Force-deformation, compression/compaction

A simple schematic of a pendulum impact device (PID) is given in Fig. 1. This equipment permits the permanent deformation pressure (H) of a compact of material to be determined [30,31]. Flat-faced tablets of the test substance are compressed at different compression forces and then subjected to impact with a... 

Some models of these tablet presses are equipped with a precompression station. This is an additional set of pressure wheels that can apply force to the material in the die prior to the final (normal) compaction step i.e., the tablet is compressed twice. When used, the force apphed is usually lower than that in the final compaction. A precompression step can densify the material, allow more time for plastic deformation, and allow air to escape rather than being trapped inside the compact. 

Schwartz [23] suggests special consideration for the compression process. Press speed for material that compact by plastic deformation, overmixing of lubricant in the force feeder, heat buildup on long compressions runs, material abrasiveness, and tooling care are important variables for consideration. Dwell time and compression and ejection forces are other variables identiLed for monitoring process. 

The deformation properties of the drug substance and excipients will have a direct influence on the strength of the tablets that are produced by direct compression. The type of deformation that occurs will depend upon the material s inherent properties and the amount of force being applied. Deformation can be described in three main ways elastic, plastic, and brittle fragmentation, but it is important to realize that these are idealized deformation mechanisms— most real materials are some combination of two or all three mechanisms. Processes such as wet granulation, melt extrusion, and roller compaction can be used to improve compaction properties and reduce formulation sensitivity to changes in raw material quality. 

After the compression and consolidation of the powder in the die, the formed compact must be capable of withstanding the stresses encountered during decompression and tablet ejection. The rate at which the force is removed (dependent on the compression roller diameter and the machine speed) can have a significant effect on tablet quality. The same deformation characteristics that come into play during compression play a role during decompression. 

Compression force is the major driving force in the powder densification process. The rate and extent of the applied force on the powder bed not only affects the way panicles physically deform but also determines the integrity of the compact formed (crushing strength/tensile strength). 

Deformability and Wet Mass Rheology The static yield stress of wet compacts has previously been reported in Fig. 21-113. However, the dependence of interparticle forces on shear rate clearly impacts wet mass rheology and therefore deformabihty. Figure 21-117 illustrates the dynamic stress-strain response of compacts, demonstrating that the peak flow or yield stress increases proportionally with compression velocity [Iveson et al., Powder Technol., 127, 149 (2002)]. Peak flow stress of wet unsaturated compacts (initially pendular state) can be seen to also increase with Ca as follows (Fig. 21-118) ... 

Compression is the most usual comminution force for brittle materials. Materials are compressed between two heavy-duty metal surfaces. Crushers based on compaction force are jaw, cone, gyratory, and roll crushers. These are most widely used to reduce the size of coarse rocks and minerals particles. In soUd waste treatment, their use is not widespread, because compression force, in most cases, simply deforms the materials but does not change their size (e.g., metals, paper, plastic, organics). 

After arriving at the maximum pressing force, pressure is released. If as shown in Fig. 8.1, compaction is performed by a punch in a die, the direction of travel of the piston reverses and, when no expansion of the densified body occurs, the pressing force should drop to zero immediately (vertical line). In reality, there is always a more or less pronounced spring-back which is caused by the expansion of compressed gas and the relaxation of elastic deformation. As mentioned before, this effect becomes more pronounced with increasing speed of densification until, at a certain compression rate, the compacted body disintegrates partially or totally upon depressurization. Therefore, it is often necessary to find an optimal compromise between densification speed (= capacity) and product integrity (= quality). 

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