We study the packing of fine glass powders of mean particle diameter in the range (4-52) μm both experimentally and by numerical DEM simulations. We obtain quantitative agreement between the ...experimental and numerical results, if both types of attractive forces of particle interaction, adhesion and non-bonded van der Waals forces are taken into account. Our results suggest that considering only viscoelastic and adhesive forces in DEM simulations may lead to incorrect numerical predictions of the behavior of fine powders. Based on the results from simulations and experiments, we propose a mathematical expression to estimate the packing fraction of fine polydisperse powders as a function of the average particle size.
Additive manufacturing processes like selective laser beam melting of polymers (LBM) are established for production of prototypes and individualized parts. The transfer to serial production currently ...is hindered by the limited availability of polymer powders with good processability.
Within this contribution the effect of powder properties, such as particle size, shape and flowability on the processability in LBM and their influence on device quality is exemplified for polybutylene terephthalate (PBT) materials. A process chain for the production of spherical polymer microparticles has been developed to obtain PBT powder materials. The process chain consists of three steps: first, polymer microparticles are produced by wet grinding. Second, the particle shape is engineered by rounding in a heated downer reactor to improve the flowability of the product. Third, a further improvement of flowability of the still cohesive spherical PBT particles is realized by dry coating with fumed silica.
Moreover, properties of the PBT powders obtained along the process chain are thoroughly characterized with respect to structure and crystallinity by infrared spectroscopy, X-ray diffraction and differential scanning calorimetry. The effect of flowability, shape and bulk density on the powders’ processabilities in LBM is assessed by characterization of the quality of thin layers built in a LBM device. It is demonstrated that the device quality is strongly determined by particle properties: powders of good flowability and high bulk density are mandatory to obtain dense devices.
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•Process chain for producing polybutylene terephthalate (PBT) powders for selective laser beam melting (LBM).•Processability of powders is assessed by characterization of sintered thin layers.•Influence of particle shape and size on processability in LBM is studied.•Good LBM processability is observed for PBT powders of good flowability.•Device quality is remarkably influenced by powder flowability and bulk density.
Melt emulsification is a top-down approach that so far has been applied in the food industries (cf. homogenization of milk) and in pharmaceutical applications. Within this contribution the ...applicability of the process for the production of spherical polymer microparticles will be outlined. Size reduction of the polymer raw melt emulsion was realized in a rotor–stator-device. The process is characterized for paraffin and polyethylene (PE) waxes in aqueous environment using Tween 85 as an emulsifier. Process temperature and dispersed phase viscosity have the largest impact on the particle size distribution (PSD): decreasing temperature and viscosity leads to smaller particles. The suitability of the melt emulsification process for technical polymers is demonstrated for polypropylene (PP) emulsified in a continuous hexadecane phase. Polymer particles obtained by the proposed method are promising candidates for advanced powders for additive manufacturing processes like selective laser beam melting (LBM) of polymers. The LBM processability of the obtained PP powders is demonstrated by production of sintered layers.
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•Melt emulsification of paraffin, PE and PP granules allows for microparticles.•Production of spherical polymer particles in a single process step.•Tailored product properties as function of process conditions (e.g. composition).•Decreasing temperature and viscosity leads to smaller particles.
High speed gas injection through nozzles is used in fluidized bed technology for controlling particle size or adding a reactant. Experiments were carried out to investigate the flow field of glass ...beads (x1,2=92μm) in multiphase jets (with respect to the applied pressure). Particle image velocimetry (PIV) was used in a semicircular fluidized bed with optical access, to analyze the particle movement. It was found that higher gas pressure at the nozzle inlet initially leads first to higher particle velocities but decreases the particle degree of mixing. In subsonic conditions the trend of the measured data could be predicted by a simple force balance model. Additionally, the transition from subsonic to choked flow conditions inside the nozzle could be distinguished by analyzing the jet opening angle. The fluidization velocity showed no significant influence in these investigations. It was also proven by solid concentration (1−ε) measurements with capacitance probes: Increasing kinetic gas energy at the nozzle inlet leads to lower solid concentration at the jet axis. At very high gas pressures there were almost no particles in the jet. Finally, a relatively new measurement technique was used for flow analysis. A fast gantry X-ray CT was used to analyze the turbulent flow without disturbing it. Thanks to a calibration the solid distribution could be made visible. This showed an entrainment zone close to the nozzle exit. The results showed that the kinetic energy of solids can be increased by applying higher gas velocities. However, very high gas velocities lead to reduction of solid entrainment into the jet.
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•High speed secondary gas injection in fluidized beds showed high particle velocities but low solids concentration in the jet•Particle velocity at the nozzle exit was estimated by a simple force balance and showed good accordance to the experiments•Fluidization showed negligible influence on particle velocity and concentration inside the jet•With an X-ray CT a region of high particle entrainment close to the nozzle exit was uncovered without disturbing the flow
This contribution addresses possibilities of plasma functionalization of different polymer particles to be used in additive manufacturing processes. The purpose of the surface manipulation is to ...enhance the wettability of single particles and, thus, the coalescence during melting of the particles in order to achieve an improved mechanical stability of parts built by laser beam melting processes. Surface functionalization is achieved by using an atmospheric pressure plasma jet in combination with a fluidized bed reactor. This allows the functionalization of temperature sensitive polymer particles in a reactor concept which can be scaled up for industrial production. To investigate the behavior of the plasma jet, which can be regarded as a secondary injection into a fluid particle bed, a semi-circle set-up was utilized to estimate treatment times of single particles with reactive plasma species. Hereby, different materials (PA12, PE-HD and PP) have been treated and the formed jet volume inside the reactor was measured. The results obtained by this set-up have then been utilized to improve the actual plasma treatment regarding treatment times. The results show that an optimum of treatment time exists for the wetting behavior of particles with water. In a last step, the influence of different plasma feed gases on the wetting behavior of the treated polymer particles has been examined.
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•Treatment of polymer particles in a plasma spouted fluidized bed is presented.•The jet as a secondary gas injection within the fluidized bed is investigated.•Effect of the treatment time on the functionalization process is evaluated.•Plasma gas composition and structure of the treated material is investigated.
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•Determination of packing fraction of copper, glass and zirconia particles.•Comparison of results from bulk density measurements, CT and DEM.•Experimentally determined packing ...fractions agree well with DEM results.•Confirmation of a simple relationship for dependence of packing fraction on size.•DEM proves that mean coordination number result is sensitive on the CT resolution.
The quantitative description of packing behaviour of particulate systems under consideration of realistic particle size distributions and material properties poses one challenging problem in powder technology, with impacts on a broad range of technological areas. Here we investigate the packing characteristics of micrometer to millimeter-sized, polydisperse spherical particle systems of glass, zirconia and copper by means of experimental measurements of the bulk density, X-ray microtomography (CT) and numerical simulations of the granular packings using a 3d discrete element method (DEM). The applied DEM method takes realistic material properties into account and considers the experimentally obtained particle size distributions as well as attractive inter-particle forces including adhesion (Johnson, Kendall, Roberts (JKR)) and non-bonded van der Waals (vdW) interactions. Good agreement of the packing densities predicted by DEM and observed in the experiments (bulk density determination & CT) is found. Moreover, we show that a simple mathematical expression for the packing fraction as a function of average particle size of the polydisperse powder system describes well our experimental and simulation results for all investigated materials, with only two fitting parameters. From the DEM simulations, the mean (first) coordination number in the respective polydisperse packing is extracted and discussed with respect to the experimentally obtained direct neighborhood detection from X-ray tomography and in context of previous works. The mean coordination number shows a rather broad variance due to the polydispersity of the samples considered. It also shows a remarkable dependency on the definition of particle contact. Therefore, caution is advised when evaluating coordination numbers with this quantity being highly dependent on the instrumental resolution.
The gradual shift of rapid prototyping towards additive manufacturing (AM) implies higher demands on the available material. So far, most of the processes as laser beam melting (LBM) rely on ...polyamide as PA12. To overcome limitations and to address this lack of suitable polymer material with a fine particle size, Schmidt et al. 1 have established a wet-grinding process which allows the production of very fine polymer particles. Since the ground product shows a rather bad flowability, further treatment is necessary to enable the use of this material in AM. A rounding of the chiseled particles can improve the flowability 2. This treatment changes the morphology of the particles by surface tension forces in the molten state 3. To accomplish rounding of the polymer powder, a downer reactor in semi-industrial scale has been established and will be characterized in this article. Main topics are the powder dispersion, the inlet geometry and its influence on the gas flow behavior inside the downer. This was achieved by combining basic simulation and experimental data. Furthermore, the heat distribution and agglomeration in the gas phase were investigated as important parameters of the process. Finally, a proof of concept by rounding wet ground PBT material was successfully conducted. The product was investigated to obtain data about a change in particle size and polymeric structure.
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•A reactor for the rounding of irregular formed polymer particles is presented.•An analysis of the flow pattern is conducted using CFD. The influence of the flow pattern on the rounding is discussed.•The rounding is described by viscous flow sintering. Agglomeration effects during rounding are addressed.
Today the modeling of the process dynamics and the complex two-phase flows in mills still remain a challenging task. In this study the dynamic comminution process in a fluidized bed opposed jet mill ...is investigated. For this purpose, well-defined model glass beads were ground in a lab-scale mill. The particle size distributions (PSDs) inside the mill and at the outlet, solid concentrations and the holdup of the mill were recorded. By comparing the morphology of the ground material with that of particles examined in single particle stressing experiments, we conclude that the predominant stressing mechanism in the mill is determined by a high number of low intensity stressing events. Furthermore, the grinding kinetics is strongly influenced by the solid holdup within the mill and the initial PSD of the feed material. A higher holdup leads to a higher product mass flow rate with slightly finer particles. The determined high solid concentration in the transport section of the mill indicates a high loading of the classifier which increases with the holdup. Our study shows that the internal recirculation is crucial for the overall performance of the mill:
Due to imperfect classification, particles accumulate inside the mill which is further enhanced by a high holdup of fine particles. Our results clearly show that the classification and transport processes must be studied in detail for any in-depth understanding of dry fine grinding in fluidized bed opposed jet mills.
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•The predominant stressing mechanism in fluidized bed opposed jet mills is related to a high number of low intensity impacts.•Grinding kinetics are influenced strongly by holdup and initial PSD of the feed material.•Solid concentration measurements in the transport zone of the mill show a high loading of the classifier.•Internal recirculation process inside the mill is crucial for the overall performance of the mill.
High speed jet penetration in fluidized beds can be used for particle size control or comminution processes. Thus, particle breakage is determined by the kinetic energy of the solids and the ...frequency of particle-particle impacts inside the flow. Both aspects are influenced by the solid concentration inside the jets and at the boundary. For understanding the fundamental breakage mechanism, the determination of the solid distribution in those complex flows is an important contribution.
In order to access the turbulent and delicate multiphase flow, a modern fast X-ray computed tomography unit was used in this work. To obtain quantitative solid concentrations, the measurement system was optimized by means of hardware filtering, exposure time and reconstruction algorithm. These methods allowed the investigation of influencing parameters like jet velocity, fluidization velocity or particle size on the solid distribution in a single jet. With the obtained experimental data, correlations for the axial and radial solid distribution were developed. In addition, the influence of multiple jet injection with a common focal point is presented.
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•Solid distributions can be determined by using an optimized X-ray CT system.•Increased jet speed delivers lower solid concentrations with thinner jet boundaries.•Higher fluidization velocities lead to broader jet boundaries.•Lower solid concentrations are observed when particle size is reduced.•Multiple jet injection just influences the single jet close to the focal point.
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