Tablet measuring applied force

To monitor and control a tablet press, certain sensors must be installed at specific locations on the machine. These sensors are called transducers. In general, a transducer is a device that converts energy from one form to another (e.g., force to voltage). Tablet press transducers typically measure applied force, turret speed, or punch position. Because the signals coming from such transducers are normally in millivolts, they need to be amplified and then converted to digital form in order to be processed by a data acquisition system. 

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. 

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. 

Looking closer at the tableting process it becomes clear that the applied forces during tableting are not only the result of machine movement. When punches are moving in a machine without material, no force can be measured. The force only develops when the punches come into contact with the powder bed. The materials resist pressure deformation, and while the punches are moving, a counterforce builds up which is the measured force at the punches. In conclusion punch forces are determined by the material and as a result materials can be characterized by the measured punch forces. 

In order to compress a powder or granulation product into a tablet of specific hardness, a defined compression force must be applied. As pointed out by Shlieout et al. [3], by compressing a constant mass of powder, any variation in the applied force causes a change in the measured force. In addition, the substance itself plays an important role, that is, if it is of good compressibility, then the force needed for compression would be low. It is well known that this compressibility will depend on powder characteristics such as crystal habit and thermodynamic behavior. 

Hardness is determined using a hardness tester, where the tablet is placed between two jaws that crush the tablet. The instrument measures the force applied to the tablet and detects when the tablet fractures. Usually 10-20 tablets are tested and the mean value is calculated. Test results can be affected by speed of the testing, the geometry of the tablet contact points, and debris in the testing area, as well as by variation in temperature, humidity, and the age of the tablets. Therefore, the tablets must be oriented consistently in the hardness tester. 

The punch pressure required to form a compact for tableting indices measurements is measured at the end of a long dwell time, typically 1.5 minutes, during which the punches remain in fixed positions and stress relaxation within the compact brings about a decay in the applied load. The reported pressure or CS is calculated from the relaxed force and it is dependent on the compact SF. A sample s CS at a standard SF, such as 0.85, can be interpreted to indicate the ease (i.e., the magnitude of the pressure) of forming compacts under standardized conditions. 

Oates, R. J., and Mitchell, A. G. (1994), A new method of estimating volume during powder compaction and the work of compaction on a rotary tablet press from measurements of applied vertical forces, J. Pharm. Pharmacol., 46, 270-275. 

After application of the precompression force, the punches move toward the main compression rollers where the final (main) force is applied. As the punches impact the rollers, the compression force increases until the punch head flat is tangent to the compression roller and maximum force is applied (Fig. 1). The applied compression force is a measured value and depends on the distance between the punches and the quantity of material in the die. After main compression, the upper punch is pulled out of the die cavity while the lower punch impacts the ejection cam to begin the ejection process. As the die table continues to rotate, the lower punch raises the tablet out of the die cavity to eject the tablet to the point of scrape-off. 

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). 

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