Selective Laser Sintering Powder Research Articles

Rheology and agglomeration behavior of semi-crystalline polyamide powders for selective laser sintering: A comparative study of PA11 and PA12 formulations

In this study, we investigated the rheological behavior of various polyamide powders as a function of temperature, shear rate, and relative humidity. Near the glass transition temperature (Tg), powders exhibited an abrupt increase in static flow threshold, indicating caking due to particle agglomeration. At higher temperatures, a transition from elastic to plastic flow was observed around 80∘C, accompanied by changes in the plateau value of the shear modulus (G0′) for different powders. In the dense flow regime, a critical temperature value dictated the transition between caking (powder’s tendency to form lumps or masses) and free-flowing behavior, with temperature promoting agglomeration and shear rate causing dispersion. The critical temperature increased with the strain rate, highlighting the delicate balance between agglomeration and dispersion. Also, it is found that Relative Humidity (RH) plays a crucial role in reducing the stress developed within the sample and counteracting the temperature effect. Additionally, the slow compaction dynamics of powders were studied, revealing an inverse logarithmic law with a characteristic relaxation time (τ) that strongly correlated with the angle of repose.

Ensemble learning based defect detection of laser sintering

AbstractIn rapid development, Selective Laser Sintering (SLS) creates prototypes by processing industrial materials, for example, polymers. Such materials are usually in powder form and fused by a laser beam. The manufacturing quality depends on the interaction between a high‐energy laser beam and the powdered material. However, in‐homogeneous temperature distribution, unstable laser powder, and inconsistent powder densities can cause defects in the final product, for example, Powder Bed Defects. Such factors can lead to irregularities, for example, warping, distortion, and inadequate powder bed fusion. These irregularities may affect the profitable SLS production. Consequently, detecting powder bed defects requires automation. An ensemble learning‐based approach is proposed for detecting defects in SLS powder bed images from this perceptive. The proposed approach first pre‐processes the images to reduce the computational complexity. Then, the Convolutional Neural Network (CNN) based ensembled models (off‐the‐shelf CNN, bagged CNN, and boosted CNN) are implemented and compared. The ensemble learning CNN (bagged and boosted CNN) is good for powder bed detection. The evaluation results indicate that the performance of bagged CNN is significant. It also indicates that preprocessing of the images, mainly cropping to the region of interest, improves the performance of the proposed approach. The training and testing accuracy of the bagged CNN is 96.1% and 95.1%, respectively.

Flowability of polymer powders at elevated temperatures for additive manufacturing

The flowability and spreadability of three different polymer powders for selective laser sintering were investigated by analysing the powder at different temperatures. These measurements were performed using both the Revolution Powder Analyzer and the Schulze Ring Shear Tester. The latter device was modified with a self-developed heating system to allow testing at temperatures up to 170 °C. Overall, the results of both tests showed that the powders' flowability decreases as the temperature increases, with the largest decrease observed near the materials' melting point. Afterwards, recoating tests were conducted on an EOS P395 machine, which utilized a self-built AI monitoring system to detect and evaluate the severity of defects in the powder bed. The tests revealed that the number and severity of defects also increases with temperature. When approaching the sintering window, the number and severity of defects greatly decreases, and the powder bed becomes stronger and more resistant to defects.

Recycling of Selective Laser Sintering Waste Nylon Powders into Fused Filament Fabrication Parts Reinforced with Mg Particles.

This paper presents recycling of selective laser sintering (SLS) waste nylon into printable filaments and parts reinforced with Mg particles. Waste nylon and waste–Mg powder mixture with 2%, 4%, and 8% Mg to nylon were extruded into the filaments. Moisture absorption, differential scanning calorimetry, and melt flow index experiments were conducted to determine the thermal characteristics, while tensile and flexural tests were conducted to evaluate mechanical properties and failure mechanisms. The results were compared with off-the-self (OTS) nylon. Waste powder was found to be extrudable and printable as FFF filament. Waste filament diameter closely matched standard filament size, while exhibiting reduced moisture absorption. High melting and crystallisation temperature for the waste nylon demonstrated a degradation of the plastic during the SLS process. Young’s modulus and ultimate tensile strength for the waste filament increased by 1.6-fold compared to that for OTS, while Mg-composite filament surpassed the waste and OTS. Waste and Mg composite dog bone results showed an increase in strength and stiffness, but the ductility deteriorated. Both flexural strength and modulus for the waste nylon increased by 13% and 26%, respectively, over OTS, and the addition of Mg enhanced flexural strength by up to 5-fold at 8% Mg over the waste. Printed surface topography demonstrated that the waste and Mg composite filaments can print the parts with desired geometric shapes and acceptable surface texture. The findings showed that recycling waste SLS powder into FFF prints would be a viable and useful alternative to disposal, given its abundance.

Selective Laser Sintering of Polystyrene: Preserving Mechanical Properties without Post-processing

This study investigated the possibility of improving strength and density of polystyrene (PS) parts produced by selective laser sintering (SLS) without additives or post-processing. Post-processing methods are used by researchers to achieve optimal mechanical properties. An efficient method was proposed to systematically evaluate the PS powders and optimize the SLS process, in order to overcome the challenges of sintering the PS parts. The tailored SLS powder was produced from PS granule, and its properties including particle size and particle morphology, powder bed density and Hausner ratio (HR) were analyzed in order to evaluate powder quality. The FT-IR test was also utilized to investigate the possible effects of cryogenic milling and SLS processes on the material’s chemical compounds. Then, the effects of input parameters, including powder bed temperature as well as power and speed of the laser on the parts’ mechanical properties, were investigated. The sample’s maximum tensile strength and elongation were, respectively, 17.06 ± 0.9 MPa and 2.75% ± 0.55. The results revealed that it is possible to strengthen PS samples without additives and post-processing. However, due to the powder particle burning and confinement in the molten pool, the increase in energy density beyond the optimum level created spherical pores in the structure that weakened the sample’s strength. It is indicated that the energy density of the laser can be increased to achieve higher strength of specimens and dimensional accuracy can be controlled. TGA analysis of the SLS samples demonstrates when the energy density exceeds 0.4 J/mm2, degradation and decomposition take place in the specimens.

Weld seam morphology and bond strength of infrared and vibration welded SLS parts of polyamide 12 as a function of the layer build-up direction and the welding process

Selective laser sintering (SLS) offers the possibility of manufacturing complex parts without tools and thus enables the cost-effective production of individualized products. In contrast to this advantage, the producible component dimension is restricted due to the dimensions of the building chamber of commercial SLS systems. Furthermore, a great amount of cost-intensive powder is necessary for the production of large SLS parts. In order to overcome these limitations, reliable joining processes, which enable high mechanical bond properties in SLS assemblies, are indispensable. Especially welding processes are suitable in this case. However, there is only limited knowledge available about the welding of SLS parts. In particular, the influence of the characteristics of the SLS process on the resulting bond quality has not been analyzed so far. Within this paper, the influence of the SLS layer build-up direction on the resulting weld seam morphology and bond strength as a function of the welding process is investigated. Infrared and vibration welding, which differ in the method of energy input into the joining zone, are used.The investigations carried out show that high weld seam strengths can be achieved in SLS assemblies for both welding processes. The resulting bond strength is thereby largely independent of the SLS build-up direction. Vibration welded SLS parts exhibit high bond strengths in the range of the base material strength. SLS joints produced via infrared welding have lower bond strengths, which is expected to be caused by an already beginning recrystallization in the generated melt layer during the changeover phase. Furthermore, the investigations show that the weld seam morphology for SLS parts deviates from the characteristic structure of polymeric weld seams, as welded SLS samples do not show the typical separated multi-layer structure. The weld seam shows a very fine spherulitic, partly optically amorphous structure. This is probably the case because of additives contained in the SLS powder (e.g. pyrogenic silicon dioxide), which have an accelerating effect on crystallization. Deformed spherulites in the transition area to the base material are only visible for vibration welded SLS specimens. Due to larger occurring spherulites within the SLS samples compared to injection molded parts, their deformation takes place over a much wider area. The analysis of the weld seam morphology further substantiates the assumption that recrystallization already begins during the changeover phase in the infrared welding process. A highly oriented area with growing spherulites can be detected in the area of the joining plane. In summary, however, it can be noted that both welding processes are well suited for joining SLS parts.

Flow properties of polymeric powders for selective laser sintering

The effect of temperature on the flow properties of three different polymeric powders was assessed in order to properly take into account real temperature conditions relevant to Selective Laser Sintering (SLS). Shear tests were carried out at temperatures from ambient conditions to values close to the melting temperature of the polymers. Tested flowability significantly worsens at temperatures 20–30 °C lower than the powder melting point. These results are consistent with the working temperatures identified by SLS machine users, but powder flow properties estimated at finite consolidation cannot be directly used to infer on powder spread-ability. Instead, when the average intensity of the interparticle forces is calculated from the powder flow properties, the derived granular Bond number correlates well with the most suitable process conditions experienced for these powders. In particular, Bond numbers lower than 100 are required for a satisfactory powder layer formation after spreading with the use of a blade.

Development of organically modified montmorillonite/polypropylene composite powders for selective laser sintering

Organically modified montmorillonite/polypropylene (OMMT/PP) composite powders for selective laser sintering (SLS) were developed using four powder preparation methods including melt extrusion combined with cryogenic grinding, thermally induced phase separation (TIPS), surface coating, and mechanical mixing. Evaluations were conducted on the shape and size, powder-bed surface roughness, and thermal behavior of the developed composite powders as well as the mechanical properties of their injection-moulded tensile specimens. Melt extrusion proved to be the most favorable as the powders exhibited the highest thermo-oxidative stability (Td,max = 378 °C) and enhanced mechanical strength and ductility. TIPS could produce powders with reduced crystallization temperatures and increased melt enthalpies that were beneficial for SLS, and with powder morphology ideal for SLS without post-processing. It was found that at low loadings, nanofillers with sheet-like structures such as OMMT could limit the increase in crystallization temperatures of the composite powder, thus ensuring their wide sintering windows for SLS.

Electrophotographic Multilayer Powder Pattern Deposition for Additive Manufacturing

The use of electrophotographic polymer powder transfer for the preparation of patterned powder layers is discussed with respect to a possible multimaterial application in powder bed-based additive manufacturing technologies such as selective laser sintering (SLS). Therefore, an experimental setup with a two-chamber design was realized, enabling the electrophotographic transfer of SLS powder materials at typical process conditions. The powder development (pick-up) step was investigated thoroughly for different powder materials to provide deep understanding of the underlying electrostatic effects and the influence of distinct powder properties. While using two different development modes, differences in the development results were correlated to differences in their particle size distribution, bulk density, and relative permittivity. Moreover, a new strategy was invented, allowing the residual electrophotographic powder deposition to be in general independent from the already produced part height. This is known to be a huge challenge but is mandatory for the buildup of three-dimensional multimaterial components.

Modifying surface properties of polyamide powders for selective laser sintering

AnnotationThe purpose of this work was determination of the changes in surface properties of Polyamide-12 powder used in Selective Laser Sintering (SLS). The powders of original polyamide Duraform PAtm (3D Systems) were investigated. Also the powders remaining around the product after the sintering process were investigated. The difference in the chemical structure of the Polyamide-12 powder surface layers has been established. The reason is the increased content of oxygen-containing groups in the surface layers. To a greater extent, this difference was revealed for powders left after sintering and intended for reuse. Aliphatic chains are subject to oxidative degradation in the surface layers of the original powder. In the powder remaining after sintering, terminal carbonyl and amine groups are subject to oxidative degradation.

Analysis of Aluminum Alloy Powders for Additive Manufacturing Fabricated by Atomization

Powders for selective laser sintering based on commercial aluminum and some cast and wrought aluminum alloys as well as three experimental alloys have been prepared by atomization technique. The micromorphology of the powders is characterized by nearly spherical main particles distributed with the maximum in the region of 50 μm as well as existence of satellites. It is demonstrated that the adhesive strength between satellites and the main particle revealed by static light scattering before and after ultrasonic treatment can be attributed to peculiarities of its solidification.

Production and Processing of a Spherical Polybutylene Terephthalate Powder for Laser Sintering

This work describes the production of a spherical polybutylene terephthalate (PBT) powder and its processing with selective laser sintering (SLS). The powder was produced via melt emulsification, a continuous extrusion-based process. PBT was melt blended with polyethylene glycol (PEG), creating an emulsion of spherical PBT droplets in a PEG matrix. Powder could be extracted after dissolving the PEG matrix phase in water. The extrusion settings were adjusted to optimize the size and yield of PBT particles. After classification, 79 vol. % of particles fell within a range of 10–100 µm. Owing to its spherical shape, the powder exhibited excellent flowability and packing properties. After powder production, the width of the thermal processing (sintering) window was reduced by 7.6 °C. Processing of the powder on a laser sintering machine was only possible with difficulties. The parts exhibited mechanical properties inferior to injection-molded specimens. The main reason lied in the PBT being prone to thermal degradation and hydrolysis during the powder production process. Melt emulsification in general is a process well suited to produce a large variety of SLS powders with exceptional flowability.

Preparation of spherical polymer powders for selective laser sintering from immiscible PA12/PEO blends with high viscosity ratios

One key issue restricting the rapid application of selective laser sintering (SLS) technique is the lacking of an ideal method in preparing powder materials with good flowability and excellent sintering properties. This work demonstrates that by carefully controlling over material and processing parameters, spherical polymer powders with suitable size which meet the requirements of SLS process can be prepared from immiscible polymer blends with a large viscosity contrast by using a melt blending-extraction (MBE) method. By changing the viscosity of water-soluble poly(ethylene oxide) (PEO) matrix phase via mixing PEO with different molecular weights, the optimal viscosity ratio (p) for producing spherical polyamide 12 (PA12) powder with particle size ranging from 10 μm to 100 μm is determined to be about 50. In contrast, processing parameters such as mixing intensity were found to only have a marginal effect on the particle size. Available empirical or semiempirical equations based on shear flow cannot make a reasonable prediction for the particle size in such high p systems. Instead, the elongational flow presented during melt mixing is suggested to be crucial in determining the particle size. The spherical polymer powders prepared via the MBE method possess a good flowability and a wide sintering window, and can be good material candidates for the SLS technique.

Production of polyamide 11 microparticles for Additive Manufacturing by liquid-liquid phase separation and precipitation

Within this contribution liquid-liquid phase separation (LLPS) and precipitation from ethanol was studied as an approach to produce polyamide 11 (PA11) powders for selective laser sintering (SLS). To this end, the cloud point and solution temperature curve of the PA11-ethanol system was determined experimentally via turbidity measurements. The proper range of system composition and temperature for particle formation was deduced. The dependence of particle characteristics on process parameters (polymer concentration, stirring conditions and temperature regime) during LLPS and precipitation was assessed and the products were characterized with respect to their size and morphology. Furthermore, structural, rheological and thermal characteristics were analyzed and correlated with process parameters. Molecular weight distributions were determined. After removal of fines and dry coating with hydrophobic fumed silica, an optimized PA11 powder with mean particle size of 91 µm showing good flowability for SLS was obtained. SLS processability of this optimized PA11 powder was demonstrated by building multi-layered test specimens in a laser sintering machine. With this contribution, we present a comprehensive workflow for the process development, product characterization and product application of a SLS powder manufactured via precipitation.

Understanding and improving optical coherence tomography imaging depth in selective laser sintering nylon 12 parts and powder

Optical coherence tomography (OCT) has shown promise as a process sensor in selective laser sintering (SLS) due to its ability to yield depth-resolved data not attainable with conventional sensors. However, OCT images of nylon 12 powder and nylon 12 components fabricated via SLS contain artifacts that have not been previously investigated in the literature. A better understanding of light interactions with SLS powder and components is foundational for further research expanding the utility of OCT imaging in SLS and other additive manufacturing (AM) sensing applications. Specifically, in this work, nylon powder and sintered parts were imaged in air and in an index matching liquid. Subsequent image analysis revealed the cause of “signal-tail” OCT image artifacts to be a combination of both inter and intraparticle multiple-scattering and reflections. Then, the OCT imaging depth of nylon 12 powder and the contrast-to-noise ratio of a sintered part were improved through the use of an index matching liquid. Finally, polymer crystals were identified as the main source of intraparticle scattering in nylon 12 powder. Implications of these results on future research utilizing OCT in SLS are also given.

Roadmap to sustainable plastic additive manufacturing

As additive manufacturing (AM) is heading towards mass production and mass customization, the process needs to be environment-friendly and energy-efficient in order to be self-sustainable. Selective laser sintering (SLS) and fused deposition modeling (FDM) are two processes contributing massively towards plastic additive manufacturing. In SLS, the powders that do not contribute to the mass of products become unusable (after some number of reuse) and finally turn into waste. As high energy is required for production of powders, generation of these wastes impacts environment sustainability. A contrivance needs to be developed to convert these waste powders into high-value products to make the process sustainable. FDM has the largest market share in terms of number of AM systems sold and is the most popular for fabricating low-value products. Its usefulness will be further expanded if inexpensive high-value products could be made through this process. In the present work, waste SLS powder is employed to prepare feedstock for inexpensive high-value FDM products, which demonstrates that SLS refuse could be utilized for mass production of FDM feedstock. If two processes (SLS & FDM) will be connected as proposed here, it will make plastic additive manufacturing energy-efficient, self-sustainable, and will contribute to environment sustainability, in general.

Material Evaluation and Process Optimization of CNT-Coated Polymer Powders for Selective Laser Sintering.

Multi-walled carbon nanotubes (CNTs) as nano-reinforcements were introduced to facilitate the laser sintering process and enhance the thermal and mechanical properties of polymeric composites. A dual experimental-theoretical method was proposed to evaluate the processability and predict the process parameters of newly developed CNT-coated polyamide 12 (CNTs/PA12) powders. The thermal conductivity, melt viscosity, phase transition and temperature-dependent density and heat capacity of PA12 and CNTs/PA12 powders were characterized for material evaluation. The composite powders exhibited improved heat conduction and heat absorption compared with virgin polymer powders, and the stable sintering range of composite powders was extended and found to be favourable for the sintering process. The microstructures of sintered composites revealed that the CNTs remained at the powder boundaries and formed network architectures, which instantaneously induced the significant enhancements in tensile strength, elongation at break and toughness without sacrificing tensile modulus.

New Materials for SLS: The Use of Antistatic and Flow Agents

Selective laser sintering (SLS) is a process based on the principle of a locally confined energy input by a laser into a powder bed, producing highly complex parts without the use of moulds or any other tools. In order to ensure good results for the processing behaviour of a new material, the powder must perform well during the phase of feeding the material into the process chamber which majorly influences the quality of the spread of the powder into the part bed and thus the mechanical performance of the final parts. In the present study, the principle of modification of fine powders with flow agents is applied aiming to enable the use of powders for SLS which are otherwise unsuitable due to poor flowability. In addition, the influence of antistatic agent on the powder flow and processing behaviour is discussed. The additives are found to strongly improve the flow behaviour at already very small contents and thus allow for processing of the composite material. The development of determining factors shares insight into the mechanisms of dry particle coating and its implementation into a growing market of material development.

Development of Powders for Selective Laser Sintering

The selective laser sintering is a technique used to quickly fabricate a scale model of a physical part or assembly using three-dimensional computer aided design CAD data. This technique is used to obtain complex pieces in different domains like machine building industry, automotive industry, foundry and medicine. The selective laser sintering process is used to obtain different prototypes, medical instruments or personalized implants in medicine. The powders used to obtain the medical instruments and for implants must to be very resistant to corrosion, non-toxic and present good fatigue resistance. The powders used are in generally, stainless steel, alloy titan TA6V, alloy Co-Cr and different polyamides.

Rounding of Irregular Polymer Particles in a Downer Reactor

In the past additive manufacturing processes relying on beam-based methods such as selective laser sintering (SLS) of polymers were almost exclusively applied for the generation of prototypes. In recent years, however, these methods are increasingly applied in the serial production of e.g. special parts to be used in the automotive industry. With this increased relevance in industrial applications, the demands on the powder material increased as well [1]. The so far used polymer materials show weak performance regarding the mechanical stability of processed parts. In consequence, there is a need for fine powders of different polymers showing good powder flowability and high bulk density. Currently, the commercial availability of polymer powders for SLS is very limited and the production processes are still not reliable enough due to poor powder characteristics. Within this contribution a process for the production of spherical polymer micro particles showing good powder flowability is presented [2].It has been recently shown [3] for different materials that commercially available polymer granules can be ground down to the micron size range by a wet grinding process. Due to the grinding process the produced micron-sized particles are in a chiseled state, which leads to a bad flowability of the powder material. To overcome this problem the single particles are melted in a heated downer reactor and spherical particles are obtained by using the effect of the surface tension [3]. The rounding of ground PS microparticles was performed in a downer reactor with a length of 1,600 mm and a diameter of 25 mm. The reactor temperature was set 100°C above the melting temperature of the treated material. The laminar flow regime in the downer reactor allows a well-defined residence time of particles and the surrounding gas [4]. The design of the downer reactor allows transferring the polymer particles in the molten state whilst avoiding contact between the single particles and between molten particles and the reactor wall. The mechanism of the rounding process in dependence of particle size, interfacial tension and melt viscosity will be shown within a sintering model [6]. The influence of residence time and overall solids density in the downer reactor on the particle shape and on the flowability of the bulk material is shown. Estimates from the sintering model allow for a reliable reactor design.