What are the stages involved in Tablet compression /Powder compaction? how these stages can be measured & overcome compression issues?

Tablet compression is one of the general processes for oral solid dosage form, during this dosage development generally formulator changes granulation parameters or formula to get the desired outcome, but during tablet compression developed formula/ powder goes through several stages which ideally depends upon the excipients, binder property, type of binder & concentration of binder and vehicle. Following compression stages will help understand tablet behaviour to design a formula, select excipients, or selection process.

1)?????Filling (Apparent density): The first phase during compression to occur is a consolidation of the powders from the hopper to the compression die. During this consolidation phase, the powder particles adopt a more efficient packing order.

2)?????Close packing (Tapped density): The second phase of the compression process is elastic, or reversible deformation. If the force were removed during this phase, the powder would completely recover to the efficiently packed state. This phase is very short in duration for most pharmaceutical powders and very difficult to identify on most instrumented compression machines.

3)?????Deformation: The third phase of compaction is plastic, or irreversible, deformation of the powder bed. It is this phase that is the most critical in tablet formation. If too much force is applied to the powder, brittle fracture occurs.

4)?????Fracture (Plastic flow): The fourth phase where If the force is applied too quickly, fracture and de-bonding during stress relaxation can occur. if a material has significant plastic flow under compression, it will be more likely to form a compact. The Maxwell model of viscoelastic behaviour was evaluated, in an attempt to quantify the rate of plastic deformation of some direct compression excipients. The term “contact time” (Dwell time) was used to describe the total time for which a moving punch applies a detectable force to the die contents during the compression and decompression event, excluding ejection.

5)?????Fusion: the fifth phase where the final product after fused powder going through different phases the outcome pf tablet property is measure using a different tool.

Measurement tools:?

1)?????Change in bed densification:

For low cohesive material if applied force increases, elastic and plastic deformation of the particles occurs, which results in plastic flow and a reduction in inter-and intra-particulate void spaces. This lowers the overall compact density.

For highly cohesive material, the reduction in void space may yield a compact of sufficient strength for insertion into a tablet die. However, the inherent cohesiveness for most drugs and excipients is not suitable alone for tablet manufacture. ?Heckel equation will help to calculate :

? Log [1/(1 ? D)] = KP + A

?D= relative density at pressure P

K= Constant (K can be considered equal to the reciprocal of the mean yield pressure)

?A = Constant (K is a function of the original compact volume and is related to the densification and particle rearrangement prior to bonding)?

2)?????Changes in Surface Area During Compression

High bulk powder changes its packing, fracture phase is during compression this can be measure by conventional nitrogen technique this will help to reduce the lamination of the product.

3)?????Force relaxation technique:

This technique is an experimental way where the different forces applied with different speeds i.e dwell time will help to where the duration help in plastic flow is maximized in powder or not.

4)?????Stress Transmissions:

During Compression these techniques where forces can be plotted for upper, lower punches against a die wall can be related.?

Ref: Remington , Essentials of pharmaceutics , Pn: 586-587.