Powder flow

Poly etrafluoroethylene is manufactured and sold in three forms granular, fine powder, and aqueous dispersion each requires a different fabrication technique. Granular resins are manufactured in a wide variety of grades to obtain a different balance between powder flows and end use properties (Pig. 1). Pine powders that are made by coagulating aqueous dispersions also are available in various grades. Differences in fine powder grades correspond to their usefulness in specific appHcations and to the ease of fabrication. Aqueous dispersions are sold in latex form and are available in different grades. A variety of formulation techniques are used to tailor these dispersions for specific appHcations. 

Lubricants protect die and punch surfaces from wear and bum-out of the compact during sintering without objectionable effects or residues. They must have small particle size, and overcome the main share of friction generated between tool surfaces and powder particles during compaction and ejection. They must mix easily with the powder, and must not excessively impede powder flow (see Lubrication and lubricants). 

When a sample of ca 100 g has been obtained, a representative sample for use in size characterization equipment must then be taken. Some of the more modem methods of size characterization require as Htde as 1 mg of powder, thus obtaining a representative sample can be quite difficult. If the powder flows weU and does not contain too many fines, a device known as the spinning riffler (Fig. 4c) can be used. A spinning riffler consists of a series of cups that rotate under the powder supply. The time of one rotation divided into the time of total powder flow should be as large a number as possible. Although this device has been shown to be very efficient, problems can be encountered on very small (1 mg) samples, and the powder must be processed several times. Moreover, in order to avoid cross-contamination, cleanup after each of the sampling processes can be quite difficult. Furthermore, if the powder is cohesive and does not flow weU, the equipment is not easy to use. A siUca flow agent can be added to the powder to enable the powder to flow... 

Directions for reconstitution Tap bottle until all powder flows freely. Add approximately 2/3 of the 90 mL of water shake vigorously to wet powder. Add remaining water and shake vigorously again. 

The bulk density of a powder is calculated by dividing its mass by the volume occupied by the powder (Abdullah Geldart, 1999). Tapped bulk density, or simply tapped density, is the maximum packing density of a powder achieved under the influence of well-defined, externally applied forces (Oliveira et al., 2010). Because the volume includes the spaces between particles as well as the envelope volumes of the particles themselves, the bulk and tapped density of a powder are highly dependent on how the particles are packed. This fact is related to the morphology of its particles and such parameters are able to predict the powder flow properties and its compressibility. 

Water jetting Powder flow on plastic Water settling... 

When the proper crystal size has been reached, stop powder flow by shutting off the oxygen flow to the hopper. Continue flame operation and pedestal rotation. 

The near-UV illuminated titanium dioxide (anatase) powder flow reactor, as well as the gas chromatographic methods for analysis of effluents, has been described in detail earlier [2]. The procedvu-es followed were substantially those of Luo and OUis [1], and Sauer et al. [2]. 

ZT Chowhan, YP Chow. Powder flow studies I. Powder consolidation ratio and its relationship to capsule-filling weight variation. Int J Pharm 4 317, 1980. 

Electrostatic Liquid velocity Surface charge Personal charge Rubbing of plastic or rubber Liquid spray Mist Water jetting Powder flow on plastic Water settling... 

Tardos, G. I., Garcia, J., Ahart, R., and Ratuiste, F., How and Why Some Flow Aids Promote Bulk Powder Flow, AIChE Symp. Senes, 296(89) 18-34 (1993)... 

When powders flow, they do so either in a steady controlled fashion (as in the case of dry sand), or in a uncontrolled gushing manner (as would damp sand, for which the entire bulk tries to move in a solid mass). This latter condition is termed floodable flow it is most characteristic of the flow of cohesive, sticky... 

Table 1 lists a number of factors that may influence powder flow and compaction. The list is long and includes physical and mechanical properties as well as... 

Table 1 Factors That Influence Powder Flow and Compaction...

As we just suggested, particle size and shape are important physical properties influencing powder flow and compaction. Particle size is a simple concept and yet a difficult one to quantitate. Feret s diameter, Martin s diameter, projected area diameter, specific surface diameter, Stokes diameter, and volume diameter are but several of the measurements that have been used to quantify particle size using a variety of methods. 

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