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measured and is typically expressed in kilograms per cubic meter (kg/m 3 ). The major consolidation stress describes the stress state that was used to consolidate the sample during the pre-shear step; it is a combined stress state that includes the normal and shear stresses present during steady flow. The unconfined yield strength is a measure of the cohe- sive strength that the powder gained due to its consolida- tion during the pre-shear step. The effective angle of fric- tion is related to the friction between powder particles during flow. Figure 2 shows the yield locus of a blend containing 67 percent acetaminophen (APAP), 29 per- cent microcrystalline cellulose (MCC), and 4 percent hydroxypropyl cellulose (HPC). By conducting the test over a range of consolidation states, the relationship between the powder's major con- solidation stress and its cohesive strength can be estab- lished. This relationship is commonly called the mater- ial's flow function. The flow function relates the cohesive strength that a powder develops due to its consolidation in a hopper to the major consolidation stress on the material. The powder's compressibility is the relationship between its bulk density and the major consolidation stress. The flow function, effective angle of friction, and compressibility of the APAP-MCC-HPC blend are shown in Figure 3. Testing wall friction entails measuring the shear stress required to allow a sample of powder to slide along a coupon, or section, of wall material. A plot of the shear stress versus normal stress provides the wall yield locus. The angle of wall friction ϕ' is the angle that is formed when a line is drawn from the origin to a point on the wall yield locus. Figure 4 presents the wall yield locus deter- mined from wall friction tests of the APAP-MCC-HPC blend on a section of 314 stainless steel with a 2B finish. Permeability is determined by measuring the pressure drop that results when gas is fed into a bed of powder at 12 October 2017 Tablets & Capsules Figure 2 Yield locus of APAP-MCC-HPC blend, ss = 1.63 kPa Normal stress (kPa) Shear stress (kPa) 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 δ=44.3 f c =1.01 σ=3.30 Figure 3 Calculated properties of an APAP-MCC-HPC blend a. Flow function b. Effective angle of friction c. Effective bulk density 5 4 3 2 1 0 50 45 40 35 30 25 1000 900 800 700 600 500 0 2 4 6 8 10 0 2 4 6 8 10 0 2 4 6 8 10 Major consolidation stress (kPa) Unconfirmed yield strength (kPa) Major consolidation stress (kPa) Effective angle of friction (deg) Major consolidation stress (kPa) Bulk density (kg/m 3 )