Seminar on

Effect of Process Parameters on Flexo Printability

By

Mr. Girish Chintaman Labade

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 (Also known as flexo) is a printing process using flexible relief plates and pressure to create an image.  Each plate, is adhered to a cylinder and inked by an anilox roll.  The substrate is passing between plate cylinder and impression cylinder to achieve ink transfer.  As the plate cylinder rotates, an impression cylinder applies pressure to the substrate, causing it to make contact with the inked relief of the plate (Fig.1).  The selection of engravings and cell volumes depends on the printing requirements. The choice of the accurate  anilox roll is of central importance for optimal result. Low viscosity, water-based inks passing between dryers, for instance, while UV flexo inks are cured with ultraviolet light.  Flexographic printing process can be used on almost all type of substrate and uses quick-drying, semi-liquid inks.  With flexography, high press speeds can be achieved; long runs are well-suited; and all printing, var­nishing, laminating and die cutting is performed in a single pass.  A variety of products can be printed on flexographic process – including pressure sensitive labels, shrink sleeves, pouches, multi-panel labels and more [1].

The noun “Printability” can be broken down into two parts: “print” and “ability”. A first and logical approach would give as definition “the ability to be printed”. Many factors affect printability, such as ink transfer in the printing press, ink transfer to the print carrier, ink setting and ink drying. Important quality aspects are: ink distribution, ink drying, image resolution, colour gamut, density range, dot gain, rub resistance. In short, one could say that printability is factors that influence the print quality.

“Achievable print quality resulting from interaction between the critical properties of  substrate and printing parameters.” The quality of printing by flexography method depends on the printing plates, the mounting of the plates, the ink, the different substrates, the theoretical volume of the anilox roller, the pressure at the plate/substrate nip and the printing speed [2].

Fig. 1: Flexography Printing Process

Image copyright © 2021 Orbo NV. All Rights Reserved

                           The most significant parameters which to have the greatest effect on image reproduction in flexo process are plate-to-substrate impression pressure, line ruling and anilox specification. The specification of the cell engravings of the anilox roll provided 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 that there is an interaction between the line ruling. 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 are the dominant factor on the print quality, but other aspects such as the cell shape are also to have an affect It is that cells of similar volume, but of different line rulings, produced different results on the printed image in terms of the volume of ink transferred. Increasing the impression pressure between the plate and the substrate improved the uniformity of the ink film, which is an increase in solid density. It also when increases in engagement produced smaller improvements in the solid density. 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 did not affect the printed ink roughness [3].

                 Flexography printing technology underwent rapid development recently. For a long time the main objective of technological innovation are improvement of print quality. As the result of the development of the past decades achievable quality enabled flexography printing process to became the competitive of offset printing in certain areas of application. Recent innovations in flexography technology are related to setting printing pressure. Flexography technology is based on the principles of relief printing. In packaging printing 40% of  market uses flexography, while offset holds only 35%. Many recent innovations in flexographic technology are related to printing pressure(F). Printing happens in the presence of printing pressure in traditional (impact) printing process; there is physical contact between the printing form and the ink, as well as between the ink and the substrate. Ink transfer to substrate is one of the key parameters during the flexographic printing process. Printing pressure a process parameter are change gradually during in the range of visual acceptability. It is significant variations in factors determining the quality of reproduction of colour [4].

                           Fig. 2: Flexography Printing Plate

Image  Copyright © 2021 Elsevier B.V. or its licensors or contributors. ScienceDirect All Rights Reserved

                         The paper with low values of grammage gives low print density with higher values of grammage shows higher values of density. The measurement of properties of papers using three different papers- Paper A: double layer coated, Paper B: uncoated, Paper C: coated. The grammage of the different papers affects print density. The thickness of papers does not show to be a significant factor that influence on the print density of them, as papers B and C have almost the same thickness, but they appear very different values of print density. However, paper A with high thickness show also high print density values. The density appeared to be affected by the porosity of papers. As the porosity increase, the print density decreases. Because of the ink penetrates into the paper and does not remain on its surface. That why papers are not gloss and bright. Hence it is not appear high values of print density. The surface roughness of papers should affect in a larger extent print density. As the surface roughness increases, the print density decreases dramatically. Paper B appears high value of roughness and low one of print density, while papers A and C have lower values of roughness, as they are coated, show better print density. The print density depends on the smoothness of the papers. Because of the coating Paper A and C showing high value of smoothness. The characteristics of the anilox cylinder (i.e., cell volume) appeared to affect considerably print density, as expected. Therefore, the print density of the papers it is proportional to the cell volume of anilox cylinder [5].

                  

                             Fig. 3: Anilox cell volumes                                    

 Image copyright © 2019 Simec Group Srl Via Verga. All Rights Reserved

                             One of the importances of flexographic process is its ability to print image at low pressures. The impact of the pressure changes between the anilox chamber, the printing plate and the central impression cylinder on the print quality are assesses using orthogonal array techniques. The anilox to ink chamber are shown to affect the density most significantly, while it is the plate to impression pressure that affect of the tonal reproduction. The greatest(Fig.3) the pressure at three locations that are: ink chamber to anilox, anilox to printing plate and printing plate to central impression cylinder. The impact of changing pressure setting in the ink transfer it is affect on print quality. The density is significantly affect by the anilox to ink chamber pressure. As this is increase so the print ink density also increases. The ink spread or ink penetration into the paper it is affect the printed tone gain because of the mechanical deformation of the plate and the pressure between plate and CI nip pressure it affects the printed tone gain [6].

                        Advanced digital technologies produce a flat top dot structure and the dots size is almost the same as the target dot size. Standard digital platemaking system causes a large difference between nominal and actual coverage; therefore, the compensation curve is used to correct the tonal values. Minimum dot-size grows as the line screen increases. Standard LAMS technology can reproduce dots as small as 40-50 μm, while advanced digital technology can reproduce dots as small as 19 μm using all grey levels. The smallest dot a printing process can print is independent of the screen count used. Coverage has no effect on dot gain as an individual parameter but, has through the interaction with line ruling and pressure settings during the printing process. Line ruling proved to be a significant parameter because with line ruling increasing, dot gain is increased, too. Higher line rulings give a better image quality through the better dot definition, but the quality may be lost due to higher dot gain. Therefore, it is important to correctly apply compensation curve during prepress. The pressure between printing plate and impression cylinder has the most significant influence on mechanical dot deformation (expansion and barrelling) and thereby on print quality. Round top dots show a high sensitivity to change in pressure, while flat top dots remain more consistent with over-pressure. Ink spreading on the substrate has a larger effect on the final dot size at round top dots because of bullet top shape. The pressure is larger at dot centre thus the ink is squeezed out and creates an outline of "halo" effect. However, full characterisation of the proportion of dot gain due to the dot deformation on the printing plate requires a further development. Therefore, better define the print surface of dot. It is not possible to reliably determine the impression level at round top dot and thus the interaction between top printable area and shoulder of a dot are larger. The deviation of opacity of pre-printed opaque white ink are large according to specification of ISO 12647-6 and amounts to 6.2 %. However, the tolerance of cyan printed over white ink is within the limits of tolerance [7].                

References 

1.   Flexography Printing Retrieved from: https://www.gintzlerinternational.com/blog/2017/08/23/flexographic-printin     
2. Measurement and Prediction Procedures for Printability in Flexography. Retrieved from: https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.124.3799&rep=rep1&type=pdf
3.    Bould D., Hambiyn S., Gethin D. and Claypole T. (2010). Effect of impression pressure and anilox specification on solid and halftone density. Proc. IMechE Vol. 225 Part B: J. Engineering Manufacture 2010 pp. 699-709 Retrieved from: https://journals.sagepub.com/doi/abs/10.1177/2041297510394072?journalCode=pibb#:~:text=Increasing%20the%20pressure%20between%20the,and%20impression%20cylinder%20was%20changed.&text=The%20results%20also%20showed%20that,effect%20on%20the%20tonal%20reproduction.
4.   Akos B., Rozalia S. (2011) Colorimetric Properties of Flexography Printed      Foils: the effect of Impression. Obuda university e- Bulletin vol. 2, No. 1, 2 pp. 31-36 Retrieved from https://www.semanticscholar.org/paper/Colorimetric-Properties-of-Flexographic-Printed-the-Borb%C3%A9ly-Szentgy%C3%B6rgyv%C3%B6lgyi/e9a242853ce372fe7f5771464eeecfe5c7bffd99
5.  Stamatina T., Eftihis F., Diana T., Marios T. and Dimitris M. (2014). Effect of paper properties on print quality by flexographic method. Nordic Pulp and paper Researched journal   Retrieved from https://www.researchgate.net/publication/263847022_Effect_of _paper_properties_on_print_quality_by_flexographic_method/link/0046353c0435178b3b000000/download
6.     Bohan M.,Townsend P., Hamblyn S., Claypole T., and Gethin D. (2003) Evaluation of pressure in flexographic printing. In proceedings of the TAGA Conference, Montreal, Canada, , pp. 311-320 (Technical Association of the Graphi Arts, Sewickley,PA). Retrieved from https://www.researchgate.net/profile/Tim-Claypole-2/publication/265409485_Evaluation_of_Pressures_in_Flexographic_Printing/links/54ccbb2a0cf298d6565ac1fa/Evaluation-of-Pressures-in-Flexographic-Printing.pdf
7.   Dean v., Igor Z., Marin M.(2013)The influence of variable parameter of flexographic printing on dot geometry of pre-printed printing substrate Technical Gazette 20, 4, 659-667 Retrieved from: https://www.researchgate.net/publication/288390466_The_influence_of_variable_parameters_of_flexographic_printing_on_dot_geometry_of_pre-printed_printing_substrate