TY - CHAP A1 - Laumer, Tobias A1 - Stichel, Thomas A1 - Bock, Thomas A1 - Amend, Philipp A1 - Schmidt, Michael T1 - Characterization of temperature-dependent optical material properties of polymer powders T2 - AIP Conference Proceedings N2 - In former works, the optical material properties of different polymer powders used for Laser Beam Melting (LBM) at room temperature have been analyzed. With a measurement setup using two integration spheres, it was shown that the optical material properties of polymer powders differ significantly due to multiple reflections within the powder compared to solid bodies of the same material. Additionally, the absorption behavior of the single particles shows an important influence on the overall optical material properties, especially the reflectance of the powder bed. Now the setup is modified to allow measurements at higher temperatures. Because crystalline areas of semi-crystalline thermoplastics are mainly responsible for the absorption of the laser radiation, the influence of the temperature increase on the overall optical material properties is analyzed. As material, conventional polyamide 12 and polypropylene as new polymer powder material, is used. By comparing results at room temperature and at higher temperatures towards the melting point, the temperature-dependent optical material properties and their influence on the beam-matter interaction during the process are discussed. It is shown that the phase transition during melting leads to significant changes of the optical material properties of the analyzed powders. KW - laser beam melting KW - material qualification KW - Optical Material Properties KW - Polymers Y1 - 2014 U6 - https://doi.org/10.1063/1.4918508 IS - 1 PB - AIP Publishing ER - TY - JOUR A1 - Laumer, Tobias A1 - Stichel, Thomas A1 - Raths, Max A1 - Schmidt, Michael T1 - Analysis of the Influence of Different Flowability on Part Characteristics Regarding the Simultaneous Laser Beam Melting of Polymers JF - Physics Procedia N2 - Powder based Additive Manufacturing technologies offer huge potential for building parts with almost no geometrical restrictions, but both the process controlling as well as the part properties are strongly dependent on different material characteristics of the material, like the flowability. In this work, different weight percentages of nano-scaled silica dioxide particles (Aerosil®) are admixed to pure polyethylene and polypropylene powder and the resulting flowability is determined. Besides using the Hausner ratio as standardized value, the degree of coverage is introduced as a new characteristic to quantify the powder flowability. The degrees of coverage are compared to the Hausner ratios to allow a discussion and evaluation about the different characteristic values. Additionally, tensile bars consisting of polypropylene are generated to determine the porosity by cross sections and the mechanical part properties by tensile testing. As mechanical part properties, the tensile strength and elongation at break are determined and the effects of different powder flowability on these properties are analyzed. KW - additive manufacturing KW - laser beam melting of polymers KW - material qualification Y1 - 2016 U6 - https://doi.org/10.1016/j.phpro.2016.08.098 SN - 1875-3892 SN - 1875-3884 VL - 83 SP - 937 EP - 946 PB - Elsevier ER - TY - JOUR A1 - Stichel, Thomas A1 - Frick, Thomas A1 - Laumer, Tobias A1 - Tenner, Felix A1 - Hausotte, Tino A1 - Merklein, Marion A1 - Schmidt, Michael T1 - A Round Robin study for selective laser sintering of polymers: Back tracing of the pore morphology to the process parameters JF - Journal of Materials Processing Technology N2 - The mechanical properties of polymer parts built by Selective Laser Sintering are strongly related to the internal microstructure which differs with the applied production parameters. The paper focuses on the back tracing of the pore morphology of laser sintered polyamide-12 samples to the process parameters. Therefore, a data base is used which is supplied by a Round Robin initiative and includes mechanical tensile tests and the microstructural analysis of the pore morphology of several different sample charges built with different machines. The pore morphologies (porosity, pore density, pore shape and pore arrangement) measured by X-ray computed tomography are compared and discussed regarding the employed parameters and the resulting mechanical properties. The investigations point out that pore density is a superior indicator than porosity for mechanical issues. This is especially valid along the build direction since pore morphology has shown to be strongly anisotropic. Moreover, the analysis revealed that pore density is strongly affected by the process temperature, which is proved to be essential for the fabrication of mechanical robust parts using Selective Laser Sintering. Y1 - 2018 U6 - https://doi.org/10.1016/j.jmatprotec.2017.10.013 VL - 252 IS - February SP - 537 EP - 545 PB - Elsevier ER - TY - CHAP A1 - Schmailzl, Anton A1 - Geißler, Bastian A1 - Maiwald, Frederik A1 - Laumer, Tobias A1 - Schmidt, Michael A1 - Hierl, Stefan ED - Esen, Cermal T1 - Transformation of Weld Seam Geometry in Laser Transmission Welding by Using an Additional Integrated Thulium Fiber Laser T2 - Lasers in Manufacturing - LIM 2017, Conference Proceedings Y1 - 2017 N1 - CD-ROM SP - 1 EP - 10 CY - München ER - TY - CHAP A1 - Schmailzl, Anton A1 - Hierl, Stefan A1 - Schmidt, Michael ED - Schmidt, Michael T1 - Gap-Bridging during Quasi-Simultaneous Laser Transmission Welding T2 - Physics Procedia N2 - Tightness is often the main requirement for quasi-simultaneous laser transmission welds. However, remaining gaps cannot be detected by the used set-path monitoring. By using a pyrometer in combination with a 3D-scanner, weld seam interruptions can be localized precisely while welding, due to temperature deviations along the weld contour. To analyze the temperature signal in correlation to the progress of gap-bridging, T-joint samples with predefined gaps are welded. The set-path is measured synchronously. Additionally, the temperature distribution and the influence of the thermal expansion of the polymers are studied by a thermo-mechanical FEM-process simulation. On top of that, the melt blow-out of the welded samples is analyzed using μCT-measurements. The experiments have shown that closing of a gap can be identified reliably by the temperature signal and that the squeezed melt flow into the gap and the thermal expansion in the gap zone accelerates gap-bridging. Furthermore the inserted heat can be adapted in the fault zone, in order to avoid thermal damage. Y1 - 2016 U6 - https://doi.org/10.1016/j.phpro.2016.08.113 SN - 1875-3892 SN - 1875-3884 VL - 83 SP - 1073 EP - 1082 PB - Elsevier CY - Amsterdam [u.a.] ER - TY - CHAP A1 - Schmailzl, Anton A1 - Quandt, Benjamin A1 - Schmidt, Michael A1 - Hierl, Stefan T1 - In-Situ Process Monitoring during Laser Transmission Welding of PA6-GF30 T2 - Procedia CIRP N2 - Quasi-simultaneous laser transmission welding is preferably used for packaging sensors and electronics. In order to protect the components from moisture, a hermetic encapsulation is needed. However, local weld seam interruptions cannot be identified with the commonly used set-path monitoring. By using a pyrometer, coaxially integrated into a 3D-scanner, gaps between the joining partners can be allocated on basis of the measured temperature. However, the scattering of the heat radiation, especially caused by the fiber reinforcement of the plastics, leads to a reduction of the accessible heat radiation, which makes the identification of gaps considerably more difficult. The herein used experimental setup is characterized by a small detection spot and only by a slight weakening of the heat radiation inside the scanner. Hence, for welding PA6-GF30, the detection of small sized gaps is possible, even if a glass fiber content of 30 percent (wt.) and a weld seam width with approximately 1 mm are given. Y1 - 2018 U6 - https://doi.org/10.1016/j.procir.2018.08.131 VL - 74 SP - 524 EP - 527 ER - TY - CHAP A1 - Schmailzl, Anton A1 - Quandt, Benjamin A1 - Hierl, Stefan A1 - Schmidt, Michael T1 - Correlation between Joint Strength and Process Temperature in Quasi-Simultaneous Laser Transmission Welding of Polyamide 6 T2 - Proceedings of LiM2019 - Lasers in Manufacturing (23 June 2019, Munich) N2 - The joint strength is gaining importance in quasi -simultaneous laser transmission welds, especially in structural components. A correlation between the joint strength and a process characteristic is a basic requirement for selecting the best process parameter setting. In this work, the temperature is measured during welding polyamide 6 by using a scanner-integrated pyrometer with an InGaAs-detector. The filtering of the heat radiation in the upper joining partner is taken into account by calibrating the mea surement system. By this, the measured temperature signal represents the temperature in the joining zone. A correlation is found between the measured temperature and the resulting joint strength. Moreover, high joint strengths are also seen for welds with short welding times, as far as the temperature is sufficiently high. With this knowledge, a process window can be derived easily in order to produce welds with high strength and short welding times. Y1 - 2019 SP - 1 EP - 10 PB - Wissenschaftliche Gesellschaft Lasertechnik e.V. ER - TY - JOUR A1 - Osmanlic, Fuad A1 - Wudy, Katrin A1 - Laumer, Tobias A1 - Schmidt, Michael A1 - Drummer, Dietmar A1 - Körner, Carolin T1 - Modeling of Laser Beam Absorption in a Polymer Powder Bed JF - Polymers N2 - In order to understand the absorption characteristic, a ray trace model is developed by taking into account the reflection, absorption and refraction. The ray paths are resolved on a sub-powder grid. For validation, the simulation results are compared to analytic solutions of the irradiation of the laser beam onto a plain surface. In addition, the absorptance, reflectance and transmittance of PA12 powder layers measured by an integration sphere setup are compared with the numerical results of our model. It is shown that the effective penetration depth can be lower than the penetration depth in bulk material for polymer powders and, therefore, can increase the energy density at the powder bed surface. The implications for modeling of the selective laser sintering (SLS) process and the processability of fine powder distributions and high powder bed densities are discussed. KW - additive manufacturing KW - laser absorption KW - PA12 KW - polyamide 12 KW - powder bed Y1 - 2018 U6 - https://doi.org/10.3390/polym10070784 VL - 10 IS - 7 SP - 1 EP - 11 PB - MDPI ER - TY - JOUR A1 - Maiwald, Frederik A1 - Roider, Clemens A1 - Schmidt, Michael A1 - Hierl, Stefan T1 - Optical Coherence Tomography for 3D Weld Seam Localization in Absorber-Free Laser Transmission Welding JF - Applied Sciences N2 - Quality and reliability are of the utmost importance for manufacturing in the optical and medical industries. Absorber-free laser transmission welding enables the precise joining of identical polymers without additives or adhesives and is well-suited to meet the demands of the aforementioned industries. To attain sufficient absorption of laser energy without absorbent additives, thulium fiber lasers, which emit in the polymers’ intrinsic absorption spectrum, are used. Focusing the laser beam with a high numerical aperture provides significant intensity gradients inside the workpiece and enables selective fusing of the internal joining zone without affecting the surface of the device. Because seam size and position are crucial, the high-quality requirements demand internal weld seam monitoring. In this work, we propose a novel method to determine weld seam location and size using optical coherence tomography. Changes in optical material properties because of melting and re-solidification during welding allow for weld seam differentiation from the injection-molded base material. Automatic processing of the optical coherence tomography data enables the identification and measurement of the weld seam geometry. The results from our technique are consistent with microscopic images of microtome sections and demonstrate that weld seam localization in polyamide 6 is possible with an accuracy better than a tenth of a millimeter. KW - image processing KW - laser transmission welding KW - optical coherence tomography KW - process monitoring KW - transparent polymers Y1 - 2022 U6 - https://doi.org/10.3390/app12052718 N1 - Corresponding author: Frederik Maiwald VL - 12 IS - 5 SP - 1 EP - 11 PB - MPDI CY - Basel ER - TY - JOUR A1 - Maiwald, Frederik A1 - Kroth, Lea A1 - Geiger, René A1 - Schmitt, Bernhard A1 - Hierl, Stefan A1 - Schmidt, Michael T1 - Laser welding of polymer foils with spatially adapted intensity distributions JF - Joining Plastics N2 - Absorber-free laser transmission welding is characterized by its contactless energy input and geometricflexibility and enables the precise and clean joining of polymer films without absorbing additives or adhesives. It is therefore well suited for applications with high demands regarding process reliability and cleanliness such as packaging, fluid containersor as sealing film in medicaland food industry. A homogeneous weld seam temperature is necessary for a large processwindow. In this work, the naturally Gaussian-shaped intensity distributionof the laser beam is there foreconverted into a donut-shaped and a flat-top-shaped distribution. When using the donut-shape, the processwindow for welding polypropylene or polyethylene films is increased by up to a factor of 3. At the same time, the weld seam strength almost corresponds to the strength of the base material. KW - Laserschweissen KW - Kunststofffolie Y1 - 2025 U6 - https://doi.org/10.53192/JP20250146 SN - 1864-3450 IS - 1 SP - 46 EP - 52 PB - DVS Media CY - Düsseldorf ER - TY - JOUR A1 - Geißler, Bastian A1 - Laumer, Tobias A1 - Wübbeke, Andrea A1 - Lakemeyer, Patrick A1 - Frick, Thomas A1 - Schöppner, Volker A1 - Schmidt, Michael T1 - Analysis of the interaction between the temperature field and the weld seam morphology in laser transmission welding by using two different discrete laser wavelengths JF - Journal of Laser Applications N2 - Laser transmission welding is a non-contact and efficient process technology for joining thermoplastic polymers. In the conventional process, laser sources in the wavelength range of 1 μm are usually used. Therefore, most of the laser radiation is transmitted through the upper joining partner and absorbed only in the lower joining partner. As a result, the possibilities to influence the temperature field especially in the upper joining partner are limited. To overcome these limitations, an additional thulium fiber-laser with a wavelength of 1.94 μm is used in this study and coaxially aligned with a diode laser. The use of an additional thulium fiber-laser leads to a significant absorption in the upper joining partner. Through this approach, it is shown that the temperature field and the weld seam geometry can be influenced by using these two different discrete laser wavelengths. Depending on the intensity distribution of both lasers, an increase of the size of the heat affected zone in the upper joining partner can be observed. In order to develop a better process understanding, a thermal finite element model is built up and verified by comparing the calculated size of the heat affected zone for different process parameters with the experimental data. The model is able to represent the influence of both laser sources on the temperature field and is used to calculate characteristics of the temperature field, such as maximum temperatures or cooling rates. The characteristics are then used to explain the weld seam morphology, such as occurrence and size of spherulitic structures in the weld seam. Y1 - 2018 U6 - https://doi.org/10.2351/1.5040617 SN - 1938-1387 VL - 30 IS - 3 PB - AIP Publishing ER - TY - JOUR A1 - Geißler, Bastian A1 - Laumer, Tobias A1 - Wübbeke, Andrea A1 - Frick, Thomas A1 - Schöppner, Volker A1 - Schmidt, Michael T1 - Analysis of the Weld Seam Morphology of Polypropylene in Laser Transmission Welding JF - Journal of Manufacturing Science and Engineering N2 - Laser transmission welding is a well-known joining technology for welding thermoplastics. Although the process is already used industrially, fundamental process-structure-property relationships are not fully understood and are therefore the subject of current research. One aspect of these mentioned process-structure-property relationships is the interaction between the temperature field during the welding process, the weld seam morphology of semi-crystalline thermoplastics, and the weld seam strength. In this study, the influence of the line energy on the weld seam morphology of polypropylenes is analyzed. For this purpose, the size of spherulites in the weld seam is investigated, as well as different occurring phases of polypropylene (a- and b-phase). It is shown that both the spherulite size of the a-phase and the amount of b-phase increase with increasing line energy. For the explanation and discussion of the results, a temperature-dependent thermal simulation model is used to derive characteristic attributes of the temperature field (maximum temperatures, cooling rates, temperature gradients). Y1 - 2018 U6 - https://doi.org/10.1115/1.4040876 VL - 140 IS - 11 PB - ASME ER - TY - CHAP A1 - Amend, Philipp A1 - Mrotzek, Tino A1 - Laumer, Tobias A1 - Wolf, Michel A1 - Roth, Stephan A1 - Gude, Maik A1 - Schmidt, Michael T1 - Experimental Investigations on Laser-based Hot-melt Bonding and Injection Molding for Laser-structured Metal Plastic Hybrids T2 - Laser in Manufacturing (LIM 2017), Munich, Germany N2 - The use of thermoplastics in lightweight construction is continuing to grow. This implies the need for suitable joining techniques to combine thermoplastics with other materials, such as metals, to gain tailored multi-material parts. In this paper latest results of experimental investigations on laser-based hot-melt bonding and injection molding for laser-structured metal plastic hybrids are presented. As materials stainless steel and short-fiber reinforced polyamide are used. The stainless steel surface is structured with a nanosecond pulse laser before joining to improve the mechanical adhesion between the dissimilar materials. Thereby, different structure depths in the range between 16.6 ± 1.2 µm and 66.5 ± 2.5 µm as well as different hatch distances between 70 and 300 µm are realized. The laser-based joining process is carried out irradiating the metallic surface multiple times. Positioned below the metal in T-joint configuration, the thermoplastic melts as a result of heat transfer and acts as hot-melt cohesive. Besides, hybrid joints are manufactured using injection molding. For experiments, the mold temperature as well as the melt temperature are varied. Regardless of the joining process, the hybrid joints are mechanically characterized by tensile tests. The results demonstrate that for both joining processes strong laser-structured metal plastic hybrids can be realized. KW - injection molding KW - laser structuring KW - Laser-based hot-melt bonding KW - multi-material design Y1 - 2017 UR - https://www.researchgate.net/publication/318110636_Experimental_Investigations_on_Laser-based_Hot-melt_Bonding_and_Injection_Molding_for_Laser-structured_Metal_Plastic_Hybrids SN - 978-3-87525-428-0 ER -