Dry granulators

Dry granulation converts primary powder particles into granules using the application of pressure without the intermediate use of a liquid. It therefore avoids heat–temperature combinations that might cause degradation of the product.

Two pieces of equipment are necessary for dry granulation: first, a machine for compressing the dry powders into compacts or flakes, and secondly a mill for breaking up these intermediate products intogranules.

Sluggers/ Slugging

Slugging is a pre-compression process for the formation of extra large tablets (slugs), usually of variable weight, due to poor flow of the drug powder. The dry powders can be compressed using a conventional compression tablet machine or, more usually, a large heavy-duty rotary press can be used. This process is often known as ‘slugging’, the compact made in the process (typically 25 mm diameter by about 10–15 mm thick) being termed a ‘slug’.

The resulting slugs are subsequently broken down into granules, A hammer mill or oscillating granulator is suitable for breaking the compacts, which are recompressed to obtain the final tablets. The procedure is applicable to the dry granulation of hydrolysable drugs, such as aspirin.

The slugging pressure should be controlled, as excessive pressure will produce very hard granules that are difficult to deform during the recompression stage resulting in soft tablets.

 Hammer Mill

Effect of slugging load

An increase in slugging load was associated with increase in the hardness and the degree of consolidation of the slugs. This is as a result of increased bonding strength with increased compaction load. The friability decreased with increase in slugging load. The lower friability of the granules with increase in slugging load indicates that the resulting granules became harder. This finding relates to greater interparticulate cohesion at the higher slugging load.
 Oscillating Granulator

Relationship between hardness and granule compressibility
Relationship between granule hardness and compressibility is that the softer granules produced softer tablets while the harder granules formed harder tablets, contrary to a previous finding that hard granules will yield soft tablets because they are poorly deformable during compression. Plastic deformation of particles is essential for effective particle-particle contact and cohesion during tableting.

The reason why the softer granules produced softer tablets may be that the softer granules crumbled easily to powder during compression which are less readily compressible compared to granules. This is indicated by the higher degree of consolidation (Pf values) of tablets derived from the harder granules.

Correlation between tablet hardness and brittle fracture tendency more prone to brittle fracture compared with softer tablets.

Roller compactors

Roller compaction is an alternative gentler method, the powder mix being squeezed between two rollers to form a compressed sheet. The sheet normally is weak and brittle and breaks immediately into flakes. These flakes need gentler treatment to break them into granules, and this can usually beachieved by screening alone.
Materials tend to lose bonding strength, or “re-workability,” after being roller compacted. It is not uncommon that the tablet hardness of roller compacted materials is much lower than that of the virgin stock under the same tablet compression force. Extremely high roller compaction force not only reduces the “reworkability” but may also cause compact discoloration and/or splitting. In addition, very high compaction force may reduce the drug dissolution rate, especially for poorly soluble compounds.

 Roller Compactor
Operating Principles
Roller compaction basically consists of three steps :
  1. powder feeding
  2. pre-densification
  3. ribbon formation.
During the feeding step, the powder material is fed into two counter-rotating rolls by either gravity or force-feed screws. Once the powder material is drawn into the nip angle area, it rubs against the roll surface and undergoes the pre-densification process. As the material enters the roll gap, particles are deformed or fragmented to form ribbons under hydraulic pressure. The ribbons are then sized through screens to produce granules to be compressed into tablets or filled into capsules.