Methods of Extrusion on Demand for High Solids Loading Ceramic Paste in Freeform Extrusion Fabrication

Fabrication of highly dense parts with complex geometry by paste-extrusion-based solid freeform fabrication processes requires a precise control of the extrusion flow rate to dispense material on demand, which is often referred as Extrusion-On-Demand (EOD). The extrusion process for aqueous ceramic pastes is complex and difficult to control due to their non-Newtonian behavior, compressibility and inhomogeneity. In this study, three methods of EOD (based on ram extruder, needle valve, and auger valve) are introduced and investigated for the extrusion of high solids loading (i.e., >50%, volumetric) aqueous alumina paste. Optimal extrusion process parameters for these methods are determined through printing tests and analysis. The extrusion performance in terms of extrusion start and stop accuracy, as well as flow rate consistency, is compared and analyzed for the three methods. Advantages and disadvantages of these three methods are also discussed. Introduction The freeform extrusion fabrication process deposits ceramic paste layer by layer through extrusion. The details regarding the principle of the process, equipment setup and material properties can be found in [1-3]. A schematic of the extrusion fabrication system along with a photograph of the paste extrusion is shown in Figure 1. Paste-extrusion-based solid freeform fabrication requires a precise control of the extrusion flow rate to fabricate highly dense parts with complex geometry. The conventional mechanism of paste extrusion for the freeform extrusion fabrication process is ram extruder, which consists of a syringe and a plunger. Based on the ram extruder, several methods regulating the extrusion force and plunger velocity were developed. Zhao et al. [4] designed an adaptive controller with a general tracking control law and implemented it to regulate the extrusion force. Deuser et al. [5] developed a hybrid force-velocity controller to regulate both the steady-state extrusion flow rate using a plunger velocity controller and ExtrusionOn-Demand (EOD) using an extrusion force controller. Kulkarni [6] developed a dwell method and a trajectory method to compensate the delay of extrusion start and stop to improve EOD performance. The previous ram extruder based extrusion method improved the EOD performance considerably. However, the experimental result shows that the paste extrusion control model parameters vary from batch to batch due to the variation of paste properties. Thus, extrusion parameters must be tuned for different batches of paste. Also, for the same batch of paste being extruded under the constant plunger velocity, table speed, raster width and layer thickness, underfilling and over-filling of material were observed, indicating that the paste flow rate was