Localized Densification during the Compaction of Alumina Granules: The Stage I—II Transition

Tungsten marker layers and X-ray computed tomography were used to monitor the compaction of spray-dried alumina. Local density changes and compaction curves show that the well-known Stage I—II transition indicates an alteration in the direction of transmitted pressure through the uppermost layer. Stage I pressure is dispersed in local agglomerate contacts until "pockets" of low density are eliminated. In Stage II, these deformed/fragmented agglomerates behave as a continuous body and interact with wall friction to transmit pressure nonuniformly into the bulk. Discrete element simulations reproduced the compaction curves and provided clear explanation of the density distribution development.     

X-ray Computed Tomography for Evaluation of Density Gradient Formation during the Compaction of Spray-Dried Granules

X-ray computed tomography was used to quantify the internal structure of compacts made from poly(vinyl alcohol)-containing spray-dried agglomerates at specific levels of humidity. High humidities increased the average density but decreased the uniformity of the density distribution. Lower levels of humidity generally produced more diffuse high density zones. Internal uniformity is superior at lower humidities where agglomerate fracture is more common than gradual agglomerate deformation. SEM has been used to determine zone-specific agglomerate behavior at 0% and 98% relative humidity. These results can be used to connect gradient development to compaction curve behavior.     

Effect of forming pressure on densification behavior of nanocrystalline ITO powder

Since ultrafine nano-particles easily produce agglomerates, they result in an inhomogeneous particle packing structure within a green body. Nanocrystalline indium tin oxide (ITO) powder was prepared, and the compaction behavior of the powder was investigated by using an Hg porosimetry and a scanning electron microscope (SEM) as a function of the applied cold isostatic pressing (CIP) pressure. In addition, the sintering behavior of the compact was monitored with a thermo-mechanical analyzer (TMA), and the relationship between the inhomogeneity in the green body and the sintering behavior was examined. An increase in the forming pressure enhanced the homogeneity of the powder compact by crushing the larger agglomerates. The first generation agglomerate compact showed a two-step densification process in the shrinkage rate curve of TMA. The two-step densification behavior is explained by the elimination of two kinds of pores, which are small inter-crystallite pores and large inter-first generation agglomerate pores.     

Pore evolution and compaction behaviour of spray-dried bodies for porcelain stoneware slabs

The compaction behavior of spray-dried powders has turned into concern in porcelain stoneware manufacturing due to the increasing diffusion of large slabs. It is necessary to fill a knowledge gap between the compaction behavior with conventional presses and novel technologies. For this purpose, eighteen industrially-manufactured spray dried bodies were characterized for specific properties connected to the compaction behavior (curves of bulk density, intergranular and intragranular porosity in function of applied load, apparent yield strength). In addition, the firing behavior was investigated in order to reveal any effect of dry bulk density on firing shrinkage and bulk density of fired samples. Powder compressibility is within 50% and 55% (Carr index) and is primarily controlled by moisture. Two regimes are found: low pressure (fast density increasing by granule cave in and closure of intergranular porosity) and high pressure (slow density gain by downsizing microporosity). A peculiar mechanism is unveiled: granules squeeze in the low-P regime and further densification is achieved through microfracture around individual agglomerate. A phenomenological model is illustrated for the compaction of spray-dried powders. In conclusion, the performance of spray dried bodies during compaction is crucial to control the uniformity, in terms of porosity and bulk density, which has important repercussions on the properties of final slabs, especially differential shrinkages and deformation during firing due to density gradients.     

Compaction behaviour of agglomerated alumina powders

The effects of agglomerate properties, such as the binder type, binder content, moisture level, and agglomerate size, on a model compaction process was investigated by using green density-pressure interrelationships for a range of agglomerated alumina powders. The model compaction process involved single ended nominal uniaxial stress transmission in a cylindrical die. The influences of the sample aspect ratio, die wall lubrication, and compaction rate were also investigated. Two types of water soluble polymeric agents, a poly(vinyl alcohol) (PVA) and a poly(ethylene glycol) (PEG), were used. It was shown that certain agglomerate properties have a strong influence upon the compaction behaviour of these ceramic powders. The extent of the compaction is enhanced by using agglomerates with a low agglomerate yield point. In the PVA system, the agglomerate yield points decreased with increasing moisture content. The compaction behaviour of the agglomerates showed a rate dependency, that is, the compaction is retarded with increased pressing rate. The green densities of the compacts prepared in the unlubricated die were lower than those of the compacts prepared in the lubricated die due to the higher wall frictional forces operating in the unlubricated die.     

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.     

A theoretical model of the effect of distributed loading on the tensile strength of agglomerates as measured in the diametral compression test

The tensile strength of particle agglomerates is analyzed to indicate the effect of distributed loading through contact flattening during the diametral compression test. It is assumed that only the contact regions of the agglomerate are flattened and that the free boundary maintains its original position during loading. The increased packing density so produced is related to the total loading as a reaction force through an empirical relationship used to describe die compaction of powders. Agglomerate failure occurs when the maximum tensile stress caused by the platen loading exceeds the cohesive strength of the particle assemblage. Theoretical predictions of the effects of parameters such as bulk powder properties and the extent of load distribution on agglomerate strength are presented from the analysis.     

Influence of Vibration on Density of Polymer Solid Granules in Single Screw Extruder

In an especially compaction apparatus the vibration force field is applied to the solids conveying process by the axial vibration of the piston. Through the experimental investigation of solid granule compaction, it is demonstrated that the effect of vibration field and temperature on the density of polymer solid granules is very significant. The experimental results show that the pressure exerted on the solid granules increases with the increase in the amplitude or frequency of the vibration force field, which accelerates the compaction of loosely solid granules.     

Effect of Density Gradients on Dimensional Tolerance During Binder Removal

The present work utilized X-ray computed tomography (CT) and in situ laser dilatometry to correlate internal structural changes with dimensional variation during thermally driven binder removal. Nonuniform agglomerate arrangement and deformation introduced during compaction provide density gradients that drive binder migration. Simultaneously, density undergoes a slight decrease accompanied by a 1.0% loss in dimensional tolerance. This loss and CT difference images show that capillary forces generated during binder melting can change internal density distribution. This change in density has important implications for the binder removal process.