Printing and Packaging Seminar 2020-21

Thursday, May 27, 2021

Seminar on

Impact of Screen Ruling on Flexo Dot Fidelity

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.

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.

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.

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.

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

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)
Screen Ruling. Retrieved from Screen Ruling - Pro Carton
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
HD Flexo Quality on Qualified Plates. Retrieved from Microsoft Word - HD Flexo - Quality on Qualified Plates_v5.doc (monochrom.gr)
Halftone Image. Retrieved from Halftone Image - an overview | ScienceDirect Topics
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)
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)
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)
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)
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)

Seminar on

Process Parameters affecting Printability in Gravure Printing

Mr. Rugved Upadhye

Under the Guidance of

Prof. Dr. Akshay V. Joshi

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

Academic Year 2020-21

Gravure is one of the major and oldest printing technologies. Its advantages include high speed production, high print quality, and hardwearing image cylinders which allows duplicating in millions. Hence, gravure printing is principally used for very long print runs.[1] Today, gravure printing accounts for a market share of 10–15% in industrialized nations. The process is capable of producing high-quality image work, with lpi values of 150 as standard and higher values for high-quality reproductions.[2] The peculiar feature of this technology is that the image elements are engraved into the surface of the cylinder. The non-image areas are at the same, original level and therefore the entire printing plate is inked and flooded with ink. Ink is removed from the non-image by a doctor blade before printing so that the ink remains only in the cells. The ink is transferred from the cells to the printing substrate by a high printing pressure and the adhesive forces between the printing substrate and ink.[1]

Fig.1 Gravure Printing Process

Factors affecting gravure printability include Ink viscosity, cell angle, screen ruling, printing speed, impression pressure, surface properties (smoothness, levelness, ink absorbency, cleanliness, gloss, finish, etc.) and structural properties (bulk or thickness, weight, pick resistance, dimensional stability, etc.).[2] A material with high printability will produce a high-quality print, with sharp, clean print and high print accuracy which we get from the gravure printing process.[5] Gravure ink has the lowest viscosity among all the printing processes. Gravure requires a liquid ink (h = 0.05–0.2 Pa·s), which can fill the image forming cells of the gravure cylinder at a high printing speed. In order to ensure a good transfer effect and long-time printing, it is advisable to adjust the printing ink viscosity into 11-15S/Zahn #3 Cup.[4]

Fig.2 Gravure Printed Reels

Fig.3 Gravure Ink

Doctor Blade is often referred to as the “soul” of gravure printing. It controls the amount of ink transfer from the image cylinder onto the substrate. For better print quality, it is recommended to have a minimal doctor blade pressure (0.5-1.0 bar). The lamella tip-shaped blade gives the best results.[6] The cell is the engraved image carrier used in gravure printing. The volume and shape of engraved cells are important to the performance of the gravure printing process. The impression cylinder forms a nip with the gravure cylinder through which the substrate passes. A wrap of 120º is typical, but it may vary according to the substrate and the diameter of the roller. A 4:6-inch diameter roller will typically have a wrap of greater than 90º; a larger diameter roller and less wrap is desirable for heavier stock.[3]

Fig.4 Doctor Blade

Image copyrights © Daetwyler USA

Fig.5 Impression Cylinder

There are many factors that affect the print quality and ink transfer. These are either paper factors, machine factors, or Ink factors. The ink viscosity, press speed, printing pressure and the type of ink resin have a significant impact on ink transfer in the printing nip. The amount of ink on the plate is measured by a mathematical method. The cell is viewed under a traveling microscope. The shape of the cell is found as a frustum of a pyramid. The amount of ink transferred decreases as the printing speed increases. With the increase in printing pressure, the amount of ink decreases. As the ink viscosity increases, the ink transfer also increases.[7]

To increase productivity and to decrease the manufacturing cost in lightweight electronic devices and fabricating flexible materials such as radio frequency identification tags (RFID), sensors, solar cells, and flexible displays, mostly requires gravure printing over large-areas on flexible plastic substrates at low temperatures. Other methods such as gravure-offset6 and flexographic7 printing have been used for high-resolution pattern printing. Due to the deformation of a blanket and line widths smaller than 20 µm, this process is arguably one of the most significant techniques for building patterns at high resolutions. The nip force plays a significant role in high-resolution gravure printings. The adhesive force-based ink transfer model is developed. The model explains why the ink transfer ratio increased as the nip force increased. The ratio of ink transfer is a function of the contact length. As the nip force increases the contact length also increases and therefore, the ink transfer ratio also increases. A high nip force of 754 N could increase the ink transfer ratio and was found to be favorable for compensating the over-doctoring case. It also showed that low nip forces (251 N) is preferred to obtain high-resolution images and a better fidelity of the printed pattern for ηD ≈ 1.[8]

The ink uniform thickness and dot gain is affected by ink transfer, so it is required to optimize the parameter of the plate making of gravure. Unlike letterpress and offset printing, for gravure printing, color and gradation reproduction of image rely on not only dot area coverage but also thickness of ink film, it transfers ink by a cell structure, which rely on different dwell volume to achieve reproducible. By investigating the influence of screen ruling and engraving needle tip angle on ink transfer, the conclusions can be done such as: The vary curve of ink transfer is different for different screening ruling and the amount ink transfer doesn’t always increase with screening ruling increasing ( more cells in unit area ) , but it is better with more amount of ink transfer. The value of ink transfer doesn’t always increase with screening ruling decreasing and cell deeper. It is required to select the best screen ruling. While comparing different screen rulings, the highlight and middle tone area, the ink transfer is not good, irrespective of screen rulings . But in the darkened part, amount of ink transfer decreases with the screen ruling increasing, so it is better for ink transfer. For the same screen ruling, the amount of ink transfer increases with engraving needle tip angle increasing, and the opening of cells have a major impact on the ink transfer.[9]

Gravure printing is an intaglio printing process in which the image is engraved on the surface of the gravure cylinder. The printing ink is filled in the engraved cells of cylinder during printing and the rest of the ink is stripped by a blade and separated for reuse. The efficiency and the print quality is high in gravure printing and it is also possible to print on different substrates. The ink viscosity, surface properties of substrate material, and cell structures on the gravure cylinder are the main factors that determine the print quality. Appropriate inks are chosen depending upon the material to be printed and the finished product. Mostly solvent-based inks are used. The printability is better than polyethylene since the printing substrate with the lowest surface energy is polypropylene. As ethyl acetate dries faster than other alcohol-based inks, ink density and dot growth are better in terms of printability. Ethyl acetate gives the best results in terms of color and gloss. However, it is highly corrosive to the ethyl acetate engraving cylinder. For this reason, it is not recommended to use it for a long time and in large quantities. For this reason, it is more suitable to be mixed with either isopropyl or ethyl 36 alcohol. When examine in terms of price, the solvent with the lowest unit cost is isopropyl alcohol. For this reason, when it is necessary to choose among alcohols, isopropyl alcohol is a suitable solvent. It is recommended to use a mixture of ethyl acetate-isopropyl alcohol for both substrates in gravure printing.[10]

With the enhanced awareness of environmental protection, as water-based ink does not contain volatile organic solvents, and is non-combustible, and has some other advantages, it would be an environment-friendly printing ink with a good future. But the plastic water-based gravure ink could not meet the requirements of printability in proofing density, dryness, and adherence, therefore the application of water-based ink on plastic is not been promoted. Hence, the printability of Plastic Water-based Gravure Ink is being studied in various labs. Pigment/binder ratios are influenced by the viscosity and adherence of the inks. Inks have a good printability both on PET and BOPP when the pigment/binder ratio is 2:1. Inks have a good printability both on PET and BOPP when adding 0.7% of dispersant of 750 and 0.7% of dispersant of 760. Inks prepared by single resin do not satisfy the need of printability. In order to get the better overall performance of inks, the method of mixing different resins is used. The optical formulation of mixed resin both on the PET and BOPP was 3808:1437: 2640 = 1:0:1. Addition of isopropanol in the inks would increase the proofing density. Inks have a good printability on both PET and BOPP when the dosage of isopropanol was 1%.[11]

References

1. Handbook of Print Media | SpringerLink. Retrieved from - https://link.springer.com/book/10.1007/978-3-540-29900-4

2. Gravure printing - Rotogravure printing process | BOBST. Retrieved from - https://www.bobst.com/usen/products/gravure-printing/process/

3. Characterization of Gravure Cylinders. Retrieved from - https://www.printing.org/taga-abstracts/t010001

4. Gravure Printing Ink - Gravure Ink – Castle Ink. Retrieved from- https://castleink.com/blogs/printer-help/gravure-printing-ink-gravure-ink

5. What Is: Printability? | Print Peppermint. Retrieved from- https://www.printpeppermint.com/glossary/printability/

6. DOCTORBLADESFORPRINTI NG - Daetwyler USA. Retrieved from- https://www.daetwyler-usa.com/library/Daetwyler-Gravure.pdf

7. S. Elsayad F.M &S. El-Sherbiny, E.A. (2002). Some factors affecting ink transfer in gravure printing. Pigment & Resin Technology, Vol. 31 Iss 4 pp. 234 – 240 .Retrieved from http://dx.doi.org/10.1108/03699420210432429

8. Ho Anh Duc Nguyen, K.S, and Changwoo Lee. (2015. Effect of Nip Force on Ink Transfer in High Resolution Roll-to-Roll Printing. INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 16, No. 3, pp. 517-523. Retrieved from https://link.springer.com/article/10.1007/s12541-015-0070-9

9. Deng Pujuna , Z.G & Wang Yu and F.W (2011). Influence of screen ruling and engraving needle tip angle on ink transfer for gravure. Advanced Materials Research Vol 174 (2011) pp 215-218. Retrieved from https://www.scientific.net/AMR.174.215

10. Arif Ozcan and Rasim Zulfigaroglu (2020). An investigation on printability of different solvent based inks by gravure printing onto various substrates. Journal of Graphic Engineering and Design · July 2020. Retrieved from https://www.researchgate.net/publication/342946643

11. CHEN Yue-tao, W.X & HUANG Bei-qing (2014). Study about Influencing of Printing Process on Gravure Printing Ink Transfer. Applied Mechanics and Materials Vol 469 (2014) pp 301-304. Retrieved from https://www.scientific.net/AMM.469.30

Seminar on

Effect of Process Parameters on Flexo Printability

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

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

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

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

Flexography Printing Retrieved from: https://www.gintzlerinternational.com/blog/2017/08/23/flexographic-printin
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
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.
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
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
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
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

Wednesday, May 26, 2021

Seminar on

Role of Rheological Properties of UV Inks on Flexo Printability

Mr. Siddharth Raju Adagatla

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

UV inks are a special type of inks that have been formulated to dry only when exposed to ultraviolet (UV) light. When the substrate receives wet ink in printing press, it is immediately exposed to UV light so the ink does not have the opportunity to seep or spread. Hence the printed image will resemble an overall better dot and print effect, resulting in a high-quality finish. When UV ink is exposed under UV light, they absorb UV light and start a Polymerization reaction that employs the use of free electrons, or free radicals causing an instantaneous cross-linking of the resins. Photo-initiators generally react to wavelengths of between 200 - 400 nm.

Fig 1: Polymerization reaction

UV inks are widely used in Flexographic Printing and are most popular in the Outer-wrap food Packaging Industry and label application. UV inks are usually 100% non-volatile materials so there is no concern for the evaporation of any volatile materials like solvents or water-based inks. As 100% of the ink is applied to the substrate, UV inks have the potential to achieve greater densities than conventional inks. UV ink provides many desirable end-use qualities which includes excellent rub resistance, chemical resistance, exceptional color consistency, and superior gloss. Rheology is used to describe the deformation and flow behavior of a material. The rheology of UV inks is determined by the components that are added to the ink. Rheology is very important because the rheological properties are closely related to final print quality. The rheology of ink plays a central role in ink distribution and transfer in Flexographic printing processes. [1]

Fig 2: UV inks

Ingredients of UV inks:

Monomers -

The basic unit for polymers is formed by a molecule of monomer. Monomers are used at 30-60 weight percent of the formulation in flexographic formulations. Choosing the right monomers is critical because a wrong type of monomer can lead to plate swell and create registration issues. Monomers provide lower viscosity and can influence rheology, cure rates, and adhesion of UV inks. [2]

Oligomers -

Oligomer is the main component used as a binder in ink formulations. These are highly viscous and contain active groups for cross-linkage. Oligomers are essential intermediates of the polymerization reaction. Oligomers provide enhanced physical properties, cure speed, and adhesion. [3]

Additives -

Additives play a critical role in the formulation and production of inks, which adds necessary characteristics to the ink needs like to run more smoothly on press and also to protect the finished product. Additives re added to improve curing, increase viscosity and to prevent the ink from dropping off the screen, and improve adhesion. These additives come in many forms like waxes, surfactants, defoamers, or countless other products. [3]

Fig 3: Additives

Colorant -

Colorant comprises pigment and dyes. The purpose of pigment or dye is to provide image contrast on a substrate. Pigments are small particles that are insoluble in ink vehicles and dyes are soluble in the ink.. Pigments are usually more opaque than dyes. Inks that contain pigments tend to bleed less, are resistant to chemicals and heat. Pigments are a good option for conventional printing because of their high color strength and lightfastness. [4]

Photoinitiators -

Photoinitiators are chemical substances in which the small molecules are sensitive to light. Upon light absorption, they undergo photochemical reaction to produce reactive species that are free radicals that will interact with the active components in the liquid formulations. A photoinitiator exhibits several properties which include high absorption at the exposure wavelength and high molar extinction coefficient, high quantum yield of formation of initiating species and high reactivity of the radical towards the monomer. [5]

It is important to understand the relationship between printability and the rheological behaviours induced by the interfacial interactions between different components within the ink. Because the rheological behaviour of ink directly affects printability. Adding different components to ink determines the rheology of inks which has a great impact on Flexo printability. Low viscosity ink gives a constant ink transfer at sufficiently high press speeds, but that the ink transfer always increases with increasing pressure at a given speed. Transfer characteristics are related to the viscoelastic properties of the ink. Due to Carbon black, UV ink formulations show shear thinning behaviour. The ink formulation with DPHA monomer is considered as the best formulation for the ink transfer between the Anilox roller and plate cylinder with higher density and clearer dot structure. Also for the ink transfer from a plate cylinder to a printed substrate, the transfer fraction of ink is more in the ink formulation with DPHA monomer due to the viscoelastic properties of the ink. [6]

Mirror-coated paper is used in label printing, food packages, and other consumer goods. This substrate is printed with both water-based ink and UV ink with Flexographic Printing Press. Water-based ink used in the flexographic printing process is mainly composed of water, pigments as well as other additives like defoamers, and drying & adhesion agents, etc. Water-based ink is mainly used for absorbent substrates. Water-based ink is widely used in the corrugated packaging industry. The UV ink used in the flexographic printing process is mainly composed of monomers, oligomers, photoinitiators as well as pigments and has higher viscosity than convectional inks. So, UV ink is used for non-absorbent substrates. UV ink is widely used in the food packaging industry for outer wraps. Flexography press runs at a high speed using UV ink, as it quickly settles down completely when it passes through UV light. In flexographic printing more solid ink density is achieved by UV inks than water-based inks because UV ink is dried by polymerization process with negligible absorption. Whereas more print contrast is achieved by the water-based inks because water-based ink is dried by absorption and evaporation process and it penetrates in the pores of the substrate. In the case of 50% and 100% dot gain, UV inks have more dot gain than water-based inks due to their drying process that is polymerization. [7]

Polyvinyl Alcohol is the largest volume of synthetic water-soluble resin that produced in the world. Polyvinyl alcohol films exhibits high tensile strength, abrasion resistance, and oxygen barrier properties which are superior to those of most polymers. The excellent chemical resistance and physical properties of PVA resins have resulted in broad industrial use as an excellent adhesive and possess solvent, oil, and grease resistance properties. The effect of ink and paper board characteristics on flexographic print quality is based on print density, solid content, pH, and particle diameter of the inks. Viscosities of the ink are increased with an increasing percentage of Polyvinyl Alcohol. For water-based UV inks, reduction of ink viscosity reduces the print quality in flexographic printing. On Newsprint and Bond paper, the ink formulation with Polyvinyl Alcohol used as a sole binder resembles some air bubbles on the printed image, whereas on Art paper, much air space with uneven pigment particles distribution occurs. Therefore 8% Polyvinyl Alcohol ink formulation is the best formulation with good printability on various substrates like paper printing, board, paper board, coated and uncoated paper, and other printing services involving the use of paper materials for the packing industry. Polyvinyl alcohol can be utilized as a sole binder in flexographic printing applications. [8]

Fig 4: Micrographic Images with Varing Formulations on Different substrates

Polymer film materials are used to print labels and packaging through the flexography printing process. Polyethylene and polypropylene have low surface energy because there is a lack of molecular interactions between polymer surfaces and inks, and the resulted images are created with low optical characteristics. Good surface wettability is for ensuring good adhesive bonding. The contact angle of the liquid with the solid phase is called as wettability. UV-flexo inks have a higher viscosity than conventional inks so it requires high surface energy of polymer films. On higher surface energy and polarity polymer films, yellow inks spread is the best and black is the worst due to differing ability components of the ink structure formation. Decreasing the cosine of the contact angle reduces ink adhesion. The polarity of the surface has the presence of polar groups in intermolecular interactions provides an adequate adhesive bond. Maximum spreading and maximum adhesion of UV-inks are possible at high surface energy and a polar component of the surface film. [9]

The rheological properties of ink suspensions depends on the degree of flocculation. The flocculated structure depends on the shape, size, and surface characteristics of the dispersed particles and the interaction between the dispersed medium and the particles. The interaction between pigment particles and the continuous phase creates the formation of a network of particles and aggregates within the ink, which can form a continuous structure over time. The rheological properties of pigment–polymer matrix systems gives information on the processing behaviors of composite materials. To understand and control the rheological properties of UV flexo inks it is necessary to accurately analyze the specific rheological properties produced by the chemical and physical interactions of the ink components. The viscosity of Prepolymer increases as an increase in the shear rate and Dynamic viscosity of Prepolymer is dependent on the molecular weight of oligomer. Dynamic viscosity of the Colorant is increased as the strain amplitude increased. These factors should be known by every ink manufacturer to achieve the desired ink formulation. [10]

References

4 Types of Flexographic Inks & When to Use Them. Retrieved from https://blog.luminite.com/blog/4-types-of-flexographic-inks-when-to-use-them
Monomer Definition and Examples. Retrieved from https://www.thoughtco.com/definition-of-monomer-605375#:~:text=A%20monomer%20is%20a%20molecule,natural%20or%20synthetic%20in%20origin
UV CURING FLEXO INKS. Retrieved from https://fdocuments.in/document/uv-curing-flexo-inks.html
Ink. Retrieved from http://printwiki.org/Ink
What are Photoinitiators. Retrieved from https://www.sglinc.com/articles/what-are-photoinitiators/
Kyoung .J. & Ken’ichi k. (2007) Transfer characterization of UV flexographic ink: Relationships between printability and rheological properties, Nordic Pulp and Paper Research Journal Vol 22 no. 3/2007. Retrieved from DOI: https://doi.org/10.3183/npprj-2007-22-03-p293-298
Ravi T. Ankit. B & Bijender (2020) A Comparative Analysis of Print Quality with Water Based Ink and UV Ink Using Mirror Coated Substrate in Flexographic Printing Process, International Journal of Advanced Science and Technology Vol. 29, No. 1, (2020), pp. 1754 - 1761. Retrieved from: http://sersc.org/journals/index.php/IJAST/article/view/34465
B. Magny. E. Pezron , Ph. Ciceron & A. Askienazy (Aug 1999) Key factors for UV curable pigment dispersions, Conference: RADTECH ASIA '99: 7. international conference on Radiation Curing, Kuala Lumpur (Malaysia), 24-26 Aug 1999 Retrieved from : https://www.osti.gov/etdeweb/biblio/20051967
Vyacheslav R. (2013) Influence of Surface Energy of Polymer Films on Spreading and Adhesion of UV-Flexo Inks, Acta graphica 23(2013)3-4, 79-84 Retrieved from https://hrcak.srce.hr/index.php?show=clanak&id_clanak_jezik=203693
Kyoung J. , Sun K., Jong W., Yong. L. & Ken K. (2017) Rheological Properties of UV-curable Ink Influence of the Pre-polymer and Colorant, Journal of Photopolymer Science and Technology Volume 30, Number 6 (2017)703 －7 0 8 Ⓒ 2017SPST. Retrieved from DOIhttps://doi.org/10.2494/photopolymer.30.703