Compression or compaction is the pharmaceutical unit operation of applying pressure or force to the powder to densify it and generate the physical bonds between the powder particles. The critical parameters of this operation are (1) the material properties of the particles being compacted and (2) the equipment used for the compaction.
The basic mechanical unit of compaction/compression consists of three parts: (1) an upper punch, (2) a lower punch, and (3) a die. They work by following way:
- Compression and consolidation of a two-phase (particulate solid/gas) system by the application of an external force;
- Increase the apparent density (or a reduction in volume) by the displacement of air; and
- Increase the mechanical strength due to particle–particle interaction.
The required material/granules properties for compression/compaction are that the material/granules be:
- Free-flowing (so that the powder flows uniformly into the die cavity);
- Cohesive, (so that it possess binding properties so the powder will hold together when it is compressed);
- Lubricated (to prevent the powder/granulation and the tablet from sticking to punches and die, and to enable the formed tablet to be ejected from the die wall and released cleanly from the punch faces).
One should consider desirable tablet characteristics from the very first stages of formulation to the final stage of process scale-up. These might include:
- Physical strength
- Pharmaceutical elegance
- Biologically available drug substance(s)
- Stability (chemical and physical)
- Reproducibility (uniformity)
It should be noted that a good formulator will also consider scale-up parameters from the first stages of development.
The equipment used to perform the compaction operation is a tablet press. The most common tablet presses are:
If the tablet presses are instrumented and if they provide force readings or, more importantly, if they provide force vs. time curves, then the following parameters should be measured:
- Dwell time (which is most often defined as the time where the compaction force is at 90% of the maximum value)
- Compressional force
- Ejection force
Critical Parameters of Compression Operation
- Pre-compression force
- Main compression force
- Feeder speed
- Turret speed
- Punch penetration depth
Types of Tablet Tooling
As per TSM (Tooling Specification Manual) and EU, tablet compression tooling is divided into the following:
- B – type (has long and thin body characteristics, with smaller tip specifications, and can tolerate less pressure)
- D – type (has a flat physical profile and can tolerate high compression force)