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.     

Uniaxial compaction behaviour and elasticity of cohesive powders

The compression and compaction behaviour of bentonite, limestone and microcrystalline cellulose (MCC) — three cohesive powders widely used in industry were studied. Uniaxial compression was performed in a cylindrical die, 40 mm in diameter and 70 mm high, for three selected cohesive powder samples. The initial density, instantaneous density and tablet density were determined. The influence of maximum pressure and deformation rate was examined. The secant modulus of elasticity E_sec was calculated as a function of deformation rate v, maximum pressure p and powder sample. After compaction experiments in hydraulic press at three pressures - p = 30, 45 and 60 MPa - and two different deformation rates, the strength of the produced tablets was examined in a material strength testing machine.From uniaxial compression tests performed on the universal testing machine for loading and unloading, the modulus of elasticity E was calculated on the basis of the first linear phase of unloading. The total elastic recovery of tablets was also obtained.     

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.     

Micromechanical model for compaction characteristics of bentonite–sand mixtures

This paper presents a new methodology for predicting the relationship between compaction stress and dry density, referred to herein as the compaction curve, of composite mixtures based on the micromechanics. To examine the effect of sand content on the compaction characteristics, a series of uniaxial compaction tests (UCT) for bentonite–sand mixtures with different sand fractions (by weight) were performed. In addition, a prediction model based on micromechanics for composite mixtures was developed. The developed model can be used to predict the compaction curves of composite mixtures based on experimental compaction curve of the matrix. The accuracy of the proposed model along with its applicability was examined and validated by using experimental data. The results show that the proposed model can accurately predict the compaction curves of the bentonite–sand mixtures in both loading and unloading phases.     

A study on the sensitivity of Drucker-Prager Cap model parameters during the decompression phase of powder compaction simulations

The compaction of pharmaceutical powders can be simulated using phenomenological elasto-plastic continuum models adopted from soil mechanics. These models are typically implemented in finite element codes and have been used recently to investigate the macroscopic property distributions in powders during compaction.The present study demonstrates the importance of obtaining accurate yield surface parameters for use in such models. A commercial finite element code implementing the Drucker-Prager Cap (DPC) model was used to model the compression and decompression stages of powder compaction in a tabletting operation. The parameters used in the DPC model were obtained from the literature. Although the compression stage of the process gave expected behavior, the decompression response was unrealistic for at least one set of published data. Small values for the friction and cohesion parameters resulted in a significant elastic recovery during decompression. This study demonstrates the need to obtain accurate parameter data in order to model the decompression stage of powder compaction.     

High-pressure compaction modelling of calcite (CaCO3) powder compact

Numerical simulation of manufacturing processes with working conditions at high pressure (above 1 GPa) requires constitutive data of the powder for the whole range of pressure and density. Most of the test apparatuses commonly used to obtain such data are only working in the lower pressure regions. Because of the absence of high-pressure data, many parameters have to be guessed or extrapolated. A material used in high-pressure applications is Calcite (CaCO₃). The material can be used as an insulator in high-pressure capsules it is also a common material in the earth core. An apparatus often used to generate high pressure during compaction is the Bridgman anvil apparatus. In this work experimental tests with a Bridgman anvil set-up using Calcite powder discs with different thicknesses were done. A nonlinear elastic-plastic cap model was developed to model the behaviour of powder material from low pressure and loose state to high pressure and solid state. The constitutive model was implemented in a finite element code. The constitutive data were identified by optimization of experimental data. Validation was done by numerically reproduce the mechanical behaviour of uni-axially pressing Calcite to different pressure (up to 5 GPa) including unloading. The load-displacement curves, density distribution and the surface displacement were measured and compared to the finite element results. The results of the compaction simulations agree reasonably well with the experimental results.     

Roll compaction of maize powder

This paper presents a study of a roll compaction process as a dry granulation method for typical food materials such as maize powder. This process is widely applied in industry as it can continuously produce large quantities of granular product at comparatively low cost. The objectives of this work were to predict the roll compaction performance from a simple measurement involving uniaxial die compaction using the classical Johanson model. This involved determination of the optimum operating conditions for the production of granules as evaluated by apparent density.In the current work, a smooth counter-rotating rolling mill with a roller diameter of 0.08 m and a roller width of 0.20 m was used. The operating conditions for the rolling mill are shown to be influenced by parameters such as the roll gap, the roll speed, the feed powder amount, and the friction ratio. Material properties such as the compressibility factor and the angle of wall friction were investigated using uniaxial die compaction. The angle of wall friction was analysed using both contact mechanical and continuum mechanical approaches.The results indicated that this simplified approach can be used to provide a quantitative prediction of the extent of the roll compaction performance, and can be used to design optimal roller geometries and operating conditions.     

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.     

Simulation of high velocity compaction of powder in a rubber mould with characterization of silicone rubber and titanium powder using a modified split Hopkinson set-up

The paper introduces a method for characterization of silicone rubber and titanium powder in high velocity compaction using the split Hopkinson set-up. The impact test data has been used to estimate parameters in constitutive models for rubber and powder. A finite element study has been performed with different geometrical design of the high velocity compaction of titanium powder against an aluminium mandrel using a rubber mould as pressing medium. One goal of this study is to investigate if and how the manufacturing method can be applied for making dental copings.A conclusion of the experimental work is that it is possible to characterize rubber material and powder material for high velocity compaction of metal powder by the use of a modified split Hopkinson pressure bar set-up. The numerical simulation shows qualitatively good agreement with the experience from practical tests. In conclusion, the work shows the possibility to numerically study the geometric design and to optimize the densification behaviour of a complex high velocity compaction process.     

A validation procedure for numerical models of ceramic powder pressing

This paper describes an experimental procedure to validate numerical models used to simulate powder pressing. It consists mainly of two steps: closed die uniaxial pressing followed by isostatic pressing. The uniaxial pressing causes a non-homogeneous density distribution in the pressing direction as a consequence of friction between die walls and powder. In the isostatic pressing, less compacted regions have a larger volumetric strain, resulting in a non-trivial shape of the re-compacted part, which computes indirectly the previous density distribution. Experimental data from both steps are compared to the results from finite element models. The Drucker-Prager/Cap constitutive model was used to represent the compaction of alumina powder. Several simulations covering a range of parameters obtained from the literature were performed to calibrate the model, through an inverse analysis. The developed procedure sheds a light in the methods to calibrate and/or validate constitutive models used for powder pressing.     

Compaction behavior of Al6061 powder in the semi-solid state

Semi-solid powder processing involves compaction of metallic alloy powders in temperature ranges when both solid and liquid phases coexist. However, modeling of such process has been very scarce. In this study, compaction behavior of Al6061 powder in the presence of liquid phase was investigated. The relationship between compaction pressure and relative density of powder in the semi-solid state was modeled and verified against experimental measurements. The results showed that the powder compaction behavior can be accurately predicted by Shima-Oyane model when the liquid fraction was below 20%. In this range, all the normalized pressure-relative density curves merged into one, which could be expressed in a simple exponential form. When the liquid fraction was higher than 20%, the model failed to predict the compaction behavior because of the non-uniform distribution of the liquid phase. Squeezing-out of the liquid phase and interlocking of the irregular solid phase were speculated to occur during the compaction process.     

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.     

A small-angle X-ray scattering study of local variations within powder compacts

This work used two-dimensional small-angle X-ray scattering (2D-SAXS) to investigate the compaction behaviour of pre-gelatinised starch (PGS) and microcrystalline cellulose (MCC), which are commonly used as pharmaceutical excipients. By analysing azimuthal variations in scattering intensity, reproducible relationships were found between the compaction pressure, relative density and changes in the shapes of 2D-SAXS patterns for each material. These results indicated differences in the compaction mechanisms between PGS and MCC.The relationships also provided a means for investigating local variations in compaction behaviour within specimens prepared using different materials and compaction conditions. Relative density results from 2D-SAXS were consistent with expectations based on the effects of friction during compaction and appeared similar to data from other methods. In addition, however, 2D-SAXS measurements revealed local variations in the effective direction in which compaction occurred, with significant radial components observed near the die walls. This appeared to be consistent with the transfer of some compaction pressure to friction on the die wall. These observations represent an important advance, since other experimental methods do not easily reveal the direction of force transmission within the powder compact.     

The effect of processing parameters on pharmaceutical tablet properties

The preferred drug delivery system today is represented by tablets, which are manufactured using high speed rotary presses where the powder material is compressed in a die between rigid punches. Compression represents one of the most important unit operations because the shape, strength and other important properties of the tablets are determined at this time. These properties are dictated not only by the characteristics of the powder constituents (which are determined by the properties of the constituents, mixing and granulation), but also by the selection of process parameters imposed by production machinery. This paper focuses on the die fill and the compaction parameters.Die fill on high speed rotary tablet production presses is a complex phenomenon. On most presses the powder is deposited into the die under the effect of the gravity. Die fill is facilitated by the paddle wheels operating in the feed frame and the suction effect, whereby the lower punch is withdrawn while the die opening is exposed to powder in the feed frame. An experimental shoe-die system was developed to examine the effect of the contributing factors. High speed video observations enabled a detailed examination of the die fill process. The flowability of powders was quantified using the concept of critical velocity. It was illustrated that a detailed understanding of die fill could contribute to the design of feed frames as well as optimisation of press parameters in order to ensure consistent and efficient die fill, thus maximising the productivity of the presses.The compaction parameters are discussed with reference to tablet strength. Results generated using a compaction simulator as well as a number of rotary tablet presses are presented for a range of pharmaceutical excipients and placebo formulations.As a result of combined interactions between the material behaviour during compaction, powder-die wall friction and process parameters during die fill and compaction, the resulting tablets are in general non-homogeneous. X-ray computed tomography is employed to characterise the internal density distribution in tablets. The effect of tablet structure on friability, erosion and disintegration behaviour is examined.     

Constitutive Model for Cold Compaction of Ceramic Powder

Densification behavior of ceramic powder by pressure at room temperature was investigated using the constitutive equation based on plastic deformation theory for porous materials (the Lee-Kim model). The modified Lee-Kim constitutive model with two parameters is compared with the Shima model (four parameters) and the Cam-Clay model (three parameters and one polynomial equation). The calculated yield loci using the modified Lee-Kim model were compared with the experimental and calculated data for silicon nitride (Si₃N₄) powder. Contrary to the conclusion of some previous researchers, all three models simulate densification behavior of ceramic powder reasonably well.     

聚苯乙烯/铜粉温压成型的研究

对添加聚苯乙烯（ＰＳ）作粘结剂的铜粉进行了温压成型试验,研究了ＰＳ含量、温压工艺参数等对温压压坯密度的影响。结果表明,ＰＳ的加入可以明显提高铜粉温压压坯的致密化程度,提高压坯的相对密度;同时温压压坯致密化还同温压温度有关;温压成型时压坯密度与温压压力的关系仍可用经典的粉末压制方程来描述。     

An integrated study of die powder fill, transfer and compaction process using digital image correlation method

A major advantage of the powder metallurgical (P/M) manufacturing process is its ability to shape powder directly into a final component with a primary goal of a high quality, homogeneity of density and mechanical properties and productivity. In this research, powder die filling, powder transfer and powder compaction process have been studied in succession using a novel experimental set-up that utilizes a high strength transparent wall section to observe and record the particle movement and powder compaction during the entire sequence leading up to the formation of a green part. The natural powder pattern itself, as observed from the transparent wall section, is utilized for obtaining full-field displacement and strain measurement. The test set-up and the strain measurement technique offer a means of quickly obtaining density distribution data in select cases. In addition to the above, several powder flow characteristics during die filling, powder transfer and powder compaction under a range of test conditions have been noted through a series of high-speed photographic recordings.The observations reveal increased porosity in the die wall region due to friction and formation of shear bridges during powder transfer stages during suction filling. Spatial density data from optical strain measurements in the top, middle and bottom regions of the die are consistent with similar bulk density measurements from mass and volume of the 3 regions.     

Solid phase compaction of polymeric powders: effects of compaction conditions on pressure and density variations

In a recent communication it was shown that the pressure losses during the solid phase compaction of a polymeric powder could be predicted from a simple relationship of the form:

$\text{P}{}_2\text{P}{}_1\text{=(K)}{}^{\mathrm{<mtext>h</mtext><mtext>d</mtext>}}$

where $\text{h}\text{D}$ is a function of the geometry of the compact. This has now been explored in more detail and it has been found that for PVdC and PVC the factor K is dependent on the rate of compaction and the die surface finish but is independent of compaction pressure and die diameter. To illustrate the effect of the pressure losses on the structural uniformity of the compacts, microhardness measurements were taken at a large number of points across a section of each sample. A computer plot of hardness contours provided a picture of the homogeneity of the sample which could be related to the compaction conditions and average density of the compact.