Die-wall stress

One example most recently developed is a split die consisting of three sections (Figure 10) [64], Integrating the sensing web in a thin middle layer isolates stress measurement to a narrow band around the tablet and gives much closer approximation to the true stress. Further die wall force measurement is linear and independent of tablet height and position as it is uncoupled from all other die wall stresses and strains. Further it is designed in the shape of a conventional die and can be mounted without modification into a die table. 

Morehead, W. T., and Rippie, E. G. (1990), Timing relationships among maxima of punch and die-wall stress and punch displacement during compaction of viscoelastic solids, J. Pharm. Sci., 79,1020-1022. 

Yeh, C., Altaf, S. A., and Hoag, S. W. (1997), Theory of force transducer design optimization for die wall stress measurement during tablet compaction Optimization and validation of split-web die using finite element analysis, Pharm. Res., 14, 1161— 1170. 

Hoag, S. W., Nair, R., and Muller, F. X. (2000), Force-transducer-design optimization for the measurement of die-wall stress in a compaction simulator, Pharm. Pharmacol. Commun., 6(7), 293-298. 

Compaction simulators can be fitted with instrumented dies to measure the stress that the powder exerts on the die wall during the compaction and ejection cycle. Different types of instrumented dies have been constructed with single or multiple transducers to measure the die wall stress. The literature reports some examples of different config-urations of the pressure transducers within the instrumented die (13), as shown in Figure 7. 

Transmission of Forces As pressure is applied to a powder in a die or roll press, various zones in the compact are subjected to differing intensities of pressure and shear. Compaction stress decreases with axial distance from the applied pressure [Strijbos et al.. Powder Tech., IS, 187 209 (1977)] due to frictional properties of the powder and die wall. For example, the axialpressure experienced within a cyhndrical die with an applied axial loaa Oq may be estimated to a first approximation by... 

Insoluble Lubricants. Lubricants act by interposing an intermediate layer between the tablet constituents and the die wall. The smaller the amount of stress needed to shear the material, the better its lubricant properties will be. Since they are primarily required to act at the tooling/material interface, lubricants should be incorporated in the final mixing step after all granulation and... 

Upon exiting the die, the sheet extrudate will swell to a level determined by the polymer, the melt temperature, the die length-to-opening ratio, and the shear stress at the die walls. Additionally, flow instabilities will occur at values of the corrected shear stress at the wall, of the order of, but higher than 105 N/m2, as found by Vlachopoulos and Chan (58), who also concluded that, for PS, HDPE, and LDPE, the critical Sr in slits is 1.4 times higher than in tubes of circular cross section. Aside from these differences, the information presented in Section 12.1 and 12.2 applies to slit flow. 

Recent experimental data [28] indeed show that a weak interface can be created by lowering the surface energy with an fluorocarbon elastomer coating. On such a weakly adsorbing die wall, a macroscopic slip occurs in linear polyeth-ylenes during capillary die extrusion. However, the same surface fails to produce any observable wall slip at low stresses that can be reliably generated in a parallel-plate flow cell. This contrast emphasizes that massive polymer desorption and interfacial slip occur only beyond a critical wall stress. 

A widely accepted theory for lamination presented by Long [29] and reformulated by Ritter and Sucker [30] attributes capping to the residual die wall pressure. This pressure is said to cause internal shear stresses in the tablet causing the propagation of cracks, which results in lamination or capping. The propagation of cracks can be prevented by plastic relaxation of shear stresses. Therefore, materials having sufficient plasticity may not be susceptible to lamination. Some properties of the powder mixture, such as moisture content, type and amount of the binder, and... 

When the pol3maer is strongly attached to the die wall, adhesion prevails at low flow regimes, and slippage occur only above a critical wall stress value [10, 12, 14]. It should be underlined that it is this situation which is usually encountered in industrial poljrmer processes, where the dies used are machined in classical materials (stainless steel...) and with no special control of the wall roughness. 

If the die is considered as a simple capillary flow, the relationship between the rate of shear (y) and die wall shear stress (t ) can be described by Eq. (1) ... 

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