Tablet compaction mechanism

Figures 15 and 16 provide a summary of the compression cycles for rotary and single-punch tablet presses. The formation of the tablet compact in these two types of presses mainly differs in the compaction mechanism itself, as well as the much greater speeds achieved with rotary type presses. The single punch basically uses a hammering type of motion (i.e., the upper punch moves down while the lower punch remains stationary), while rotary presses make use of an accordion-type compression (i.e., both punches move toward each other). The former find their primary use as an R D tool, whereas the latter, having higher outputs, are used in most production operations.

In order to produce an adequate tablet formulation, certain requirements, such as sufficient mechanical strength and desired drug release profile, must be met. At times this may be a difficult task for the formulator to achieve, due to poor flow and compactibility characteristics of the powdered drug. This is of particular importance when one only has a small amount of active material to work with and cannot afford to make use of trial-and-error methods. The study of the physics of tablet compaction through the use of instrumented tableting machines (ITMs) enables the formulator to systematically evaluate his formula and make any necessary changes. 

ITMs provide a valuable service to all phases of tablet manufacture, from research to production and quality control [109 111]. As a research tool, ITMs allow in-depth study of the mechanism of tablet compaction by measuring the forces that develop during formation, ejection, and detachment of tablets. ITMs can also provide clues about how materials bond,... 

Studies involving instrumented compaction equipment can be extremely useful in the development of dosage forms, especially when the amount of drug substance is limited in quantity. Marshall has described a program in which dynamic studies of powder compaction can be used at all stages of the development process to acquire formulation information [63]. The initial experiments include a determination of the intrinsic compactability of the compound. In subsequent work, simple tablets are prepared, and tested for dissolution, potency, and content uniformity. Through studies of the compaction mechanism, it becomes possible to deduce means to improve the formulation under study. 

Indices are dimensionless parameters derived from various mechanical and physical properties of the tablet blend and resulting compacts. Mechanical properties typically measured include indentation hardness (kinetic and static), elastic modulus, and tensile strength (10,11). Physical properties include particle size, shape, and size distribution, density (true, bulk, and tapped), flow properties and cohesive properties. 

The range of compression pressures to prepare tableting indices compacts is shown in Table 3 with each considered moderate relative to other excipients such as lactose, mannitol, and calcium phosphate dibasic. Avicel PH302 required considerably less pressure than PH102 and PH105, and thus shows greater ease of compression. Their moderate compressibility indicates that a fairly substantial pressure was required to achieve the SF. The rank order of each excipient by the tableting indices mechanical properties is provided in Table 4. 

In summary, when tested, the grades of calcium phosphate dibasic discussed above exhibited mechanical properties that were very appropriate for tablet compaction and thus for formulation processing by direct compression, dry granulation, or wet granulation. With this in mind, it is easy to understand the popularity of DCP in pharmaceutical tablet formulations. 

Mullarney MP, Hancock BC, Carlson GT, Ladipo DD, Langdon BA. The powder flow and compact mechanical properties of serose and three highintensity sweeteners used in chew-able tablets. Int J Pharma 2003 257 227-236... 

It was demonstrated that dimensional analysis of the tableting process can produce a scientifically reliable way of predicting tablet properties across the range of materials and with diverse compaction mechanisms. A theoretically sound scale-up method is thus readily available for tableting equipment of different capacity. The method can be readily expanded to include other materials and tablet presses and other target quantities, such as tablet stability (disintegration) and bioavailability (dissolution). 

Mullarney, M. P., Hancock, B. C., Carlson, G. T., Ladipo D. D., and Langdon, B. A. (2003), The powder flow and compact mechanical properties of sucrose and three high-intensity sweeteners used in chewable tablets, Int. J. Pharm., 257, 227-236. 

The demonstration of the validity of the continuum-based modelling approach to tablet compaction requires familiarity with fundamental concepts of applied mechanics. Under the theory of such a mechanism, powder compaction can be viewed as a forming event during which large irrecoverable deformation takes place as the state of the material changes from loose packing to near full density. Moreover, it is important to define the three components of the elastoplastic constitutive models which arose from the growing theory of plasticity, that is the deformation of materials such as powder within a die ... 

Instrumented tablet presses with computer interfaces allow the pharmaceutical scientist to study the mechanism of compaction and the relationship of the mechanism to tablet-compaction properties and formulations. In addition, automated systems are useful to develop compression profiles for reference purposes, to control weight of tablets during development and production, and to monitor punch wear. This automation reduces the burden on personnel faced with the requirements of quality control. Merck Sharp and Dohme s major production facility in the United Kingdom is fully computerized to manufacture a high-volume tablet product as well as multiple-tablet products. " ... 

McKenna, A. McCafferty, D.F. Effect of particle size on the compaction mechanism and tensile strength of tablets. J.Pharm. Pharmacol. 1982, 34, 347-351. 

The physical strength of a tablet is dependent on the extent and strength of interparticulate bonds and these in turn are related to the compressive force which is applied. Therefore, the relationship between the applied force and some parameter related to tablet strength is a good indication of the ease with which a given substance will form satisfactory tablets, and may also give an insight into the compaction mechanism of the solid and its mechanical properties. 

Radial and axial die-wall force measurements also provide an insight into the compaction mechanism of the material and may indicate a die-wall binding (sticking) that is, in effect, a negative pull on lower punch. The radial die-wall pressure due to friction is material-specific and is more evenly distributed inside the die with an addition of a lubricant. Instrumentation of the die presents a technological challenge because pressure is distributed non-linearly with respect to tablet position inside the die and depends on tablet thickness. " ... 

Analysis of tablet compaction involves force and displacement, which can be normalized to stress and strain and the material relationships between stress and strain. The study of these factors comes under the study of solid or continuum mechanics. Many key concepts used in this chapter have their origin in continuum mechanic so in this section we will give a very brief overview of the main points needed to understand the basics of tablet... 

Production of a powder tablet, compact, or briquette can be carried out by a number of techniques. Each method results in the manufacture of different types of products with respect to size, shape, and physical properties. However, all have in common a basic compaction mechanism. 

Cunningham JC, Sinka IC, Zavaliangos A. Analysis of tablet compaction. I. Characterization of mechanical behavior of powder and powder/tooling friction. J Pharm Sci 2004 93 2022-2039. 

Tablet Press. The main components of a tablet compression machine (press) are the dies, which hold a measured volume of material to be compressed (granulation), the upper punches which exert pressure on the down stroke, and the lower punches which move upward after compaction to eject the tablets from the dies. Mechanical components deflver the necessary pressure. The granulation is fed from a hopper with a feed-frame on rotary-type presses and a feeding shoe on single-punch presses. A smooth and even flow ensures good weight and compression uniformity. Using the proper formulation, demixing in the hopper is minimized.

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