INKJET PRINTING: FACING CHALLENGES AND ITS NEW APPLICATIONS IN COATING INDUSTRY

This study is devoted to some of the most important issues for advancing inkjet printing for possible application in the coating industry with a focus on piezoelectric droplet on demand (DOD) inkjet technology. Current problems, as embodied in liquid filament breakup along with satellite droplet formation and reduction in droplet sizes, are discussed and then potential solutions identified. For satellite droplets, it is shown that liquid filament break-up behavior can be predicted by using a combination of two pi-numbers, including the Weber number, We and the Ohnesorge number, Oh, or the Reynolds number, Re, and the Weber number, We. All of these are dependent only on the ejected liquid properties and the velocity waveform at the print-head inlet. These new criteria are shown to have merit in comparison to currently used criteria for identifying filament physical features such as length and diameter that control the formation of subsequent droplets. In addition, this study performs scaling analyses for the design and operation of inkjet printing heads. Because droplet sizes from inkjet nozzles are typically on the order of nozzle dimensions, a numerical simulation is carried out to provide insight into how to reduce droplet sizes by employing a novel input waveform impressed on the print-head liquid inflow without changing the nozzle geometry. A regime map for characterizing the generation of small droplets based on We and a non-dimensional frequency, Ω is proposed and discussed. In an attempt to advance inkjet printing technology for coating purposes, a prototype was designed and then tested numerically. The numerical simulation successfully proved that the proposed prototype could be useful for coating purposes by repeatedly producing mono-dispersed droplets with controllable size and spacing. Finally, the influences of two independent piezoelectric characteristics - the maximum head displacement and corresponding frequency, was investigated to examine the quality of filament breakup quality and favorable piezoelectric displacements and frequencies were identified

NUMERICAL AND SCALING STUDY ON APPLICATION OF INKJET TECHNOLOGY TO AUTOMOTIVE COATING

A thorough literature review identified lack of precision control over quality of droplets generated by the currently available industrial sprayers and a growing need for higher quality droplets in the coating industry. Particularly, lack of knowledge and understanding in continuous inkjets (CIJ) and drop-on-demand (DOD) technologies is identified as significant. Motivated by these needs, this dissertation is dedicated to computational fluid dynamics (CFD) and scaling studies to improve existing inkjet technologies and develop new designs of efficient coating with single and/or multiple piezoelectric sensors to produce on-demand droplets. This dissertation study aims at developing a new DOD type coating technology, but it required understanding the effects of paint viscosity on droplet generation mechanism, an effective droplet delivery method to the coating surface, painted surface quality and control system of the DOD among others. Waterborne (WB) paints are chosen as the working liquid to identify three different DOD designs capable of creating a stream of mono-dispersed droplets. Volume-of-fluids (VOF) multiphase model explored the droplet creation process and effects of various parameters on the droplets’ quality. The law approach scaling analysis identified scaling laws to scale up these numerical results conduced for the laboratory-scale DOD to the large industrial scale inkjet nozzles

MULTIPHYSICS ANALYSIS AND OPTIMIZATION OF 3 DIMENSIONAL PRINTING TECHNOLOGY USING NANO FLUIDIC SUSPENSIONS

Fabrication of micro and nano devices is of prime significance to the area of Micro-Electro-Mechanical Systems (MEMS). Attempts have been made to accommodate high performance devices in compact units, thus reducing their overall size. There exist a variety of microfabrication techniques including lithography, chemical vapor deposition, and LIGA that are used today. Manufacturing costs associated with these processes can be prohibitive due to cycle time and the precious material loss that occurs during etching operations. These drawbacks become more significant problem when building curved traces and support structures that most occur in 3D space. To address the problems associated with building 3-dimensional circuits and devices in free space, a unique manufacturing process has been developed. This process utilizes conductive Nano-Particulate Fluid Jets (NPFJ) that are deposited onto a substrate by a Continuous Inkjet (CIJ) printing methodology. In this method, a fluid jet consists of colloidal suspensions of conductors and carrier fluids that are deposited onto a substrate and later sintered at high temperatures to form a homogeneous material. The major contribution of the present research is the investigation, development and optimization of the NPFJ. In this work, a Computational Fluid Dynamics (CFD) model has been developed to simulate the fluid jet and CIJ process. The modified CIJ printing process involves interaction of three domains namely, electrostatics, structural and fluidics. A coupled field analysis of the piezoelectric membrane that exists in the CIJ print head is conducted to establish the perturbation characteristics applied to the fluid. Interaction of the above three domains is captured within a single model using a (FSI) fluid-structural algorithm which staggers between domains until convergence is attained. A Design of Experiments approach was used to determine trends for the drop formations based on various exciting parameters. Results from these simulations have been validated using an ultra-high-speed camera featuring exposure/delay times from 100 nanoseconds at full sensor resolution.The results of present research will give manufacturers the freedom to construct 3D devices and circuits that conform to the desired shapes and sizes of products, rather than being limited to present 2D components such as printed circuit boards

NUMERICAL ANALYSIS OF DROPLET FORMATION AND TRANSPORT OF A HIGHLY VISCOUS LIQUID

Drop-on-demand (DOD) inkjet print-head has a major share of the market due to simplicity and feasibility of miniature system. The efficiency of droplet generation from DOD print-head is a result of several factors, include viscosity, surface tension, nozzle size, density, driving waveform (wave shape, frequency, and amplitude), etc. Key roles in the formation and behavior of liquid jets and drops combine three dimensionless groups: Reynolds number, Weber number and Ohnesorge number. These dimensionless groups provide some bounds to the “printability” of the liquid. Adequate understanding of these parameters is essential to improve the quality of droplets and provide guidelines for the process optimization. This thesis research describes the application of computational fluid dynamics (CFD) to simulate the creation and evolution process of droplet generation and transport of a highly viscous Newtonian fluid. The flow field is governed by unsteady Navier-Stokes equations. Volume of Fluid (VOF) model is used to solve this multi-phase (liquid-gas) problem

Superconducting HfO2-YBa2Cu3O7-δ nanocomposite films deposited using ink-jet printing of colloidal solutions

To reduce the fabrication costs while maximizing the superconducting and pinning properties of YBa2Cu3O7−δ (YBCO) nanocomposite films, the drop-on-demand ink-jet printing technique was used to deposit colloidal YBCO inks onto LaAlO3 substrates. These inks containing preformed HfO2 nanocrystals were carefully adjusted, prior to the jettability, as the droplet formation depends on the rheological properties of the inks themselves. After carefully adjusting printing parameters, 450-nm thick pristine YBCO films with a self-field critical current density (Jc) of 2.7 MA cm−² at 77 K and 500-nm thick HfO2-YBCO nanocomposite films with a self-field Jc of 3.1 MA·cm−² at 77 K were achieved. The final HfO2-YBCO nanocomposite films contained dispersed BaHfO3 particles in a YBCO matrix due to the Ba2+ reactivity with the HfO2 nanocrystals. These nanocomposite films presented a more gradual decrease of Jc with the increased magnetic field. These nanocomposite films also showed higher pinning force densities than the pristine films. This pinning enhancement was related to the favorable size and distribution of the BaHfO3 particles in the YBCO matrix

Adjoint-based optimization for inkjet printing

In this thesis the flow inside inkjet printhead microchannels is analysed using a two- parameter low Mach number expansion of the compressible Navier–Stokes equations and a reduced order model for the free surface flow inside the inkjet nozzle. The channel flow is separated into equations for an incompressible flow with no acoustic oscillations and equations for thermoviscous acoustic oscillations with no mean flow. This thesis concerns two types of optimal control problems. The optimal control problem of the first type is finding a velocity profile of the piezo-electric actuator that eliminates residual oscillations after a droplet is ejected. The cost function is the sum of the acoustic energy in the channel and the surface energy of the spherical cap of ink at the end of the nozzle at a given time. This problem is approached by obtaining the sensitivity of the total energy inside an inkjet microchannel with respect to boundary forcing using the adjoint method. Using gradient-based optimization algorithms, optimal waveforms are found that minimize the objective value at various final times and for geometries with increasing complexity. Physical interpretation to the optimal waveforms profiles is provided, and the exploited mechanisms are revealed. The optimal control problem of the second type is finding a shape of the inkjet printhead channel that maximises dissipation of the acoustic oscillations, without increasing the pressure drop required to drive the steady flow. Similarly, the adjoint approach is used to obtain the sensitivity of the acoustic flow eigenvalues with respect to boundary deformations in Hadamard form. Knowing the shape sensitivity of the incompressible flow viscous dissipation, the constrained optimization problem is solved to find a design that has the same viscous dissipation for the steady flow but a 40% larger decay rate for the oscillating flow. The final shape is not straightforward and would have been difficult to achieve through physical insight or trial and error. It could be improved further by adapting the parameters that describe the shape, but in this case the improvement would be small. The method is general and could be applied to many different applications in microfluidics. In summary, the methods in this thesis are promising techniques in the design and optimization of inkjet printheads. The discussed numerical techniques and the gained physical understanding can be used to automatically find the optimal design parameters, or, at a minimum, accelerate the experimental trial and error processes.This project has received funding from the European Unions Horizon 2020 research and innovation programme under Grant Agreement No. H2020-MSCA-ITN-2015

3-D jetting for enhanced functionality of thermoset elastomeric materials

The aim of this work was to assess the feasibility of 3-D inkjet printing of elastomers in latex form to create a novel material that would offer shielding against electromagnetic interference (EMI). To achieve this aim it was necessary to characterise and select suitable materials, carry out ink jetting trials, modify the materials accordingly to improve the printability and assess post jetting conditions including drying and curing behaviour. Particle size, surface tension, and viscosity measurements were made for a series of elastomer latex materials and carboxylated styrene butadiene rubber (XSBR) latex was identified as the most suitable. Latex ink optimisation included dilution with water and the addition of a humectant, triethylene glycol monomethyl ether (TGME), which delayed drying and reduced nozzle blocking. The surface energy was measured for arrange of potential substrates and PET was identified as the most suitable, due to its relatively high surface energy which allowed for an ideal level of wetting and spreading. Analysis of the cross-sectional profiles of the printed samples by white light interferometry showed that drying during printing was an important issue for the latex ink. Ink jetting of a composite material with control of filler distribution was shown to be feasible when ten layers of conductive carbon black ink were deposited alternately between ten layers of XSBR ink. Printing was successfully carried out with a latex combined with a resorcinol resin which was subsequently cured, indicating that it should be possible to 3D print a thermoset elastomer in this way. Conductive carbon black was printed in various patterns onto PET sheet and the dielectric properties measured. Results indicated that at very low carbon contents, the printed patterns could provide EMI shielding. The research has shown that it is feasible to create a cured 3D elastomeric object containing filler with a controlled distribution that is capable of providing EMI shielding