Methods of Preparation of TabletsThere are three methods by which compressed tablets can be prepared; 1. Direct Compression 2. Dry Granulation 3. Wet Granulation.

1. Direct CompressionThe materials which are available in crystalline form and have free flowing and binding characteristics can be compressed directly but the majority of the drugs can not be compressed easily in this process because they may disintegrate. Some vehicles are used for making them compressed such as – spray dried lactose, anhydrous lactose, calcium phosphate, mannitol, microcrystalline cellulose, etc.The drugs which can be compressed directly are – NaCl, NaBr, KCl, KBr, KI, KNO3, NH4Cl, KMnO4 etc.

In this method the blend of powders are compressed into slugs on specially designed tablet machines. The slugs are the flat faced large size tablets which are prepared under heavy pressure on tablet press. The slugs are then broken to suitable size granules by passing through an oscillating granulator or other suitable device fitted with no.10 or no.20 sieve. The resultant granules are mixed with lubricants and other necessary additives. This method is also known as slugging, pre-compression or double compression method.

Processing steps: Weighing of raw material – screening – mixing – compression – slugs – milling – mixing - compression to finished tablets.

2. Dry Granulation

Advantages:The main advantages of dry granulation or slugging are that it uses less equipments and space. It eliminates the need for binder solution, heavy mixing equipment and the costly and time consuming drying step required.

Disadvantages:i. It requires a specialized heavy duty tablet press to form slug.ii. It does not permit uniform color distribution as can be achieved

with wet granulation where the dye can be incorporated into binder liquid.

iii. The process tends to create more dust than wet granulation, increasing the potential contamination.

The most widely used process of agglomeration in pharmaceutical industry is wet granulation. Wet granulation process simply involves wet massing of the powder blend with a granulating liquid, wet sizing and drying. Important steps involved in the wet granulation are- 1. Mixing of the drug(s) and excipients 2. Preparation of binder solution 3. Mixing of binder solution with powder mixture. 4. Drying of moist granules 5. Mixing of screened granules with disintegrant, glidant, and lubricant.

3. Wet granulation

Advantagesi. Permits mechanical handling of powders without loss of mix quality.

ii. Improves the flow of powders by increasing particle size and sphericity.

iii. Increases and improves the uniformity of powder density.

iv. Improves cohesion during and after compaction.

v. Reduces air entrapment

vi. Reduces the level of dust and cross-contamination.

vii. Allows for the addition of a liquid phase to powders (wet process only).and makes hydrophobic surfaces hydrophilic.

Limitation of wet granulationviii. It is an expensive process because of labor, time, equipment, energy and space

requirements. Loss of material during various stages of processing.

ix. Stability may be major concern for moisture sensitive or thermo labile drugs.

x. Multiple processing steps add complexity and make validation and control difficult.

OVER ALL STEPS

Evaluation of Tablets or Standardization of TabletsThe following standards or quality control tests are carried out for this purpose. These are – 1. Diameter size and Shape. 2. Uniformity of weight. 3. Thickness. 4. Hardness. 5. Friability. 6. Percentage of medicament. 7. Rate of Disintegration.