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Mitarbeiter der Fakultät für Maschinenbau

Prof. Dr. Hans-Joachim Schmid

Kontakt
Vita
Publikationen
Prof. Dr. Hans-Joachim Schmid

Fakultät für Maschinenbau

Fachgruppeninhaber - Mitarbeiter - Studiendekan

Profilbereich Nachhaltige Werkstoffe, Prozesse und Produkte

Institutsleiter - Professor - Vorsitzender des Paderborner Instituts für Additive Forschung (PIAF)

Partikelverfahrenstechnik (PVT)

Fachgruppeninhaber - Professor - Lehrstuhlleiter

DMRC

Leiter - Professor - Wissenschaftlicher Leiter

Paderborner Institut für Additive Fertigung (PIAF)

Professor - Sprecher

Institut für Leichtbau mit Hybridsystemen (ILH)

Mitglied - Professor - Vorstand FK LEM

Telefon:
+49 5251 60-2404
Telefon:
05251 60 2410
Fax:
+49 5251 60-3207
Büro:
E3.319
Web:
Besucher:
Pohlweg 55
33098 Paderborn
Prof. Dr. Hans-Joachim Schmid
Sonstiges
Seit 01.06.2008

Wissenschaftlicher Leiter des Direct Manufacturing Research Center an der Universität Paderborn

Seit 01.10.2006

Inhaber des Lehrstuhls für Partikelverfahrenstechnik an der Universität Paderborn

15.01.2005 - 30.09.2006

Akademischer Rat am Lehrstuhl für Feststoff- und Grenzflächenverfahrenstechnik der Universität Erlangen-Nürnberg

15.06.1999 - 14.01.2005

Wissenschaftlicher Assistent am Lehrstuhl für Feststoff- und Grenzflächenverfahrenstechnik (Prof. Wolfgang Peuckert) an der TU München bzw. ab 15.10.2003 an der Universität Erlangen-Nürnberg

02.06.1993 - 14.06.1999

Wissenschaftlicher Mitarbeiter am Institut für Mechanische Verfahrenstechnik und Mechanik der Universität Karlsruhe

19.12.1998

Promotion zum Dr,-Ing. an der Universität Fridericiana Karlsruhe

Titel der Dissertation: Zum Partikeltransport in Elektrischen Abscheidern.

Gesamturteil 'Mit Auszeichnung'

01.10.2018 - 01.06.1993

Studium des Chemieingenieurwesens an der Universität Fridericiana Karlsruhe

Abschluss 'Dipl-Ing.'

22.01.1967

geboren in Waiblingen (Rems-Murr-Kreis)

Seit 01.06.2008

Wissenschaftlicher Leiter des Direct Manufacturing Research Center an der Universität Paderborn

Seit 01.10.2006

Inhaber des Lehrstuhls für Partikelverfahrenstechnik an der Universität Paderborn

15.01.2005 - 30.09.2006

Akademischer Rat am Lehrstuhl für Feststoff- und Grenzflächenverfahrenstechnik der Universität Erlangen-Nürnberg

15.06.1999 - 14.01.2005

Wissenschaftlicher Assistent am Lehrstuhl für Feststoff- und Grenzflächenverfahrenstechnik (Prof. Wolfgang Peuckert) an der TU München bzw. ab 15.10.2003 an der Universität Erlangen-Nürnberg

02.06.1993 - 14.06.1999

Wissenschaftlicher Mitarbeiter am Institut für Mechanische Verfahrenstechnik und Mechanik der Universität Karlsruhe

19.12.1998

Promotion zum Dr,-Ing. an der Universität Fridericiana Karlsruhe

Titel der Dissertation: Zum Partikeltransport in Elektrischen Abscheidern.

Gesamturteil 'Mit Auszeichnung'

01.10.2018 - 01.06.1993

Studium des Chemieingenieurwesens an der Universität Fridericiana Karlsruhe

Abschluss 'Dipl-Ing.'

22.01.1967

geboren in Waiblingen (Rems-Murr-Kreis)


Liste im Research Information System öffnen

2023

Influence of the filler matrix adhesion and the effects of conditioning on tensile properties of laser sintered parts built with polyamide glass bead dry blends

I. Kletetzka, M. Kosanke, D. Meinderink, V. Neßlinger, G. Grundmeier, H. Schmid, 2023

<jats:title>Abstract</jats:title> <jats:p>In this work, the influence of the filler matrix adhesion on the tensile properties of laser-sintered parts built with Polyamide 613 filled with glass beads was investigated. For this purpose, dry blends of glass beads with and without organosilane coupling agents and polyamide powder were prepared and processed into tensile specimens on an EOS P396. The samples were tested both in the dry state and after an accelerated conditioning in a climate chamber. Furthermore, FEM simulations were performed to model the limiting cases of optimum adhesion and no adhesion. By correlating the tensile tests with the simulation results and by analyzing the fracture surfaces, it was shown that the filler matrix adhesion is sufficient in the dry state but is strongly degraded by conditioning. Even the presence of various organosilane thin films could not prevent a strong deterioration of the filler-matrix adhesion and the associated deterioration of the mechanical properties. Since a comparison with an injection molded sample of the same polymer filler combination shows identical behavior after conditioning, it is assumed that this problem is not limited to additively manufactured parts.</jats:p>


Instabilities of Polymer Melt Suspensions under Uniaxial Extension (Poster)

M. Neukötter, S. Jesinghausen, H. Schmid. Instabilities of Polymer Melt Suspensions under Uniaxial Extension (Poster). In: International Congress on Rheology, Athens, 2023.


Experimental and simulative determination and correction of the effective gap extension in structured coaxial measuring systems

S. Josch, S. Jesinghausen, C. Dechert, H. Schmid, Rheologica Acta (2023)

<jats:title>Abstract</jats:title><jats:p>The use of structured measuring systems to prevent wall slip is a common approach to obtain absolute rheological values. Typically, only the minimum distance between the measuring surfaces is used for further calculation, implying that no flow occurs between the structural elements. But this assumption is misleading, and a gap correction is necessary. To determine the radius correction <jats:inline-formula><jats:alternatives><jats:tex-math>$$\Delta r$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:mi>r</mml:mi> </mml:mrow> </mml:math></jats:alternatives></jats:inline-formula> for specific geometries, we conducted investigations on three Newtonian fluids (two silicon oils and one suspension considered to be Newtonian in the relevant shear rate range). The results show that <jats:inline-formula><jats:alternatives><jats:tex-math>$$\Delta r$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:mi>r</mml:mi> </mml:mrow> </mml:math></jats:alternatives></jats:inline-formula> is not only shear- and material-independent, but geometry-dependent, providing a Newtonian flow behaviour in a similar viscosity range. Therefore, a correction value can be determined with only minute deviations in different Newtonian fluids. As the conducted laboratory measurements are very time-consuming and expensive, a CFD-approach with only very small deviations was additionally developed and compared for validation purposes. Therefore, simulation is an effective and resource-efficient alternative to the presented laboratory measurements to determine <jats:inline-formula><jats:alternatives><jats:tex-math>$$\Delta r$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:mi>r</mml:mi> </mml:mrow> </mml:math></jats:alternatives></jats:inline-formula> for the correction of structured coaxial geometries even for non-Newtonian fluids in the future.</jats:p>


Influence of a prolonged shelf time on PA12 laser sintering powder and resulting part properties

S.H. Klippstein, I. Kletetzka, I. Sural, H. Schmid, The International Journal of Advanced Manufacturing Technology (2023)

In the laser sintering technology, the semi-crystalline polymer material is exposed to elevated temperatures during processing, which leads to serious material ageing for most materials. This has already been investigated intensively by various authors. However, the ageing of the material at ambient temperatures during shelf life has not been the focus so far. The need to analyse the shelf life can be derived from an ecological and economic point of view. This work is focusing on the shelf life of PA2200 (PA12). To reduce the potential influences of powder production fluctuations, two different powder batches stored for 5.5 years and 6.5 years are investigated and compared to a reference powder produced 0.5 years before these investigations. Multiple powder analyses and part characterisations have been performed. A significant yellowing and molecular chain length reduction can be derived from the measurement results. Whereas the influence on mechanical part performance was minor, the parts built with the stored powders are more yellowish. As it is most likely that this is due to the consumption of polyamide stabilisers, it can be assumed that these parts will be subject to significantly faster ageing. Therefore, it is still not recommended to use the stored powders for critical parts or light intense and humid environments.


CDMA: Centrifugal Differential Mobility Analyzer - Ein Messprinzip zur Bestimmung von Multidimensionalen Partikeleigenschaften (Vortrag)

T. Rüther, H. Schmid. CDMA: Centrifugal Differential Mobility Analyzer - Ein Messprinzip zur Bestimmung von Multidimensionalen Partikeleigenschaften (Vortrag). In: DECHEMA Jahrestreffen PMT 2023, Paderborn, 2023.


Development of an adaptive coaxial concrete rheometer and rheological characterisation of fresh concrete

S. Josch, S. Jesinghausen, H. Schmid, Applied Rheology (2023), 33(1)

<jats:title>Abstract</jats:title> <jats:p>The accessibility to rheological parameters for concrete is becoming more and more relevant. This is mainly related to the constantly emerging challenges, such as not only the development of high-strength concretes is progressing very fast but also the simulation of the flow behaviour is of high importance. The main problem, however, is that the rheological characterisation of fresh concrete is not possible via commercial rheometers. The so-called concrete rheometers provide valuable relative values for comparing different concretes, but they cannot measure absolute values. Therefore, we developed an adaptive coaxial concrete rheometer (ACCR) that allows the measurement of fresh concrete with particles up to <jats:inline-formula> <jats:alternatives> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j_arh-2022-0140_eq_001.png" /> <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:msub> <m:mrow> <m:mi>d</m:mi> </m:mrow> <m:mrow> <m:mi mathvariant="normal">max</m:mi> </m:mrow> </m:msub> <m:mo>=</m:mo> <m:mn>5.5</m:mn> <m:mspace width=".5em" /> <m:mi mathvariant="normal">mm</m:mi> </m:math> <jats:tex-math>{d}_{{\rm{\max }}}=5.5\hspace{.5em}{\rm{mm}}</jats:tex-math> </jats:alternatives> </jats:inline-formula>. The comparison of the ACCR with a commercial rheometer showed very good agreement for selected test materials (Newtonian fluid, shear thinning fluid, suspension, and yield stress fluid), so that self-compacting concrete was subsequently measured. Since these measurements showed a very high reproducibility, the rheological properties of the fresh concrete could be determined with high accuracy. The common flow models (Bingham (B), Herschel–Bulkley, modified Bingham (MB) models) were also tested for their applicability, with the Bingham and the modified Bingham model proving to be the best suitable ones.</jats:p>


Flowability of polymer powders at elevated temperatures for additive manufacturing

M.J. Rüther, S.H. Klippstein, S. Ponusamy, T. Rüther, H. Schmid, Powder Technology (2023), 422, 118460

DOI


Dynamisch-mechanische Polymerpartikelverrundung zur Verbesserung der Fließfähigkeit für das selektive Lasersintern

S. Ponusamy, H. Schmid. Dynamisch-mechanische Polymerpartikelverrundung zur Verbesserung der Fließfähigkeit für das selektive Lasersintern. In: Jahrestreffen der Fachgruppen Zerkleinern / Klassieren / Agglomerations- und Schüttguttechnik, Braunschweig, 2023.


Particles as Seeds for Instabilities in Uniaxially Elongated Polymer Suspension Filaments (Presentation)

M. Neukötter, S. Jesinghausen, H. Schmid. Particles as Seeds for Instabilities in Uniaxially Elongated Polymer Suspension Filaments (Presentation). In: Annual Meeting of the German Rheological Society (DRG) , Berlin, 2023.


2022

Model droplet formation in extensional filament stretching within a Filament Extension Atomizer

M. Neukötter, S. Jesinghausen, H. Schmid, Rheologica Acta (2022)

<jats:title>Abstract</jats:title><jats:p>Further innovation in the field of selective laser sintering (SLS) is strongly connected to the availability of new materials since the market is dominated by polyamide 12 (&gt;90%). The aim of this publication is to develop a descriptive model for the droplet formation process in a Filament Extension Atomizer to predict the applicability to exploit further polymers for the SLS process. The feasibility was tested, investigated and characterized using a “Dripping out of a nozzle” setup for uniaxial extension. The droplet formation process was then observed via high-speed camera imaging and classified for certain parameters. The experiments were carried out using semi-diluted polyethylene oxide (600–4000 kg/mol), glycerol and water solutions as model fluids. Driven by the Plateau-Rayleigh instability, different types of spherical droplets were observed and various droplet formation mechanisms demonstrated and analyzed. Based on the experimental results, a predictive model is derived to describe various essential parameters.</jats:p>


A combined flue gas cleaning system with a novel entrained flow SCR using an online synthesized catalyst (Poster)

J. Beimdiek, H. Schmid. A combined flue gas cleaning system with a novel entrained flow SCR using an online synthesized catalyst (Poster). In: 1st International Workshop on Reacting Particle-Gas Systems, Bochum, 2022.


Innovative Werkstoffherstellung für das Polymer Laser Sintern – Teil 2

M.J. Rüther, S.H. Klippstein, S. Ponusamy, S. Jesinghausen, H. Schmid, Werkstoffe (2022), 6, pp. 18 - 19


Innovative Werkstoffherstellung für das Polymer Laser Sintern – Teil 1

M.J. Rüther, S.H. Klippstein, S. Ponusamy, S. Jesinghausen, H. Schmid, Werkstoffe (2022), 5, pp. 28-29


Combined filtration process for biomass combustion plants to comply with legal requirements (Presentation)

J. Beimdiek, F. Prill, M. König, M. Müller, I. Hartmann, H. Schmid, in: 13th World Filtration Congress Proceedings, 2022


A combined flue gas cleaning system with a novel entrained flow SCR using online synthesized catalyst particles (Poster)

J. Beimdiek, H. Schmid. A combined flue gas cleaning system with a novel entrained flow SCR using online synthesized catalyst particles (Poster). In: 11th International Aerosol Conference (IAC), Athens, 2022.


Novel entrained flow SCR using online synthesized catalyst particles (Poster)

J. Beimdiek, H. Schmid. Novel entrained flow SCR using online synthesized catalyst particles (Poster). In: 13th World Filtration Congress, San Diego, 2022.


Novel entrained flow SCR using online synthesized catalyst particles in flue gas cleaning systems (Presentation)

J. Beimdiek, H. Schmid. Novel entrained flow SCR using online synthesized catalyst particles in flue gas cleaning systems (Presentation). In: 25th ETH-Conference on Combustion Generated Nanoparticles, Zürich, 2022.


Triggering instabilities of polymer solution suspensions under uniaxial extension (Presentation)

M. Neukötter, S. Jesinghausen, H. Schmid. Triggering instabilities of polymer solution suspensions under uniaxial extension (Presentation). In: Joint Online Symposium on Rheology, Digital, 2022.


Instability analysis of suspensions with a polymer solution matrix (Presentation)

M. Neukötter, S. Jesinghausen, H. Schmid. Instability analysis of suspensions with a polymer solution matrix (Presentation). In: Annual European Rheology Conference, Sevilla, 2022.


Flowability of polymer powders at elevated temperatures for additive manufacturing

M.J. Rüther, S.H. Klippstein, H. Schmid. Flowability of polymer powders at elevated temperatures for additive manufacturing. In: 9th Congress on Particle Technology, Madrid, 2022.


A novel correction method for the shear rate in a couette rheometer (Vortrag)

T. Rüther, S. Jesinghausen, H. Schmid. A novel correction method for the shear rate in a couette rheometer (Vortrag). In: AERC , Sevilla, 2022.


Multidimensional particle characterization: Centrifugal Differential Mobility Analyzer (Poster)

T. Rüther, H. Schmid. Multidimensional particle characterization: Centrifugal Differential Mobility Analyzer (Poster). In: World Congress on Particle Technology 2022, Madrid, 2022.


NEW CORRECTION APPROACHES FOR DETERMINING THE TRUE SHEAR RATE IN A COAXIAL-RHEOMETER (Vortrag)

T. Rüther, S. Jesinghausen, H. Schmid. NEW CORRECTION APPROACHES FOR DETERMINING THE TRUE SHEAR RATE IN A COAXIAL-RHEOMETER (Vortrag). In: Joint Online Symposium on Rheology 2022, 2022.


Powder Spread Process Monitoring in Polymer Laser Sintering and its Influences on Part Properties

S.H. Klippstein, F. Heiny, N. Pashikanti,, M. Gessler, H. Schmid, JOM - The Journal of The Minerals, Metals & Materials Society (TMS) (2022), 74, pp. 1149–1157

Confidence in additive manufacturing technologies is directly related to the predictability of part properties, which is influenced by several factors. To gain confidence, online process monitoring with dedicated and reliable feedback is desirable for every process. In this project, a powder bed monitoring system was developed as a retrofit solution for the EOS P3 laser sintering machines. A high-resolution camera records each layer, which is analyzed by a Region-Based Convolutional Neural Network (Mask R-CNN). Over 2500 images were annotated and classified to train the network in detecting defects in the powder bed at a very high level. Each defect is checked for intersection with exposure areas. To distinguish between acceptable imperfections and critical defects that lead to part rejection, the impact of these imperfections on part properties is investigated.


Effects of spherical fillers on the processability and mechanical properties of PA613 and PP-based LS dry blends

I. Kletetzka, R. Gawlikowicz, H. Schmid, in: Proceedings of the 33nd Annual International Solid Freeform Fabrication Symposium, Laboratory for Freeform Fabrication and University of Texas, 2022

In this work, the influences of spherical fillers on the processing properties and the resulting mechanical properties of laser sintered components are investigated. For this purpose, micro glass spheres, hollow glass bubbles and mineral spheres are dry blended to the matrix polymers polyamide 613 and polypropylene with a filling ratio of 20 and 40 vol%. First, relevant properties of the blends, such as powder flowability, thermal behavior and melt viscosity, are investigated. Based on the results, processing parameters are then developed for the LS process and the mechanical properties of the components are investigated. The aim is to be able to tailor the mechanical properties of LS components by adding fillers and thus to create new application areas for additively manufactured components.


Powder Spread Flaws in Polymer Laser Sintering and its Influences on Mechanical Performance

S.H. Klippstein, H. Schmid, in: Proceedings of the 33nd Annual International Solid Freeform Fabrication Symposium, 2022

By monitoring the recoating process within polymer laser sintering production, it was shown that multiple powder-spread-flaws can be detected. Those groove-like flaws are expected to be the result of agglomerates jamming between the recoater and the last powder layer. This work is analyzing the interaction between powder-spread-flaws and part properties, showing the influence of the recoating process on the performance of laser sintering parts. Therefore, artificial powder-spread-flaws are applied to the build jobs of tensile test specimens which are measured and analyzed regarding the elongation at break, strength and fracture position. For the characteristics of the flaws, the artificial grooves are varied in depth and width. Furthermore, the position of the flaw is changed form mid part to close to surface areas. It was shown, that several flaws are visible at the part surface, resulting in stress concentration and reduced performance. But there are as well parts with flaw-layers, which are not visible after the build process on the part. Those parts can have significantly reduced mechanical properties as well.


Shelf Life of Polyamide 12 (PA2200) Laser Sintering Powder

I. Kletetzka, S.H. Klippstein, H. Schmid, in: Proceedings of the 33nd Annual International Solid Freeform Fabrication Symposium, Laboratory for Freeform Fabrication and University of Texas, 2022


A Shape Newton Scheme for Deforming Shells with Application to Capillary Bridges

S. Schmidt, M. Gräßer, H. Schmid, SIAM Journal on Scientific Computing (2022), 44(4), pp. B1175-B1194

DOI


Low Temperature Laser Sintering with PA12 and PA6 on a Standard System

D. Menge, H. Schmid, Macromolecular Symposia (2022), 404(1), 2100397

DOI


2021

Mechanical characterization and numerical modeling of laser-sintered TPE lattice structures

C. Kummert, H. Schmid, L. Risse, G. Kullmer, Journal of Materials Research (2021)

Additive Manufacturing provides the opportunity to produce tailored and complex structures economically. The use of lattice structures in combination with a thermoplastic elastomer enables the generation of structures with configurable properties by varying the cell parameters. Since there is only little knowledge about the producibility of lattice structures made of TPE in the laser sintering process and the resulting mechanical properties, different kinds of lattice structures are investigated within this work. The cell type, cell size and strut thickness of these structures are varied and analyzed. Within the experimental characterization of Dodecahedron-cell static and cyclic compression tests of sandwich structures are focused. The material exhibits hyperelastic and plastic properties and also the Mullins-Effect. For the later design of real TPE structures, the use of numerical methods helps to reduce time and costs. The preceding experimental investigations are used to develop a concept for the numerical modeling of TPE lattice structures.


Low Temperature Laser Sintering on a Standard System: First Attempts and Results with PA12

D. Menge, H. Schmid, 2021

The laser sintering process has been a well-established AM process for many years. Disadvantages of LS are the low material variety and the thermal damage of the unprocessed material. The low temperature laser sintering attacks at this point and processes powder material at a build chamber temperature lower than the recrystallization temperature. This drastic reduction in temperature results in significantly less thermal damage to the material. This work deals with the low temperature laser sintering of Polyamide 12 (PA12) on a commercial, unmodified laser sintering system to compare it to standard laser sintered PA12 and to create the basis for low temperature laser sintering of high temperature materials on such a system. First results by changing the exposure parameters and by fixing parts on a building platform show a processing of PA12 on an EOS P396 at a build chamber temperature less than 100 °C instead of standard approx. 175 °C.


Einfluss der Zerstäubung auf die Partikelsynthese in Sprayflammen

R. Tischendorf, H. Schmid, M. Bieber, M. Reddemann, R. Kneer, F. Fröde, T. Grenga, H. Pitsch. Einfluss der Zerstäubung auf die Partikelsynthese in Sprayflammen. In: Jahrestreffen der ProcessNet-Fachgruppe MPS, Paderborn, 2021.



Flowability Measurements of Different SLS Materials at Elevated Temperatures with a Modified Schulze Ring Shear Tester

M.J. Rüther, D. Schulze, H. Schmid. Flowability Measurements of Different SLS Materials at Elevated Temperatures with a Modified Schulze Ring Shear Tester. In: Solid Freeform Fabrication Symposium, digital, 2021.


Effektivität von Luftreinigern zur Reduzierung des COVID-19-Infektionsrisikos/ Efficacy of air purifiers in reducing the risk of COVID-19 infections

S. Schumacher, C. Asbach, H. Schmid, Gefahrstoffe (2021), pp. 16-28

Luftreiniger werden derzeit häufig als mögliches Hilfsmittel zur Minimierung des Infektionsrisikos im Rahmen der COVID-19-Pandemie diskutiert. Dabei taucht oft die Frage auf, ob Luftreiniger grundsätzlich in der Lage sind, Viren oder virenbeladene Tröpfchen abzuscheiden. Ziel dieses Artikels ist es, die wesentlichen Grundlagen der Wirkungsweise von Luftreinigern und Filtern zu beschreiben, Methoden zur Messung der Reinigungswirkung im Größenbereich von Viren und virenbeladenen Tröpfchen aufzuzeigen sowie typische Szenarien zum Betrieb von Luftreinigern in einem mathematischen Modell zu erfassen. Darauf basierend können die Möglichkeiten und Grenzen von Luftreinigern für reale Anwendungsfälle besser eingeschätzt werden.


Laser Sintering Design Guidelines for media transmitting Components

I. Kletetzka, C. Kummert, H. Schmid, in: Proceedings of the 32nd Annual International Solid Freeform Fabrication Symposium, Laboratory for Freeform Fabrication and University of Texas, 2021

In automotive and other fields of application media-carrying components often have complex, flow-optimized geometries and are made of plastics for reasons of weight and cost. Therefore, the laser sintering technology is predestinated to manufacture these components as it offers a very high degree of design freedom and good mechanical properties. For industrial applications the long-term properties of the SLS material in contact with liquid media are important and were therefore investigated for PA12, PP and PA613. Hereby, different media such as motor oil or Glysantin based coolant were tested with different temperatures and immersion times of up to 26 weeks. The mechanical properties were tested after immersion and compared to injection molded samples. Furthermore, laser sintering design guidelines for media-carrying components were developed. These guidelines for instance include the minimum wall thickness to ensure media tightness and the removal of powder from channels with a high length to diameter ratio.


Assessing spreading and removal of virus laden aerosols in different settings using an aerosol method (Presentation)

J. Ludwig, C. Kykal, H. Schmid. Assessing spreading and removal of virus laden aerosols in different settings using an aerosol method (Presentation). In: European Aerosol Conference, 2021.


Synthese und asymmetrische Funktionalisierung von submikronen Janus-Polymer-Partikeln in der Gasphase (Poster)

J. Ludwig, H. Schmid. Synthese und asymmetrische Funktionalisierung von submikronen Janus-Polymer-Partikeln in der Gasphase (Poster). In: Jahrestreffen der ProcessNet-Fachgruppe Aerosoltechnik, 2021.


Theoretische Beschreibung des Tropfenbildungsprozesses bei der Filament Extension Atomization: Entwicklung eines Vorhersagemodells für die Tropfengröße (Poster)

M. Neukötter, S. Jesinghausen, H. Schmid. Theoretische Beschreibung des Tropfenbildungsprozesses bei der Filament Extension Atomization: Entwicklung eines Vorhersagemodells für die Tropfengröße (Poster). In: Jahrestreffen der ProcessNet-Fachgruppen Computational Fluid Dynamics und Mehrphasenströmungen, Online, 2021.


Filament Extension Atomization – A novel Process for Powder Production? (Presentation)

M. Neukötter, S. Jesinghausen, H. Schmid. Filament Extension Atomization – A novel Process for Powder Production? (Presentation). In: Annual European Rheology Conference, Online, 2021.


2020

Additive Leichtbaustrukturen für die Flugzeugkabine

D. Menge, S.H. Klippstein, H. Schmid, 2020


An Apparatus to Synthesize Ceramic Nanoparticles with a Precisely Adjusted Temperature History and a Significant Mass Output

D.B. Rasche, L. Tigges, H. Schmid, Review of Scientific Instruments (2020), 91, pp. 055104

For gas phase nanoparticle production, hot wall reactors are widely used. In this article, we will describe the fundamental design considerations for a hot wall reactor system able to produce oxide nanoparticles. The system is outstanding in its ability to produce mostly spherical nanoparticles at particle sizes of up to 100 nm and even larger at mass outputs in the order of grams per hour by being able to rapidly quench the aerosol. While high production rates or larger particle sizes are already easily obtained with hot wall reactors, it is very challenging to produce these spherical particles at high mass rates. We will show in this research that the temperature and the particle number concentration are the major aspects influencing the particle morphology at the end of the process. Investigation on the performance of the setup shows good control over the temperature and the particle production stability. A representative particle characterization using SEM and scanning mobility particle sizer showed that particles are mostly spherical, while the particle size distribution had a geometric standard deviation close to 1.5. In addition to the aspects mentioned above, a possibility to manipulate the aggregation downstream of the reactor is to be presented as well. We found that applying electrical charges to the aerosol particles (in opposite polarity) can significantly foster aggregation.


Influence of Part Microstructure on Mechanicl Properties of PA6X Laser Sintered Specimens

C. Kummert, W. Diekmann, K. Tews, H. Schmid, in: 29th Annual International Solid Freeform Fabrication Symposium, 2020, pp. 735-744

DOI


Influence of differing nozzle configurations on the final particle characteristics and particle structure evolution in spray flames

R. Tischendorf, H. Schmid, M. Bieber, M. Reddemann , R. Kneer, F. Fröde, T. Grenga, H. Pitsch. Influence of differing nozzle configurations on the final particle characteristics and particle structure evolution in spray flames. In: 4th International Symposium Gas‐Phase Synthesis of Functional Nanomaterials, Duisburg, 2020.


Interlaboratory study on rheological properties of cement pastes and reference substances: comparability of measurements performed with different rheometers and measurement geometries

M. Haist, J. Link, D. Nicia, S. Leinitz, C. Baumert, T. von Bronk, D. Cotardo, M. Eslami Pirharati, S. Fataei, H. Garrecht, C. Gehlen, I. Hauschildt, I. Ivanova, S. Jesinghausen, C. Klein, H. Krauss, L. Lohaus, D. Lowke, O. Mazanec, S. Pawelczyk, U. Pott, N.W. Radebe, J.J. Riedmiller, H. Schmid, W. Schmidt, E. Secrieru, D. Stephan, M. Thiedeitz, M. Wilhelm, V. Mechtcherine, Materials and Structures (2020)

This paper presents the results of an interlaboratory study of the rheological properties of cement paste and ultrasound gel as reference substance. The goal was to quantify the comparability and reproducibility of measurements of the Bingham parameters yield stress and plastic viscosity when measured on one specific paste composition and one particular ultrasound gel in different laboratories using different rheometers and measurement geometries. The procedures for both in preparing the cement paste and carrying out the rheological measurements on cement paste and ultrasound gel were carefully defined for all of the study’s participants. Different conversion schemes for comparing the results obtained with the different measurement setups are presented here and critically discussed. The procedure proposed in this paper ensured a reasonable comparability of the results with a coefficient of variation for the yield stress of 27% and for the plastic viscosity of 24%, despite the individual measurement series’ having been performed in different labs with different rheometers and measurement geometries.


Absolute Rheological Measurements of Model Suspensions: Influence and Correction of Wall Slip Prevention Measures

S. Pawelczyk, M. Kniepkamp, S. Jesinghausen, H. Schmid, Materials (2020), 467

<jats:p>Since suspensions (e.g., in food, cement, or cosmetics industries) tend to show wall slip, the application of structured measuring surfaces in rheometers is widespread. Usually, for parallel-plate geometries, the tip-to-tip distance is used for calculation of absolute rheological values, which implies that there is no flow behind this distance. However, several studies show that this is not true. Therefore, the measuring gap needs to be corrected by adding the effective gap extension δ to the prescribed gap height H in order to obtain absolute rheological properties. In this paper, we determine the effective gap extension δ for different structures and fluids (Newtonian, shear thinning, and model suspensions that can be adjusted to the behavior of real fluids) and compare the corrected values to reference data. We observe that for Newtonian fluids a gap- and material-independent correction function can be derived for every measuring system, which is also applicable to suspensions, but not to shear thinning fluids. Since this relation appears to be mainly dependent on the characteristics of flow behaviour, we show that the calibration of structured measuring systems is possible with Newtonian fluids and then can be transferred to suspensions up to a certain particle content.</jats:p>


2019

FVA-Nr. 813 I - Generative Verfahren zur Herstellung von Polymerbauteilen

D. Menge, R. Walter, H. Schmid, U.P. Breuer, 2019


Understanding and Manipulation of Nanoparticle Contact Forces by Capillary Bridges

H. Schmid, G. Grundmeier, M. Dörmann, A.G. Orive, T. de los Arcos, B. Torun, in: Particles in Contact, 2019

Since, in the presence of humidity the inter-particle processes are dominated by capillary forces, a fundamental understanding of the water adsorption and the capillary bridge formation is very important. However, the adsorbed water structure and thus the capillary bridge formation is influenced by various parameters like the particle morphology (e.g. particle size, roughness) as well as the surface chemistry (surface energy, adsorbate structure) and therefore needs to be analyzed on a submicroscopic or even molecular basis. A multi-scale approach ranging from experiments on an individual particle level (AFM and liquid bridge simulation) and investigations on small particle ensembles (combined QCM-D/FTIR) up to macroscopic description of bulk behavior is presented in this chapter. In this context, the combined in situ QCM-D/FTIR experiments are bridging the gap between experiments on an individual particle level and macroscopic bulk behavior. Variation of surface chemistry by means of adsorption of functional organic molecules allows for the correlation of macroscopic particle behavior to nanoscopic effects like the presence and structure of adsorbate layers as well as the formation of capillary bridges while keeping the disperse properties constant. Besides extensive experimental work, simulations of capillary bridges formed by condensation from humid air are presented. It is clearly shown that well known approximations which have been introduced for micron-sized particles are not valid any more for nano-scaled particles. The forces between nanoparticles by static liquid bridges and their dependency on particle size, contact angle, humidity and interparticle distance are discussed in detail. Furthermore, capillary forces during separation of particles are studied thoroughly and a constitutive law based on a contact stiffness allows the transfer to DEM simulations of wet powders. Finally, it is demonstrated by comparison to Molecular Dynamics simulations, that the used continuum approach to simulate capillary bridges might even be used down to particle sizes of a few nanometers, if some additional effects are considered correctly.


Methodik zur Qualifizierung des Lasersinter Prozesses für die Serienfertigung

S.H. Klippstein, H. Schmid, in: Proceedings of the 16th Rapid.Tech Conference, 2019

Zuverlässige, wiederholbare Bauteileigenschaften sind unabdingbar um das Herstellungsverfahren Polymer Lasersintern im industriellen Prozess-Portfolio vieler Firmen aufnehmen zu können. Einige Unternehmen und Institute haben sich daher in jüngster Zeit mit dem Thema der reproduzierbaren Bauteileigenschaften beschäftigt. Mit der hier vorgestellten und angewandten Methodik wird nicht nur der Prozessablauf vom Bauteil bis zu Nachbearbeitung betrachtet, sondern auch die Maschinenperformance in einem Ringversuch und über einen längeren Zeitraum geprüft. Rückgrat dieser Untersuchung bildet hierbei der aus der Six Sigma Lehre stammende DMAIC (Define - Measure - Analyse - Improve - Control) Verbesserungszyklus. Hierfür wird ein Standard-Prozess definiert. Diesem folgend werden die für die Industrie oder den Anwender interessanten Messungen aufgenommen und analysiert. Anschließend wird der Prozess sowie die Messmethodik optimiert und auch Kontrollmethoden definiert. Für die Anwendung der entwickelten Methodik wird exemplarisch der Maschinentyp EOS P396 mit PA2200 untersucht. Daten für die Bestimmung der Mechanik, der Optik und der Haptik sowie für die Dimensionen und die Bauteildichte werden als Qualitätskriterium aufgenommen und über einen längeren Zeitraum analysiert. Weiteres Ziel ist es, den Messaufwand zu reduzieren und die Qualitätssicherung im Serienbtrieb zu gewährleisten.


Representative Sample Extraction from Spray Flames for SMPS and HR-TEM Measurements

R. Tischendorf, M. Bieber, R. Kneer, H. Schmid. Representative Sample Extraction from Spray Flames for SMPS and HR-TEM Measurements. In: PARTEC 2019 International Congress on Particle Technology, Nürnberg, 2019.


Devolopment, Production and post-processing of a topology optimized aircraft bracket

S.H. Klippstein, A. Duchting, T. Reiher, F. Hengsbach, D. Menge, H. Schmid, in: 30th Annual International Solid Freeform Fabrication Symposium, 2019, pp. 1932-1945

Structural parts for aviation have very high demands on the development and production process. Therefore, the entire process must be considered in order to produce high-quality AM metal parts. In this case study, a conventional part was selected to be optimized for AM. The process presented includes component selection, design improvement with a novel approach for topology optimization based on the AMendate algorithm as basis of MSC Apex Generative Design,component production on a SLM 250 HL and post-processing including heat treatment and surface smoothing. With the topology optimization a weight reduction of ~60 % could be realized, whereby the stress distribution is more homogeneous. Furthermore, the challenges of support optimization and post-processing have to be addressed, in order to produce competitive parts.


Water adsorption and capillary bridge formation on silica micro-particle layers modified with perfluorinated organosilane monolayers

I. Giner, B. Torun, Y. Han, B. Duderija, D. Meinderink, A.G. Orive, M.T. de los Arcos de Pedro, C. Weinberger, M. Tiemann, H. Schmid, G. Grundmeier, Applied Surface Science (2019), pp. 873-879

Monodisperse micron-sized silica particle monolayers deposited onto plasma-grown SiOx-ultra-thin films have been used as reference systems to investigate wetting, water adsorption and capillary bridge formation as a function of silica surface functionalization. 1H,1H, 2H,2H perfluorooctyltriethoxysil (FOTS) monolayers, have been deposited on the respective surfaces by means of chemical vapor deposition resulting in macroscopically low energy surfaces. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) reflection absorption spectroscopy confirmed the monolayer formation. Water adsorption isotherms were studied by a combination of in-situ FTIR reflection spectroscopy and quartz crystal microbalance (QCM) while macroscopic wetting was analysed by contact angle measurements. The comparative data evaluation indicates that the macroscopic wetting behaviour was changed as expected, however, that water nanodroplets formed both at intrinsic defects of the FOTS monolayer and at the FOTS/SiOx interface. Capillary bridges of liquid water are dominantly formed in the confined particle contact areas and between surface asperities on the particles. The comparison of wetting, adsorption and capillary bridge formation shows that the hydrophobization of porous materials by organosilane monolayers leads to the formation of morphology dependent nanoscopic defects that act as sites for preferential capillary bridge formation.


2018

Representative domain size for the simulation of coalescence filtration in nonwoven and foam media

S. Abishek, A. King, J. Schuler, G. Kasper, H. Schmid, B. Mullins, Separation and Purification Technology (2018), pp. 344-352

Pore-scale filtration simulations require high spatio-temporal resolutions and significant computational effort, hence, keeping the domain size to a minimum is desirable. Previous studies have considered domains based on Brinkman length, or are limited by computing power, and little information is available for conditions involving high fluid saturation – typical of steady state mist filtration. In this study, simulations are performed to characterize the effect of domain size on pressure drop, residual saturation, liquid film thickness and interfacial area concentration, using virtual nonwoven and foam filters with similar micro-structural properties. Further, experiments using micro-CT are performed to validate the present computational simulations. It is found that two phase flow through filters are more sensitive to local geometric variations or mesh resolution in the porous media than single phase flow. Statistical uncertainties in the steady state quantities of less than +-10% can be expected to cope with the increase in computing power required for practical mesh sizes. A computational domain size of about 50–100xd (where d is the strut or fibre diameter) was found to be required for CFD for the operating conditions considered.


Topology Optimized Heat Transfer Using the Example of an Electronic Housing

D. Menge, P. Delfs, M. Töws, H. Schmid, in: 29th Annual International Solid Freeform Fabrication Symposium, 2018, pp. 687-697

Function integration is a key issue for an efficient and economic usage of Additive Manufacturing. An efficient heat transfer by topology optimized structures is a rarely considered approach which will be outlined with an exemplary electronic housing which has been newly designed. A commercial projector unit, whose electrical components in total produce 38 W, shall be integrated in the closed housing and passively cooled by natural convection. Topology optimized structures shall be generated in the inner part of the housing to transfer the heat homogenously from the projector components to the housing wall while simultaneously minimizing the mass. At the outside of the housing walls, lattice and rib structures are applied to increase the effective surface for heat transfer by natural convection and radiation. Furthermore, the housing geometry is optimized regarding a minimization of support structures to reduce the post-processing effort. Finally, the housing shall be built of AlSi10Mg by SLM.


Thermoplastic Elastomer Part Color as Function of Temperature Histories and Oxygen Atmosphere During Selective Laser Sinterung

C. Kummert, S. Josupeit, H. Schmid, Journal of Minerals, Metals and Materials Society (2018), 70(3), pp. 425-430

The influence of selective laser sintering (SLS) parameters on PA12 part properties is well known, but research on other materials is rare. One alternative material is a thermoplastic elastomer (TPE) called PrimePart ST that is more elastic and shows a distinct SLS processing behavior. It undergoes a three-dimensional temperature distribution during the SLS process within the TPE part cake. To examine this further, a temperature measurement system that allows temperature measurements inside the part cake is applied to TPE in the present work. Position-dependent temperature histories are directly correlated with the color and mechanical properties of built parts and are in very good agreement with artificial heat treatment in a furnace. Furthermore, it is clearly shown that the yellowish discoloration of parts in different intensities is not only temperature dependent but also influenced by the residual oxygen content in the process atmosphere. Nevertheless, the discoloration has no influence on the mechanical part properties.


Bestimmung von Nanopartikeleigenschaften eines Aerosols

H. Schmid, D. Rasche. Bestimmung von Nanopartikeleigenschaften eines Aerosols, Patent 10 2017 101 766.6. 2018.

Zusammenfassung: Bei einem Verfahren und einer Vor- richtung zur Bestimmung von Nanopartikeleigenschaften ei- nes nanopartikelbeladenen Aerosols (17), mit einer Test- kammer (1) zur Bestimmung der Nanopartikeleigenschaften des Aerosols (17), wobei die Testkammer (1) einen Aero- soleinlass (9) zum Einleiten des Aerosols (17) und ein Hül- lenluftstromeinlass (10) zur Einleitung eines Hüllenluftstroms (16) aufweist, die Oberseite (2) und die Unterseite (3) der Testkammer (1) jeweils eine flächige Elektrode (4, 5) auf- weist, so dass zwei im Abstand zueinander angeordnete Elektroden (4, 5) bereitstehen, zwischen denen ein elektri- sches Feld (6) erzeugbar ist, soll ein kompaktes Messgerät für die Vermessung der Eigenschaften von Aerosolnanopar- tikeln zur Verfügung gestellt werden, bei dem es ermöglicht wird, im selben Gerät die zweidimensionale Verteilung be- züglich des aerodynamischen und des volumenäquivalen- ten Durchmessers zu bestimmen, wodurch eine Lösung ge- schaffen wird, die es ermöglicht Aussagen über die Größe, die Masse, die Form und die geometrische Struktur einer Partikel zu treffen. Dies wird dadurch erreicht, dass der Inhalt der Testkammer (1) in Rotation versetzt wird, wobei die Ro- tationsachse (18) parallel zum elektrischen Feld (6) verläuft.


2017

Aufbau einer Lasersinter-Versuchsanlage und Verarbeitung neuer Materialien am Beispiel von Polyamid 6 und Polyamid 613

J. Lohn, H. Schmid, in: Rapid.Tech – International Trade Show & Conference for Additive Manufacturing, 2017

Selective Laser Sintering (SLS) is an Additive Manufacturing technology, which allows the production of functional polymer parts. Conventionally mainly Nylon 12 (PA 12), Polyamide 11 (PA 11), glass- or aluminum filled materials enable rapid prototyping during the product development process. Those materials do not always meet the requirements for direct production of functional parts by laser sintering. For Direct Manufacturing of functional parts out of required materials and in high quality, the laser sintering process and the material palette needs to be further developed. During this research a laser sintering test equipment for material qualification has been built up. The forthcomings are an optimized software solution, an innovative optical system, an alternative powder coating system and an improved temperature control. The functionality of the test equipment is proved with the standard material PA2200 and two new laser-sintering-materials, Nylon 6 (PA 6) and Polyamide 613 (PA 613), are investigated. The process parameters are derived and mechanical properties are determined by tensile tests.


Dependencies of the Adhesion Forces between TiO2 Nanoparticles on Size and Ambient Humidity

J. Laube, M. Dörmann, H. Schmid, L. Mädler, L. Colombi Ciacchi, The Journal of Physical Chemistry C (2017), pp. 15294-15303

We study the variation of the adhesion forces between wet TiO2 nanoparticles as a function of their size and the ambient relative humidity. Combining all-atom molecular dynamics and capillary simulations we demonstrate that the linear scaling of the interparticle forces with the particle diameter, well established for microscopic and macroscopic particles, can be extended down to diameters of a few nm. At this size scale, however, the molecular nature of the water adsorbates dictates the adhesion forces both via solvation effects and influencing parameters of analytical capillary models such as the equilibrium particle–particle separation distance and the water/particle contact angle. Moreover, the water surface tension becomes considerably larger than the macroscopic bulk value due to combined effects of thin-film confinement and tight curvature, in a way that strongly depends on humidity and particle size. Taking these effects into proper account, classical capillary equations can be used to predict the interparticle forces even of the smallest particles considered here (4 nm), although the circular approximation fails to reproduce the distance at which the water meniscus breaks. Finally, the transition between the dominating effects at the nanoscopic scale and conventional capillary theory valid at microscopic size scales can be only rationalized if the presence of roughness asperities on the surface of the large particles is explicitly taken into account.


Distance-dependency of capillary bridges in thermodynamic equilibrium

M. Dörmann, H. Schmid, Powder Technology (2017), pp. 175-183

Capillary forces are very important considering the handling of powders as they, in general, exceed other adhesion forces. These capillary forces are dependent on several different parameters. Especially the distance between the particles is an important parameter. For example, in moving bulk solids a large variety of distances between particles will occur. Therefore, the distance-dependence of capillary bridges was investigated with a numerical simulation method, assuming thermodynamic equilibrium which is attained very fast for small particles. This method uses the Kelvin equation and the Young-Laplace equation to calculate numerically the shape of the capillary bridge without any assumption regarding the shape. The force is eventually derived from the meniscus shape. The distance becomes extremely important when the capillary liquid bridge between two surfaces is only determined by capillary condensation depending on relative humidity. Only a slight increase of the distance within the fraction of a nanometer changes the behaviour of the capillary force significantly. Furthermore, the influence of the force on the separation of particles will be presented. The force decreases almost linearly with increasing distance for a wide range of distances and consequently, a contact stiffness for capillary bridges could be derived. These results may e.g. be used in DEM models. Also, the maximum separation distance of capillary bridges in thermodynamic equilibrium and the correlation with the according bridge volume was investigated. As two limiting cases for capillary bridges at varying distances one can assume either a constant curvature, i.e. infinitely fast attainment of equilibrium, or a constant volume, i.e. infinitely slow attainment of equilibrium. Therefore, a comparison of these two possibilities will be presented and discussed as well.


Herstellbarkeit und mechanische Charakterisierung von lasergesinterten Gitterstrukturen

D. Menge, S. Josupeit, P.. Delfs, H. Schmid, in: Additive Fertigung von Bauteilen und Strukturen, Springer Vieweg, 2017, pp. 105-120

DOI


Experimental analysis and modeling of local ageing effects during laser sintering of polyamide 12 in regard to individual thermal histories

S. Josupeit, H. Schmid, Journal of Applied Polymer Science (2017), 134(42)

Polymer laser sintering (LS) is an important additive manufacturing (AM) technology. Individual and complex parts are directly produced from CAD data without the need of specific tools. The raw material is a polymer powder, which is deposited layerwise and melted selectively with a laser. Built parts are embedded in residual unmolten powder, the so-called part cake, which undergoes thermal ageing effects due to the exposure to high temperatures for long times during the manufacturing process. Hence, the recyclability of the unmolten powder is limited. This article focuses on a fundamental analysis of the ageing kinetics dependent on time, temperature, and oxygen content in the gas atmosphere. A model is developed and applied to measured, position-dependent process temperature histories to successfully predict the ageing distribution within a part cake. The results can be used to optimize the thermal process management in LS and to develop new efficient powder recycling methods.


2016

Modelling of temperatures and heat flow within laser sintered part cakes

S. Josupeit, L. Ordia, H. Schmid, Additive Manufacturing (2016), pp. 189-196

Temperature effects in the polymer laser sintering process are an important aspect regarding the process reproducibility and part quality. Depending on the job layout and position within the part cake, individual temperature histories occur during the process. Temperature history dependent effects are for example part warpage, the crystallization rate and powder ageing effects. This work focuses on temperatures and heat flow within laser sintered part cakes. Therefore, a thermal Finite Element (FE) model of a part cake is developed based on experimental in-process temperature measurements. Thermal boundary conditions and properties of the used bulk polymer powder are analyzed and relevant parameters are identified. The model is validated and optimized considering different job heights and ambient conditions during the cooling phase. It is finally possible to predict position-dependent temperature histories as a function of significant job parameters. The model allows a transfer of the results for varied boundary conditions during cooling. In combination with an implementation of built parts, this model will be an important tool for the development of optimized process controls and cooling strategies.


Guard ring induced distortion of the steady velocity profile in a parallel plate rheometer

H. Schmid, S. Pieper, Applied Rheology (2016)

The shape and fracture of the free surface frequently limits the measuring range and impedes the use of optical velocimetric techniques in parallel plate and cone plate setups. To prevent this, various kinds of edge guards are often employed. In the present study, we elucidate how an edge guard distorts the steady velocity profile in a parallel plate setup. To this end, we analyzed the velocity field of a strongly shear-thinning fluid, a Newtonian fluid and a set of suspensions via particle image velocimetry in a parallel plate device. Several guard ring sizes were studied. The distortion is described by a simple three parameter model. These parameters are mostly constant for different fluids and suspensions with particle volume fractions below 45%. With increasing radius, the guard ring.s influence approaches a limiting value that we attribute to the influence of the fluid surrounding the gap. Our results indicate a limiting ratio of the difference between plate radius and guard to gap size that should always be exceeded. In the presence of a guard ring, even Newtonian fluids do not exhibit a constant shear rate for most radial distances within the gap. This distortion of the velocity field challenges the simple superposition approach of unguarded device and guard influence that is prevalent in the literature.


Geometrische Genauigkeit von Lasersinter-Bauteilen: Einflüsse und Maßnahmen / Dimensional accuracy of polymer laser sintered parts: Influences and measures

W. Kniffka, M. Eichmann, G. Witt, S. Josupeit, P. Delfs, T. Lieneke, G. Adam, H. Schmid. Geometrische Genauigkeit von Lasersinter-Bauteilen: Einflüsse und Maßnahmen / Dimensional accuracy of polymer laser sintered parts: Influences and measures. 2016.

In the polymer laser sinter process, part quality depends on many influencing factors along the process chain. For application of the technology in series production and an integration of laser sintered parts into a technical environment, the dimensional accuracy of parts has to be taken into account. Therefore, occurring deviations of the process have to be known to define tolerances for part design. Dimensional deviations and their scattering have to be reduced and homogenized based on process parameters and build job layout. In this work, the dimensional accuracy of laser sintered parts is analyzed for varied parameter values. Influences of different process and geometrical build job parameters on dimensional deviations are figured out. The experimental results allow an evaluation of more and less important influences. Finally, measures are deduced to reduce and homogenize dimensional deviations.


Layer-formation of non-colloidal suspensions in a parallel plate rheometer under steady shear

S. Pieper, H. Schmid, Journal of Non-Newtonian Fluid Mechanics (2016), pp. 1-7

Suspensions are subject to confinement induced structuring, i.e. layering, at the confining surfaces. While most of the previous work focused on layering in Couette cells, the present study aimed to characterize the resulting layers at the plates of a parallel plate rheometer with regard to their relative particle concentration. The particle concentration profile over the radial distance was characterized for various mean concentrations and gap heights. To this end, we mapped the distribution of fluorescently dyed tracer particles in density and refractive index matched suspensions. The results indicate that layering at the surfaces stabilizes as the ratio between gap height and particle diameter increases. For lower gap heights, i.e. as the suspension approaches a two dimensional state, the layer concentration was non-uniform over the plates. In general, results were quantitatively different for the upper and lower plate and the concentration profiles were noticeably asymmetric. We conclude that this is probably the result of the rheometer loading or the start-up process. The stable layers as well as the inhomogeneous particle distribution in general offer an explanation for the lack of transferability of viscosimetric results between different setups.


Additive Fertigung

J. Gausemeier, M. Schmidt, R. Anderl, H. Schmid, C. Leyens, G. Seliger, M. Karg, M. Kage, acatech - Deutsche Akademie der Technikwissenschaften, 2016


Manufacturability and Mechanical Characterization of Laser Sintered Lattice Structures

S. Josupeit, P. Delfs, D. Menge, H. Schmid, in: 27th Annual International Solid Freeform Fabrication Symposium , 2016, pp. 2077-2086

The implementation of lattice structures into additive manufactured parts is an important method to decrease part weight maintaining a high specific payload. However, the manufacturability of lattice structures and mechanical properties for polymer laser sintering are quite unknown yet. To examine the manufacturability, sandwich structures with different cell types, cell sizes and lattice bar widths were designed, manufactured and evaluated. A decisive criterion is for example a sufficient powder removal. In a second step, manufacturable structures were analyzed using four-point-bending tests. Experimental data is compared to the density of the lattice structures and allows for a direct comparison of different cell types with varied geometrical attributes. The results of this work are guidelines for the design and dimensioning of laser sintered lattice structures.


Optimized build orientation of additive manufactured parts for improved surface quality and build time

P. Delfs, M. Tows, H. Schmid, Additive Manufacturing (2016), 2(12, Part B), pp. 214-320

The layered structure of Additive Manufacturing processes results in a stair- stepping effect of the surface topographies. In general, the impact of this effect strongly depends on the build angle of a surface, whereas the overall surface roughness is additionally caused by the resolution of the specific AM process. The aim of this work is the prediction of the surface quality in dependence of the building orientation of a part. These results can finally be used to optimize the orientation to get a desired surface quality. As not all parts of the component surface are equally important, a preselection of areas can be used to improve the overall surface quality of relevant areas. The model uses the digital AMF format of a part. Each triangle is assigned with a roughness value and by testing different orientations the best one can be found. This approach needs a database for the surface qualities. This must be done separately for each Additive Manufacturing process and is shown exemplarily with a surface topography simulation for the laser sintering process.


Extended Analysis of the Surface Topography of Laser Sintered Polymer Parts

P. Delfs, H. Schmid, in: Fraunhofer Direct Digital Manufacturing Conference, 2016, pp. 411-414

DOI


Thermal properties of polyamide 12 powder for application in laser sintering

S. Josupeit, H. Schmid, in: International Congress on Particle Technology (PARTEC) , 2016


Dimensional accuracy of polymer laser sintered parts: Influences and measures

S. Josupeit, P. Delfs, T. Lieneke, G. Adam, M. Gessler, H. Pfisterer, H. Schmid, in: Rapid Tech - International Trade Show & Conference for Additive Manufacturing , 2016, pp. 107-120

In the polymer laser sinter process, part quality depends on many influencing factors along the process chain. For application of the technology in series production and an integration of laser sintered parts into a technical environment, the dimensional accuracy of parts has to be taken into account. Therefore, occuring deviatons and their scattering have to be reduced and homogenized based on process parameters and build job layout. In this work, the dimensional accuracy of laser sintered parts is analyzed for varied parameter values. Influences of different process and geometrical build job parameters on dimensional deviatons are figured out. The experimental results allow an evaluation of more and less important influences. Finally, measures are deduced to reduce and homogenize dimensional deviations.


Additive Manufacturing of a Lightweight Rotor for a Permanent Magnet Synchronous Machine

S. Lammers, G. Adam, H. Schmid, R. Mrozek, R. Oberacker, M. Hoffmann, F. Quattrone, B. Ponick, in: EDPC 2016, 2016

Additive Manufacturing (AM), also known as 3D printing, is a relatively new technology which enables the toolless production of components and entire assemblies directly from a CAD file. Today, the technology is still not widely used in industrial production. It is mainly limited to special applications, although it shows great potential. In this paper, first approaches are shown to apply AM to the production of rotors for permanent magnet synchronous machines (PMSM). The possibilities of a lightweight design with a low moment of inertia as well as the influence on the magnetic anisotropy for an improved sensorless control of PMSM are pointed out. The results clearly demonstrate the great potential of additive manufacturing in electrical engineering applications.


Influences on the dimensional Accuracy of Laser Sintered Parts along the Process Chain

S. Josupeit, P. Delfs, T. Lieneke, H. Schmid, in: 27th Annual International Solid Freeform Fabrication Symposium , 2016


Direct measurement of wall slip and slip layer thickness of non-Brownian hard-sphere suspensions in rectangular channel flows

S. Jesinghausen, R. Weiffen, H. Schmid, Experiments in Fluids (2016)

Wall slip is a long-known phenomenon in the field of rheology. Nevertheless, the origin and the evolution are not completely clear yet. Regarding suspensions, the effect becomes even more complicated, because different mechanisms like pure slip or slip due to particle migration have to be taken into account. Furthermore, suspensions themselves show many flow anomalies and the isolation of slip is complicated. In order to develop working physical models, further insight is necessary. In this work, we measured experimentally the wall slip velocities of different highly filled suspensions in a rectangular slit die directly with respect to the particle concentration and the particle size. The slip velocities were obtained using a particle image velocimetry (PIV) system. The suspensions consisting of a castor oil–cinnamon oil blend and PMMA particles were matched in terms of refractive indexes to appear transparent. Hereby, possible optical path lengths larger than 15 mm were achieved. The slip velocities were found to be in a quadratic relation to the wall shear stress. Furthermore, the overall flow rate as well as the particle concentration has a direct influence on the slip. Concerning the shear stress, there seem to be two regions of slip with different physical characteristics. Furthermore, we estimated the slip layer thickness directly from the velocity profiles and propose a new interpretation. The PIV technique is used to investigate the viscosity and implicit the concentration profile in the slit die. It is shown that the particle migration process is quite fast.


Layer-formation of non-colloidal suspensions in a parallel plate rheometer under steady shear

S. Pieper, H. Schmid, Journal of Non-Newtonian Fluid Mechanics (2016), pp. 1-7

DOI


2015

Evaluation of the Efficiency of Filtration Processes Using Precoat Materials

S. Schiller, C. Hellmich, H. Schmid, Chemical Engineering & Technology (2015), pp. 491-498

The highly efficient filtration of ultrafine dust emitted by biomass combustion processes with a baghouse filter has been successfully tested in the last years. To protect the filter material from the very small and sticky fine dust particles and to guarantee very high total collection efficiencies (> 99 %) in a long-term stable process, the use of a precoat is necessary. Tests done in a laboratory and a real-application plant show that the reuse of precoat materials can lead to significant savings. Considering the influences of different combustion processes, the associated precoat efficiencies could be calculated. With these characteristic ratios, it is possible to evaluate different process settings. Hence, the amount and the cost of the needed precoat could be reduced significantly.


Process Development of a Liquid-Liquid Phase Transfer of Colloidal Particles for Production of High-Quality Organosols

J.V. Erler, S. Machunsky, S. Franke, P. Grimm, H. Schmid, U.A. Peuker, in: Colloid Process Engineering, 2015

The emphasis of the study presented is on a new process of particle extraction to transfer magnetite nanoparticles from an aqueous into an immiscible organic phase directly through the liquid-liquid interface. For the production of high-quality organosols, stabilized colloidal and functionalized particles are required in a liquid organic phase. The mechanism of phase transfer is initiated by adsorption and chemical binding of surfactants (fatty acids) at the particle surface. The resulting physico-chemical dispersion of the hydrophobically modified particles leads to the formation of the stabilized organic colloid, or organosol. The aim here is to demonstrate the entire chain of the transfer process in a continuous miniplant, which comprises particle synthesis, conditioning, and transfer, and which uses a drop column for extraction and as a transfer device. Based on the investigation of the governing principles and the material parameters, the results obtained for the transfer kinetics in the individual contact devices (centrifuge, single-drop column, and drop column for different operations) are used for the dimensioning of the entire process chain.


Bipolar charge distribution of a soft X-ray diffusion charger

L. Tigges, A. Wiedensohler, K. Weinhold, J. Gandhi, H. Schmid, Journal of Aerosol Science (2015), pp. 77-86

The conditioning of the aerosol particle population into a bipolar charge equilibrium is an essential prerequisite to calculate the particle number size distribution using mobility particle size spectrometers. This is commonly realized by diffusion charging of bipolar air ions generated by e.g. a 85Kr source. Because of strict legal regulations on radioactive sources in several countries, soft-X-ray (SXR) appears as a suitable alternative. However, multiple measurements showed a systematical and significant difference between the particle charge distribution delivered by a radioactive source and an SXR charger, respectively. In this investigation, a calibrated particle charge distribution, suitable for the SXR chargers, was calculated based on the Fuchs model. An approximation analogous to the commonly used Wiedensohler approximation formula (Wiedensohler, 1988) was computed. The use of the new SXR approximation of the bipolar charge equilibrium for the inversion of an electrical mobility distribution to a particle number size distribution improves the comparability of these results, compared to measurements involving a 85Kr charger or to bipolar chargers using radioactive material in general. A systematic error in case of using the SXR charger could be eliminated and hence the root mean square deviation could be reduced from 13% using the common parameters for both charger types to 7% using the new SXR approximation for the SXR bipolar charger.


On the bipolar charge distribution used for mobility particle sizing: Theoretical considerations

L. Tigges, A. Jain, H. Schmid, Journal of Aerosol Science (2015), pp. 119-134

An essential part of mobility particle size spectroscopy is the prediction of the aerosol charge distribution in a highly concentrated bipolar ion environment. This charge distribution can be readily calculated, but is depending on several environmental conditions. These influences are investigated theoretically. The first part of this work deals with a sensitivity analysis using the Fuchs model (Fuchs, 1963) to determine the variation of the resulting charge distributions depending on the input parameters. It is demonstrated, that the main influencing variable is the difference between the positive and negative ion mobility. A sensitivity analysis reveals that a reasonable variation of the ion mobilities may lead to variations of the particle density distribution up to±20%. The second part investigates the evolution of the charge distribution along the tubing downstream of the bipolar charger exit starting with equal ion concentrations for positive and negative ions. Due to wall losses depending on ion mobility a non-equilibrium charge distribution is developing along the plumbing. The evolution of the particle charge distribution is determined using a coupled population balance model. Even though the non-equilibrium character is clearly shown, it turns out that this effect is negligible at conditions typical for particle size measurements.


Highly efficient filtration of ultrafine dust in baghouse filters using precoat materials

S. Schiller, H. Schmid, Powder Technology (2015), pp. 96-105

The precipitation of sticky and ultrafine particles has become increasingly important. Biomass burners are one important example for ultrafine dust emission sources with ever growing importance. Therefore, a baghouse filter has been developed, which combines excellent separation efficiency (> 99%, clean air dust loading of < 1 mg/m3) with convenience in operation. However, in order to prevent clogging of the filter cloth by sticky and ultrafine particles, it is necessary to use a precoat layer (e.g. hydrated limestone powder). If this technology is applied to larger scale processes, e.g. biomass burning for industrial drying processes, the reuse of the precoat material can generate significant savings. Therefore, extended tests on recycling of used precoat material have been performed. Particularly, the influence of precoat injection parameters and various mixing strategies of used and virgin powder for refreshing the precoat material have been investigated. Different mixtures have been characterised by their ability to disintegrate, flowability and filtration behaviour. It is clearly demonstrated that upon redispersing the used precoat fine dust mainly adheres to the coarse precoat with only a limited number of dust agglomerates being produced in addition. For each kind of precoat a minimum amount is determined in order to ensure a long-term stable process. This way a saving potential of between 40–67% has been found. An economic and ecologic process has been developed to precipitate ultrafine dust in a baghouse filter system using precoat materials.


Simulation of Capillary Bridges between Particles

M. Dörmann, H. Schmid, Procedia Engineering (2015), pp. 14-23

Capillary forces between particles often dominate other adhesion forces. However, the calculation of the shape of capillary bridges and the resulting force is complex, so often assumptions and approximations are used. These assumptions are not useful for nanoscale particles. Therefore, a simulation method was established to calculate numerically the shape of the meniscus and derive the corresponding capillary bridge force. The main focus are nanoscaled particles with liquid bridges formed by condensing air humidity. The dependence of the capillary force on various parameters such as particle sizes, contact angle and humidity was investigated and it is demonstrated that often-used assumptions cannot be used for nanoscaled particles without remarkable mistake.


Mass finishing of laser sintered parts

P. Delfs, Z. Li, H. Schmid, in: 26th Annual International Solid Freeform Fabrication Symposium, 2015, pp. 514-526

DOI


Temperature History within Laser Sintered Part Cakes and its Influence on Process Quality

S. Josupeit, H. Schmid, in: 26th Annual International Solid Freeform Fabrication Symposium, 2015, pp. 190-199

The temperature distribution and history within laser sintered part cakes is an important aspect regarding the process quality and reproducibility of the polymer laser sintering process. Especially the temperature history during the build and cooling phase is decisive for powder ageing effects and the development of part quality characteristics. In this work, a measurement system for three-dimensional in-process temperature measurements is set up and the influence of different parameters on the inner part cake temperature distribution and history is analyzed. Important factors are not only geometrical build job parameters like the part packing density and build height, but also process parameters like the layer thickness and bulk powder density. Individual in-process temperature profiles at different positions within a part cake are finally correlated with powder ageing effects. The results of this work help to understand the temperature history dependency of powder and part properties and can therefore be used to develop optimized process controls.


Material Properties of Laser Sintered Polyamide 12 as Function of Build Cycles Using Low Refresh Rates

S. Josupeit, J. Lohn, E. Hermann, M. Gessler, S. Tenbrink, H. Schmid, in: 26th Annual International Solid Freeform Fabrication Symposium, 2015, pp. 540-549

Due to long process times at high temperatures, unmolten polyamide 12 material ages during the manufacturing process. Hence, it needs to be refreshed with new material for further build cycles. In application, refresh rates of about 50 % are commonly used. In this work, the recycling optimized material PA 2221 from EOS is analyzed along a series of 13 build and refresh cycles using a reduced refresh rate of 32 %. Before and after every build, the powder is analyzed regarding melt properties determined by MVR and DSC measurements. Thereby, in-process ageing effects are investigated and the steady-state conditions are determined accordingly. In addition, powder properties are directly linked to resulting mechanical and geometrical part properties. Key findings are a robust DSC measurement method for polyamide 12 powder, constant “circulated” material properties after three build/refresh cycles and robust tensile properties along the whole tested powder life cycle. As a result, process conditions of PA 2221 using reduced refresh rates can be derived from this work.


Surface roughness optimized alignment of parts for additive manufacturing processes

P. Delfs, M. Toews, H. Schmid, in: 26th Annual International Solid Freeform Fabrication Symposium, 2015, pp. 1334-1344

The layered structure of Additive Manufacturing processes results in a stairstepping effect of the surface topographies. In general the impact of this effect strongly depends on the build angle of a surface, whereas the overall surface roughness is caused by the resolution of the specific AM process. The aim of this work is the prediction of the surface quality in dependence of the building orientation of a part. These results can finally be used to optimize the orientation to get a desired surface quality. As not every area of a part can be optimized, a predetermination of areas can be used to improve the surface quality of important areas. The model uses the digital STL format of a part as this is necessary for all AM machines to build it. Each triangle is assigned with a roughness value and by testing different orientations the best one can be found. This approach needs a database for the surface qualities. This must be done separately for each Additive Manufacturing process and is shown exemplary with a surface topography simulation for the laser sintering process.


Powder ageing and material properties of laser sintered polyamide 12 using low refresh rates

S. Josupeit, S. Tutzschky, M. Gessler, H. Schmid, in: Proceedings of the Rapid Tech 2015, Springer Vieweg, 2015, pp. 63-78

Powder ageing effects due to thermal load usually lead to a limited recyclability of unmolten polyamide 12 powders in the laser sintering process. In this work, the recycling optimized material PA 2221 is analyzed regarding its ageing behavior along a series of more than ten build/refresh cycles using a refresh ratio of about 30%. Thereby, the melt viscosity is measured before and after every cycle and the “steady-state” condition is determined accordingly. In addition, the material properties of parts manufactured with PA 2221 are discussed dependent on different process parameters and compared to parts made of standard PA 2200 material with a refresh rate of about 50%. These parameters include the mechanical properties as well as for example the impact strength and geometrical properties as function of layer thickness and testing temperature. As a result, the optimal process conditions and achievable material properties for PA 2221 can be derived from this work.


Tutorenprogramm und Vertiefungsberatung stellen Weichen in entscheidenden Phasen des Student Life-Cycles in Maschinenbaustudiengängen

I. Koke, K. Dibblee, F. Hankeln, S. Seidel, A. Trächtler, H. Schmid, in: 44. Jahrestagung der Deutschen Gesellschaft für Hochschuldidaktik - Hochschuldidaktik im Dialog, Paderborn, 2015


2014

Absence of pressure sensitivity of apparent wall slip in pressure-driven flow of non-colloidal suspensions

S. Pieper, N. Kirchhoff, H. Schmid, Rheologica Acta (2014), pp. 69-75

We investigated an explanation for the length dependence of apparent wall slip in capillary and slit die experiments that had been implied in earlier studies. Firstly, we used a very long slit die fitted with six equidistant pressure transducers and an adjustable outlet. It was found that, all other parameters being equal, the impact of pressure on the flow behavior of suspensions was not different from the impact on the unfilled matrix fluid. Particle-related properties, such as apparent wall slip, were therefore not influenced by pressure as assumed by other authors. Secondly, we determined wall slip velocities by both the Mooney method and numerical simulation, based on an implementation of Phillips constitutive equation for particle migration. This led to a similar length dependence of wall slip as had been reported in an earlier work. We concluded that this was due to the transient nature of particle migration and its interrelationship with apparent wall slip, rather than an influence of pressure.


Ultrafine Dust Filtration Using Precoat Materials Considering the Influence of Filter Media

S. Schiller, H. Schmid, Chemical Engineering & Technology (2014), pp. 1009-1020

Ultrafine dust separation from different sources like industry, traffic, or private households has become increasingly important in the last decade. A compact baghouse filter system has been developed which is suitable for pellet heaters. For filtration, a precoat material is required to prevent clogging of the filter media by the ultrafine dust particles. In order to ensure the best performance of this filter system, different combinations of filter media and precoat materials, e.g., grade efficiencies and the cake area load for the various filter media, were investigated in a special test rig. With this highly efficient technology, extremely high separation efficiencies of > 99 % and ultrafine dust concentrations of > 1 mg cm−3 could be reached in a long-term stable process.


Simulation of Capillary Bridges between Nanoscale Particles

M. Dörmann, H. Schmid, Langmuir (2014), pp. 1055-1062

Capillary forces are very important as they exceed in general other adhesion forces. But at the same time the exact calculation of these forces is very complex, so often assumptions and approximations are used. Previous research was done with regard to micrometer sized particles, but the behavior of nanoscale particles is different. Hence, the results for micrometer sized particles cannot be directly transferred when considering nanoscale particles. Therefore, a simulation method was developed to calculate numerically the shape of a rotationally symmetrical capillary bridge between two spherical particles or a particle and a plate. The capillary bridge in the gap between the particles is formed due to capillary condensation and is in thermodynamic equilibrium with the gas phase. Hence the Kelvin equation and the Young–Laplace equation can be used to calculate the profile of the capillary bridge, depending on the relative humidity of the surrounding air. The bridge profile consists of several elements that are determined consecutively and interpolated linearly. After the shape is determined, the volume and force, divided into capillary pressure force and surface tension force, can be calculated. The validation of this numerical model will be shown by comparison with several different analytical calculations for micrometer-sized particles. Furthermore, it is demonstrated that two often used approximations, (1) the toroidal approximation and (2) the use of an effective radius, cannot be used for nanoscale particles without remarkable mistake. It will be discussed how the capillary force and its components depend on different parameters, like particle size, relative humidity, contact angle, and distance, respectively. The rupture of a capillary bridge due to particle separation will also be presented.


Simulation of the Surface Topography on Laser Sintered Polymer Parts

P. Delfs, A. Herale, Z. Li, H. Schmid, in: 25th Annual International Solid Freeform Fabrication Symposium, 2014, pp. 1250-1258

One barrier of laser sintering (LS) to become the main process for Direct Manufacturing (DM) is the surface quality of LS parts. Hence, the property which has to be improved is the rough surfaces of LS parts due to the layered structure. Another additional effect is the incomplete melting of powder particles on the surface due to the high process temperature. In this paper we demonstrate our approach of a theoretical model for the topography of LS part surfaces. We investigated the surface roughness as a function of surface orientation. Considering that the model involves further variables as layer thickness, particle density and particle size distribution to describe the topography precisely. Experimental results were used to optimize and check the results of the model.


Three-dimensional in-process temperature measurement of laser sintered part cakes

S. Josupeit, H. Schmid, in: 25th Annual International Solid Freeform Fabrication Symposium, 2014, pp. 49-58

An uneven temperature distribution and varying cooling rates at different positions within the part cake are two of the most important challenges regarding the part quality and reproducibility of the polymer laser sintering process. In the presented work, a temperature measurement system is implemented within an EOSINT P395 laser sintering system. It allows the determination of a three dimensional temperature distribution and history during the full build and cooling process. The influence of important job parameters, for example the packing density, job height and layer thickness, can be figured out. In combination with a finite element simulation of the cooling process, the temperature measurement will be the basis for optimized process controls.


Thermal ageing of polyamide 12 used for polymer laser sintering - influence on part quality characteristics

S. Josupeit, S. Rüsenberg, N. Rupp, M. Gessler, H. Schmid, in: 72nd Annual Technical Conference of the Society of Plastics Engineers (ANTEC 2014), 2014, pp. 2383-2385

Polymer laser sintering is one of the most important additive manufacturing technologies for the tool-less production of three-dimensional prototypes and end-use parts. In this process, parts are manufactured layerwise out of a polymer powder by laser exposure. After the building process, these parts are located within a loose bulk powder cake. Due to long process times and high process temperatures, this powder ages thermally, which reduces the recyclability of the material. As a result, mixtures of used and virgin powder ("refreshed" powder) with a mixture ratio of approximately 50% are commonly used in the industry. The goal of this work is to determine the exact influence of different powder ages on resulting part quality characteristics, especially the mechanical behavior and the surface quality. Therefore, refreshed powder with different qualities adjusted by the melt volume rate (MVR) was processed along a defined process quality chain. To analyze the part qualities, mechanical tensile and profilometer tests were performed. The focus is on an application-oriented test set-up to ensure the usability of the results in the industry. The material used is polyamide 12 (PA 2200) processed on an EOSINT P395 laser sintering system from EOS GmbH, Krailling, Germany.


A method to characterize the quality of a polymer laser sintering process

S. Rüsenberg, S. Josupeit, H. Schmid, Advances in Mechanical Engineering (2014), 6, pp. 1687-8140

The reproducibility and reliability of quality aspects are an important challenge of the polymer laser sintering process. However, existing quality concepts and standardization activities considering influencing factors along the whole process chain have not been validated experimentally yet. In this work, these factors are analyzed and kept constant to obtain a reliable material data set for different layer thicknesses and testing temperatures. In addition, material qualities regarding powder ageing effects are analyzed using different build heights and layer thicknesses: while an increase of the layer thickness reduces mechanical part strength and density, it also results in a less intense thermal ageing of unmolten powder due to shorter build times.


Development of a Basic Model to Simulate the Laser Sintering Cooling Process

S. Josupeit, L. Ordia, H. Schmid, in: International Conference on Additive Technologies, 2014, pp. 222-227

The process-related occurrence of varying cooling rates at different positions within the part cake is an important challenge regarding the part quality and reproducibility of the polymer laser sintering process. Temperature history dependent parameters are for example the part warpage, the crystallization behavior or powder ageing effects, which have to be considered for optimized process controls. Nevertheless, the inner temperature distribution and history during the cooling process is difficult to measure and less known yet. In this work, a Finite Element (FE) model is developed to understand and predict the temperature distribution and history within the part cake during the cooling process. Therefore, the thermal boundary conditions of a laser sintering system are analyzed and relevant parameters are identified. A basic FE model is set up in ABAQUS CAE software considering a part-free powder cake. Important thermal parameters of the bulk powder and the environment are adjusted and verified in relation to experimental in-process measured data. With this model it is possible to predict position-dependent cooling rates as a function of significant job parameters, for example the job height or the environmental conditions during the cooling phase. In combination with extended in-process temperature measurements and a consideration of built parts, this model will be an important tool for the development of optimized process controls.


Simulation of the Surface Topography on Laser Sintered Polymer Parts

P. Delfs, H. Schmid, in: 25th Annual International Solid Freeform Fabrication Symposium, 2014, pp. 1250-1258

One barrier of laser sintering (LS) to become the main process for Direct Manufacturing (DM) is the surface quality of LS parts. Hence, the property which has to be improved is the rough surfaces of LS parts due to the layered structure. Another additional effect is the incomplete melting of powder particles on the surface due to the high process temperature. In this paper we demonstrate our approach of a theoretical model for the topography of LS part surfaces. We investigated the surface roughness as a function of surface orientation. Considering that the model involves further variables as layer thickness, particle density and particle size distribution to describe the topography precisely. Experimental results were used to optimize and check the results of the model.


2013

Liquid–liquid phase transfer of magnetite nanoparticles — Evaluation of surfactants

J. Erler, S. Machunsky, P. Grimm, H. Schmid, U.A. Peuker, Powder Technology (2013), 247, pp. 265-269

Because of the large surface area of colloids interface effects are dominant in contrast to volume effects. The study presents experimental results of the direct transfer of magnetite nanoparticles from an aqueous to a non-miscible organic phase. The starting point is a water-based colloid that is synthesized through a precipitation reaction. The transfer is based on the adsorption of surfactants onto the particle surface at the liquid–liquid interface. While penetrating the liquid–liquid interface, the particles are covered with surfactants and a partial de-agglomeration is initiated. The intention is to produce a stable organic colloid, which has important applications in industry. The optimized process parameters for the successful phase transfer process, the adsorption reactions at the liquid–liquid interface and the stabilization of primary particles in the organic phase are demonstrated.


Hocheffiziente Feinstaubabscheidung aus Kleinfeuerungsanlagen mit einem Schlauchfilter

S. Schiller, H. Schmid, Chemie Ingenieur Technik (2013), pp. 1324-1328

Zur Erfüllung der Anforderungen der novellierten ersten Bundesimmissionsschutzverordnung (1. BImSchV: Verordnung über kleine und mittlere Feuerungsanlagen) wurde ein Schlauchfiltersystem für den Gebrauch in kleinen und mittleren Feuerungsanlagen entwickelt. Ein zur Filtration benötigtes Filterhilfsmittel wird vollautomatisch portioniert, dispergiert und dem Prozess zugeführt. Mit dieser Technik ist es möglich, den Feinstaubgehalt im Abgas auf unter 1 mg m^3 i.N. zu senken und damit die geforderten Grenzwerte weit zu unterschreiten.


A material-based quality concept for polymer laser sintering

S. Josupeit, S. Rüsenberg, H. Schmid, in: 24th Annual International Solid Freeform Fabrication Symposium , 2013, pp. 44-54

In this work, the quality of laser sintered parts is investigated along a defined process chain for a nylon 12 material (PA 2200) on an EOSINT P395 laser sintering system. Important influencing factors are figured out. Rheological powder characterization methods are investigated as well as mechanical, physical and other chosen part properties. The concept allows reproducible part quality characteristics and is used to obtain (testing) temperature dependent material data. It can also be extended on further materials based on nylon 12: PA 2241 FR, which is convenient for the aircraft industry due to its flame-retardant properties, and PA 2221, which has economic advantages due to a lower material consumption.


Advanced characterization method of Nylon 12 materials for application in laser sinter processing

S. Rüsenberg, H. Schmid, in: International Conference of the Polymer-Processing-Society (PPS), 2013

Laser Sintering is a powder based additive manufacturing technology. The amount of used powder, which is not sintered, can be refreshed and used for further building jobs. Due to cost reduction, better mechanical properties and a simpler processing it is reasonable to combine virgin and used powder for part manufacturing. During the building time the powder is thermally loaded and its structure is changed. State of the art in laser sintering is a defined powder ratio. Since the powder ageing depends on the individual history of sintering, this procedure cannot ensure a defined powder quality. However, for a serial production it is necessary to characterize the raw material, as well as all other steps along the process chain, in order to ensure a high reproducibility and reliability. Rheological, physical and particle properties are considered to correlate powder and chosen material properties. Different powder qualities adjusted by the Melt Volume Rate (MVR) are tested using promising rheological properties like viscosity or molecular weight. Investigations about powder bed density, bulk properties as well as thermal characterization complement the experimental setup. These parameters are correlated with mechanical, electrical, thermal and physical material properties of laser sintered parts. The most important influencing factors along the process chain are kept constant. At the end a procedure is shown to obtain a defined powder quality. This procedure will be transferred to other materials and will be tested using other laser sintering machines of the same type as well as other types.


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