Seminar on

Effect of Anilox Profiles on Flexo Print

 Quality 

By

Mr. Shyam Santosh Laturiya

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

Flexography printing is a roll-feed web printing process. Anilox roller the heart of the flexography printing. An engraved metal or ceramic roller utilized in flexographic printing presses to transfer ink from the fountain roller to the printing plate. A flexographic inking system is also known as an anilox system. The purpose of the anilox roller is to pick up ink from the fountain roller and deliver a predetermined, metered, uniform amount of ink to the rubber printing plate. The anilox roller is additionally called a metering roller, a knurled roller, and an engraved roller. It is used for the transfer of a precisely specified quantity of ink onto the printing plate. Therefore, the quality of the anilox roll has a tremendous influence on printing quality. It is a special transfer roller for flexographic printing presses. The surface is made up of numerous recesses of the same size, shape, and depth. These recesses are called cells or ink holes. The cell can store ink, through the irregular shape, size, and depth of the anilox roller, the quantity of ink will be controlled to attain the specified thickness of the ink layer. The anilox roller plays a vital role in flexographic printing [1].

    Anilox Line Per Inch (LPI) Line count or screen count refers to the amount of cells per inch/centimeter as measured along the engraving angle because that's where the cells line up in closest proximity to each other. L/cm stands for lines per centimeter L/in stands for lines per inch, this refers to the number of cells per inch L/cm. Line count is the number of cells per linear inch, at the angle of engraving, typically 30°, 45° or 60° about the axis of the anilox roll. Once engraved, anilox cell count does not change. Lower anilox line counts accommodate higher volumes for solid coverage and coatings. A Line Screen is that the measure of what percentage halftone lines are printed during a linear inch [2]

Fig 1: Anilox Line Per Inch

Anilox Cell Structure are typically one among five types- Trihelices, Pyramid, Quadrangular, Hexagonal or Hexagonal Channel Screen. A Trihelices screen could be a line engraved at a 45-degree angle. These are primarily utilized in coating applications of viscous fluids. A Pyramid cell essentially an inverted pyramid. This engraving is incredibly commonly utilized in the industry. They are typically utilized in metered roll systems that don't require a doctor blade. The ink transferred may be a vital function within the printing. A quad engraving is very similar to a pyramid engraving, it’s just cut short, these have more versatile and more positive release characteristics, this engraving type most commonly appears with a doctor blade system. Finally, we have the hexagonal cell. Hexagonal cells have superb release qualities and may be utilized in both chrome and ceramic engravings. Each cell is linked together be a shallow channel. Due to the spectrum line, it will be utilized with a range of inks, adhesives, etc. Hexagonal cells typically hold more ink than a quad engraved pattern and generate a more even and predictable flow throughout the printing. The 60 degrees hexagonal pattern allows for more cells to be compacted in a locality and provides the most efficient distribution of volume. There is less wasted space if compared to diamond shape patterns. This is the explanation, the most uniform distribution of ink of the 60-degree hexagonal engraving became the norm in flexo printing [3].

Fig 2: Anilox Cell Structure

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Cell volume means the capacity of the engraved surface per square inch, expressed in billions of cubic microns (BCM). Higher volume translates to higher solid ink density, more colour, or a heavier coating thickness. Lower volumes apply thinner ink films directly related to higher print quality and process efficiency. Anilox cell volume changes over time, because of wear, plugging and cleaning procedures. Dirty or plugged cells are temporary and may be corrected to revive cell volume and colour. Wear is irreversible and permanent, resulting in weaker or less colour due to lower volume [4].

Fig 3: Anilox Cell Volume

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Anilox screen angle the 60° hex pattern is usually recommended for many flexo printing applications for the subsequent reasons: Cells will be nested more tightly together, permitting approximately 15% more cells in each area. Post areas are eliminated because of the position of the cells during laser engraving. Cells will be shallower while still releasing a similar amount of ink because more cells fit into a sq in area. Channeling is avoided (as compared to the 30°) because the straight side of the cell wall isn't positioned horizontally. The 60° hex pattern is most easily reproducible from a producing standpoint. 45° and 30° patterns could also be preferable in non-printing flexo applications (such as coatings, laminations, and specialty printing), but ordinarily don't seem to be recommended for general flexo printing applications [5].

Fig 4: Anilox Screen Angle

The ink transfer ratio to the upper plate for different contact angle (30°,50°,70°,90°), while the contact angle of the trapezoidal groove is 70° and the contact angle of the upper plate is increases, the ink transfer rate decreases. The wetted area of the trapezoidal groove is the same but, with the contact angle increasing, the upper plate wetted area decreases, so the rate of ink transfer is reduced. Ink transfer ratio to the trapezoidal groove for different contact angle (30°,50°,70°,90°), and the contact angle of the upper plate is 70°, it will see that the trapezoidal groove contact angle increases, the ink transfer rate increases. The increasing contact angle of the trapezoidal groove, the adhesion of upper plate is reduced and the other parts unchanged, so that the transfer rate is increases. The contact angle of the trapezoidal groove remains constant 70°, and with the increase of the contact angle of the upper plate, the adhesion of upper plate reduces, so that the transfer rate decreases. Changing in the aspect ratio of trapezoidal groove, less ink is transferred with the increasing of aspect ratio, that is to say trapezoidal groove with large open width transferred more ink. If ink is transferred out from anilox, it is not only by adhesive forced from the upper plate and trapezoidal groove, but also by capillary force which block printing ink transferred. Because of the anilox roller mesh point size is small, the capillary force is inversely proportional to the opening width. The larger the opening width, the smaller the capillary force, the greater the ink transfer rate [6].

The flexography inking system of high-line screen engraving technologies is anilox rollers, this delivers the accuracy and consistent amount of ink during the process of flexography, making it possible to produce the high-resolution print quality. The cell clogging of anilox roller five level of cell clogging from which the first level of cell clogging and the bottom of all cells is almost uniform in shape and depth and there is no sign of clogging, in second level bottom of less than half of cells is narrow and the depths of less than half of cells is reduces (slightly red, widening yellow outline) and average ink transfer reduces, in the third level shape of the bottom of about half of cells is change. Half of cell depths decrease (red is almost invisible, yellow predominates), and the average ink transfer reduces, in the fourth level more than half of cell bottoms is narrow and no longer visible, more than half of the cell depths decrease (red is no longer visible), and average ink transfer reduces, and in the fifth level bottom of almost all cells is narrowed and no longer visible, depths of almost all cells reduces (yellow and green predominate) and average ink transfer reduces. The higher the level of clogging, the higher the minimum line screen of the anilox rollers. This means the rollers of the lowest liners are more difficult to clog. The engraving line screen increases, the average cell depth to opening ratio also increases. The line screen distribution of anilox rollers engraved at angle of only 60° by the level of clogging as the level of pollution increases, the average line screen also increases [7].

The specification of the cell engravings of the anilox roll provide the largest influence on the amount of ink deposited on to the substrate, and hence on the optical density and halftone reproduction. The density of the halftone dot structures shows that there is an interaction between the line ruling on the plate and the cell volume of the anilox roll. The rate of density increase for increasing anilox volume is greater at higher line rulings than for lower line rulings. This is attributed to a greater perimeter of the dots at the higher ruling, per unit area on the plate. The anilox cell volume is the dominant factor on the print quality, but other aspects such as the cell shape also have an effect. Cells of similar volume, but of different line rulings, produces different results on the printed image in terms of the volume of ink transfer. The roughness of a printed ink film is dependent on the volume of ink carried by the anilox roll. The cell pitch of the anilox roll does not affect the printed ink roughness. The cell volume changed, the anilox engraving with the highest cell volume produces the highest optical density. The anilox specifications have the large overall effect on the optical density [8].

Vibration and sound signals were analysed for patterns relating to cell wear on anilox rolls used in flexo printing. The major defect of the anilox roll is the cell wear. The surface engraving cell of an anilox roller determines the ink film thickness. The depth and angle (volume) of each cell determine the amount of ink transfer, cell wear makes the walls lower, and the amount of ink carried by the worn spots will increase reducing the uniformity and quality of print. Using new, lightly used, and heavily used rollers, the cells show more rough and rounded surfaces (walls), some pitting, chipping, and fracture of cell surfaces. If the wear of the roll cells is increased, the spikes number and amplitudes decreases, and the location shift along the frequency axis occurs. Cell wear symptoms is clearly revealed to the short time frequency transform (STFT). The vibration signals do not lead to any consistent results relativity to cell wear. The power spectral density (PSD) patterns reveal to the number of high-energy spikes and their magnitudes decrease as cell wear increases. The spikes spread out with cell wear. Great potential for designing an on-line anilox roll testing and evaluation system that will optimize its useful life and enhance the print quality [9].

There are so many process parameters involved in flexography that effect on the print quality. The printing quality is usually defined by the line width, thickness and ink transferred. The print quality is determined by anilox cell geometry and volume, printing plate properties, nip pressure, printing angle, printing speed, doctor blade, contact angle, ink type and viscosity. Dot dipping is a printing defect where the dot enters the anilox cell, leading to bad printing results, anilox volume plays a significant role in ink release, an increases in anilox volume shows result in an overall increases in cross-sectional area, reducing sheet resistance and improving the conductivity of the printing network. Increases in anilox and printing forces leads to more ink transfer onto the anilox roller, that produces a thicker printed line width. Increasing the anilox speed cause a decrease in average printed line width, this is due to the amount of ink picked up by the anilox roller, at a lower speed, more ink is picked up by the anilox, as there is more time for the ink to enter the anilox cells, with more ink in anilox cells, the ink transfer over to the flexographic printing plate and substrate is more is result in higher line width due to more ink spreading. Increasing the anilox also increases the average printed line width as the anilox force is increases, the nip pressure between the printing plate and anilox roller is increases. A higher nip pressure increases the ink transfer onto the printing plate, as more ink is push out of the anilox cells, so that it is resulting in a higher line width due to more ink spreading. This increases in line width is due to the increase in ink transfer, a higher anilox volume is made possible by having larger anilox cells. Anilox volume and printing force have a dominant effect on the printed line width; printing speed and relief depth have a mild effect on printed line width; anilox speed and anilox force have an almost negligible influence on the printed line width, the trend with anilox force is different. Anilox volume and printing force are the primary factors, while printing speed and relief depth are secondary factors affecting the printed line width and the print quality [10].

References

1. Anilox Roller. Retrieved from http://printwiki.org/Anilox_Roller#:~:text=A%20flexographic%20inking%20system%20is,to%20the%20rubber%20printing%20plate
2. MAIN PARAMETERS OF LASER ENGRAVED CERAMIC ANILOX ROLLERS. Retrieved from https://cheshireanilox.co.uk/wp-content/uploads/2017/09/anilox-parameters.pdf
3. Anilox Roll Specifications: Line Count or Volume? Retrieved from https://www.labelandnarrowweb.com/issues/2008-07/view_features/anilox-roll-specifications/
4. Effect of Anilox Cell Geometries to Print Quality Flexographic Printing Technology. Retrieved from https://www.materials-academy.co.uk/dwnlds/seminar/posters/Foulston.pdf
5.  ANILOX ROLL. Retrieved from https://fdocuments.in/document/anilox-roll.html
6.  Chen J., Tang Z., Wang S., and Miao B. (2015). Simulation of Ink Transfer Characteristics for the anilox. Applied Mechanics and Materials Vols 713-715 pp 2859-2863. Retrieved from https://www.scientific.net/AMM.713-715.2859
7.  Eugenijus J. (2020). Investigation of anilox roller cell clogging. Journal of Graphic Engineering and Design, Volume 11 (2). Retrieved from https://www.researchgate.net/publication/347741073_Investigation_of_anilox_roller_cell_clogging.
8. Bould D., Hamblyn S., Gethin D., and Claypole T. (2010). Effect of impression pressure and anilox specification on solid and halftone density. Journal of Engineering Manufacture. Retrieved from https://www.semanticscholar.org/paper/Effect-of-impression-pressure-and-anilox-on-solid-Bould-Hamblyn/2fe464fc596106ef233675256f4f493555b89c2f
9. Uwe T., and Issam A. (2007). Using sound and vibration signals to monitor cell wear on anilox rolls. Nondestructive Testing and Evaluation. Retrieved from https://www.tandfonline.com/doi/abs/10.1080/10589750412331290596
10. Zhong Z., Ee J., Chen S., and Shan X. (2020.). Parametric investigation of flexographic printing processes for R2R printed electronics. Materials and manufacturing processes. Retrieved from https://www.tandfonline.com/doi/abs/10.1080/10426914.2020.1732411?journalCode=lmmp20.