avoid the interference patterns or screen clash [5].

            The quality of the image which is scanned depends on different parameters such as resolution, screen ruling, contrast, screen shapes etc. of the scanned image. The variation in the image quality is observed by the histogram representation of the image. Histogram gives a graphic representation of the tonal distribution in an image. The number of pixels are plotted at each brightness level of the image. The image quality is improved by selecting proper screen ruling and screen shape. The discernibility of screen pattern produces an adverse effect on the quality of the image which is printed and since the pattern does not provide any information about the object itself but it acts as a disturbance. The fine screen images are difficult to produce when we are printing them on cheap grades of paper. The amplitude of the first harmonic and consequently, the discernibility of the pattern is greatest in midtones but fades away gradually with the approach of highlights or shadows. It shows that the discernibility of the dot formation patterns should be subdued within the midtones of the images. By changing the configuration of elementary dot cells or by varying the orientation and shape of dots, the satisfactory result can be obtained. This helps the human visual system to perceive contours which are more closely packed. By using different screen ruling, the shades can be distributed in a manner to give better detail. The image quality for the diamond type screen shape loses the details in extreme highlight zone during printing and the brightness level varies from 250 to 255. Hence the diamond screen is discarded for this particular image. The image quality for other screen shapes, i.e., round, cross, ellipse, line and square are more or less identical with a little improvement for line screen [6].

            The dot gain increases with the increase in screen ruling irrespective of the other factors for the chosen self-adhesive glassine liner. It is attributed that more number of dots at higher screen ruling that cause a merging effect. The dot gain is low at lower screen rulings, but smooth change of tones is possible with high screen rulings. During pre-printing stages, the compensation should be carried out to get good results at high screen rulings. The dot gain slightly decreases with speed in the chosen range. The reduced time of contact at high speed is the reason for reduced dot gain. As the screen ruling increases the dot gain increases. It is seen that there is increase in dot gain from lower to higher LPI. The dot measurement is related by the dot area, density of the tint and the solid density. Dot gain is the difference between intended dot area and the actual dot area printed. Reflective density is the percentage of the reflected light. It means the percentage of light that is reflected from the substrate and the ink. Density is preferred over reflectance. The equal increase in density correlate more closely with the human vision. The substrate also has a major effect on the amount of dot gain. Surface characteristics affect the dot gain property. The low probability value (P) shows minimum possibility of response dot gain not getting affected by the factor screen ruling [7].

            The combination of CTP process and finer-than-normal screen rulings effectively improve the printing quality. The screen ruling has little impact on solid ink density. It also has some influences on print contrast. The finer the screen ruling is, the higher the print contrast is, the sharper the images is, and the better the details reproduction is. Screen ruling has a great influence on dot gain of the image. The finer the screen ruling is, the more serious the dot gain is. Screen ruling also has a little influence on the color reproduction. The brightness of the prints of 220lpi is higher than that of 175lpi, but its saturation is relatively lower. Screen ruling has a little influence on color gamut as well. For light areas and middle tone areas, the influence of screen ruling on color gamut can be neglected, but for shadow areas finer screen ruling results smaller gamut. High printing quality is not always achieved by fine screen ruling. This is because finer screen ruling always means more serious dot gain, which needs strict and effective process control measurements. In the digital prepress process of CTP, the excessive dot gain of prints is compensated by reducing the dot values on plates. When CTP meets with finer-than-normal screen rulings, sophisticated prints will come. The printing quality is evaluated mainly according to the reproductions of color and tone. The choice of screen ruling is limited by imaging resolution and tone value levels. Higher is the screen ruling, more serious dot expansion is and more is the degree of compression in shadows. Due to this reason the finer-than-normal screen ruling for printing process always requires harsh requirements. CTP technology is just qualified for this requirement. When the output resolution is regular, the higher the screen ruling is and the smaller the gray level is. Small gray level would be disadvantageous to tonal reproduction[8].

            The HD Flexo technologies are suitable for testing standard production conditions and allow for increasing the print quality of the image. By using HD Flexo technology, the reproduction in highlights and transitions to zero is improved. Increased screen ruling allows better reproduction in mid tones, neutralizing the negative effect of visible rosettes in grey and full color areas of the image. The digital printing plates reproduce the tonal range better, with finer details and with a decrease in the dot gain. Screen ruling selection is based on particular print conditions, design characteristics, screen ruling for the image and for aniloxes, and are specified in a way to achieve the best results when HD Flexo is used. If the screen ruling of the image is high then it will lead to a higher image quality. This is possible only if proper screen ruling along with proper anilox ruling is selected and the correct HD Flexo settings are applied. After the implementation of the new technology HD Flexo, the quality of the received imprints is significantly ameliorated, and the smooth reproduction of the bright tones and the transition to zero is ameliorated as well. For polyethylene and for paper, the best results are achieved for screen ruling 174 lpi and HD C46 when printed with solvent plates. For thermal plates, the best results are achieved for screen ruling 161 lpi and HD C27 for both substrates [9].

          The use of different screen rulings is of great significance in the curve adjustment of the printing plate. There is a difference in coverage value between the control fields made with different screen rulings of the same nominal value. The correction with the bump-up curve shows more influence on coverage values of higher screen ruling in the lower nominal values. The correction with the compensation curve decreases coverage value mainly in the middle tones, as it is expected, in order to adjust the tone reproduction to the printing system. The relief i.e. the difference between the printing and the non-printing areas is smaller in the control fields with higher screen ruling. It is stated that higher screen rulings can increase the imprint quality, but at the same time the correction curves which are used for lower screen ruling are not applicable. The printing process factors, the pressure between the printing plate and the anilox roller and the printing plate and the printing substrate has to be precisely defined so as to eliminate ink transfer from the non-printing areas, as the relief on the higher screen ruling is lower. The screen ruling is an important factor which is to be taken into consideration in the digital file correction procedure. The different screen rulings have a profound influence on the relief depth and cross-section of the printing elements. This also has a significant impact on the quality of the final product [10].

REFERENCES

1. A Beginner’s Guide to Flexography Printing and it’s Uses. Retrieved from What Is Flexography Printing and What Is It Used For? (lifewire.com)
2. Screen Ruling. Retrieved from Screen Ruling - Pro Carton
3. Impact of Screen Ruling of Screen Ruling on the Formation of the Printing Elements on the Flexographic Printing Plate. Retrieved from Impact of Screen Ruling on the Formation of the Printing Elements on the Flexographic Printing Plate | Semantic Scholar
4. HD Flexo Quality on Qualified Plates. Retrieved from Microsoft Word - HD Flexo - Quality on Qualified Plates_v5.doc (monochrom.gr)
5. Halftone Image. Retrieved from Halftone Image - an overview | ScienceDirect Topics
6. Swati B. (1999). Effect of Screen Ruling and Screen Shape on Image Quality. IS&T’s PICS. Retrieved from Effect of Screen Ruling and Screen Shape on Image Quality (imaging.org)
7. Rashmi B. (2012). Effect of Screen Ruling on Dot Gain in Self Adhesive Labels. International Journal of Advanced Engineering Research and Studies. Retrieved from Microsoft Word - rashmiapb@yahoo.com (technicaljournalsonline.com)
8. Jiang W. and Li X. (2011). Influences of Screen Ruling in Printing Quality in CTP Workflow. Advanced Materials Research Vol. 174 pp 247 - 250. Retrieved from Influences of Screen Ruling on Printing Quality in CTP ... citeseerx.ist.psu.edu › viewdoc › download (google.co.in)
9. Tatiana B., A. Ganchev and Jana K. (2014). Optimizing HD Flexo for Different Plate Technologies and Substrates. International Circular of Graphic Education and Research, No. 7. Retrieved from Optimizing HD Flexo for Different Plate Technologies and Substrates (internationalcircle.net)
10. Tamara T., Sanja P. and Tomislav C. (2013). Impact of Screen Ruling on the Formation of the Printing Elements on the Flexographic Printing Plate. ACTA GRAPHICA. Retrieved from Impact of Screen Ruling on the Formation of the Printing Elements … (srce.hr)