On the Analysis of the Printing Nip on Sheetfed Lithographic Presses and its Implications on the Runnability of Common Substrates

This dissertation is based on the thesis that: Adequate analysis of the offset printing nip, under proper solutions of the Navier-Stokes equations---NSE---for the ink flow and, every day common sense, must provide answers to relevant questions still pending response in the offset printing process. Among these questions are: the influence of wall pressure on the flow, the presence or lack thereof of cavitation, the influence of the Newtonian and non- Newtonian conditions of the ink and, the validity of Stefan's law in the flow as ink and substrate split from the blanket/ink interface.;As those questions are properly studied and, solutions are provided on the influence of the ink flow on common substrates (mainly focused on coated papers). Answers are presented on ink velocity profiles, influence of ink viscosity and ink layer height (under full ink coverage), and adequate release of the substrate at common SFO---Sheet Fed Offset---printing conditions.;In order to understand the flow, a model is proposed by using a thin channel, high aspect ratio, based on the very thin ink layer running at 2 microns (2 x 10--6 m) through a long contacting channel S, of 0.012 m, while the walls move at a set velocity of 3 m/s and known contacting pressure (peaking at the center at 1 MPa).;The problem is divided in two regions with similar geometry (high aspect ratio) but, with different boundary conditions. Inside the nip, the problem is a Couette-like flow. Once it leaves the nip, the problem turns into an adhesion matter. Due the high aspect ratio, NSE can be simplified to the Lubrication Approximation Equations---LAE.;The printing process is absolutely symmetric in the depth direction of the flow---considered here as y-direction in accordance with paper physics literature---, validating the simplification of the study from 3D to 2D. The physics and geometry of the flow indicate as well that: the fluid is incompressible, the interaction of the ink and substrate happen very rapidly and prevent any change in the ink viscosity, the temperature of the system is constant and heat transfer has no influence on the analysis. In addition, curvature is negligible and finally, wall pressure is known (through work developed here using Finite Element Methods). Since wall pressure is known, its derivative is also known and NSE can be solved swiftly by successive integration under the printing boundary conditions. Each of these assumptions is discussed in this dissertation and proven to be valid for the analysis of the flow in the offset printing nip.;As a result of the analysis, it is found that solutions for the NSE, under Newtonian and non-Newtonian (Power Law viscosity model), provide a very similar profile for the fluid's velocity field. This is also true for the region of adhesion at nip's exit. The analysis presented here supports the simplification of treating offset ink as a Newtonian fluid since the results for ink flow velocity are rather similar.;The influence of porosity at the bottom wall, treated as a boundary conditions for the solution of the flow, proves to be very minimal. Similarity simplifications such as Berman-Blassius are not instrumental in the analysis of the printing nip, since the result is completely impervious to porosity. The solution through LAE provide adequate and reasonable solutions to the ink flow. LAE serve at nip's exit, under adhesion solutions in cylindrical coordinates (offered by other researchers) and, in rectangular coordinates developed in this dissertation under the Reynolds adhesion equation to find answers for ink internal pressure and extension.;The action of the ink extension serves as a means to predict adequate sheet release from the blanket. It is used as well to determine the ability of the ink flow to release without surface damage (eventual surface picking or delamination). Ink extension provides adequate understanding of the definition for Runnablity in z-direction as proposed here.