Pressure compaction

The evidence obtained in compaction experiments is of particular interest in the present context. Figure 3.22 shows the results obtained by Avery and Ramsay for the isotherms of nitrogen on compacts of silica powder. The hysteresis loop moved progressively to the left as the compacting pressure increased, but the lower closure point did not fall below a relative pressure of 0-40. Similar results were obtained in the compaction of zirconia powder both by Avery and Ramsay (cf. Fig. 4.5), and by Gregg and Langford, where the lower closure point moved down to 0-42-0-45p° but not below. With a mesoporous magnesia (prepared by thermal decomposition of the hydrated carbonate) the position of the closure point... 

Compression ratio is the dimensional relationship between the loose and compacted powder at a given compacting pressure. 

Fig. 9. Effect of density, where d is density as compacted and is density as siatered, for (a) particle size and (b) compacting pressure.

Working Treatments. Restricted plastic deformation takes place entirely within the confines of a closed die cavity. A sintered part may be placed in the die cavity and pressure appHed to the part. This pressure generally is of the same magnitude as the original compaction pressure. This second apphcation of pressure can be categorized as follows. 

FIG. 20-71 Classification of agglomeration processes by agitation intensity and compaction pressure. Relative density is with respect to primary particle density and equals (1 — e) where e is the solid volume fraction. Reprinted from Granulation and Coating Technologies for High-Value-Added Industties, Ennis and Litster (1996) with permission of E G Associates. All rights reserved. 

A powder s strength increases significantly with increasing previous compaction. The relationship between the unconfined yield stress/, or a powder s strength, and compaction pressure is described by the powder s flow function FE The flow function is the paramount characterization of powder strength and flow properties, and it is calculated from the yield loci determined from shear cell measurements. [Jenike, Storage and Flow of Solids, Univ. of Utah, Eng. Exp. Station Bulletin, no. 123, November (1964). See also Sec. 21 on storage bins, silos, and hoppers.]... 

Pai+icle size enlargement equipment can be classified into several groups, with advantages, disadvantages, and applications summarized in Table 20-36. Comparisons of bed-agitation intensity, compaction pressures, and product bulk density for selected agglomeration processes are highlighted above in Fig. 20-71. 

A carbon rod is used as a current collector for the positive electrode in dry cells. It is made by heating an extruded mixture of carbon (petroleum coke, graphite) and pitch which serves as a binder. A heat treatment at temperatures of about 1100 °C is used to carbonize the pitch and to produce a solid structure with low resistance. For example, Takahashi [23] reported that heat treatment reduced the specific resistance from 1 Q cm to 3.6xlO"1Qcm and the density increased from 1.7 to 2.02 gem- 1. Fischer and Wissler [24] derived an experimental relationship [Eq. (1)] between the electrical conductivity, compaction pressure, and properties of graphite powder ... 

Boride Binder nature and content (wt%) Compacting pressure X 10 (N m ) Sintering T (°C) Holding time (min) Sintering atmosphere or vacuum V (pressure in torr) Relative density (%) Remarks Ref. 

We have recently tested the Tx model described above by obtaining T, measurements in powder samples with known S/V. Samples used were constructed from fumed silica (CAB-O-SIL M-5 and TS-500, Cabot Corp.), and were either hydrophilic (M-5) or treated by the manufacturer to be hydrophobic (TS-500). Powder of each type was pressed into a polycarbonate cylinder, with a degree of compression controlling the pore space volume of each sample. These materials have a very high specific surface area (200 m2 g 1 for M-5, 212 m2 g-1 for TS-500), which is not expected to change significantly even at the maximum compaction pressure used. 

The outcome of dissolution tests on pharmaceutical formulations can be influenced by a nnmber of factors including the nature of the excipients and lubricants, blending method and granulation procedure used. When the solubility of the API is not the limiting factor, the compaction pressure used to produce tablets is inversely proportional to their dissolution rate and, as noted in the previous section, directly proportional to the slope of the NIR spectra for intact tablets. Blanco et have developed various approaches for... 

It can also identify texture of the semi coke formed as illustrated in Figure 6. If a binder is used with a coal, the Plastofrost technique can determine the coal-binder interaction and the texture of coke formed from the binder phase. Although not considered in studies undertaken at Waterloo, the axial location of the thermocouples in the sample holder makes the Plastofrost procedure capable of measuring coal-coke conductivity as a function of coal, temperature and compaction pressure, with just a modest redesign of the heating slab. 

An important observation is that the fiber volume fraction at zero compaction pressure can differ significantly between different fabrics [8], This represents a lower limit to the range of fiber volume fractions that are acceptable. A lower nominal fiber volume fraction can result in movement of the reinforcement during filling and incomplete impregnation. Another typical feature of the compaction behavior is that all fabrics behave like nonlinear (stiffening) springs and that the possible increase in fiber volume fraction from the value at rest is limited [8] (Fig. 12.4). 

In practice, most molds are flexible and will deform when the fiber volume fraction becomes too high (which corresponds to a high compaction pressure). The forces associated with the compaction can be so high that the mold surface becomes dented or even that the entire mold breaks (if the clamping device is powerful enough). 

High compaction pressure is a particularly difficult problem when the thickness of the laminate in a part is varying in different positions (stepping of the thickness). In this case even a small error in the placement of the fabric with a corresponding increase in local fiber... 

The necessary stiffness of the tool is easy to estimate with standard engineering methods and should therefore always be calculated. The loads on the mold are primarily the internal overpressure from resin and reinforcement. It should be kept in mind that the compaction pressure necessary for closing the mold with dry reinforcement can be as high as or higher than the injection pressure [8]. The mold must be dimensioned for the maximum injection pressure in the whole cavity because this will be the internal over pressure if the outlets are closed while pressure is maintained at the inlet. 

Maximum pressure in the mold (i.e., injection+compaction pressure)... 

Regardless of matrix or fiber materials, the key process variables for filament winding are temperature, compaction pressure/fiber tension, and laydown rate. Typical measures of final cylinder quality include degree of cure/crystallinity, void volume fraction, fiber volume fraction, and residual stresses and strains. 

Groves and Teng (1992) investigated the effect of compactional pressure on biologically active proteinaceous enzymes such as a-amylase, P-glucuronidase, lipase, and urease. Assaying the activity of these enzymes before and after the compaction... 

Tye CK, Sun C, Amid on GE. Evaluation of the effects of tableting speed on the relationship between compaction pressure, tablet tensile strength and tablet solid fraction. J Pharm Sci 2005 94(3)465-472. 

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