The outcome of a print in production run plays a crucial role in commercial and packaging printing. In the growing packaging industry, colorfulness and saturated prints with high chroma attract the eye of the consumer. The design and layout of a packaging carton comprise of images that consist of halftones in the print process, which demand attractiveness and visibility using bright colors. In this research, an effort has been made to identify and analyze various parameters involved in offset lithography affecting color attributes of prints. This study also focused on the investigation of the best process conditions that would yield optimum color values through multiresponse factors such as chroma and lightness. A general full-factorial Design of Experiments (DOE) approach was used to evaluate the effect of prepress parameters such as screen ruling and dot shape and press parameters such ink viscosity and paper smoothness. These parameters were then optimized using a customized response surface design. From the experiment, it was observed that viscosity of the ink was a significant factor that majorly controls the color attributes. The surface smoothness of the paperboard was one of the factors influencing the improvement of color reproduction. A smoother surface makes even contact during ink transfer in the offset printing machine and hence reflects color with a higher chroma. The optimum parameters were as follows: 15 Pa s ink viscosity, 0.77 μm paper smoothness, and 200 lines per inch (lpi) screen ruling that resulted in increasing chroma (C*) in the middle and shadow tones in the halftones. K E Y W O R D Schroma, ink viscosity, lightness, response surface design

In the recent years, the demands for offset inks with better flow ability and viscosity have risen higher with the improvements of printing techniques. To ensure uniformity in printability sheet after sheet it is very important to maintain certain print conditions for that print job as approved by customers and use this data for future reference of printing. The quality of offset printing process depends on many chemical and physical specifications of the ma terials and components involved in the process. Most important being printing inks and its rheology. In this work, three process color cyan inks have been formulated with varying levels of viscosity with use of certain rheology modifiers. Trials on the printing machine were conducted using a systematic layout of test elements on a fully automatic offset lithography printing machine using a Solid Bleached Sulphite Board (SBS) and the print results were correlated to rheological parameters such as viscosity and thixotropy. The tone value increase (TVI) was measured and was correlated to viscosity and index of thixotropy. Higher viscosity yields lower dot gain and better color reproducibility. A mathematical relation has been established between ink viscosity, dot area and tone value increase. As the demands for packaging increases, the study about the ink rheology and its effect on print performance can help printers and ink manufacturers with better ink formulations to achieve precise print results.