My Research Interests are Additive manufacturing, composite materials
High level research area
Development and optimization of a novel method of additively manufacturing polymer-ceramic-metal (cermet) composites by fused granular fabrication
Specific problems and challenges
There is little understanding related to how a screw extruder can be used to produce highly-filled polymer parts via fused granular fabrication, and therefore, there is no consensus regarding printing process parameters and their relation on the feasibility of 3D printing such parts or on the quality of these parts. A major challenge is addressing the problem of segregation between the polymer and cermet feedstock due to the major size, shape, and density differences between these materials. Additionally, a polymer material that is environmentally friendly and non-toxic needs to be selected for this project.
Polyvinyl alcohol (PVA) was identified as the polymer binder material to be used in this project, as it is biodegradable, biocompatible, and non-toxic in nature. When dissolved in water, it can be biodegraded by organisms in wastewater and does not leave behind microplastics. It also evaporates as a result of thermal decomposition, and therefore has potential to be thermally debound too.
The screw extruder selected is a single-screw extruder producted by RobotDigg Shanghai. It is driven by a 400W servo motor powering a 35mm diameter extrusion screw, with an L/D ratio of 13:1.
A feeding system that separately feeds the PVA pellets and cermet powders and that can be mounted onto the screw extruder will be designed and characterized to allow for calibrated feeding of materials at different compositions. Additionally, a cartesian manipulator will be designed and built to control the movement of a heated print bed where material will be deposited on from the stationary screw extruder. DoE will be used to optimize print quality on a thin wall, and optimized parameters will then be used to print a small flange.
First, a rheometer is used to characterize the rheological properties of the as-received PVA pellets. This will help to inform how some printing parameters, such as barrel and nozzle temperature, can be tweaked to improve print quality, as well as the limits they can go.
Next, the volume throughput of the screw extruder will be characterized at a fixed temperature and various servo motor speeds. This will help to inform the design and required motor speeds for the feeding screws for the PVA and cermet components for different PVA-cermet compositions. The entire feeding system consisting of the feeding screws and funnel are then tested for homogeneity and accuracy of desired compositions of PVA and cermet.
The screw extruder is then tested to verify whether the cermet is mixed evenly throughout the PVA binder matrix. After verifying the homogeneity of extrudate, DoE will be conducted to optimize printing parameters on a thin wall. Using these optimized printing parameters, a small flange will be printed to verify the proof of concept.
Results and deliverables
The characterization of the rheological behavior of PVA was completed, wherein the PVA has shown a decrease in viscosity and thixotropism with an increase in temperature from 195°C to 215°C.
The volume throughput of the screw extruder has also been characterized from a servo motor speed from 100 to 500 rpm. Fixing the servo motor speed at 300 rpm, the design of the feeding screws and the required stepper motor speeds for the PVA and the cermet is finalized. An algorithm for the PVA feeding screw is also designed to address the issue of PVA pellets jamming in the feeding screw.
The feeding system is also tested on a moving conveyor where it is verified that the feeding system is able to feed the 2 materials at the desired composition into the screw extruder.