An investigation of the applicability of Walker and Fetsko ink transfer equation on and the influence of ink viscosity on heat set ink used on the web offset process

The Walker and Fetsko ink transfer equation is the most used ink transfer equation to predict the ink and paper behavior on the ink transfer step in the printing process. All previous studies have shown that this equation is mostly applicable in the three major printing processes, letterpress, lithography and gravure. A study by Schaeffer, Fisch and Zettlemoyer reported extensive measurements for several oil-base ink and paper combination over a range of proof-press printing conditions. Yuri Bery did a series of studies for modifying the Walker and Fetsko equation in gravure inks for Weyerhaeuser Company. All the studies showed that although generally the Walker and Fetsko ink transfer equation can be applied to all three major printing processes, there are always some modifications needed for different rheological characteristics and printing conditions . The trends for the lithographic process is toward Web Offset printing. The components of Web Offset ink is quite different from conventional sheetfed lithographic ink in the pigment and vehicle used. This paper is to find out if the Walker and Fetsko ink transfer equation can be also applied to the Web Offset ink. By investigating the effect of viscosity - one of the most important characteristics in rheology of ink - on the transfer parameters, the ink transfer mechanism model can be examined to see whether it is the same for oil base ink as for the heat-set Web Offset ink. The result of this experiment showed there is a significant different absorption behavior between coated paper and uncoated paper. This difference is effecting the applicability of the Walker and Fetsko ink transfer equation in this particular type of ink and paper combination

The rheology and roll-to-roll processing of shear-thickening particle dispersions

Particle dispersions are ubiquitous in our daily lives ranging from food and pharmaceutical products to inks. There has been great interest in the recent years in formulation of functional inks to fabricate myriad flexible electronic devices through high-throughput roll-to-roll technologies. The formulations often contain several functional additives or rheological modifiers that can affect the microstructure, rheology and processing. Understanding the rheology of formulations is important for tuning the formulation for optimal processing. This thesis presents investigations on the rheology of particle dispersions and their impact on roll-to-roll technologies. Shear-thickening behavior is common in particle dispersions, particularly, concentrated particulate inks. We have investigated the elongational rheology of a shear-thickening particle dispersions. We find that, an optimal polymer concentration resulted in a strong strain-hardening behavior with a magnitude that is significantly larger than the shear-thickening magnitude. Understanding the non-linear viscoelastic behavior of materials is very important, as deformations in applications are typically large. The rheology of three shear thickening dispersions with different thickening mechanisms was studied. Strong differences in the viscoelastic non-linearities in the three shear-thickening dispersions were observed. We next study the impact of rheology of shear thickening particle dispersions in gravure printing and slot die coating processes. In gravure printing, an efficient transfer of ink from gravure cell to the substrate is desired to print patterns with good print quality for desired product performance. We have conducted a study on how a shear thickening rheology impacts the amount of ink transfer. Distinct ink transfer behaviors associated with an extensional thickening, and an extensional thinning of the test fluids were observed. In a slot-die coating process, obtaining highly thin uniform defect free films at high speeds is a challenge. We have investigated the role of shear-thickening rheology on the stability of coating and the process window. The onset of shear-thickening was found to reduce the maximum web speeds for stable coating through the evolution of a ribbing defect. The presented investigations on the rheology of particle dispersions and their influence on roll-to-roll processing behavior would enable tuning of ink formulations and optimizing the process parameters towards fabrication of flexible electronics

Scalability of carbon-nanotube-based thin film transistors for flexible electronic devices manufactured using an all roll-to-roll gravure printing system.

To demonstrate that roll-to-roll (R2R) gravure printing is a suitable advanced manufacturing method for flexible thin film transistor (TFT)-based electronic circuits, three different nanomaterial-based inks (silver nanoparticles, BaTiO3 nanoparticles and single-walled carbon nanotubes (SWNTs)) were selected and optimized to enable the realization of fully printed SWNT-based TFTs (SWNT-TFTs) on 150-m-long rolls of 0.25-m-wide poly(ethylene terephthalate) (PET). SWNT-TFTs with 5 different channel lengths, namely, 30, 80, 130, 180, and 230 μm, were fabricated using a printing speed of 8 m/min. These SWNT-TFTs were characterized, and the obtained electrical parameters were related to major mechanical factors such as web tension, registration accuracy, impression roll pressure and printing speed to determine whether these mechanical factors were the sources of the observed device-to-device variations. By utilizing the electrical parameters from the SWNT-TFTs, a Monte Carlo simulation for a 1-bit adder circuit, as a reference, was conducted to demonstrate that functional circuits with reasonable complexity can indeed be manufactured using R2R gravure printing. The simulation results suggest that circuits with complexity, similar to the full adder circuit, can be printed with a 76% circuit yield if threshold voltage (Vth) variations of less than 30% can be maintained

Impact of Electrostatic Assist on Halftone Mottle in Shrink Films

Gravure printing delivers intricate print quality and exhibit better feasibility for printing long run packaging jobs. PVC and PETG are widely used shrink films printed by gravure process. The variation in ink transfer from gravure cells on to the substrate results in print mottle. The variation is inevitable and requires close monitoring with tight control on process parameters to deliver good dot fidelity. The electrostatic assist in gravure improves the ink transfer efficiency but is greatly influenced by ESA parameters such as air gap (distance between charge bar and impression roller) and voltage. Moreover, it is imperative to study the combined effect of ESA and gravure process parameters such as line screen, viscosity and speed for the minimization of half-tone mottle in shrink films. A general full factorial design was performed for the above mentioned parameters to evaluate half-tone mottle. The significant levels of both the main and interactions were studied by ANOVA approach. The statistical analysis revealed the significance of all the process parameters with viscosity, line screen and voltage being the major contributors in minimization of half-tone mottle. The optimized setting showed reduction in halftone mottle by 33% and 32% for PVC and PET-G respectively. The developed regression model was tested that showed more than 95% predictability. Furthermore, the uniformity of dot was measured by image to non-image area (ratio) distribution. The result showed reduction in halftone mottle with uniform dot distribution

Characterization and Investigation of Large-Area, Ultra-Thin Gravure Printed Layers

Graphical gravure printing is a very reliable process to transfer smallest amounts of fluid droplets to a substrate. Nevertheless, enabling this printing technique to produce large-area, ultra-thin layers for applications such as organic light emitting diodes (OLEDs) is a challenging task. This application not only imposes strong requirements on the printing technology but also on large-area measurement methods. Characterizing the homogeneity of sub-100 nm thin layers across the total printing area is one of the two central topics of the present investigation. Utilizing optical interference from the thin film samples I developed and evaluated a method which successfully determined the thickness of thin, organic semiconductor layers with an accuracy better than 5 nm. Sample sizes of up to 150 × 150 mm² could be characterized within seconds using two hardware setups. I enabled a microscope and a modified flatbed scanner to acquire conventional RGB-images of the thin film samples. These images were then compared to a corresponding physical model using MATLAB resulting in a laterally resolved thickness map. The method is predestinated for being part of an inline process control. In the second part of the thesis, I deduced a physical understanding of gravure printing to produce ultra-thin, homogeneous layers from low viscous ink solutions which are based on small molecules dissolved in toluene. To this purpose, I processed on two consecutively mounted 150 × 150 mm² ITO-coated glass substrates with varying process parameters, resulting in a total number of 128 different gravure printed fields, each 30 × 30 mm² in size. Applying the large-area characterization method developed in the first part, I measured the thicknesses of all sub-100 nm printed layers (with a total area of ~1800 cm²). This thickness data was analyzed regarding several surface parameters, such as roughness, dominant lateral wavelength, skewness and kurtosis. These surface parameters were referred to the physical models of fluid and thin film dynamics with respect to the underlying process parameters. As a consequence, two distinct process windows for the gravure printing process to produce homogeneous, ultra-thin layers were identified. The process windows were defined by two types of ink transfer mechanisms, namely single cell transfer and film splitting transfer, as well as appropriate film leveling and drying times. The two process windows for producing homogeneous, ultra-thin layers using gravure printing have been reported in the literature and were demonstrated through the present experiments. By combining the two different topics, for the first time, these experimentally observed process windows were theoretically verified

Use of the Vandercook Proof Press to Predict Rotogravure Print Quality

The object of this thesis is to investigate a new idea in predicting printability for rotogravure printing. Present proof presses in the gravure industry are clumsy, expensive, and in most cases, impractical for regular use. In order to understand the relationship between ink, paper, and plate, it is necessary to consider many variables such as smoothness, ink receptivity, porosity, moisture content, formation, surface strength, and opacity. The laboratory work performed and presented in this paper shows that a Vandercook proof press may be used to predict printability in the gravure process

Physical mechanisms governing pattern fidelity in microscale offset printing

We have studied the offset printing of liquid polymers curable by exposure to ultraviolet light onto flat and unpatterned silicon and glass substrates. The interplay of capillary, viscous, and adhesion forces dominates the dynamics of ink transfer at small feature sizes and low capillary number. For smooth and nonporous substrates, pattern fidelity can be compromised because the ink contact lines are free to migrate across the substrate during plate separation. Using a combination of experiments and equilibrium simulations, we have identified the physical mechanisms controlling ink transfer and pattern fidelity. In considering the resolution limit of this technique, it appears that the dynamics of ink flow and redistribution during transfer do not explicitly depend on the absolute feature size, but only on the aspect ratio of film thickness to feature size. Direct printing holds promise as a high-throughput fabrication method for large area electronics

Investigation of Roll-to-Roll Gravure Printing for Printed Electronics with Fine Features

Gravure printing is known to be cost competitive in manufacturing of printed electronic devices due to its capability to mass produce at lower costs. Current standard of gravure printed feature sizes is in a range of around 50 μm down to sub-10 μm, predominantly through small scale setups and specialized engraving. However, reliance on gravure cell design limits the scalability of printing over a large area due to the setup cost. In this study, ink viscoelastic behavior was modified to improve replication of gravure printed features over a large printing area of 300 mm web-width without a reduction in gravure cell dimension. Fine lines were printed using a high viscosity ink with a good replication of the nominal line width. Control over the printed features was performed through the variation of printing speed and the alteration of ink viscosity. The effects of ink viscosity and printing speed on the printed ink particle distribution and size were also examined. New methodologies of characterizing ink transfer were also developed to help understand the ink transfer processes: mass transfer and particle transfer. A deeper understanding of the thixotropic effect and shear recovery behavior of inks was achieved through simulations of shearing conditions

Direct printing of polymer microstructures on flat and spherical surfaces using a letterpress technique

We have developed a letterpress technique capable of printing polymer films with micrometer scale feature sizes onto flat or spherically shaped nonporous substrates. This printing technique deposits polymer only in desired regions thereby eliminating subsequent developing and subtraction steps. Flat or curved printing plates, which are fabricated from either rigid or deformable materials, are used to transfer thin molten polymer films onto flat target substrates. By deforming the printing plates into a spherical shape, it is also possible to print patterned films onto the concave side of a spherically deformed target substrate. These printed films serve as good resists for both wet chemical etching and reactive ion etching. Interferometric measurements of the polymer film thickness are used to probe physical mechanisms affecting printing instabilities, pattern fidelity, and edge resolution. Our experimental study indicates that this letterpress technique may prove suitable for high-throughput device fabrication involving large-area microelectronics