Freeform Extrusion of High Solids Loading Ceramic Slurries, Part I: Extrusion Process Modeling

A novel solid freeform fabrication method has been developed for the manufacture of ceramic-based components in an environmentally friendly fashion. The method is based on the extrusion of ceramic slurries using water as the binding media. Aluminum oxide (Al2O3) is currently being used as the part material and solids loading as high as 60 vol. % has been achieved. This paper describes a manufacturing machine that has been developed for the extrusion of high solids loading ceramic slurries. A critical component of the machine is the deposition system, which consists of a syringe, a plunger, a ram actuated by a motor that forces the plunger down to extrude material, and a load cell to measure the extrusion force. An empirical, dynamic model of the ceramic extrusion process, where the input is the commanded ram velocity and the output is the extrusion force, is developed. Several experiments are conducted and empirical modeling techniques are utilized to construct the dynamic model. The results demonstrate that the ceramic extrusion process has a very slow dynamic response, as compared to other non-compressible fluids such as water. A substantial amount of variation exists in the ceramic extrusion process, most notably in the transient dynamics, and a constant ram velocity may either produce a relatively constant steady-state extrusion force or it may cause the extrusion force to steadily increase until the ram motor skips. The ceramic extrusion process is also subjected to significant disturbances such as air bubble release, which causes a dramatic decrease in the extrusion force, and nozzle clogging, which causes the extrusion force to slowly increase until the clog is released or the ram motor skips.Mechanical Engineerin

Freeform Extrusion of High Solids Loading Ceramic Slurries, Part II: Extrusion Process Control

Part I of this paper provided a detailed description of a novel fabrication machine for high solids loading ceramic slurry extrusion and presented an empirical model of the ceramic extrusion process, with ram velocity as the input and extrusion force as the output. A constant force is desirable in freeform extrusion processes as it correlates with a constant material deposition rate and, thus, good part quality. The experimental results in Part I demonstrated that a constant ram velocity will produce a transient extrusion force. In some instances the extrusion force increased until ram motor skipping occurred. Further, process disturbances, such as air bubble release and nozzle clogging that cause sudden changes in extrusion force, were often present. In this paper a feedback controller for the ceramic extrusion process is designed and experimentally implemented. The controller intelligently adjusts the ram motor velocity to maintain a constant extrusion force. Since there is tremendous variability in the extrusion process characteristics, an on-off controller is utilized in this paper. Comparisons are made between parts fabricated with and without the feedback control. It is demonstrated that the use of the feedback control reduces the effect of process disturbances (i.e., air bubble release and nozzle clogging) and dramatically improves part quality.Mechanical Engineerin

Aqueous-Based Extrusion Fabrication of Ceramics on Demand

Aqueous-Based Extrusion Fabrication is an additive manufacturing technique that extrudes ceramic slurries of high solids loading layer by layer for part fabrication. The material reservoir in a previously developed system has been modified to allow for starting and stopping of the extrusion process on demand. Design pros and cons are examined and a comparison between two material reservoir designs is made. Tests are conducted to determine the optimal deposition parameters for starting and stopping the extrudate on demand. The collected test data is used for the development of a deposition strategy that improves material deposition consistency, including reduced material buildup at sharp corners. Example parts are fabricated using the deposition strategy and hardware design.Mechanical Engineerin

Adaptive Control of Freeze-Form Extrusion Fabrication Processes

Freeze-form Extrusion Fabrication (FEF) is an additive manufacturing process that extrudes high solids loading aqueous ceramic pastes in a layer-by-layer fashion below the paste freezing temperature for component fabrication. Due to effects such as the air bubble release, agglomerate breakdown, change in paste properties during extrusion as a result of liquid phase migration, etc., the extrusion force is difficult to control. In this paper, an adaptive controller is proposed to regulate the extrusion force. Recursive Least Squares is used to estimate extrusion force model parameters during fabrication and a low-order control scheme capable of tracking general reference trajectories is designed and implemented to regulate the extrusion process. The controller is implemented to regulate the extrusion process. The controller is implemented fro sinusoidal, triangular, and square reference trajectories and the results demonstrate excellent tracking performance of the adaptive extrusion force controller. Several parts were fabricated with the adaptive extrusion force controller. These results illustrate the need for extrusion force control and that variable reference extrusion

Modeling, Analysis and Simulation of Paste Freezing in Freeze-Form Extrusion Fabrication

During the freeze-form extrusion fabrication process for aqueous-based pastes, the sub-zero temperature environment aids the part in maintaining its shape by freezing the water present in the paste. The paste freezes very quickly when deposited on a substrate in a freezing environment. However, as the part’s height increases, the freezing time increases as the heat conduction rate to the substrate decreases. The freezing time can exceed the time required to extrude one layer of paste due to water’s high latent heat, thus leaving the extruded paste in its semi-liquid state and causing the part to deform or even collapse. Therefore, dwell time is needed between layers, which may substantially increase the build time of the part. In this paper, the effects of the paste material, paste solids loading, convection coefficient, initial paste temperature, ambient temperature, total time between layers, and layer thickness on the freezing time of paste are investigated. The paste temperature and paste freezing time are computed for various process parameters via numerical simulation using the commercial code Fluent

Diffusive MASS NMR Studies of Transport in Porous Materials

NMR methods are widely used to probe the structure and fluid dynamics of porous materials including such diverse materials as cheese and chocolate, cosmetics and pharmaceuticals, solvents in resins and soft matter, biological tissue, and for oil exploration. NMR measurements are uniquely suited to these studies since it records the correlation of changing local magnetic fields over a time scale of ms to seconds. The local magnetic fields are established by local variations in the bulk magnetic susceptibility of the sample (and so are directly tied to the sample's local structure). The fluctuation in field that a spin sees is due to molecular transport (including molecular diffusion) through these local fields, and so reports on the length scales of structures and impediments to transport. In the past this information has primarily been employed via empirical relations that relate bulk measurements of relaxation times or diffusion to some microscopic property (pore size, throat size, S/V, and surface relaxivity, etc.). These empirical relationships, while useful, hide the underlying complexity of spin dynamics in confining geometries. We have developed a new set of methods to provide a means of systematically varying the reflective time scale of the measurement and thus the reflective length scale. This new handle permits a detailed, microscopic picture of the structure and dynamics

Interactions of an Additive Manufacturing Program with Society

Additive Manufacturing (AM) has shown considerable promise for the future but also proposes some challenges. Many AM barriers tend to be non-technical and instead are human-centric issues such as lack of education of practitioners in AM capabilities, cultural differences, vested interests, and potentially lack of imagination. It is highly desirable for all research and educational institutions to help address these issues. This paper summarizes the additive manufacturing research and education program at the Missouri University of Science and Technology (Missouri S&T) and its interactions with various constituents, including K-12 students, undergraduate and graduate students, distance students, and industry

Development of Extrusion-on-Demand for Ceramic Freeze-Form Extrusion Fabrication

In the Freeze-form Extrusion Fabrication (FEF) process, extrusion-on-demand (EOD) refers to the ability to control the start and stop of paste extrusion on demand and is vital to the fabrication of parts with complex geometries. This paper describes the development of EOD for ceramic FEF through modeling and control of extrusion force, selection of appropriate process parameters, and a dwell technique for start and stop of extrusion. A general tracking controller with integral action is used to allow tracking of a variety of reference forces while accounting for the variability in the paste properties. Experiments are conducted to model the process and tune the controller. The developed technique for EOD is demonstrated to fabricate a number of cross sections and three-dimensional parts from alumina paste

Development of Extrusion-on-Demand for Ceramic Freeze-Form Extrusion Fabrication

In the Freeze-form Extrusion Fabrication (FEF) process, extrusion-on-demand (EOD) refers to the ability to control the start and stop of paste extrusion on demand and is vital to the fabrication of parts with complex geometries. This paper describes the development of EOD for ceramic FEF through modeling and control of extrusion force, selection of appropriate process parameters, and a dwell technique for start and stop of extrusion. A general tracking controller with integral action is used to allow tracking of a variety of reference forces while accounting for the variability in the paste properties. Experiments are conducted to model the process and tune the controller. The developed technique for EOD is demonstrated to fabricate a number of cross sections and three-dimensional parts from alumina paste.Mechanical Engineerin