Computer aided studies of the tablet compression process
The compactional behaviour of a range of materials has been investigated by using an instrumented, 25.4 mm diameter punch and die set in conjunction with a Mayes Hydraulic Testing Machine. A CBM microcomputer with a multi-channel 3D-ADC was employed to monitor applied force of up to 45kN, the radially transmitted pressure to the die wall, die base reaction and punch displacement. Avicel and Sta-Rx compacts exhibited greater contact time, energy absorbtion and higher rate of radial pressure transmission to the die wall than was shown by Emdex, Emcompress and Paracetamol DC. It was deduced from the form of Heckel plots that time dependent bases Avicel and Sta-Rx underwent further densification when held under a constant stress. Densification of the mate~ials were usually greater if a slower speed of compression was used. Time dependent strain movements of the formed compacts held under a constant stress within the die have been investigated. The elastic and viscoelastic recoveries observed on load release have also been studied. The effects of various parameters on these movements and the feasibilty of using an elastic recovery index to predict the quality of a material are discussed. Avicel and Sta-Rx exhibited an extended plastic flow when held under a constant stress. Poorly compressible materials such as magnesium stearate and paracetamol exhibited the least strain movements and the greatest elastic expansion. It was found that the elastic recovery indices of the compressible materials were always lower than those of poorly compressible powders. Viscoelastic behaviour of the compacts after ejection has been studied following the changes in depth of penetration of an indenter over a long period into the surface of the compacts. Creep compliance curves obtained from the above two techniques have been compared. Both tests enable important viscoelastic parameters of the materials to be determined. Viscoelastic properties deduced from indentation tests were brought1y similar to those obtained by compression within the die. Throughout the work all data were recorded, processed and presented using a microcomputer and disk storage. Compressional criteria which are defined in the thesis have been found useful in evaluating some commercial granulates.