Direct Three-Dimensional Visualization and Characterization of Microstructures Formed by Printing Particles

Additive manufacturing performed with solid particles can form microscopic structures based on the properties of the particles used. These microstructures will likely impact the performance of the product produced. Characterizing these microstructures is difficult since the interior of powders is a challenge to image. Most imaging techniques are limited to surface visualizations. We have established a methodology to directly visualize the resulting particle structure by mapping powder particle positions in the interior three-dimensionally. We use Confocal Laser Scanning Microscopy (CLSM) to capture stacks of cross-sectional images of micron-sized poly-dispersed electro-photographic printing particles. Assisted with image analysis tools, we have obtained the co-ordinates and deduced the radius for each particle in selected sampling volumes. With this information, we are able to recreate the particulate structure in three-dimensional space and to determine the microstructural parameters, such the packing fraction for this powder system. I n t roduct ion Micron-size particles have been used in numerous industries for a broad range of applications. One example is electrophotographic printing where polymeric particles have been the carrier of color pigments and the enabler for deposition and permanency of the pigments on substrates for the production of digital prints in graphic communication. Most recently the manufacturing world has adopted the graphic communication industry’s digital printing processes, and consequently the use of micron and sub-micron particles. The manufacturing adaptation of printing takes the form of adding materials to build parts, in contrast to the traditional subtractive process where materials are removed. Additive manufacturing, also known as functional printing or threedimensional (3D) printing, has been advanced for the purpose of improving manufacturing capability in form (such as shape), fit (such as processes and tolerances), and function (such as high performance material systems). For powder-based additive manufacturing, similar to electrophotographic (EP) printing, the quality of a product will be influenced by particle structures resulting from the print process and materials used. For instance, a missing placement of a particle aggregate such as a halftone dot, will result in a missing “mark” in the manufactured product, which consequently leads to a defect in the product and a potential failure in its application. Many micron-size powders are cohesive [1-3]. Cohesive particles will likely form highly porous structures [1, 2]. It has been demonstrated that the porosity of particle structures can directly impact the properties of images in electrophotographic printing [2, 3]. Visualizing particle microstructures can be difficult since it requires imaging the interior of powders. Much of the quantification of particle microstructures is through their packing fractions (defined as the ratio of the volume occupied by solid particles to the total volume) which can be estimated through inference from other measurements [1] or simulations [4, 5]. Visualization of microscopic structures has been limited to surface techniques, typically electron microscopies such as Scanning Electron Microscopy (SEM) [3, 6-9]. High Resolution SEM imaging of powder highlights the structures and voids on the surface within view [3, 6-9]. However, such reconstructions do n o t reflect the actual microstructures in the interior. Confocal Laser Scanning Microscopy (CLSM) is a highresolution optical microscope with depth selectivity and is capable of imaging interior structures for samples that are transparent or fluorescent. 3D imaging with CLSM has been conducted for 1-2 micron particles in colloidal systems to study the atomic nucleation process with dispersed mono-dispersed “hard spheres” [10-12]. Experimentation on these systems is performed with thin (a few layers of particles) and dilute samples for the transmission efficiency of light [13, 14]. The works on particle tracking in real space with the CLSM have yielded an unprecedented level of information on crystal nucleation [15] and phase transitions [11, 12]. The CLSM has also been employed for the studies of biological [16] systems. Included among the many CLSM applications for biological systems is the mapping of shape through the tracing of signals from fluorescence dyes that have been intentionally instituted on the surfaces of the sampling objects. The goal of this research is to develop a methodology that is capable of identifying particle positions in the interior of a powder to illustrate and characterize the structure it forms. The methodology extends the Confocal Laser Scanning Microscopy (CLSM) to larger size and higher density powder systems. The methodology involves the capture of stacks (in the depth direction) of planar images of powder particles with CLSM and the analysis of the stacks of images through their pixelated values. Electrophotographic printing (EP) particles are used for the study because they possess the florescent property and because they have been characterized extensively in print quality studies [2, 3]. This work demonstrates the feasibility of using CLSM to threedimensionally map and visualize interior structures of a particle sediment. This study will shed light to understand the performance of parts produced from additive manufacturing as well as characterize image quality in digital EP toner printing. Met hodol ogy In this study, yellow toners were employed which were extracted from an HP3700 cartridge. The particle sediments were created by simply dropping a small quantity of toner onto a microscope glass slide using a spatula. Samples of sediments were enclosed with double-sided tapes and another glass slide on top to prevent toner contamination of the CLSM imaging system. There are two reasons for utilizing sediments on glass slides instead of printed unfused samples on transparencies: (1) a desire to generate thick but image-able specimens to represent individual volumetric units resulting from additive manufacturing and (2) CLSM calls for