Study of cold powder compaction by using the discrete element method

The discrete element method (DEM), based on a soft-sphere approach, is commonly used to simulate powder compaction. With these simulations a new macroscopic constitutive relation can be formulated. It is able to de-scribe accurately the constitutive material of powders during the cold compaction process. However, the force-law used in the classical DEM formulation does not reproduce correctly the stress evolution during the high density compaction of powder. To overcome this limitation at a relative density of about 0.85, the high density model is used. This contact model can reproduce incompressibility effects in granular media by implementing the local solid fraction into the DEM software, using Voronoi cells. The first DEM simulations using the open-source YADE software show a fairly good agreement with the multi-particle finite element simulations and experimental results.     

Roller compaction/Dry granulation: Use of the thin layer model for predicting densities and forces during roller compaction

The thin layer model is based on the assumption that the deformation of powder during tableting can be transferred to the roller compaction process, provided that it was established with sufficient accuracy in the tableting experiments. In particular, the process of compaction between the rolls is presumed to consist of three parts, a rearrangement, an “exponential” and an elastic recovery phase. The rearrangement and “exponential” phases are used to calculate the densification of the material. The forces between the rolls during elastic recovery, the third phase, proved to be essential to the prediction, because 20% to 30% of the total roller compaction force is required to counteract ribbon recovery. Four different excipients and one powder blend were tested in the model. For two materials, the density and force predictions turned out to be accurate within ± 2.5% and ± 10%, respectively. For one excipient and the model blend, the predictions deviated systematically whereas those for the remaining excipient were within the above mentioned limits in ca. 50% of the experiments. For explaining these differences, we evaluated both the influence of the course of the force-time profile, at comparable densification times, and the influence of different compression times, for comparable force-time profiles. Finally, the impact of density distributions within ribbons on the prediction was estimated.     

Bulk density of solid polymer resins as a function of temperature and pressure

In a plasticating extruder, solid polymers are heated and are subjected to high pressures before they are melted and delivered to a die. In both the solids conveying and melting sections, these temperature and pressure increases will compact the unmelted polymer bed as it moves down the screw channel. Performance of the extruder depends in part on how well the screw design matches the compaction behavior of the resin for a given set of process conditions. The design of these screw sections, however, is often done based on past experience and with little knowledge of the resin compaction behavior. A much improved design would include screw performance prediction using variable bulk density and computer simulations. Computer simulations, however, are often performed using constant solid bulk density because of the lack of reliable density data as a function of both pressure and temperature. An instrument was developed for studying the compaction behavior of pellet and powder resins. Bulk densities and storage friction coefficients are reported for several important thermoplastic resins as a function of temperature and pressure. The bulk density data were fitted to a semi-empirical model.     

Reaction-based Processing of Textured Alumina by Templated Grain Growth

Highly textured, dense alumina ceramics were fabricated by a new processing route which utilizes a mixture of Al metal powder, alumina powder, alumina platelet (template) particles and a liquid phase former. The process involves dry forming the powder mixture (e.g. uniaxial pressing, and roll compaction) to align the plate-like template particles. The addition of a calcium aluminosilicate glass reduces constrained densification by the template particles and allows attainment of high density at ∼1450°C. The degree of orientation (i.e. r is 1 for a random sample and 0 for a perfectly textured material) and volume fraction of textured material, f, were measured by X-ray-based rocking curve technique and SEM-based stereological analysis, respectively. It has been shown that texture quality (the r parameter) is controlled by initial strain during forming, sintering time and temperature. In addition, alumina ceramics with the volume fraction of textured material ranging from 1 to ∼100% can be obtained.     

同向双螺杆挤出过程中聚合物颗粒熔融问题探讨Ⅰ—相变分数法与实验验证

在双螺杆挤出过程中，聚合物颗粒挤出熔融过程中较为常见的一种固、液两相共存的形式可以用“海－岛”模型来描述。提出了“海－岛”模型的一种简化模型－中间模型，通过引入“相变分数”对粘性耗散热进行分配，得出了固相分数及熔体平均温度沿螺槽方向的变化规律。通过实验对相关结果进行了验证，对比表明该方法基本能反映聚合物颗粒熔融的真实过程。     

Wall friction in the compaction of pharmaceutical powders: measurement and effect on the density distribution

In this paper, the axial density profile of tablets of microcrystalline cellulose (MCC) powder compacted in nonlubricated die is investigated by finite element modelling (FEM). The Drucker-Prager/Cap model was adopted for the compaction behavior of powder. The material parameters of the model, including the die wall friction coefficient, were estimated from experimental data of die compaction where the initial density of powder is taken uniform. Changes of Young's modulus with density was measured with a four-point beam bending test. The results of the simulation of the compression and the decompression steps were used to calculate the axial density distribution. Comparison with the measured data presented in [A. Michrafy, M.S. Kadiri, J.A.D. Dodds, Wall friction and its effects on the density distri-bution in the compaction of pharmaceutical excipients, Chem. Eng. Research and Design, Vol. 81, Part A, September (2003)] is discussed.     

Investigation of the mechanism of low‐density particle and liquid mixing process in a stirred vessel

In order to investigate the mechanism of the low‐density solid particle and liquid mixing process, a specialised agitator structure was used. Both computational fluid dynamics simulation and experiments were carried out to study the two‐phase mixing characteristics in the stirred vessel. The mixing process was captured by snapshots. The flow field and solid phase volume fraction evolution were analysed. Experimental and numerical results agreed well with each other. Solid particles floating on the liquid surface were gradually transported to the bottom through the centre of the vessel and the mixing time was predicted and tested. Results indicate that the agitator structure used in this study is able to form an obvious axial circulation in the vessel and then achieve a good performance in low‐density solid and liquid mixing operations. The study provides a valuable reference for the design and optimisation of solid–liquid mixing equipment. © 2011 Canadian Society for Chemical Engineering     

Densification of silicon nitride powder with various additives

Cold compaction behavior of a sub-micron size silicon nitride powder with additives has been studied at various loading rates up to about 450 MPa. Liquid lubricants are found to be more effective than the solid lubricants. A marked increase in relative density has been obtained with the aid of 10 weight percent polyethyleneglycol after surface treatment of the powder with a dispersant solution. The loading rate between 0.8 and 800 MPa per minute has no effect on the densification process. Results are analysed with various theoretical equations available in the literature. The Cooper and Eaton's equation and also its modified form are used to determine the mechanism of densification during cold compaction.     

Synthesis and microstructure evolution of WCoB based cermets during spark plasma sintering

WCoB based cermets were prepared by spark plasma sintering at sintering temperature among 600°C-1200 °C. The phase evolution was investigated by detecting density behavior, phase composition, microstructure and mechanical properties during sintering process. The sintering process can be divided into three stages: powder densification, solid phase reaction and liquid phase sintering. WCoB hard phase forms at 1000 °C during solid phase sintering, showing better mechanical properties than Co₂B, especially on Vicker's hardness. WCoB hard phase forms on the basis of Co₂B binary boride and its content increases in liquid phase sintering stage with high density. The Vicker's hardness and transverse rupture strength (TRS) reach the maximum value of 1262 Hv and 1212 MPa at 1200 °C and 1170 °C, respectively. The fracture toughness reaches the maximum value of 21.8 MPa m^1/2 at 1050 °C, and the inter-granular fracture is the main fracture mechanism.     

Densification behavior of nanocrystalline titania powder under cold compaction

Densification behavior of nanocrystalline titania powder was investigated under cold compaction. Experimental data were obtained from triaxial compression with various loading conditions. Lee and Kim proposed the Cap model by employing the parameters involved in the yield function of sintered metal powder and volumetric strain evolution under cold isostatic pressing. The parameters in the Drucker/Prager Cap model and the proposed Cap model were obtained from experimental data under triaxial compression. Finite element results from the models were compared with experimental data for densification behavior of nanocrystalline ceramic powder under cold isostatic pressing and die compaction. The proposed model and the Drucker/Prager Cap model agreed well with experimental data under cold compaction. Finite element results and experimental data also, show that relative density distribution of nanocrystalline ceramic powder compacts is nonuniform compared to the conventional micron powder compacts at the same averaged relative density.     

Cold compaction study of Armstrong Process® Ti-6Al-4V powders

The Armstrong Process® developed by Cristal US, Inc./International Titanium Powder, is an innovative, low-cost technology for producing Ti and Ti alloy powders in a one-step, continuous process. In this work, Armstrong Ti-6Al-4V powders were characterized and the cold compaction behavior of the powders were investigated in detail. As-received as well as milled powders were uniaxially die-pressed at designated pressures up to 690 MPa to form disk samples with different aspect ratios. Samples with high aspect ratio exhibited non-uniform density along the pressing axis and the density distribution was consistent with the result predicted by finite element analysis. The model developed from the linear regression analysis on the experimental density data can be used to predict density of compacts with different aspect ratios. In the studied pressure range, an empirical powder compaction equation was applied to linearize the green density — pressure relationship. Cold compaction parameters were obtained for the as-received and milled Armstrong Ti-6Al-4V powders.     

Compaction and Sintering Behavior of Bimodal Alumina Powder Suspensions by Pressure Filtration

The compaction behavior of fine alumina powders with different particle sizes or bimodal particle-size distributions that are undergoing pressure filtration was investigated. Three alumina powders—average particle sizes of 0.2—0.86 μm—were compacted to a solids fraction of 62—65 vol% from suspensions at pH 3, which was the pH level at which the suspensions showed their lowest viscosity. When the powders of different average sizes were mixed, the suspensions showed better flowability, and the lowest viscosity was obtained when the fraction of fines was ∼30 vol% and pH = 3. The mixed-sized powder suspensions were compacted to higher density than the suspensions of unmixed fine or coarse powders, and the maximum density was obtained for mixed suspensions that had the lowest viscosity, despite the different particle-size ratio. Maximum densities of 72.5% and 75.0% were attained when the size ratios were 2 and 5, respectively. The compacts that were pressure-filtered from mixed suspensions exhibited a single-peaked pore-size distribution and a homogeneous microstructure, whereas the pore-size distributions of dry-pressed compacts were double-peaked. The sintering behavior of the compacts that were pressure-filtrated from bimodal powders exhibited significantly better sinterability and much-less linear shrinkage than the coarser powders and the dry-pressed powder compacts.     

A combined experimental-numerical study of the compaction behavior of NaCl

The compaction behavior of NaCl as a model substance is investigated by an integrated experimental and computational approach. The method for characterization of this granular material employs convenient experiments: load-displacement measurements of compaction; measurements of strain on outer circumference of an elastic tubular die; load on bottom and top of the powder compact, as well as compressive strength tests. Related equations for identification of material parameters are derived and are used to characterize powder behavior and powder-die friction. Subsequently, these material parameters are used in simulations with the Drucker-Prager-Cap (DPC) model. For the verification of the computations density distributions are determined based on micro X-ray computer tomography. Good agreement between the spatial density distributions from measurements and simulations is obtained. Restrictions of computer tomography in powder compaction applications are specified. While the study employs NaCl as a model substance, the approach is applicable to a wider array of granular substances.     

Dispersion and densification of nano Si–(Al)–C powder with amorphous/nanocrystalline bimodal microstructure

The dispersion behavior and densification of nano Si–(Al)–C powder with amorphous/nanocrystalline bimodal microstructure were investigated. The Si–C powders synthesized by a mechanical alloying (MA) process had a near‐spherical shape with an average particle size of 170 nm. A solid loading of 62 vol% was achieved using polyethyleneimine (PEI) as a dispersant. The optimum dispersant amount was 1 wt% based on zeta potential, sedimentation, and viscosity analysis data. The high zeta potential value (73 mV) compared with that of the commercially available SiC (65 mV) was caused by modified surface properties and consequent promotion of the cationic dispersant adsorption. A Si–Al–C slurry containing 6.5 wt% of sintering additives with a solid loading of 60 vol% was also prepared. The relative density of the dried Si–Al–C slurry was 63.3% without additional compaction, which could be densified at 1650°C at a pressure of 20 MPa using a spark plasma sintering furnace.     

Liquid velocity in a high‐solids‐loading three‐phase external‐loop airlift reactor

BACKGROUND: Airlift reactors are of interest for many different processes, especially for three‐phase systems. In this study the behavior of a high‐loading three‐phase external‐loop airlift reactor was examined. In particular, the effect of parameters such as airflow rate (riser superficial gas velocities between 0.003 and 0.017 m s^?1), solids loading (up to 50%, v/v) on liquid circulation velocity in the air‐water‐alginate beads system as a crucial hydrodynamic parameter was studied. RESULTS: It was observed that increase of the airflow rate resulted in increase of the liquid velocity in the system. The same result but less pronounced was observed by introducing small amounts of solid particles up to 7.5% v/v. However, further introduction of solids caused decrease of the liquid velocity. Laminar regime for the liquid circulation was observed for low gas velocities. Minimum gas velocities for recirculation initiation in the reactor were determined for all solid loadings and linear dependence on the solid content was found. Gas holdups for the three‐phase system were larger than for the two‐phase system in all experiments. A simple model for predicting the liquid circulation velocity in the three‐phase system with high solid loading of low‐density particles was developed. This model is based on the viscosity of integrated medium (solid + liquid) which is a new aspect to analyze this phenomenon. CONCLUSIONS: The developed model shows very good agreement with the experimental results for all solid loadings. It also includes the influence of reactor geometry on the liquid circulation velocity thus enabling optimization. Copyright © 2012 Society of Chemical Industry     

A study of the applicability of the capillary rise of aqueous solutions in the measurement of contact angles in powder systems

The rate of wetting of a powder bed was studied in terms of the wetting parameters, liquid surface tension and powder/liquid contact angle, using three carbon black powders and aqueous solutions of surfactants and organic liquids. The rate of capillary rise of pure organic liquids through a powder bed can be described by the Washburn equation, and when compared with the behavior of aqueous surfactant solutions, it showed that deviations from linearity of the latter are attributable to adsorption of surfactant on the solid surface with resultant depletion of solute from the liquid phase. Agreement between Washburn capillary rise results and sessile drop results was observed whenever adsorption was absent.     

Solution miscibility and phase‐change behavior of a polyethylene glycol–diacetate cellulose composite

Polyethylene glycol (PEG) and diacetate cellulose (CDA) exhibit good miscibility in acetone solution. The miscibility is related to the molecular weight of PEG, which increases as miscibility decreases. The phase‐change behavior of PEG in composite with CDA prepared from the miscible solution was found to be completely different from that of pure PEG. When the PEG fraction in the composites was less than 85%, PEG within the composite did not melt into liquid; even when the temperature was 40°C higher than the melting point of PEG, the PEG–CDA composite exhibited solid–solid phase‐change behavior. Thermal analysis indicated that the PEG–CDA composite had greater enthalpy and exhibited good thermal stability. The PEG–CDA composite that exhibited solid–solid phase‐change behavior can be used as a new kind of phase‐change material for thermal energy storage and temperature control. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 652–658, 2003     

Dynamic Compaction of Aluminum Nitride Powder: Hugoniot Measurement and Comparison with Static Behavior

A limited number of shock Hugoniot measurements were performed on two unsintered aluminum nitride powder compacts having initial densities of 1.30 and 1.53 g/cm³. Stresses achieved ranged from 0.25 to 1.8 GPa, corresponding to linear compaction rates of 0.2 to 0.6 km/s. Densification is incomplete behind the first shock wave, proceeding only to ∼70% of the solid density, regardless of the initial density. On reshock, however, significantly higher densities are achieved. Initial compaction of the powder to a relative density of 65% to 70% occurs readily at stresses below 0.25 GPa. For greater stresses, however, densification is slight. Comparisons with static compression data for the same powders suggest that this resistance to compaction is an effect of compaction rate.