TABLET COMPRESSION:
Reducing the bulk volume of a powder by applying mechanical force so as to form a defined shape of desired weight and strength is called ‘Compression’.
Tablet compression is the process of squeezing powders together and driving out the air between the particles, resulting in a compressed Tablet.
WHY COMPRESSION?
- Granules or powder cannot be taken in correct quantity.
- Handling of Tablets is easier than powder
- Deliver the accurate dosage
- Different material have different bulk density and the bulk volume may be more or less so if we take such type of powders the dose may not be accurate.
TABLET PRESS OR TABLET COMPRESSION MACHINE:
The machine used for making Tablets is called as “Tablet press” or “Tablet compression machine”.
The compression is done either by single punch machine (stamping press) or by multi station machine (rotary press).
The Tablet press is a high-speed mechanical device, it squeezes the ingredients into the required shape with extreme precision.
Each Tablet is made by pressing the granules inside a die, made up of hardened steel. The die is a disc shape with a hole cut through its center. The powder is compressed in the center of the die by two hardened steel punches (Upper and lower punch) that fit into the top and bottom of the die.
The punches and dies are fixed to a turret that spins round. As it spins, the punches are driven together by cam tracks (upper and lower cam track). The top of the upper punch (the punch head) sits on the upper cam edge. The bottom of the lower punch sits on the lower cam edge.
The shapes of two cams determine the sequence of movements of the two punches. The sequence is repeated over and over because the turret is spinning round. The force exerted on the ingredients in the dies is very carefully controlled. This ensures each Tablet is perfectly formed.
STAGES OF COMPRESSION:
- Filling
- Weight Control
- Re-arrangement (pre-compression)
- Consolidation (main compression)
- Ejection
Filling:
Top punch is withdrawn from the die by the upper cam, bottom punch is low in the die so powder falls in through the hole and fills the die.
Weight Control:
Bottom punch moves up to adjust the powder weight-it raises and expels some powder.
Re-arrangement (pre-compression):
Compression begins with particle re-arrangement, the reduction of interparticulate void volume, bringing particles into closer contact. Pre-compression used to take out of the air from granules.
Consolidation (main compression):
Continued volume reductions leads to consolidation. Consolidation refers to particle interactions of sufficient strength that the formed mass persists after the compressive force is removed.
Top punch is driven into the die by upper cam, bottom punch is raised by lower cam. Both punch heads pass between heavy rollers to compress the powder.
Ejection:
Top punch is withdrawn by the upper cam, lower punch is pushed up and expels the Tablet. Tablet is removed from the die surface by scraper plate.
TABLET COMPRESSION MACHINE AND PARTS:
A rotary press is used to press various kinds and shapes of Tablet. There are various types of rotary Tablet presses available in the market and are custom-made as per the requirements.
Compression machine consists of several parts:
- Hopper
- Feeder
- Tablet press punches (Upper and Lower punch)
- Tablet press Die system (Die)
- Turret
- Cam Track
- Filling station and weight control
- Pre-compression roller
- Main compression roller
- Ejection cam
- Take off blade and discharge chute
- Control Panel
- Machine sensors
- Support system (Lubrication system)
Hopper:
The main function of hopper in compression machine is to facilitate the powder from container to machine feeder. Hopper shape and design play an important role in flowability of powder. Hopper consists of sensor which indicate the powder level. Butterfly valve in hopper controls the flow of powder in feeder.
Feeder:
Feeder system feeds the powder into the die. Tablet press machine feeder system is made up two critical components:
- Feeder Housing
- Feed Peddles
Feeder housing:
Material from the hopper will enter the dye system through the housing. The feeding process should be consistent and accurate to produce high quality tablets.
The feeding housing is made of stainless steel 316 since it is in contact with the product. Also, the product must not stick on the feeder housing as it will cause inconsistencies during the feeding process.
Feed peddles:
Quite a number of high-speed rotary tablet press machines have a feed peddles. The feed peddles ensures consistent and accurate material feeding into the die systems.
Without a feed peddle, especially if the machine is operating at a high speed, there could be chances of some dies being filled halfway. This may result in tablets with varying thickness or the degree of compaction.
Tablet press punches (Upper and Lower punch):
Punches are used for compressing the granules / powder within the dies. To produce the desired Tablets, punches move within the die, thereby compressing powder into the desired Tablets.
Tablet press die system (Die):
Die defines the size and shape of Tablet. It is the die cavity where the powder is compressed into desired Tablet of definite thickness and size. The movement of Tablet press machine punches takes place within the die bore or cavity. Therefore, the punch and die must be machined together to ensure compatibility.
Turret:
Turret is the heart of compression machine; it holds the punches and dies. It precisely machined to ensure both die pockets and punch guides are fully aligned for optimal Tablet making process. Turret determines the number of stations in the machine, this helps to determine the production capacity.
Cam Track:
Cam track guides the movement of the punches (upper and lower). This helps to control filling, compression and ejection of already processed Tablets.
Filling station and weight control:
A critical procedure in Tablet compression process is the weight control by controlling the depth of die filling. This ensures the die cavity is filled to a required depth according to required weight of Tablet before any compression process begins.
Pre-compression roller:
It generates the force to squeeze the air out of the granules / powder. These rollers apply a small amount of force on the upper and lower punches. This gives initial compression force.
Main compression roller:
It generates the force for compression of Tablet. Main compression rollers exert a predetermined amount of force (final compression force) for the formation of Tablets. The compression force at this stage is higher than the pre-compression force.
Ejection cam:
The ejection cam steadily and slowly pushes the bottom punch upwards. At the same time, the top cams move up and so are the upper punches. As a result, the fully compressed Tablet leaves the die cavity i.e., the compressed Tablet remains just at the top of the die.
Take off blade and Discharge chute:
Take of blades are fitted just above the feeder housing. Their main role is to deflect the fully compressed Tablets into the discharge chute and then are collected in containers.
Control Panel: Controlling various functions of machine.
Machine sensors: Measures a physical quantity and coverts it into a signal which can be read by electronic instrument.
Support system (Lubrication system): It reduces friction between moving parts
TYPICAL COMPRESSION CYCLE:
