A comparison of three methods of mounting a linear variable displacement transducer on an instrumented tablet machine

Estimation of Elastic Recovery, Work of Decompression and Young's Modulus Using a Rotary Tablet Press

Capping and lamination occur when the bonds within a tablet cannot withstand the elastic recovery during decompression. To estimate tablet recovery, it is necessary to subtract the machine recovery from the total recovery during decompression. A direct relationship between the force and machine deformation is applied in a novel fashion to calculate in-die tablet recovery on a Manesty Betapress and to estimate the Young's modulus, E, of various solids from the recovery data. The E values of about twenty-five pharmaceutical solids were found to be in reasonable agreement with literature values. This procedure allows in-process analysis of tablet recovery and gives E without the use of specialized equipment.

The tabletting machine as an analytical instrument: qualification of the tabletting machine and the instrumentation with respect to the determination of punch separation and validation of the calibration procedures

The quality of the determination of punch separation in an eccentric tabletting machine equipped with two inductive displacement transducers was carefully investigated, since this tabletting machine is used as an 'analytical instrument' for the evaluation of the compression behaviour of pharmaceutical materials. For a quasistatic calibration procedure using gauge blocks, the repeatability under standard conditions and the robustness against variations in machine settings, installation conditions, equipment and methods were examined. The readings during calibration can be easily influenced by machine parameters as a result of deficiencies in the construction of the machine and in the mode of instrumentation. The poor plane-parallelism of the punch faces has a further negative effect on the accuracy of punch separation. In addition, the response at loading to lower and higher forces as during calibration was investigated. While at loading up to 100 N, the response of the system to the gauge blocks is systematically influenced by punch separation, for slow manually applied punch-to-punch loading up to 16.5 kN at a broad range of penetration depths, no significant effects were observed in the region of interest for tabletting. To get an indication of the transferability of the calibration and the determination of punch deformation to normal operating conditions, the lateral tilting of the punches during dynamic idle runs, punch-to-punch loading, and compression of microcrystalline cellulose was analyzed. A transfer of the response derived from punch-to-punch compression to tabletting conditions seems to be possible, although this must be questioned on grounds of theoretical considerations. From all the experiments performed, a total error of about +/- 30 microns must be assessed for the determination of punch separation.

On the accuracy of a new displacement instrumentation for rotary tablet presses

The Portable Press Analyzer (PPA; Puuman Oy, Finland), a commercially available instrumentation for rotary tablet presses, was tested for accuracy of determination of force and displacement. The calibration of the force transducers (strain gauges) was tested under a static condition. The calibration of the displacement transducers (plastic film potentiometer) was compared for static and dynamic recordings. Force measurement was found precise (deviation < 1.1%) after alterations in the calibration procedure. Displacement measurement was affected by punch tilting and the application of the transducers. If tilting of punches was not considered, the deviation of displacement measurement from the true value (using steel tablets as a reference) was found up to 110 microns. By modifying the original PPA system by supplementing additional displacement transducers in the adjacent turret positions of the punches and adding a custom electronic device (Tilting Compensation Device), the accuracy of distance measurement was improved to 18.1 microns (+/- 3.64). Furthermore, machine and tooling deformation were recorded and found different under static and dynamic conditions. Correction of punch displacement for elastic deformation therefore should preferably be made from dynamic recordings.

Deformation of the Stokes B2 rotary tablet press: quantitation and influence on tablet compaction

Deformations that affect the vertical punch displacement of a Stokes B2 rotary tablet press were characterized with a cathetometer. The press deformation was found to be elastic for both the upper and lower compression roller assemblies. However, the upper and lower compression roller assemblies have different Hookian spring constants: 8.58 x 10(4) and 5.18 x 10(4) kN/m for the upper and lower assemblies, respectively. Using two-way analysis of variance, the Hookian spring constants were shown to be independent of compaction phases and lower punch penetration setting. To study the influence of press deformation on tablet compaction, the Hookian spring constants were factored into the caculation of the incremental work of compaction for dibasic calcium phosphate dihydrate and microcrystalline cellulose. As the peak compression pressure increases, the force-displacement work done on the tablet during the loading phase decreases relative to calculations that neglect press deformation. This decrease in force-displacement work was attributed to elastic press deformation, which absorbs energy during the loading phase and then releases this energy later in the compaction cycle, altering the punch-displacement profile. The rate at which the press stores and releases elastic energy depends in part upon the viscoelastic properties of the tablet. Based upon these results, the coupling between press elasticity and a tablet's viscoelastic properties should be accounted for when analyzing tablet compaction or trying to simulate the punch-displacement profile of a tablet press that deforms during compaction.

Calculation of punch displacement and work of powder compaction on a rotary tablet press

To calculate the work of compaction during tableting it is necessary to have accurate values of force and punch displacement. The direct measurement of punch displacement on a rotary press is both costly and complicated but calculated displacements will be in considerable error unless deflections in the press during compression, are taken into account. By analysing the physical restraints imposed on the punches during tablet compression, an expression for punch displacement was derived. From preliminary measurements made on the table press of machine deflections and punch displacement under static conditions, the terms of this expression were evaluated for dynamic conditions. This analytic solution was then used to determine the true punch displacement and work of compaction from direct measurements of vertical force and turret position.

The use of strain gauges for radial stress measurement during tableting

The influence of size, configuration and positioning of die wall strain gauges on the measurement of radially transmitted stress developed during tableting was investigated. Calibration of strain gauges attached to a cutaway die wall was achieved by compression of rubber-like materials in the die, Breon Polyblend 504 being more effective than red rubber for this purpose. Hysteresis of response observed when calibrating a thin-walled die was possible due to excessive distortion of the weakened die wall although the extent of this hysteresis varied with calibration material. The use of full bridge arrangements produced an increase in response when the compression site was moved away from the gauges. The opposite effect was seen when half bridge arrangements, using two active gauges, were used. The paradoxical effect observed when full bridge arrangements were used was shown to be due to straining of the compensating gauges. The dependence of die wall response on compact position was substantially reduced by the use of multiple gauges mounted along the die length and connected together to form a single gauge on each side of the die.