Dwell time plays an important role during tablet compression, and it has a direct impact on the product quality. The hardness and strength of tablets are dependent on the dwell time, and tablet compression defects like tablet capping and lamination can also be prevented by the phenomenon of dwell time.
We are familiar that the tablet compression process is completed in four stages, which are filling, weight adjustment, compaction/compression and ejection.
The dwell time phenomenon is observed during the compaction or compression stage, and in this article, we will learn
- What is dwell time?
- How to calculate dwell time?
- Importance of dwell time?
- What factors affect dwell time?
What is dwell time?
- The duration of time spent by the punch head flat under the main compression rollers during the process of tablet compression is known as dwell time.
- The amount of time for which the punch head flat remains in contact with the main compression rollers is known as dwell time.
- Dwell time can also be defined as the duration of time for which maximum compression force is applied over powder or granules during tablet compression.
- Tablet compression dwell time is the duration of time at which maximum punch penetration in the die takes place.
- We can also define dwell time as the duration of time for which the punches are not moving vertically.
Explanation
To understand dwell time in a better way, just imagine a set of punches having a flat head & a die.
When powder or granules are filled in the die cavity, it is subjected to the pressure which is applied through the punches by the main compression rollers.
The pressure is applied when the heads of punches come under the main compression rollers, and the duration of time for which these heads remain under the rollers is known as dwell time.
Unit of dwell time
The dwell time during tablet compression is measured in milliseconds.
How to calculate dwell time?
The dwell time in pharmaceutical industries during tablet compression is calculated by dividing the punch head flat by the tangential velocity of the turret.
Dwell time = punch head flat diameter (mm) ÷ turret tangential velocity
The above Formula requires more simplification by solving the concept of tangential turret velocity.
The tangential turret velocity expressed as TV is measured in millimetre/second, and it is the function of the revolution per minute of the turret & pitch circle diameter (PCD)
What Is a Pitch Circle Diameter?
PCD or pitch circle diameter is the measurement of the turret from the centre of a die to the exact opposite die centre.
So
The tangential velocity of the turret is
TV= (turret rpm × π × PCD) × 60 ×1000
By putting the value of tangential velocity in above mentioned formula, it becomes as,
Dwell time= punch head flat diameter (mm) ÷ (turret rpm × π × PCD) × 60 × 1000
Officially, it is written as
dt =Punch head flat diameter (Dhf) ×60 × 1000
Dpc × π × rpm
dt = dwell time (ms)
Dhf = punch head flat diameter (mm)
Dpc = turret pitch circle diameter (mm)
rpm=turret speed
1000= constant
Example
Calculate the dwell time if we have the following values
- Punch head flat = 12 mm
- Pitch circle diameter = 250 mm
- RPM of turret= = 50
Put values in the following formula
dt =Punch head flat diameter (Dhf) ×60 × 1000
Dpc × π × rpm
dt = 12 × 60 ×1000
250 ×3.14 × 50
dt = 18.34 milliseconds
Result
Dwell time for the given values will be 18.34 ms.
Importance of compression machine dwell time?
- The hardness of the tablet depends on the dwell time, meaning higher dwell time results in higher application of pressure.
- High dwell time helps to overcome tablet capping and lamination issues due to the complete removal of entrapped air.
- Dwell time helps to scale up batches for comparison of small-scale compression machines to large-scale scale.
Factors affecting dwell time
- Turret speed
- Head flat diameter
Turret speed
If turret speed is high, the dwell time will be short, so to increase the dwell time, the turret speed is reduced, and by doing so, the contact between the punch head flat and the main compression rollers is increased.
Head flat diameter
If the diameter of the punch head flat is high, the dwell time will also be high as the contact time will be increased for a larger diameter.
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