Tablet press instrumentation force

The use of compaction simulators was first reported in 1976. Since then, a variety of simulators have been developed. Hydraulic simulators, as well as mechanical simulators, are available to characterize raw materials, drug substances, and formulations, as well as to predict material behavior on scale-up. The appeal of simulators is due to the fact that they purport to provide the same compaction profile as experienced on a tablet press while using only gram or even milligram quantities of powders. Compaction simulators can achieve high speeds, as would be experienced on a production tablet press, and can be instrumented to measure a variety of parameters, including upper and lower punch force, upper and lower punch displacement, ejection force, radial die wall force, take-off force, etc. Summaries on the uses of simulators and tablet press instrumentation can be found in (19,20). 

In general, one should be aware that tablet press instrumentation involves the use of strain gauges or piezoelectric transducers to provide a voltage signal proportional to the force applied for the compaction operation. Let us say we can measure forces, such as those applied to the granulation by the punches, that applied to the die wall, that required for tablet ejection, etc. With tlie use of other transducers we can also measure distance. With the measurement of force and distance, we can calculate work, energy, etc. 

There are several uses of tablet press instrumentation in the scale-up process itself. One of these involves obtaining a sample of the scale-up batch and compacting that sample on the pilot-plant or research instrumented tablet press on which the formulation has been previously evaluated. Similarity of the fingerprint or the various research plots (Heckel, force-displacement, radial vs. axial plots) is evidence that the scale-up batch is similar to the previously evaluated research batch [2]. 

Measurement of the punch and die forces plus the relative displacement of the punches can provide raw data which, when suitably processed and interpreted, facilitate the evaluation of many tableting parameters. Many of the workers first involved in instrumenting tablet presses concentrated on deriving relationships between the applied force (FA) and the porosity (E) of the consolidating mass. 

A major development in pharmaceutical technology has been the application of instrumentation techniques to tablet presses. The ability to monitor the forces that develop during the compaction, ejection, and detachment of tablets has brought about new insights into the physics of compaction, facilitated formulation development, and provided a means for the in-process control of tablet weight in manufacturing [62,63], In... 

The details of instrumentation for tablet presses is thoroughly described in several texts and review papers [1,2,9,10] and will not be repeated here. The concepts, however, are important. The first and most important result of the instrumentation results in a plot of force vs. time (see Fig. 1). This shows the maximum force (or pressure) used for compaction of a tablet and such plots become even more important on a rotary press, when this measurement can be made on each tablet produced. 

If the tablet presses are instrumented and if they provide force readings or, more importantly, if they provide force vs. time curves, then the following parameters should be measured ... 

Force measurements made without displacement values are still useful in identifying the dependency of tablet hardness (and other associated characteristics) on compaction force, and also the effect of the tablet press compaction speed on tablet strength (influence of dwell time/effective contact time). Dwell time dependency is an important scale-up factor for the tableting process, and evaluation of the sensitivity of a formulation to dwell time at small scale is useful, although the actual commercial dwell time is not always achievable on instrumented, small-scale tablet presses. 

The key to progress in this field was the introduction of the so-called instrumented tablet press by Higuchi and others in the mid 1950s, in which force transducers were fitted to the press to measure the applied load. This revolutionized research into the tabletting process and in addition has lead to the development of presses with automatic tablet weight control, since the mass of particles in the die governs the force detected by the transducer (see the article on Automation of Tablet Presses in this encyclopedia). 

Fig. 8 Force and displacement data from an instrumented excentric tablet press.

The compression force is measured using an instrumented tablet press as described earlier. The physical strength of the tablet is measured with crushing apparatus such as that described in most current pharmacopoeias. The results are conventionally presented graphically with compression force as the abscissa and strength as the ordinate. 

In 1966, a U.S. patent was granted to Knoechel and co-workers for force measurement on a tablet press. This patent was followed by two seminal articles in Journal of Pharmaceutical Sciences on the practical applications of instrumented rotary tablet machines. A number of other patents related to press instrumentation and control followed from 1973 on ward. ... 

Much of the current body of knowledge about compaction properties of pharmaceutical materials came from instrumented tablet presses. Many tablet properties, such as tensile strength (hardness) and porosity can be predicted from force profiles. Work of compaction (a scale-up parameter) can be obtained with proper instrumentation. Information about the plasticity of materials can be derived from force-time curves. ... 

The major use of the instrumented press in the operations or production area is for tablet weight monitoring and control. Early research in this field was able to show that the measured force of compression was proportional to the mass of material in the die cavity. This, of course, led to systems that could monitor the uniformity of the peak heights measure, send a signal to a servo motor on the press to adjust the weight control if necessary, and finally turn off the press or... 

Punch displacement measurements are easily done on a single station press by attaching LVDT to the punch. On a rotary press, such measurements can be done by means of slip ring, telemetry, or instrumented punch. Punch displacement profiles may be used in conjunction with compression force to estimate work of compression and work of expansion (measure of elasticity). Because capping tendency increases with the punch penetration depth, it may be desirable to monitor actual punch movement into the die. The shape of a force-displacement curve is an indication of the relative elasticity or plasticity of the material whereas plastic deformation is desirable for stronger tablets, excess plasticity usually results in tablets that tend to cap and laminate. ... 

In one study, the formulation efficiency of several direct compression materials was evaluated using instrumented press methodology. It was found that subtle changes in the structure of the component particles could lead to observation of significantly different behaviors upon compression. The tablet hardness and compressibility of differently sourced sucrose materials, obtained at comparable compressional forces, was found to vary significantly with the source of the compound. 

Tabletting research and development can be carried out in single station punch-and-die presses, which use different drive mechanisms. If used for high precision tabletting or development work, instrumentation is added with which data can be recorded, stored, and processed. For tabletting research, determination of the pressing force over displacement (densification) diagram is of great value [B.97].

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