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The mechanical properties of polymer parts built by Selective Laser Sintering are strongly related to the internal microstructure which differs with the applied production parameters. The paper focuses on the back tracing of the pore morphology of laser sintered polyamide-12 samples to the process parameters. Therefore, a data base is used which is supplied by a Round Robin initiative and includes mechanical tensile tests and the microstructural analysis of the pore morphology of several different sample charges built with different machines. The pore morphologies (porosity, pore density, pore shape and pore arrangement) measured by X-ray computed tomography are compared and discussed regarding the employed parameters and the resulting mechanical properties. The investigations point out that pore density is a superior indicator than porosity for mechanical issues. This is especially valid along the build direction since pore morphology has shown to be strongly anisotropic. Moreover, the analysis revealed that pore density is strongly affected by the process temperature, which is proved to be essential for the fabrication of mechanical robust parts using Selective Laser Sintering.
In this paper, the use of electrophotographic polymer powder transfer for the preparation of multi-material layers is discussed with respect to the application in powder bed-based additive manufacturing technologies as selective laser sintering (SLS). Therefore, the challenges of this task are considered verifying the critical process steps in order to develop a concept for an electrophotograhic laser sintering machine. On that basis, an experimental setup with a two-chamber design is realized which enables the investigation of the electrophotographic powder transfer at typical process conditions of SLS. Using this setup, transfer tests of polypropylene powder patterns were performed and qualitatively analyzed by photographic imaging. The results confirm the high potential of the application of electrophotography for multi-material powder deposition and show how a residual electrophotographic powder deposition can be achieved in general, which is independent from the already produced part height, in order to build up three-dimensional multi-material components.
The mechanical and microstructural investigation of polymer parts (polyamide 12) fabricated by Selective Laser Sintering as part of a Round Robin initiative is presented. The paper focuses on the microstructural analysis of the Round Robin samples and their evaluation regarding their effect on mechanical properties with respect to each other. Therefore optical microscopy on microtomed samples, X-ray computed tomography and Differential Scanning Calorimetry is used to determine the morphology of residual particle cores and of internal pores.
In former works, the optical material properties of different polymer powders used for Laser Beam Melting (LBM) at room temperature have been analyzed. With a measurement setup using two integration spheres, it was shown that the optical material properties of polymer powders differ significantly due to multiple reflections within the powder compared to solid bodies of the same material. Additionally, the absorption behavior of the single particles shows an important influence on the overall optical material properties, especially the reflectance of the powder bed. Now the setup is modified to allow measurements at higher temperatures. Because crystalline areas of semi-crystalline thermoplastics are mainly responsible for the absorption of the laser radiation, the influence of the temperature increase on the overall optical material properties is analyzed. As material, conventional polyamide 12 and polypropylene as new polymer powder material, is used. By comparing results at room temperature and at higher temperatures towards the melting point, the temperature-dependent optical material properties and their influence on the beam-matter interaction during the process are discussed. It is shown that the phase transition during melting leads to significant changes of the optical material properties of the analyzed powders.
The generation of multi-material components by laser beam melting (LBM) is a challenge which requires the invention of new coating devices for preparation of arbitrary powder patterns. One solution is the usage of vibration-controlled nozzles for selective deposition of polymer powders. Powder flow can be initiated by vibration enabling a start-stop function without using any mechanical shutter. In this report, the delivery of polymer powder by vibrating nozzles is investigated with respect to their application in LBM machines. Therefore, a steel nozzle attached to a piezo actor and a weighing cell is used in order to measure the stability and time-dependence of the powder mass flow upon vibration excitation with the usage of different kind of powder formulations. The results show that precompression of the powder inside the nozzle by vibration excitation is essential to realize a reliable start-stop function with reproducible discharge cyles and to prevent a initial flush of powder flow. Moreover, the use of different powder materials showed that mass flow is even possible with powders which are not optimized regarding flowability, but is readily enhanced with a factor of 2 to 3 by admixing Aerosil® fumed silica.
Simultaneous Laser Beam Melting of polymers (SLBM) allows the generation of multi-material components,consisting of different thermoplastic polymers, within one additive building process. Besides the common advantagesof conventional Laser Beam Melting (LBM), multi-material components built by SLBM can fulfill different productrequirements like different chemical resistances or haptic material properties within a single part. To achieve suchparts, different powder materials are deposited next to each other and preheated a few degrees below their meltingtemperatures by infrared emitters and laser radiation (λ = 10.60 μm), before in the last step the preheated powdersare molten simultaneously by an additional laser source (λ = 1.94 μm). In this paper, different polymer powders likepolypropylene (PP) and polyamide 12 (PA12) are used for the generation of multi-material specimens. By varyingdifferent building parameters according to a specified design of experiments, their influence on the part properties isanalyzed. Important building parameters are the intensity and the irradiation time of the laser beam used for meltingthe preheated powders. Besides using tensile tests to determine the tensile strength and the elongation at break, theaverage part height in dependence of the energy input is analyzed. The overall aim is to specify the correlationbetween different building parameters regarding the energy deposition on the resulting part properties.
Powder based Additive Manufacturing technologies offer huge potential for building parts with almost no geometrical restrictions, but both the process controlling as well as the part properties are strongly dependent on different material characteristics of the material, like the flowability. In this work, different weight percentages of nano-scaled silica dioxide particles (Aerosil®) are admixed to pure polyethylene and polypropylene powder and the resulting flowability is determined. Besides using the Hausner ratio as standardized value, the degree of coverage is introduced as a new characteristic to quantify the powder flowability. The degrees of coverage are compared to the Hausner ratios to allow a discussion and evaluation about the different characteristic values. Additionally, tensile bars consisting of polypropylene are generated to determine the porosity by cross sections and the mechanical part properties by tensile testing. As mechanical part properties, the tensile strength and elongation at break are determined and the effects of different powder flowability on these properties are analyzed.
The generation of multi-material components using Laser beam melting (LBM) is a challenge which requires the invention of new coating devices for the preparation of arbitrary powder patterns. One solution is the usage of vibration-controlled nozzles for selective deposition of polymer powders. Powder flow can be initiated by vibration even when using powders with low flowability. In this report, the selective deposition of polymer powder by vibrating nozzles is investigated with respect to their application in LBM machines. Therefore, a steel nozzle attached to a piezo actor is applied, whereas the nozzle itself features internal channels which allow the precise control of the powder temperature using heat transfer oil. The setup is used to study the influence of temperature on the powder mass flow. The results show that, next to the vibration mode, the temperature strongly influences the powder mass flow which is done by affecting the moisture and thus the particle-particle adhesion forces. This shows that a precise control of the powder temperature inside the nozzle is required in order to achieve a constant mass flow and thus a successful application of vibrating nozzles inside LBM machines.
By Laser Beam Melting of polymers (LBM), parts with almost any geometry can be built directly out of CAD files without the need for additional tools. Thus, prototypes or parts in small series production can be generated within short times. Up to now, no multi-material parts have been built by LBM, which is a major limitation of the technology. To realize multi-material parts, new mechanisms for depositing different polymer powders as well as a new irradiation strategy are needed, by which polymers with different melting temperatures can be warmed to their specific preheating temperatures and be molten simultaneously. This is achieved by simultaneous laser beam melting (SLBM). In the process, two different materials are deposited next to each other and preheated a few degrees below their melting temperatures by infrared emitters and laser radiation (λ = 10.60 µm), before in the last step the two preheated powders are molten simultaneously by an additional laser (λ = 1.94 µm). So far, multi-material tensile bars have been realized and analyzed regarding their boundary zone between both materials. The experiments showed that the temperature gradients in the boundary zone and along the building direction seem to be of great importance for the process stability and the resulting part properties. Therefore, a detailed analysis of the occurring temperature gradients during the process is needed to identify adequate process adjustments regarding the temperature controlling. To analyze the temperature gradients, thermocouples positioned inside the powder bed are used. By varying the temperature of the building platform, the influence of different temperature gradients on the resulting part properties is shown.
In this paper, first results regarding the realization of multi-material parts by Simultaneous Laser Beam Melting (SLBM) of polymers are presented. This new approach allows the layerwise generation of parts consisting of different polymer materials within one building process. Besides the typical advantages of additive manufacturing technologies, such parts can fulfill different product requirements concomitant and therefore could enlarge the overall field of application. The powder materials used for this paper are polyethylene (PE) and a polyamide based thermoplastic elastomer (TPE). After depositing the powder materials next to each other, infrared-emitters heat the lower melting polymer and a CO2 laser provides the preheating temperature of the higher melting polymer. In the last step, a thulium fibre laser melts the two preheated powders simultaneously. The realized specimens are characterized by cross sections and their tensile strengths are determined. Additionally, the new approach of the simultaneous energy irradiation is investigated using a Finite Element Analysis in order to gain a more profound process understanding. In that sense, the influence of the size of the exposure area on the reachable maximum temperatures inside that area was analyzed by the simulation and compared to experimental studies.
By simultaneous laser beam melting (SLBM), parts consisting of different polymer powders can be additively manufactured within one building process. Besides the advantages of conventional LBM, e.g., not needing additional tools and being able to realize parts with almost any geometry, different product requirements can be achieved within a single part. Product requirements may be different chemical resistances or haptic material properties. Therefore, SLBM enlarges the application field for additive manufacturing in general. In the process, two different materials are deposited on the building platform and preheated a few degrees below the melting temperature of the lower melting polymer by infrared emitters. Afterward, a CO2 laser (λ = 10.6 μm) provides the energy for the temperature difference between the preheating temperatures of both materials. Finally, a digital light processing chip is used to achieve simultaneous and flexible energy deposition for melting both preheated polymers. By illuminating the chip with a laser, parts of the beam can be flexibly guided onto the powder bed or into a beam trap. As laser, a single mode thulium laser (λ = 1.94 μm) is used. After melting the layer, a new layer is deposited and the process starts anew. In this paper, polypropylene and polyamide 12 are used as materials. After analyzing the material and melting behavior during the process by a high-resolution thermal imaging system, the parts are qualified regarding their material compatibility at the boundary zone and porosity by cross sections.
In this report, the delivery of polyamide 12 (PA 12) powder and powder layer preparation by vibrating steel nozzles is investigated and discussed with respect to its application for laser beam melting. Therefore, a setup was realized which includes a steel nozzle attached to a piezo actor as well as a positioning system. In order t o investigate the mass flow characteristics in dependency on the applied vibration state, a weighing cell is used enabling time-resolved mass flow measurements. Moreover, single-layer patterns consisting of colored and uncolored polyamide 12 were created and characterized regarding surface homogeneity and selectivity before as well as after the melting of the powder layers by a hot plate.
By using Additive Manufacturing technologies, like Laser Beam Melting (LBM) of polymers, parts can be realized within single days and necessary modifications can be quickly adapted. With increasing complexity, products are often made out of different polymer materials and the need for multi-material parts is an increasing industry requirement, which cannot be fulfilled by the single material parts realizable by LBM. Therefore, Simultaneous Laser Beam Melting (SLBM) as a new Additive Manufacturing technology offers the possibility to build parts consisting of different polymer materials. The realizable parts combine different material properties, like differing stiffness or chemical resistances, within a single part. Up to now, different materials are deposited next to each other on the building platform, thus the boundary surface between the different polymers is orientated perpendicular to the building direction. For this paper, the polymer powders are alternated in building direction. Thus, the boundary surface is orientated horizontally and is larger, both influencing the boundary surface and resulting part properties, which are analyzed by a high-resolution thermal imaging system and by cross sections.
In this paper, the use of electrostatic polymer powder transfer methods for the preparation of multi-material layers is discussed with respect to the application in Simultaneous Laser Beam Melting (SLBM). Therefore, the basic principles of the single process steps as well as the challenges in combination with SLBM are considered verifying the critical process steps. On that base, process concepts are developed which might enable the fabrication of high quality multi-material parts in the future. Moreover, since the polymer powders typically used with Laser Beam Melting differ strongly from common toners for e. g. electrophotographic printing, an experimental setup was built to study the powder transfer with an electrically chargeable transfer plate using polyamide 12 powder. The results of this study show that transfer of powders usable for Laser Beam Melting can be achieved, but depends on the electric field strength which is a function of the gap between transfer and substrate plate and the intermediate electric potential.
Abstract — In this report, the dry delivery of polyamide 12 (PA 12) powder for the preparation of powder layers in laser beam melting (LBM) is investigated. Therefore, an experimental setup was built which consists of a glass nozzle assembled on a piezo-electric actuator. By applying a sinusoidal voltage signal to the actuator, the nozzle is set into vibration mode which enhances the powder delivery through the nozzle. By using this effect, powder mass flow is controlled and a valve-like start and stop function is realized. In order to identify suitable process parameters, a broad range of vibration modes were investigated using two nozzles made from glass with different orifice diameters. Therefore, the vibration frequency and the voltage signal of the actuator were varied and the resulting mass flow was detected by a balance. It was found that both the frequency and the voltage signal affect the mass flow and its stability but with different impact. Moreover, powder lines were deposited with different velocities in order to characterize the setup regarding applicability for highly selective powder deposition for LBM.
In this report, the dry delivery of polyamide 12 powders by vibrating capillary steel nozzles is investigated and discussed regarding its potential for powder layer preparation in Laser Beam Melting. Therefore, a setup including a steel nozzle assembled on a piezoelectric actuator is presented, which enables the precise control over very small powder quantities by vibration excitation. An analysis reveals that the mass flow through the nozzle can be adjusted by the vibration modes in a certain range depending on the nozzle's specifications, whereas the vibration modes themselves show a complicated behaviour. Using a positioning system in combination with the vibrating nozzle, single-layer patterns consisting of polyamide 12 are produced and characterized regarding surface homogeneity and selectivity using a laser stripe sensor.
By simultaneous laser beam melting (SLBM), different polymer powders can be processed to multi-material parts, which offers the potential to enlarge the field of application for conventional LBM. In a SLBM process, a powder bed consisting of different polymers and therefore with different melting and crystallization temperatures is deposited. Besides the use of infrared emitters for preheating the lower melting polymer, a CO2 laser distributes the necessary preheating temperature of the higher melting polymer. In the last step, a thulium fibre laser distributes the energy necessary for melting the two preheated powders simultaneously. In order to analyze the temperature gradients of the process on the powder surface and in deeper layers, a high-resolution thermal imaging system and thermocouples are used.
Die Energieeinbringung in Form von elektromagnetischer Strahlung in das Pulverbett beim selektiven Laserstrahlschmelzprozess von Kunststoffen wird maßgeblich durch das Absorptionsverhalten der Pulverwerkstoffe bestimmt. Aufgrund von Vielfachreflexionen an den einzelnen Pulverpartikeln in den Poren weisen Pulverwerkstoffe im Vergleich zu makroskopischen Festkörpern andere Absorptionseigenschaften auf. Im vorliegenden Beitrag wird daher ein geeigneter Messaufbau zur Bestimmung der optischen Materialeigenschaften von verschiedenen Pulverwerkstoffen vorgestellt. Mittels eines Ulbricht-Kugel-Messaufbaus wird der Reflexions-, der Transmissions- und der Absorptionsgrad von pulverförmigen Werkstoffen bei einer Wellenlänge von 10,6 µm eines CO2-Lasers und unterschiedlichen Schichtstärken ermittelt. Neben den Werkstoffen Polyamid 12 und Polyethylen im reinen Zustand werden Mischungen mit Additiven, wie beispielsweise Graphit, analysiert und verglichen. Durch die Beimischungen kann eine für den Schmelzprozess zu geringe Absorption des reinen Kunststoffes wie im Fall von Polyethylen-Pulver bei einer Wellenlänge von 10,6 µm gezielt erhöht werden, wodurch die Entwicklung neuer Pulverwerkstoffe erleichtert wird.