Maximum powder loading

As the volume fraction of powder in a feedstock is increased, the viscosity of the feedstock rises exponentially until a critical volume fraction of powder is reached. Figure 2 shows the mixer torque (which is directly proportional to the viscosity of the feedstock) as a function of silicon carbide powder concentration in a polyethylene/wax binder system [Cle 05]. It is assume that by extrapolating the data to zero reciprocal torque, the maximum powder loading for the feedstock formulations can be determined. 

With this method, a maximum powder loading of 62% by volume was calculated for a SiC/polyethylene/wax-feedstock prepared with carbon black sintering additive. For the feedstocks without carbon black, a value of 57 vol% was calculated. This result corresponded with the fact that it was not possible to produce an extrudahle carbon black-free SiC feedstock with a powder loading higher than 52 vol%. The feedstocks prepared at 53 and 54 vol% were powdery, very brittle and did not exhibit any plastic behavior. 

Fig. 2 Reciprocal torque as a function of the binder SiC powder volume ratio. Extroplation to zero reciprocal torque (i.e. infinite torque) permits an estimate of the maximum powder loading for the feedstock to be made [Cle 05]...

Sfo is the Stokes number based on initial nuclei diameter do [Adetayo et al.. Powder Tech., 82, 37 (1995)]. Extent (/cf), depends logarithmically on binder viscosity and inversely on agitation velocity. Maximum granule size depends hnearly on these variables. Also, (/cf ), has been observed to depend hnearly on liquid loading y. Therefore, the maximum granule size depends exponentially on liquid loading. Fig. 20-73 illustrates this normahzation of extent (/cf), for the drum granulation of hmestone and fertilizers. 

Powder Mechanics Measurements As opposed to fluids, powders may withstand applied shear stress similar to a bulk solid due to interparticle friction. As the applied shear stress is increased, the powder will reach a maximum sustainable shear stress T, at which point it yields or flows. This limit of shear stress T increases with increasing applied normal load O, with the functional relationship being referred to as a yield locus. A well-known example is the Mohr-Coulomb yield locus, or... 

Fluorocarbons (PTFE, FEP, PVF2) Powder, emulsions Excellent high temperature properties. TFE to 500 F. FEP is easier to mold, but maximum use temperature is 400 F. Nearly inert chemically. Nonflammable. Loading with conductive filler improves creep resistance. Low coefficient of friction. High-temperature cable shielding, gaskets, heat-shrinkable tubing. 

Double-base and CMDB powders are loaded on shallow trays which are stacked in dry houses and subjected to a stream of hot air (140°F.) for several days. Results of a typical drying run for a double-base casting powder are shown in Figure 5. Initial drying is quite rapid however, extended cycles are required to get consistent values below the specification maximum, in this case 0.50% total volatiles (TV). 

Grosse [55] indicated that a typical NG double base powder on static testing should produce a maximum stress of 200 kg/cm2. When the loading rate is increased to 0.23 sec and 0.15 sec until fracture, the stress is 360 and 442 kg/cm2, respectively. 

Lu Packed Density. Accdg to MIL-STD-1233, Method 400, the packed d of a powdered material is the weight per unit volume of material which has been packed until it has attained the most compact form. Packed d indicates the loading d of loose pyrotechnic powders (See also item E) Relative Density (d ). It is the ratio of the absolute d at t°C to abs d of water at 3.98°C (0.999973 g/cu cm, its maximum d) and is expressed as g/ml. It is also called "density (or specific graviry) relative to water at 4°C ... 

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