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Original scientific paper Madhura P. Mahajan, Swati Bandyopadhyay Correlation of ink viscosity and printability in offset lithography process on paperboard used in packaging Correlation of ink viscosity and printability in offset lithography process on paperboard used in packaging Madhura P. Mahajan1, Swati Bandyopadhyay2 1 Department of Printing Engineering, Pune Vidyarthi Griha’s College of Engineering and Technology Parvati, Pune Maharashtra, India 2 Department of Printing Engineering, Jadavpur University, Kolkata, India Corresponding author: Madhura P. Mahajan, [email protected] Abstract 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 specifi- cations 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 formula- tions to achieve precise print results. Keywords: Ink viscosity, Tone Value Increase (TVI), Shear Stress, Dot area 1. Introduction tone value increase (TVI) and color are some Offset lithography print process is the most of the important response factors for quality common and most widely used process for control in printing. Dot area has the size of ink printing on various paper and paperboards area per unit printing area with unit of percent and is widely used to print on higher gram- (%). There are two reasons for increase in dot mage paperboard used for cartons or box size of the halftone: impact of the mechanical making. A continuous tone photograph or im- action during the ink transfer between print- age is reproduced in a print process with help ing cylinders and other is due to the scattering of halftone dots of varying sizes to represent of light beneath the dot that results into high- the variations in visual impression of lightness er color intensity coming from them [2]. Phys- or darkness. Printing by offset lithography is ical dot gain is a characteristic of its process governed by number of factors. A change in or due to raw material in the ink that causes any of the factors results into a change in the change in the flow properties. Understand- print parameters [1]. The layout, design and ing and calculating physical extension of dot color reproductions for a carton or packaging from its nominal value can be useful to cali- box form crucial basis for acceptance or re- brate printing presses for halftoning 3[ ]. This jection of the cartons. Solid ink density (SID), work covers the mechanical part of tone value Acta Graphica Vol 29, No 3 (2018) 37 Original scientific paper Madhura P. Mahajan, Swati Bandyopadhyay Correlation of ink viscosity and printability in offset lithography process on paperboard used in packaging increase due to one of the important raw ma- infinite shear rate were calculated by the Cas- terial in the print process which is ink. Print- son equation in their work [13]. In the offset ing is the application of a thin film of a dye or print process the splitting of ink film occurs pigment dispersion in liquid with desired vis- with the help of rollers rotating at same speed cosity to paper or polymer film [4]. As the con- which is governed by phenomenon of shear centration of ink increases, the flocculated na- thinning and viscoelasticity [11]. Ink splitting ture also increases that results into high shear on the rollers is an interesting phenomenon. thinning which occurs due to interparticle at- The passage of ink takes place between every traction [5]. Inks with higher concentration of two rollers in contact. As the rollers rotate the pigments are short inks having shorter ink fila- ink film is forced from the entry side of the nip ment size and possess higher yield value. With contact of the rollers due to high pressure. But good rheology, it helps in better transfer down due to pressure lower than atmospheric pres- rollers [6]. The rheological properties includ- sure at the exit side of the nip, cavitation oc- ing viscosity, thixotropy and viscoelasticity are curs and splits the film of ink into small fila- contributing factors in determining the quali- ments [14]. Previous work deals with study of ty. At lower shear rates, viscosity decreases and viscoelastic behavior due to varying levels of reaches a plateau which is termed as zero shear binder polymers, pigments and various rheol- viscosity (η0). Due to breakage of flocs at high ogy modifying additives added in inks. Stress shear, orientation of molecules and non-spher- sweep was conducted in the dynamic test and ical shape of particles in ink dispersions, vis- higher critical stress indicated a stronger struc- cosity approaches another plateau named in- ture of the inks [15]. The rate of breakdown finite shear viscosity (η∞). The ink transfer on and rate of build-up during the up and down- the printing machines is controlled by infinite curves respectively play a significant role es- shear viscosity, as the particle flocs present in pecially in printing inks where it is important the inks segregate at high shear rate. The hys- that the inks should have a slow rate of break- teresis loop is another important parameter to down on the rollers and faster build-up on pa- evaluate thixotropic nature of materials. Dot per after printing [16]. As the viscosity of ink spread resulting into TVI occurs if thixotro- increases, in our case due to the addition of py is too weak and it is difficult to transfer ink additives like fumed silica, the cohesive force if thixotropy is too strong [7]. Many printing increases and so does the thixotropy of the ink processes including screen printing especially [17]. Yield stress is the minimum stress re- using silver paste also recommend higher vis- quired for the ink to flow. Below this stress the cosity during printing which resulted into bet- material is purely elastic [18]. It is an import- ter print resolution [8]. Color changes in print ant characteristic which explains the structure result can be attributable to changes in ink vis- of the ink. Higher viscosity has higher yield cosity rather than process parameters [9]. Pre- stress, thus giving a stronger network struc- vious research shows a number of rheologi- ture to the ink [19]. Oscillating strain or stress cal models being fitted to the flow curves of is applied and corresponding stress or strain is inks. Bingham model used to fit flow curves measured. Dynamic strain sweeps are usually of emulsified inks and un-emulsified inks [10. carried out from 0.1 to 100%. The strain sweep The experiments of shear stress and shear rate helps to check the strain % at which the mate- was also studied using Bingham Exponen- rial behaves linearly. Oscillatory tests are done tial Decrease model in which they studied the over a range of frequency [20] [21]. Printing rheological properties of highly pigmented inks are subjected to high shear rates during inks at different temperatures [11]. The effect the printing operation. These shear forces de- of decreasing shear rate and increasing shear velop high shear stresses. Hence it is neces- rates on viscosity and parameters such as zero sary to characterize the inks for their behavior shear viscosity and infinite shear viscosity was on the printing presses [22]. Clay thickeners studied using the Carreau model [12]. Casson and fumed silica are added to inks to increase model was used to evaluate the flow behav- yield and viscosity of inks [24]. Use of clay in ior of emulsified and unemulsified offset inks inks gives smoothness and gloss due to their by rotational tests. The yield point, viscosity at good rheology [29]. Offset inks are dispersions 38 Acta Graphica Vol 29, No 3 (2018) Original scientific paper Madhura P. Mahajan, Swati Bandyopadhyay Correlation of ink viscosity and printability in offset lithography process on paperboard used in packaging having high viscosity. The solid part of inks 64741-91-9 and Rosin Modified Phenolic Res- comprises of pigments which constitutes 10- in has been used in the inks. The composition 20% by weight, resins 20-40% by weight, dis- of flow improver used in different percent is persed in an oil-based medium which usu- castor oil ester. The base ink composition as in ally constitutes 30-50% by weight. Additives Fig. 1 has been modified to obtain variation in such as waxes, plasticizers are added to 10% by the viscosities of the 3 inks and they were fur- weight [32]. ther analysed for their flow behavior and print A lot of work has been done on the chemical results. and physical behavior of inks. But very less has Table 1 Base inks modified to achieve different flow been done on the effect of rheology and flow of properties ink on the print behavior in terms of dot gain Raw Material Ink Ink Ink and color etc.

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