Effect of process variables on the Drucker-Prager cap model and residual stress distribution of tablets estimated by the finite element method

Moisture Behavior of Pharmaceutical Powder during the Tableting Process

The moisture content of pharmaceutical powder is a key parameter contributing to tablet sticking during the tableting process. This study investigates powder moisture behavior during the compaction phase of the tableting process. Finite element analysis software COMSOL Multiphysics^® 5.6 was used to simulate the compaction microcrystalline cellulose (VIVAPUR PH101) powder and predict temperature and moisture content distributions, as well as their evolution over time, during a single compaction. To validate the simulation, a near-infrared sensor and a thermal infrared camera were used to measure tablet surface temperature and surface moisture, respectively, just after ejection. The partial least squares regression (PLS) method was used to predict the surface moisture content of the ejected tablet. Thermal infrared camera images of the ejected tablet showed powder bed temperature increasing during compaction and a gradual rise in tablet temperature along with tableting runs. Simulation results showed that moisture evaporate from the compacted powder bed to the surrounding environment. The predicted surface moisture content of ejected tablets after compaction was higher compared to that of loose powder and decreased gradually as tableting runs increased. These observations suggest that the moisture evaporating from the powder bed accumulates at the interface between the punch and tablet surface. Evaporated water molecules can be physiosorbed on the punch surface and cause a capillary condensation locally at the punch and tablet interface during dwell time. Locally formed capillary bridge may induce a capillary force between tablet surface particles and the punch surface and cause the sticking.

Finite Element Analysis and Modeling in Pharmaceutical Tableting

Finite element analysis (FEA) is a computational method providing numerical solutions and mathematical modeling of complex physical phenomena that evolve during compression tableting of pharmaceutical powders. Since the early 2000s, FEA has been utilized together with various constitutive material models in a quest for a deeper understanding and unraveling of the complex mechanisms that govern powder compression. The objective of the present review paper is to highlight the potential and feasibility of FEA for implementation in pharmaceutical tableting in order to elucidate important aspects of the process, namely: stress and density distributions, temperature evolution, effect of punch shape on tablet formation, effect of friction, and failure of the tablet under stress. The constitutive models and theoretical background governing the above aspects of tablet compression and tablet fracture under diametral loading are also presented. In the last sections, applications of FEA in pharmaceutical tableting are demonstrated by many examples that prove its utilization and point out further potential applications.

Numerical study for tableting process in consideration of compression speed

A numerical study of the tableting process using a finite element method (FEM) is important to quantitatively understand the structural change inside the tablet and the mechanism of tableting failures such as capping, picking, lamination, and sticking. In the pharmaceutical field, the Drucker-Prager Cap (DPC) model is used most widely to demonstrate the mechanical behavior of the powder during tableting. The DPC model, however, cannot consider compaction speed, although the compaction speed has a large impact on the tablet strength and tableting failures. In the present study, a combined novel model using both the DPC and Perzyna models, which incorporates a visco-plastic behavior considering the compression speed, was proposed and numerical simulation was conducted. Cellulose, lactose, and acetaminophen were selected as model powders. The DPC-Perzyna model parameters were determined from experimental compaction tests, unconfined compression tests, and tension tests. The calculated loading curves agreed with the experimental data under different compaction speeds, in addition the high compression speed resulted in less plastic deformation and much residual stress. It was demonstrated that the DPC-Perzyna model proposed in the present study was useful to analyze the tableting process when considering compression speed.

Determining the Influence of Granule Size on Simulation Parameters and Residual Shear Stress Distribution in Tablets by Combining the Finite Element Method into the Design of Experiments

The influence of granule size on simulation parameters and residual shear stress in tablets was determined by combining the finite element method (FEM) into the design of experiments (DoE). Lactose granules were prepared using a wet granulation method with a high-shear mixer and sorted into small and large granules using sieves. To simulate the tableting process using the FEM, parameters simulating each granule were optimized using a DoE and a response surface method (RSM). The compaction behavior of each granule simulated by FEM was in reasonable agreement with the experimental findings. Higher coefficients of friction between powder and die/punch (μ) and lower by internal friction angle (α^y) were generated in the case of small granules, respectively. RSM revealed that die wall force was affected by α^y. On the other hand, the pressure transmissibility rate of punches value was affected not only by the α^y value, but also by μ. The FEM revealed that the residual shear stress was greater for small granules than for large granules. These results suggest that the inner structure of a tablet comprising small granules was less homogeneous than that comprising large granules. To evaluate the contribution of the simulation parameters to residual stress, these parameters were assigned to the fractional factorial design and an ANOVA was applied. The result indicated that μ was the critical factor influencing residual shear stress. This study demonstrates the importance of combining simulation and statistical analysis to gain a deeper understanding of the tableting process.

Determination of Density Distribution of Tablets Using Synchrotron X-ray Computed Tomography

The purpose of this study was to determine the density distribution of scored and round-faced tablets using synchrotron X-ray computed tomography. The tablets were made by direct compression of standard formulations. The density distribution of scored flat-faced tablets was uniform in the whole cross-sectional image. However, the tablet formulated using microcrystalline cellulose (MCC) was very dense at the tip of the score only. It is caused by the poor fluidity of MCC particles. In the case of round-faced tablets, the density in the central section of the tablet was relatively low, compared with those of peripheral areas. These observations correlated well with the results obtained by the finite element method simulation using appropriate material models.