Jan-Uwe Reinhard Schmidt

Weight- and cost-optimized extruders are used in additive manufacturing. Material-flow pulsation is a major challenge in screw-based material extrusion additive manufacturing, as it reduces dimensional accuracy and print quality. This study investigates the relationship between internal nozzle pressure and material flow in a polymer-feeding single-screw extruder and evaluates pressure-based closed-loop control as a means of reducing extrusion pulsation. An experimental setup comprising an integrated nozzle-pressure sensor, a synchronized conveyor belt, and a laser-line scanner was developed to simultaneously measure nozzle pressure and the cross-sectional area of the extruded strand. Based on volumetric setpoints derived from the G-code, corrected extruder motor commands were generated and implemented using proportional (P) and proportional–integral (PI) pressure control. The experimental results reveal a strong correlation between internal nozzle pressure and the resulting strand cross-section. The strand modulation correlated with screw rotation could be almost completely eliminated. At the investigated medium extrusion speed, pressure feedback reduced the remaining strand cross-sectional modulation to approximately 33% of the uncontrolled reference condition. Using PLA (Polylactic Acid) material the presented approach demonstrates that pressure-based feedback control is a practical method for improving process stability and extrusion quality in screw-based additive manufacturing and provides a foundation for future advanced control strategies.

Ultrasonic power stimulation of polymers has been employed to modify their properties for various industrial applications. It is used to disperse particles in polymers or to impregnate continuous fiber-reinforced filaments. These processes operate under extreme conditions, involving temperatures exceeding 400 °C and pressures reaching up to 60 bars. Traditional sound introduction systems rely on moving horns within cavities, which pose significant challenges in maintaining clean environments. The moving horn makes it inconceivable to seal such sound introduction systems. To address these limitations, an innovative tool that incorporates only sealed components capable of introducing sound energy without employing cavities was developed. This technology focuses on optimizing eigenfrequencies to efficiently transfer motion into the polymer channel while minimizing disturbing vibrations and sealing issues. The simulation results reveal the mode of operation between the fourth and fifth resonance of the tool. The measurements of the tool validate this theory, indicating a range of operation between 18.5 kHz and 19.5 kHz. With an amplitude of 15 μm, resulting in a minimum power introduction of 224.03 W/cm^² in PLA, the average flow rate gain is 13.6%. This new design reduces the risk of blockages and damage in the processed goods and minimizes the force required to process the polymer.

Residual stresses in fiber metal laminates (FML) inevitably develop during the manufacturing process. The main contributor to these stresses is the difference in the coefficients of thermal expansion (CTE) between fibers and metal in combination with high process temperatures. To quantify these stresses, the use of specimens with an asymmetric layup is an easily adaptable method. The curvature that develops after the manufacturing of flat laminates with an asymmetrical layer stack is a measure of the level of residual stresses evolving during cure. However, the accuracy of the curvature evaluation is highly dependent on specimen design and other influencing parameters. This leads to deviations when compared to other methods for residual stress quantification as can be seen from the literature. Therefore, in this work a large set of FML specimens is comprehensively investigated to identify relevant influencing parameters and derive conclusions about specimen design and evaluation techniques. For certain layups and process parameters, there is a good correlation between the curvature and the stress-free temperature, which is further covered by analytical solutions for bimetals. This correlation is the basis to transfer curvature into a stress-free temperature that can consequently be used for the quantification of residual stress levels in more complex FMLs. The transfer is validated by in situ strain measurements during cure using a strain gauge technique. Based on the results, the application of asymmetric specimens for residual stress characterization in more complex laminates is presented in the form of a workflow. The work shows the basic considerations and procedures necessary to use asymmetric specimens for residual stress quantification in FML. Furthermore, the results obtained can also be transferred to other composite materials.

Residual stresses in fiber metal laminates (FML) inevitably develop during the manufacturing process. The main contributor to these stresses is the difference in the coefficients of thermal expansion (CTE) between fibers and metal in combination with high process temperatures. To quantify these stresses, the use of specimens with an asymmetric layup is an easily adaptable method. The curvature that develops after the manufacturing of flat laminates with an asymmetrical layer stack is a measure of the level of residual stresses evolving during cure. However, the accuracy of the curvature evaluation is highly dependent on specimen design and other influencing parameters. In this work a large set of FML specimens is investigated to identify relevant influencing parameters and derive conclusions about specimen design and evaluation techniques. For certain layups and process parameters, there is a good correlation between the curvature and the stress-free temperature, which is further covered by analytical solutions for bimetals. This correlation is the basis to transfer curvature into a stress-free temperature that can consequently be used for the quantification of residual stress levels in more complex FMLs. The transfer is validated by in-situ strain measurements during cure using a strain gage technique. Based on the results, the application of asymmetric specimens for residual stress characterization in more complex laminates is presented in the form of a workflow.