Tablet force-displacement curve

Fig. 18 A typical force-displacement curve. WF= work done overcoming die wall friction WD, work of elastic recovery Wp/, net work involved in tablet compact formation.

Antikainen, O. K., and Yliruusi, J. K. (1997), New parameters derived from tablet compression curves. Part 2. Force-displacement curve, Drug Dev. Ind. Pharm., 23, 81-93. 

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. ... 

To investigate the effect of molecular weight on the mechanical behavior and the binder activity of the excipient under compression, interactive excipients with different molecular weight PVP were mixed with paracetamol and compressed into tablets. The tensile strength of the tablets was determined. The elastic recovery can be calculated using a force-displacement curve using this equation ... 

The most basic analysis is the presentation of force versus time or displacement versus time. These curves are different for eccentric and rotary tableting machines. The data given in Figures 12 and 13 are valid for the contact time of the compaction cycle of one single tablet. Due to the eccentric-driven movement of the punches, the force-time curve can be described by a sharp peak at the maximum force evolving at the punches and the displacement-time curve can be described with a sharp peak at the minimum height of the powder bed. For curves of eccentric tableting... 

From force-displacement-time curves some parameters can be directly deduced for example, the power during tableting and the maximum power can be determined. Another important parameter which serves as a measure for viscoelasticity is the peak offset time [101,103], For single-punch tableting machines it is defined as the time difference between maximum displacement and maximum force. 

When one plots force vs. displacement, the area under the curve thus represents work. In practice, the compression/decompression data take the form shown in Fig. 18. The area under the upward line represents the work done on the tableting mass during compaction, while that under the downward line arise from the fact work is done on the punch by the tablet as a result of the latter s elastic recovery on decompression. 

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