Welcome to a new column dedicated to helping you understand the science behind printing. Before the topic of ink viscosity is addressed, let us begin with a background of your team of authors. RYETAGA (Ryerson Technical Association of the Graphic Arts) is Ryerson University’s official Student Chapter of TAGA (Technical Association of the Graphic Arts). As Canada’s only TAGA student chapter, RYETAGA student members take part in every aspect of our journal production. We will be submitting our student journal publication to the competition at the TAGA Annual Technical Conference in March 2011. The TAGA conference provides industry and student members the opportunity to learn about the latest research and technology in the graphic arts industry, through conference presentations, seminars and networking opportunities. RYETAGA’s student journal publication will be competing against student chapters from other universities to keep the grand prize for best overall student technical journal, the Helmut Kipphan Cup, and the Harvey Levenson prize for the undergraduate research paper at Ryerson University. Dr. Martin Habekost, professor at the Graphic Communications Management School, advises the team. To learn more, visit our website: www.RYETAGA.com. We hope you enjoy learning about print.
What is Viscosity?
Viscosity is a measure of a liquid’s ability to resist flow. A thick liquid that does not flow easily has high-viscosity; a thin liquid that readily flows has low-viscosity. The viscosity of ink strongly affects how it behaves on the press and is ultimately transferred to the sheet.
What Influences Ink Viscosity?
Thixotropy
Ink viscosity dictates many aspects of a job. To control your ink’s viscosity, one must understand what causes the inks to have a high or a low viscosity. Liquids such as printing inks have thixotropic qualities. Inks are highly viscous under normal conditions, but will gradually become less viscous as force or stress is applied. To illustrate this phenomenon, consider what happens to an ink over the course of an average press run. Before it is removed from the can, an offset ink behaves more like a semi-solid than a liquid. However, as the press’ rollers work the ink, it becomes less viscous and more fluid in state. We can clearly see that as the ink is subjected to force, it becomes less viscous. This can negatively impact print quality, as low-viscosity inks are less capable of reproducing sharp halftone dots.
Effect of Temperature on Viscosity
Fluctuations in temperature can have a drastic impact on the viscosity of ink. For example, a temperature increase of 5.5°C can reduce the viscosity of a printing ink by 50 percent or more (DeJidas and Destree, 2005). This strong correlation between temperature and ink viscosity can have serious implications. Colour changes on paper can occur over the course of a press run if the ink is not at its optimum temperature. Similar colour variations can occur between printing units if temperature is not kept stable. A cold ink, which will be higher in viscosity than an ink at the proper temperature, is also more likely of promoting picking, where ink pulls fibres from the press sheet, resulting in wasted sheets.
Impact of Viscosity
By understanding what influences ink viscosity, you can be more aware of what impact viscosity has on your printed job. The viscosity of ink characterizes multiple aspects of how well the ink will transfer onto its substrate, thus affecting the quality of the printed image. Throughout a press run, the properties of the ink’s viscosity can have several effects, on ink absorption, colour strength and drying. Highly viscous inks can be problematic as a result from the amount of tackiness it has; higher ink tack causes difficult image transfer to the substrate. With low viscosity, inks become more mobile and tend to flow on the printed substrate; this will often result in issues such as a growth in size of halftone dots, which causes the image to lose its sharpness and dirty printing (Akshay, 2007). Therefore, the viscosity of the ink on press dictates the quality of your final printed image.
Conclusion
The ideal ink viscosity depends on the press configuration, since the ink viscosity also has an influence on the ink/water balance. Your ink supplier can recommend the correct ink for your press. The viscosity of ink increases when it is not being worked and decreases when there is a constant applied force. Temperature also plays a factor affecting viscosity. As the temperature of the ink increases, the viscosity decreases. If the ink is not at the suitable printing standard, the ink’s viscosity increases or decreases to the point where the ink will not transfer properly. That is why it is important to keep the press at a constant temperature to minimize fluctuations in ink viscosity, which could lead to fluctuations in print quality and unacceptable print products.
I wish the authors would have followed-up with a discussion of how ink mananufacturers assess the required level of vicosity in an offset printing ink, and not just leave it at “your ink supplier will know”.