Integration of a needle valve mechanism with cura slicing software for improved retraction in pellet-based material extrusion

Fused granular fabrication (FGF) is a cost-effective and increasingly popular additive technology that enables the production of parts using a wide range of exotic materials that are not typically available with material extrusion additive manufacturing. This paper presents a novel needle valve approach for flow control in a FGF system that uses flexibly connected screw extruders. The addition of a needle valve to the existing system allows complex travel moves to be performed without stringing, greatly increasing the available part complexity of the printer. The needle valve is actuated by one of the 3D printer's extruder stepper motors and can be controlled by g-code generated by off-the shelf slicer software. This paper also proposes a mathematical model for predicting the oozing volume of the valve, which is the small quantity of material that escapes through the nozzle during valve actuation. This model is important for calibration and g-code generation, which could be integrated with commercial slicers to use this needle valve design without writing custom code. The model is tested experimentally, and results show a strong correlation between predicted and measured oozing volumes. The performance of the valve is also characterized in terms of flow rate versus needle position. Experiments showed that the needle stroke (the vertical distance that the needle travels between open and closed states of the needle valve) is the most important parameter in minimizing oozing. An optimized stroke of 0.4 mm was used, with a maximum flow rate of 346 mm3/min when using a nozzle inner bore diameter of 0.8 mm. Using the optimized valve parameters, parallel cube structures were printed to demonstrate the reduction of stringing, along with various demonstration parts including a 3Dbenchy benchmark print and a soft robotic actuator. Overall, this study demonstrates that needle valves present a reliable and low-cost approach for controlling the flow of polymer in FGF systems with a remotely connected screw extruder.