Powder compaction compact strength

The high elastic modulus, compressive strength, and wear resistance of cemented carbides make them ideal candidates for use in boring bars, long shafts, and plungers, where reduction in deflection, chatter, and vibration are concerns. Metal, ceramic, and carbide powder-compacting dies and punches are generahy made of 6 wt % and 11 wt % Co ahoys, respectively. Another apphcation area for carbides is the synthetic diamond industry where carbides are used for dies and pistons (see Carbon). 

For the two explosive loading systems used, the initial pressure wave into the powder is relatively low, varying from perhaps 1.5-4 GPa. In such cases the most relevant compression characteristic of the powder compact is its crush strength , i.e., the pressure required to compress the porous compact to solid density. In the simulations, this strength can be varied over a wide range with the P-a model. The wavespeed of the initial waves was modeled on the basis of shock-compression data on rutile at densities from 44% to 61% of solid density [74T02]. 

Fig. 6.8. The peak mean-bulk temperatures predicted in one-dimensional numerical simulation are investigated for powder compacts of different crush strengths. For the explosive loadings of the Bear fixtures, no difference in temperature is predicted for crush strengths up to about 2 GPa. This value is about that of the initial loading wave into the samples. Above that pressure the crush strength has a strong effect on temperature. The predicted behavior can be understood in terms of the various loading paths.

No binder or a minimal amount, needs to be added to the powder to impart strength to the compact. 

A method of measuring tensile strength has been developed by Ashton et a/.(13) who also used a cylindrical cell split diametrically. One half of the cell is fixed and the other, which is movable, is connected to a spring, the other end of which is driven at a slow constant speed. A slowly increasing force is thus exerted on the powder compact and the point at which failure occurs determines the tensile strength this has been shown to depend on the degree of compaction of the powder. 

A. Gupta, G.E. Peck, R.W. Miller and K.R. Morris, Nondestructive measurements of the compact strength and the particle-size distribution after milling of roller compacted powders by near-infrared spectroscopy, J. Pharm. Sci., 93(4), 1047-1053 (2004). 

Masaki H, Akinobu O, Fukuji H. Effect of particle size of lactose on the compaction properties of powder and tensile strength of tablets. Yakuzaigaku 1986 46(l) 50-57. 

It has to be mentioned that the chemical analysis of the impurities often is insufficient for an adequate powder characterisation because of their very specific influence on the properties of the powder compacts (sintered bodies). On the one hand, particle size and particle distribution of the impurity inclusions are important for instance it has been shown that the same impurity contents of Fe cause a drastic decrease in strength of the compacts if concentrated in few inclusions with diameters >50 pm, but have nearly no influence as inclusions <5-10 pm [238]. On the other hand, most metallic impurities form silicides which cause a remarkable volume increase [238]. 

In a demonstration of the pharmaceutical advantage that can be realized through the use of a cocrystal form of a substance, it was shown that the 1 1 cocrystal of caffeine and methyl gallate exhibited significantly improved powder compaction properties [64], The compression characteristics of the cocrystal were reported to be excellent over the entire pressure range studied, with the tablet tensile strength of the cocrystal being twice that of caffeine at pressures less than 200 MPa. The superior compaction properties of the cocrystal product were attributed to the presence of slip planes in crystal structure. 

Johanson predicted theoretical compactor operating conditions to handle air entrapment effects in materials. In general, he concluded that the critical compacting pressure level is dependent on a number of factors roll speed, roll diameter, powder permeability, compressibility, and compact strength. Johanson indicated, when applying these principles in commercial application, a compactor operator would have to operate the press at slightly less than maximum pressures to allow for material inconsistencies and variable in-feed flow rates. ... 

Precompression is often used to tamp or apply a small compression force prior to the main compact compression cycle. Rotary tablet presses are often equipped with a separate precompression station, which is positioned between the die-filling feed frame and the main compression station. Typically, precompression is used to improve the quality of tableted products, where it increases the strength of the compact and/or decreases the incidences of capping and lamination. The compact strength is enhanced through the increase in the effective contact time in which the powder particles are in contact under an applied force. During this extended contact time, stronger interparticulate bonds form and stress relaxation occurs. 

Flow Functions and Flowabilily Indices Consider a powder compacted in a mold at a compaction pressure Oi. When it is removed from the mold, we may measure the powder s strength, or unconflned miiaxial compressive yield stress L (Fig. 21-38). The unconfined yield and compaction stresses are dietermined directly from Mohr circle constructions to yield loci measurements (Fig. 21-36). This strength increases with increasing previous compaction, with this relationship referred to as the powder s flow function FF. 

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