2.1 Offset print standards

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1 Printing is one of the growing fields of technology and it has travelled a long way over the years towards transforming itself from art to science and finally technology. Except digital printing technology, all other printing systems are well established and being accepted widely for various purposes. In the recent times, various digital printing systems are gaining popularity over the conventional printing systems due to personalization, print-on-demand and cost, very minimal material waste and environmental aspects. As the recent trend is towards greener printing so the digital printing systems addresses to this point more effectively. Being a matured technology, sheet fed offset printing technology has devised various standards over the years to take care of raw material to the final printing and other related activities associated to it, but at the same time there is a different scenario for the digital printing systems. Most of the digital printers till to date are following the print related quality factors close to the sheet fed offset printing system. As the growth and development of this sector is continually increasing, it cannot be ignored completely when it comes to quality printing, as because standardization essentially helps to reduce the cost of production and at the same time ensures consistency into the print production system. As the hybrid presses are becoming popular day by day in the printing industry due to various reasons, development of a common substrate is the need of the hour. Hence the print quality factors which are applicable to sheet fed offset press and digital press needs to studied and analysed carefully, so that these points can be incorporated into the papermaking system for producing a common printing substrate which will be used for both the conventional sheet fed offset and digital presses very successfully for taking care of runnability and printability related issues for these presses. The print quality factors which are very common to both the sheet fed offset and digital printing systems which are discussed in detail in the subsequent pages; 2.1 Offset print standards In a paper written by Ian C. White entitled The Print Quality Index, A Management Tool, three categories of defects is defined: minor, major and critical (White 1975). Minor defects are slight imperfections which, if noticed, would not be the source of any complaint. A minor defect does not fall outside of any specified numerical tolerances. Major defects are defects which seriously affect the overall visual appearance of the product. Examples of these defects are hickies, streaks, and mottle. Major defects fall outside of specified tolerances. Critical Fed Offset Printing and Digital Printing 28

2 defects are serious deviations from specifications which jeopardize the integrity of the product (e.g., the cover of a book tears away from the spine when it s laid flat). When the number of print demerits exceeds a specified tolerance level, particularly with critical defects, there is sufficient cause to reject the job. In the offset printing system it is very easy to deal with conformance of standard, because over the years numbers of standards have been developed to take care of such issues and moreover offset printing is a matured technology. Materials used in the offset process also undergo various check process to confer with the offset standards prior to the use in the production process. These standards only fail in their ability to address the visual significance of any print defects. ISO 12647:2 clearly defines various standard procedures and characteristic of the sheet fed offset printing and is being used and adopted by the sheet fed offset process worldwide. 2.2 Digital print standards Digital printing, when compared to offset printing, is a hands-off process. Digital printing is comparatively cheaper than offset for short run printing jobs. An offset press operator who has tested the inks and substrates can make modifications (e.g. by adding surfactants, defoamers, primers, etc., or adjusting plate pressure, press speed, ink coverage, etc.), either prior to a pressrun or on the fly, to make up for any problems that might occur on the press. On the flip side, most production digital printing processes depend on the use of certified paper to perform to their best capacity. Any adjustments that need to be made to the actual press require the intervention of technical support personnel other than the press operator. While digital printing materials are not yet standardized, some work has been done exploring the impact that materials have on digital printing, with the intention of developing material standards. 2.3 Solid ink density (SID) Density is basically defined as the ability of a material to absorb light. It plays a major role in any printing process, as because the density of the ink on the printed sheet should be within the specified density range for producing quality print products. For the success of printing of halftones and process colours, it is highly important to measure, monitor and control the amount of ink to be deposited on to the printing substrate during the printing cycle. In a more technical aspect, this can be expressed as the ink film thickness on the printed substrate. Fed Offset Printing and Digital Printing 29

3 Effective control of thickness of printed ink onto the printing substrate is highly important for all the available printing processes. By maintaining proper solid ink density (SID) during the printing cycle it is very much possible to control related print quality factors like; the hue, dot gain, ink trapping, ink consumption in the press and a number of other related factors which are highly important to maintain consistent print quality throughout the print run. It is a wellknown fact in the printing industry that, high density of the printing ink is basically resulted by high thickness of printed ink layer during the printing cycle. The amount of ink to be deposited on to the substrate is highly dependent upon the target densities of each process colour within a given printed area. High roughness and porosity substrates requires high ink feed and as the ink drying mechanism in this type of substrates is mainly by penetration and absorption, there is the possibility of higher dot gain in this type of papers also. It is generally observed that, for a given volume of printing ink, the higher the roughness and porosity of the printing substrate the lower will be the final print ink density on the printing substrate. Ink density is basically measured by a suitable measuring instrument (i.e. densitometer) on the solid ink patches printed (preferably outside the print area) on the substrate. There is no unit of solid ink density (SID); and it is denoted by a number. They are two concepts related to solid ink density; absolute density and relative density. Absolute density denotes the measured density of the ink sample including the substrate to be printed or the base. Relative density is basically calculated by the density of the ink sample minus the density of the substrate or the base paper. It should be taken in to account that for ensuring a consistent print quality level throughout the print run, the variation in solid ink density should be in the range of ± Care must be taken to avoid variation of SID beyond this density range for ensuring consistent print quality throughout the whole print run. While measuring the solid ink density, the ink film thickness should be maintained at micro-meter on the substrate. Two print quality factors; dot gain and print contrast are highly dependent upon solid ink density. As per the general rule, dot gain and print contrast values are highly dependent upon the changes in solid ink density. For maintaining consistent print quality in a press, the solid ink density should be measured, monitored and controlled as per the standards at a regular interval of time. As per the general rule, the higher the density of a solid ink area the thicker the printed ink film layer and vice-versa. Care should be taken to control the thickness of ink film because excess ink feed may result into spaces between half-tone dots begin to fill. When the ink film Fed Offset Printing and Digital Printing 30

4 thickness is too high than the standard targeted value for the particular printing system, it may leads to ink tailing/misting and subsequently the non-image area/background of the image carrier may start taking up ink. When the ink film thickness is too thin, it may results in to breakdown of the ink on the sheet causing low contrast, loss of sharpness, and mottle. 2.4 Dot gain Dot gain is a general phenomenon in any printing system, and moreover it can be controlled effectively but cannot be avoided entirely. It should be kept in mind that dot gain is a measurable, predictable and controllable effect in any of the printing process for effecting quality printing output. When the dot grows in size during the pre-press/press operations (i.e. from the preparation of image carrier to the final printing) basically due to some of the unavoidable situations during the successive stages of print production, it is termed as the dot gain. Total dot gain can be defined as the difference between the dot size on the image carrier/plate and the corresponding printed dot size on the printed substrate. Surface characteristics of paper and ink absorption rates on the surface of the substrate are the basic factors which lead to accelerate the process of dot gain. While comparing the surface of the coated paper and uncoated paper, uncoated papers show higher dot gain, because of its surface roughness and porosity. In the conventional sheet fed offset presses, the squeeze pressure between the printing cylinders may also help the dot size to grow. There are two different dot gain phenomena which are commonly in use: physical dot gain/mechanical dot gain and optical dot gain. The first is related to the printing process, and it is commonly known as the physical dot gain and it is basically comes into picture when the printed halftone dots differ from their nominal size/original size dots. On the other hand, optical dot gain comes into picture when the light scatters inside the substrate which is to be printed thereby causing the light to exchange between the different chromatic areas. Physical dot gain is the difference between the film printing dot area and the physical area of the dot on the printed sheet. This change in dot size (increase or decrease) results from both exposure effects in plate making and physical deformation of the dot during the printing process (CGATS 1994). Optical dot gain is the difference between the apparent dot area and the physical area of the dots printed on the sheet. This apparent optical increase in dot area occurs because the dot is on the surface of the sheet but light is scattered throughout the Fed Offset Printing and Digital Printing 31

5 substrate. As a result, some of the light that enters the paper between dots is not totally reflected but is trapped under the dots (CGATS 1994). Some amount of dot distortion will always be present in the printed images because of the physical nature and elements of the current lithographic process. Understanding the types of dot gain, why and when they occur, and how they can be discovered, measured, and controlled can give the printer a winning hand in quality printing. Dot gain beyond the standard acceptable limit leads to loss of image detail and definition, variation of colours from print to print, print contrast, ink density, trapping of colours, and hue error of the process colours. Electronic printing processes have taken some of the guess work out of dot gain, but controlling dot gain is still essential if one is to improve quality in printing (Killeen 1995). Most of printing defects in the offset lithography are either the results of poor operating skills, the use of defective materials, and/or improper techniques or control. They are usually correctable. Dot gain, on other hand, is a characteristic defect of offset lithography and also a built-in/ integral part of the offset lithographic printing process (Bruno 1986).Optical dot gain is a visual phenomenon created because of the light-absorbing characteristics of ink and the light-scattering characteristics of the substrate. When light hits the non-image area, or "white space," it is scattered and some of the light is absorbed below the halftone dots. This light cannot be reflected back to the eye and is said to be "absorbed" (Killeen 1995). Dot gain variation has the greatest influence on colour variation, it is important to understand how to control and compensate for it (Southworth & Southworth 1989). If controlled, dot gain is not necessarily bad which is just as well, because dot gain is inherent to every printing process (Schlapfer 1988).There is one-to-one relationship between the dot spread and dot gain; the greater the spread of the dot, greater the dot gain. In any printing system, care must be taken to control the dot gain within the targeted limit/standard range, so as to produce consistent print quality throughout the whole print run. It is very difficult to print half tone dots at true its true value (in comparison of half tone dot on the printed sheet to that of the half tone dot in original supplied for printing). In practice, for maintaining effective gray balance between the printed colours, care must be given to maintain balanced dot gain between the three process colours (cyan, magenta, and yellow).figure 7 indicates the concept of dot gain. Figure 8 shows a typical densitometer which is used in the graphic arts industry for quality control purpose. Fed Offset Printing and Digital Printing 32

6 Fig.7. Concept of dot gain (Lawer 1997) Fig.8. Densitometer (Dot gain in process technology 2004) The densitometer, one of the most widely used instruments to measure dot areas, provides important information that helps to control and improve the printing process (GATF 1995). Generally, dot gain is measured from solid and tint values by densitometers. A densitometer can measure either incident light reflected from a substrate (reflection density) or light transmitted through a film (transmission density) or both (Killeen 1995). Most modern densitometers use the Murray-Davies and/or Yule-Nielson equations to calculate dot gain. A unique aspect of dot gain is that the gain is not the same across the scale of halftone values; the gain is generally greatest around the 50 % dots, tapering off at the highlight and shadow ends of the tone scale (Rinehart 1983). Fed Offset Printing and Digital Printing 33

7 Dot area (%) is defined as the % of an area covered by halftone dots, ranging from 0% to 100%. The lightest areas of an image are represented by the smallest coverage, at or near 0%, while the darkest image areas consist of dots near maximum coverage of 100% (CGATS 1994). Some of the common dot areas selected for the measurements of dot gain are; 30%, 40%, 50%, 70%, 75%, and 80%. It is very important to identify various factors that are responsible for the dot gain in various printing systems. In order to produce optimum print quality, the very first step is to identify the factors followed by understanding them in detail before reducing their effects in the printing system. The mechanisms of both the optical and physical/mechanical dot gain in relation to various print factors affecting them can be summarized under the following distinct heads: pre-press factors, press factors, ink, and paper. Prepress factors and dot gain; prepress factors that can contribute to dot gain range from such basics as the shape of the dot and the fineness/coarseness of the screen ruling through the method of making the halftones and negatives to plate exposure and processing. Dot shape, generally, dots could be round, square, elliptical, or other special shapes. Dot shape is an important factor in tonal reproduction. Shape distortion during ink transfer from plate to blanket and blanket to substrate causes poor colour and a shift in gray balance (Killeen 1995). Screen rulings are described in lines per inch (lpi) and refer to the number of halftone dots per linear inch in the halftone or colour separation. As a general guide, dot gain is less with a lower screen ruling (Killeen 1995). Press factors and dot gain; press factors such as press speed, fountain solution, blanket, and rollers all contribute to dot gain (Killeen 1995). It is a basic fact that when the printing press runs at high speed the sharper will be the printing, and the lower and more consistent will be the dot gain. Press speed is another variable that should be controlled and monitored for controlling dot gain in the press. When discussing on the characteristics of blankets, there are basically two types of offset blankets used in the printing industry: conventional (noncompressible) and compressible (Bruno 1986). Compressible blankets are more popular today than conventional ones because they feature an additional rubber layer that functions similar to a shock absorber, allowing a blanket to rebound after it has taken a hit. On conventional blankets the deformation, especially in the nip, can cause slippage of the blanket particularly on coated paper in dot areas from middle-tones to solids, resulting in directional distortion of the dot gain called slurring. Compressible blankets, on the other Fed Offset Printing and Digital Printing 34

8 hand, compress under impression and do not exhibit the accumulation of rubber in the nip even under moderately excessive impression pressure. Therefore, compressible blankets are more tolerant to pressure changes, do not distort image elements, and maintain more consistent dot gain and dot shapes during printing (Bruno 1986). In the sheet fed offset printing, proper setting between the three cylinders is highly crucial for the transfer of ink film from the plate to the blanket and finally from the blanket on to the substrate (GATF 1995). Pressure between the cylinders in the printing unit is commonly known as the squeeze. The squeeze between the plate and blanket cylinder and the blanket and impression cylinder decides the amount of dot gain to be observed on the printed substrate. High amount of squeeze between the cylinders will result into more ink spread which finally results in to high dot gain, if the squeeze is low than the required amount of squeeze between the printing cylinders then the inked image may not be transferred on to the substrate effectively. In order to ensure overall contact of the plate, blanket, and impression cylinders in sheet fed offset presses, a squeeze of at least inch (0.076 cm) must be used. This is equivalent to an impression band of about 3/16 inch (4.8 cm), which amounts to an impression pressure of about 50 pounds per lineal inch (8.9 Kg/cm) or about 200 pounds per square inch (0.36 Kg/cm^) (Bruno 1986). The fountain solution system on a sheet-fed offset lithographic press provides a water-based fountain (or dampening) solution to the printing plate before it is inked. Its major objective is to provide fast and complete separation of the image and nonimage areas of the plate; i.e., to prevent ink from becoming deposited in non-image areas (DeJidas & Destree 1990). High-quality offset printing depends on fountain solutions formulated to a precise level of acidity, and this level is bracketed by a narrow range on the ph scale. It is fundamental that the fountain solution and printing ink are in balance to achieve high quality prints at maximum production levels. The ph of fountain solutions is one of the major factors to consider in reaching a balance between ink and fountain solutions (Brothers 1988). In offset lithography, one of the important factors to be taken in to consideration is the amount of ink to be applied on to the printing substrate. Success of offset lithography is highly dependent on the proper ink film thickness on the printed substrate. As per the theoretical part thick ink film results in to more saturated colours. But excess feed of ink will help the white spaces between the halftone dots to fill and it is a serious problem for the light Fed Offset Printing and Digital Printing 35

9 colours i.e. it looks dirty (Measuring ink density 1986).The more ink supplied, the thicker the ink film, and therefore, the greater the dot gain (Takahashi, et al. 1983). Dot gain increases as the square of the ink film thickness (Johnson 1980). Ink should be carried at the thinnest film possible conductive to achieve good runnability, and water, in turn, should be kept to the minimum required to keep the plate clean (Scarlett 1989). Ink and dot gain; the effect of ink rheology on offset printing quality is a complex subject. One of the key issues is the effect that ink has on dot gain. Many elements of ink, such as pigment, viscosity, tack, stability, and strength, contribute to overall dot gain (Takahashi, et al. 1983). Strength of ink is an indication of the amount of pigment contained in an ink (Scarlett 1988). Due to the high cost of pigment in ink, it is easy to reduce cost by reducing the amount of pigment. Reducing the pigment reduces the strength of the ink, causing the press operator to run a thicker ink film, which can cause dot gain problems on press (Killeen 1995). However, an optimum level of strength should be determined by experimentation, which varies from case to case. Tack of ink is the stickiness of an ink (Southworth & Southworth 1989). The higher the ink tack, the lower is the dot gain (Scarlett 1989). If tack is too high, the ink will pick up the paper; if it is too low, the ink will not print a sharp dot. It is also indicated that the normal rule of thumb for sharp printing is to use the highest tack possible within the limitations imposed by the paper and press speed (Scarlett 1988). Ink viscosity is different from ink tack. Viscosity is a property of fluids resulting from molecular attraction which makes them offer a resistance to flow (Scarlett 1989). Ink viscosity is affected by ink temperature. If the press is cold, the viscosity of the ink will be higher and the dot gain lower; if the press is too warm, the viscosity will be lower and dot gain higher. In offset lithographic printing, a delicate ink-water balance is essential for printing quality. It is not completely true to state that the lithographic process is based on the fact that ink and water do not mix (Killeen 1995). In fact inks do absorb some water, normally at the % level, but this should level out when they reach their limit. The water pick-up of an ink will reduce the viscosity and tack of the ink, which, in turn, creates dot gain problems (Scarlett 1989). Paper and dot gain; generally, there are two major categories of paper types: uncoated, which includes newsprint, some magazine papers, bound, and most book paper; coated, which includes most magazine papers and high finish and glossy papers (Bruno 1986). Printers can, Fed Offset Printing and Digital Printing 36

10 however, rely on assistance from their suppliers in making selections of both coated and uncoated stocks. The printability of each of the paper types is determined by its surface properties such as acceptability of ink transfer and amount of absorption. Absorbency is the property that determines at what rate and in what amount the ink penetrates the paper. The more absorbent the paper, the higher will be the dot gain %. The rate of absorption plays a key role in the amount of dot gain on press. When ink is set on an absorbent paper, it will penetrate the paper and spread (Killeen 1995). It is obvious that uncontrolled dot size change will seriously distort the tone and colour reproduction. Better control of dot size change means better control of colour. Better control of colour during the printing cycle will certainly reduce the rework and scrap, and thus, lower the cost and also increase the productivity. Therefore, it is necessary to monitor the dot gain on press closely, establish a standard that will match the chosen proofing system, and then maintain that standard in the pressroom (Prince & Adams 1994). Uncoated papers have a rough surface that absorbs and lets the ink spread, and therefore, produce greater dot gain than coated paper (MacPhee & Lind 1991). The surface of uncoated papers is filled with much irregularity, intertwined cellulose fibres that create peaks and valleys (Adams, et al. 1996). The essential difference between the uncoated paper and the coated paper is the surface roughness/surface irregularity. Uncoated papers are generally having the more surface roughness and for creating the coated paper, the surface of the uncoated paper has to be coated with the help of suitable coating material like clay. Surface of the uncoated papers shows more deviations from an ideal plane than that of the coated papers. This is the reason behind achieving high quality half tone reproduction on coated papers than the uncoated papers. Coated papers are basically less absorbent and having smooth surface than the uncoated papers (Southworth & Southworth 1989). As a common practice, smoothness of a paper is usually visually examined before and after drying. Under microscopic view of the printed image it is observed that in the case of uncoated papers, the ink penetrates into the pores and the cavities of the paper. One of the basic problems with the coated paper is the large coverage areas of the printed dots which results into larger physical dot gain on the printed sheets. Coated papers also results into more homogenous dot shape and hence lower dot gain. Uncoated papers always show larger total dot gain because optical dot gain is higher in this type of paper. Some of the possible causes Fed Offset Printing and Digital Printing 37

11 behind optical dot gain can be summarized as; the lateral light scattering within the substrate, the size of the halftone dots, and the halftone dot shape (especially the dot perimeter) (Mahziar, et al. 2014). One of the major factors that affect dot gain is the solid ink density of more precisely the ink film thickness of the printed substrate. For maintaining a consistent dot gain throughout the complete print run, it is essential to monitor and control ink density during the press run. At a regular interval of time, the possible changed variables (in relation to the benchmarking process) in a typical press that affects dot gain is required to be checked and rectified without any delay. It is very difficult if too much unwanted delay is there before testing and it may leads to change of several variables in between. This will make the work more difficult for bringing the process in to control (US Ink). Coated papers are to be selected along with lower blanket to paper pressures, higher plate to blanket pressure, and higher fountain solution ph values for the situations which demands for print products with high amount of emphasis on highlight areas and high fidelity factors. Coated papers are also to be selected along with lower blanket to paper pressures, lower plate to blanket pressure, and lower fountain solution ph values for the situations which demands for print products with high amount of emphasis on mid tones and shadow areas and high fidelity factors (Yung-Cheng 1997). 2.5 Print gloss Gloss is an aspect of the visual perception of objects. Gloss is the attribute of surfaces that causes them to have shiny or lustrous, metallic or matt appearances. Gloss is a visual impression that is caused when a surface is evaluated by either visually or by suitable measuring instrument. The more direct light is reflected from the surface of the object the more will be the impression of gloss. The gloss of a printed image is highly dependent upon the gloss of the paper, with a matt coated paper giving lower print gloss and a gloss coated paper giving higher print gloss. However, as a secondary effect, the coating porosity will impact on the rate that the ink solvents are removed from the ink and that the ink sets, which will also influence the print gloss. A fast ink setting coating generally has a lower print gloss and this occurs because of the following two mechanisms: (a) ink filaments produced in a printing nip do not have time to level before being immobilized as the ink sets. This leaves a macro rough surface which Fed Offset Printing and Digital Printing 38

12 reduces the print gloss. (b) A very micro-porous and fast ink setting coating paper surface may pull more of the ink oils and some resins from the ink into the coating layer, leaving ink pigments and extenders sticking out from the surface of the paper. This will result in to an increase in micro roughness and a reduced print gloss effect. Print gloss is one of the important factors influencing print quality and determined by surface smoothness other than refractive indices of the materials formulated in paper and inks. In offset printing the viscous inks transfer to the surfaces of a paper sheet passing through the printing nips and solidify on the surfaces and in electrophotography the solid toner particles are fused and adhere to paper surfaces. In offset printing, when an ink split pattern stays on an inked surface of paper immediately past to the printing nip and if the ink vehicle absorbed quickly by the coating of the paper surface then the ink film loses fluidity and the split pattern is left even on the dried inked surface of the paper. This split pattern result in to a low print gloss. The effects of the formulated amount and chemical structure of latex in coatings, pigment properties, smoothness and porosity of coatings and printing conditions are the basic factors to be taken in to account for print gloss. Offset-like (where print gloss changes with paper gloss) electrophotography has been established by controlling toner movement (Kitano, et al. 2008). In offset printing, the formation of printed surface topography occurs through the following processes. The ink is transferred to the paper, and the topographical unevenness of the printed surface results from ink layer splits. This unevenness is smoothed out during the initial several tens of seconds. Furthermore, the ink flows along the paper surface, and the printed surface becomes smoother than the paper surface. For the next several hundreds of seconds, the ink vehicle penetrates the paper and causes the printed surface to gradually become increasingly similar to the paper surface. In other words, the roughness initially hidden by the ink layer becomes apparent as a result of the ink vehicle penetration. In electrophotography, the formation of printed surface topography takes place through the following processes. Before the toner is fused on the surface of the paper, low-frequency roughness, that is, roughness greater than the size of the toner particles distributed over the un-fused surface, is characteristically similar to that of the paper surface. The pressure and heat applied to the toner provide the toner layer with a shape different from that of the paper surface topography through processes such as coalescence and flow. The change in the shape Fed Offset Printing and Digital Printing 39

13 of the toner on the printing substrate is highly dependent on the applied heat energy and the viscosity of the fused toner during printing cycle (Kitano, et al. 2008). Gloss together with colour, brightness and opacity are critical optical properties of the papers. Gloss is an important optical parameter in evaluation of print quality. The psychophysiological sensation of gloss of a paper is a measure of the surface reflection of light in the specular direction and it essentially decides the gloss of the paper. In general, a paper surface reflecting light in all directions produces a sensation of a 'nonshiny' or matt surface having low gloss. The appearance of a surface depends on the way the incident light is reflected, absorbed or transmitted by the surface. Hunter (1937, 1952) tried to characterize gloss and led to the conclusion that at least six different visual criteria exist for ranking gloss; (a) specular gloss: perceived brightness associated with the specular reflection from a surface (b) contrast gloss: perceived relative brightness of specular and diffusely reflecting areas (c) distinctiveness-of-images gloss: perceived sharpness of images reflected in a surface (d) absence-of-bloom gloss (haze): perceived cloudiness in reflections near the specular direction (e) sheen: perceived shininess at grazing angles in otherwise matt surfaces (f) absence-of-texture gloss: perceived surface smoothness and uniformity. Paper gloss and print gloss are important optical qualities for the paper along with brightness. These attributes are the primary quality indicators for the end-use performance of papers. Gloss for rough surfaces like papers was described as a function of surface roughness, refractive index of constituent materials, and the wavelength of light (Chinmayanandam 1919). Since most of components in coating have similar refractive indexes, one can readily presume that gloss is a function of roughness (Lee 1974). As like gloss itself, print gloss could be determined by final surface roughness of the ink film on substrates given that ink or ink components has similar refractive indexes too. It is accepted that, if a paper surface is not smooth, some areas locally have orientations that differ from the average plane of the surface. These reflect light at angles differing from the average specular angle, lowering the gloss of the surface. Each constituent of the coating colour, as well as the roughness of the substrates, affects the final roughness of the coating. Researchers have usually preferred to measure gloss rather than roughness. Because it is well accepted that a high roughness results in a low gloss and vice versa, data available about gloss may be used to identify the parameters influencing the coating surface roughness. Porosity and surface roughness of base paper are of major importance for the quality of Fed Offset Printing and Digital Printing 40

14 coated paper. There are many of factors which influence roughness and porosity of coated paper: presence of fines, coarseness of fibres, ratio of softwood and hardwood, fibre size and size distribution, filler content effect, formation, pre-calendering, and so on. The composition of the coating colour and the absorption potential of the base paper are crucial. Gloss increases with increasing coating weight and then levels off. The rougher the raw stock, the higher the coat weight to apply to reach the maximum gloss. As the coat weigh increases, surface defects are filled up and the surface becomes smoother. For a certain critical coat weight, all surface defects are leveled and the roughness is then only defined by the coating components and gloss is only a function of the coating formulation (Kent, et al. 1986). Print gloss is a complex function of the ink composition and the ink film roughness. The print gloss was known to increase with ink film thickness. In fact, print gloss was found to decrease again at higher inking level since a split pattern was appearing in the ink film. Ink rheology affects strongly print gloss since it influences the ink film splitting in the nip and the ability of the ink to level before drying. Roughness is therefore a cause of reduced gloss. Most of the previous research work attributed to the macro scale roughness to poor fibre dispersion, ionic destabilization or flocculation, roughness of base paper, and insufficient coating or calendering (Fugett, et al. 1991). Pigment particle size distribution, particle shape, binder type (film shrinkage), drying temperature and time, pigment wetting, coating holdout (especially at low coating weights), and varying coating weights contribute to micro scale roughness (Lee 1974).Properties of the paper coating and the characteristics of the ink affect printed gloss. Greater paper gloss and greater ink holdout both increase printed gloss. Depending on choice of paper and ink, different authors have found differences in the relative importance of paper gloss and ink holdout (Borchers 1988).Current paper and printing researchers agree that excessive ink solvent absorption can strongly reduce printed gloss (Aspler & Lepoutre 1991 and Zang & Aspler 1994). 2.6 Print mottle Print mottle is without doubt one of the most important factors regarding visual impression of print quality in any printing system. It is usually the result of uneven ink layer or non-uniform ink absorption across the paper surface and it is more prominently visible in middle tone Fed Offset Printing and Digital Printing 41

15 images or areas of uniform colour such as solids and continuous tone screen images. Mottle may be the result of differential ink gloss, density, or colour of the printed ink film. It is often described in terms of its more specific root cause by mottle type. Some of the very common printing mottles are; Printer s mottle, ink trap mottle, back-trap mottle, water interference mottle and paper surface mottle. Two offset prints showing different amount of mottle: figure 9 shows low amount of mottle and figure 10 shows high amount of mottle on printed sheets. Fig.9. Low amount of mottle (STFI-Mottling 2012) Fig.10. High amount of mottle (STFI-Mottling 2012) During the wet on wet printing, ink from the surface of the printed substrate gets deposited on the subsequent printing unit blankets and if the substrate fails to absorb ink evenly on its complete surface area, then the subsequent blankets take ink unevenly giving cloudy appearance to the final print on the printed substrate which is commonly known as back trap mottling. Some of the possible causes of such type of mottle are; paper uniformity and setting Fed Offset Printing and Digital Printing 42

16 characteristics, ink set rates, poor ink trap, and blanket surface characteristics. Figure 11 shows back trap mottle. Figure 12 shows paper which is unsuitable for printing and figure 13 shows paper which is suitable for printing. Fig.11. Back trap mottling (Mottling 2004) Fig.12. Unsuitable press paper (Mottling 2004) If the water in the fountain solution in the conventional offset printing system at some of the places stays on top of paper surface and eventually refuses the ink layer to be deposited on it, then it is termed as water interference mottle. Ink and water imbalance may also affect the uniformity of ink layer transfer on to the surface of the paper thereby resulting in a non-sharp, hollow, or weak dot structure in subsequent printing units. Some of the possible causes of this type of mottle include; inconsistency in the metering of the fountain solution or irregularity in the fountain solution mix (such as high conductivity or ph, etc.), that may demand running heavy layer of fountain solution to the plate surface so as to keep the non-image areas clean. Fed Offset Printing and Digital Printing 43

17 This excessive fountain solution can over emulsify the ink thereby leading to water interference mottle. Figure 14 shows a print with water interference mottle. Fig.13. Suitable press paper (Mottling 2004) Fig.14. Water interference mottle (Mottling 2004) In paper surface mottling, the surface of the paper to be printed does not allow homogeneous transfer of ink layer on to its surface. Some of the possible causes related to paper surface mottle include paper manufacturing process variables such as uneven coat weight, binder migration, wire/felt patterns, poor base stock formation, non-uniform surface absorption of the paper, and other few factors. Figure 15 shows a print with paper surface mottling. Printer s mottle is the result of a misconfigured printing press that transfers an inconsistent or uneven layer of ink film on to the paper surface. It is generally caused by the printing press including insufficient printing pressure, contamination of the blanket, wrong printing Fed Offset Printing and Digital Printing 44

18 sequence, and excess ink quantities applied on to the paper surface which leads to a muddy and mottled print image, cloudy structures may be generated due to faults in film and printing plate production. Figure 16 shows a print with printer s mottle. Fig.15. Paper surface mottling (Mottling 2004) Fig.16. Printer s mottle (Mottling 2004) Poor or inconsistent ink traps in various printing units which eventually transfers nonuniform ink layer on to the surface of the paper and/or previous ink films. This type of mottle is commonly known as the ink trap mottle. Trap requires one wet ink film to capture or trap subsequent ink films. Incorrect ink tack grading, wrong ink sequence, screens over solids, and paper absorbency are the basic causes behind the ink trap mottle. It is still common to use visual assessment method to quantify the amount of mottle in the printing industry. However, the drawback of using visual evaluation method for assessing mottle is high cost and time consumption factors involved in it. A minimum printing pressure Fed Offset Printing and Digital Printing 45

19 is required for a good print, a good ink transfer from cylinder to cylinder. Under ideal conditions, the pressure between plate and blanket must be 0.08 mm and the pressure between blanket and substrate must be at least 0.10 mm. The above pressure setting between the cylinders in the printing unit will result in to print having no sign of cloudiness or mottling. As a general rule rough/uneven surfaces of the printing substrate may need a higher printing pressure as mentioned above to take care of mottling effectively. 2.7 Print sharpness Sharpness is considered as one of the most important print quality factors because it helps to determine the amount of detail in a particular printing system. It is expressed by the boundaries between zones of different tones or colours. Print sharpness should be given immediate importance for the specific printing system in use, so as to take care of possible risk of lateral spread of the inks which are used for effecting printing. As per the basic rule, high print sharpness is related equally to the high print quality. Print sharpness is better judged by visually. It is determined by visual appearance of the sharpness of the edge of the particular image. As per the general practice, an ideal edge is formed when the image consists of a straight line of some density followed by an immediate changeover to paper white. Sharpness is one of the print quality factors which draw attention of the print users in a greater extent and prints with low level of sharpness not only degrades the final quality of print but also spoils or rejects the jobs. Printing sharpness is an indication of how well a printing process can reproduce and maintain detail. Due to mechanical differences in printing press/paper and differences in the flow properties of printing inks, sharpness will vary from one colour ink to another colour ink. Therefore it is important to determine sharpness for each colour. Sharpness is calculated by taking density measurements of a solid patch and a middletone tint patch of a colour and plugging them into the following formula: density of tint/density of solid = sharpness. The sharpness value is a relative number and there is no specific number to be used as a guideline. However, once satisfactory printing conditions are achieved, the sharpness number must be maintained throughout the press run. 2.8 Print consistency Consistency is a very hard question. The perfect state of the consistency may only exist in the theory. One of printing industry s biggest challenges is ensuring colour consistency Fed Offset Printing and Digital Printing 46

20 throughout the print run and also in the repeat order printing consignments. In the printing industry, the term print consistency is related to the press consumables, the press calibration and the colour conversion techniques. Practically, the ink pigment varies from batch to batch, the surface characteristics of paper may also vary and the printing condition may not be same all the time. Combining all the above issues, colour consistency comes in to picture. But despite of all of the above mentioned conditions, the basic purpose in a print shop is to get printed sheets as much closer to each other. Colour should be consistent across all printed materials; an experienced printer will be able to provide printed materials that are spot-on every time. Some of the points which help to maintain print consistency include; paper quality, paper base colour, standard press environment, etc. The way each person sees a colour can vary, depending on the structure of the individual s eye. The type, quality and amount of light in the room can greatly affect the shade and tone of colour as perceived by the human eye. Digital printing technology simplifies the printing process and it essentially eliminates the need to create plates/image carrier and hence it leaves less room for error. In other words, once the printer has printed as per the specification and requirements of the customer s requirements, then it becomes very easy to reproduce the same results again and again with same consistency. The digital printing process is highly different from the conventional offset printing in several ways. First of all, when a print product is run on an offset press, there is the possibility of the density of the ink to vary across one sheet and/or vary sheet-to-sheet, which leads to colour inconsistency or variation of print quality from one print sheet to the other. This essentially due to number of variables that affect print quality in sheet fed offset printing. At the same time, it is very difficult to modify digital inks because these inks are mostly proprietary in nature. It is not required to add additives to the ink and the ink is supplied to the press in sealed can. These inks are highly stable and are not subject to any possible changes due to humidity, evaporation, etc. The colour can only be changed if the art file values are changed to create a different colour. Digital printing provides consistency both within a run and between different orders and which is a bit complicated task in the traditional sheet fed offset press environment. Fed Offset Printing and Digital Printing 47

21 2.9 Runnability Runnability is generally understood to encompass the performance of papers in press operation such that the sheets will run smoothly through the print engine without jamming, and thereby minimizing possible stoppage of the press during the print production cycle. In a more simplified language, it is related to the ease with which a paper/substrate moves through a printing press (from feeding unit to the delivery unit through various printing units). Paper affects print productivity, quality and profitability as because paper constitutes 50-70% of the total printing costs. Two properties of paper are highly important from a printer s point of view i.e. runnability and printability. Within runnability, properties such as MD: CD, paper tension, paper travel, temperature resistance is highly important (Wilson 1998). A poor running sheet will cause problems or necessitate slow operation of the presses which adversely affects the economics of printing. It is also equally important from the runnability point of view that the pallet of paper should arrive in the press room in good mechanical condition well before the printing operation. Paper strength, uniformity and freedom from defects are the some of the important properties which affects the runnability of paper also. The offset process of printing demands that paper should withstand following the following factors; nip pressure of the blanket, the tack of the ink, stress and strain from the feed board position to the delivery board through the different printing units, the moisture by the fountain solution, the heat that may be applied during the drying of the printing ink. Some of the causes for poor paper runnability are; paper is generally hygroscopic in nature which means it expands when exposed to moisture, paper with very low moisture content tends to absorb moisture when exposed to the environment for extended period of time. This may results in to wavy edges that may lead to runnability problems. The trend towards short-run, variable data printing and personalization are some of the basic factor for the wide acceptability of digital printing technology. Electrophotography printing technique is basically targeted for the marketing applications which require robust paper runnability. Machine downtime is a serious issue in variable data printing system, where the loss of a single sheet can disrupt the whole print run. Runnability related issues are very common across all printing processes, but some of them are highly specific to digital printing. A leading cause of paper jams in digital printing is out- of-plane deformation (such as curl or cockle), a problem that is occurred at the higher toner levels and fuser temperatures used in full colour printing. Fed Offset Printing and Digital Printing 48

22 Runnability is also one of the major points to be taken in to account in the digital printing machines. Two of the more common causes include; high paper curl caused by poor storage of paper or by poor control of press room conditions. If the rate and degree of change in paper moisture content is large then the paper is likely to curl in one or both planes and may in extreme cases, develop a wavy edge. The best possible solution of this problem is to keep the paper in moisture proof wrappings until the same is required for printing. The paper should be conditioned in the press room temperature in its original packaging for at least 24 hours before printing (Guide for digital printing). Compared with many offset press requirements, paper properties for digital printing systems must be controlled critically in terms of paper stiffness, moisture level, edge quality and dimensional stability in order to meet the jam free requirements of complex high-speed paper paths of the digital press and uniform charging characteristics for toner transfer efficiency on to the printing substrate. The chemical composition, spatial distribution of components, and thickness uniformity of paper used in the digital presses are therefore more critical than the traditional printing papers. The design and production of high quality digital papers requires significant skill and expertise and care must be taken to incorporate all the related issue that may affect runnability of paper in a digital press. Different printing processes are designed to produce optimum result with certain amounts of moisture either in paper or in the press room. In digital printing the moisture content can affect seriously to the toner adhesion on to the printing substrate, paper jams in the feeding/printing cycle and finally the fuser roller temperature. In offset printing machines, it can seriously affect the interaction between the ink and the printing press, the paper and the press, and finally the ink and paper. Moisture in paper varies from 2-12% depending on various factors during the manufacturing of paper and the chemicals/additives added to the pulp. The moisture content of a print shop is measured as relative humidity and it affects the amount of moisture in paper, but once unpacked the paper can either absorb or dissipate moisture. Printing papers are manufactured to work best in a 45-55% relative humidity (RH) at 72 F environment for producing high quality printing and being stable in the press. A non-uniform moisture profile of the sheet, in turn, may create areas of stress concentration which are prone to tearing during printing. Utmost care must be given to have the paper surface with uniform moisture profile. Tear strength of paper is most often used as a predictor Fed Offset Printing and Digital Printing 49