Compression is a process in which powders or granules are converted into a solid unit dosage form or compact mass called tablet by applying pressure. The equipment used for this process is called a compression machine. The process of compression is most widely used in pharmaceutical industries for the manufacturing of tablets of different shapes and sizes. Compression machines are of two types.
- SINGLE PUNCH COMPRESSION MACHINE
- ROTARY/MULTI-STATION COMPRESSION MACHINE
1.0SINGLE PUNCH COMPRESSION MACHINE
Single punch compression machine is the most conventional type of compression machine and is based on old technology. It compresses one tablet at a time because it has only one die and one upper and one lower punch and speed of tablet compression is very slow.powder or granules are filled in die through the hopper and compressed into a tablet by applying pressure through the upper punch.
Single punch compression machine is also called Eccentric type or single station machine. Now a days Single punch machine is only used in research and development laboratories or teaching laboratories
2.0 ROTARY/MULTI-STATION COMPRESSION MACHINE
Rotary type compression machine is also called a multi-station compression machine and is a high-speed machine. it consists of many dies and multiple upper and lower punches. It produces large no of tablets in a single round and it is called rotary because all the punches and dies rotate during the process of compression.
In multi-station compression, machine pressure is applied by a set of wheels or rollers called upper compression roller and lower compression roller.
Mechanism of Rotary Compression Machine.
Rotary compression machine works on a very simple principle of application of pressure. Powders or granules are filled in the die cavity and then pressure is applied through a set of punches i.e upper and lower punches when these punches pass-through space between two compression rollers.
One wheel or roller is present on the upper side of the upper punches and one wheel is located below the lower side of lower punches. These two compression rollers apply pressure on punches which transmit this pressure to powders and granules and convert powders or granules into tablets.
Stages Of Compression:
The process of compression is completed in 4 different stages which are as follow,
- Weight Adjustment/Metering
First, stage of compression is filling and in this stage powders or granules are filled in the die cavity. During this stage the position of the lower punch in the die estimate filling weight. The powders or granules are overfilled to ensure complete filling of the die cavity. In this stage powders and granules from the hopper are delivered to feeders and from feeders powders or granules are filled in the die cavity.
2. Weight Adjustment/Metering
The second stage of compression is weight adjustment of the tablet according to specification mentioned in Batch manufacturing record. In this stage, the extra powder is removed by the tail over die which removes the extra granules when lower punch passes over the weight adjustment cam.
Weight of tablet is determined by the position of the lower punch in the die cavity. Position of lower punch is adjusted by the weight adjustment cam by rotating the knob/rotating dial attached to the shaft of weight adjustment cam. This knob can be rotated in the clockwise or anti-clockwise direction. In one direction weight adjustment cam moves upward and lift the lower punch in the die cavity so filling space is decreased and the weight of the tablet is also decreased because fewer granules are filled in less space.
In the opposite direction lower punch comes down in the die cavity and space in the die cavity is increased so the weight of the tablet is also increased.
The third stage is compression in which powder or granules are converted into a tablet by applying pressure. The pressure is applied by two compression rollers that are an upper roller and lower roller. When punches pass-through space between upper and lower rollers, the pressure of roller is transmitted to granules or powders by the tips of upper and lower punches and tablet is formed.
The pressure exerted by these rollers also determines the hardness and thickness of the tablet. Lower wheel is attached to a knob or a rotating dial and can be moved up or down by rotating the knob in a clockwise or anti-lock wise direction. In one direction the lower wheel is lifted upwards and pressure is increased and thickness is decreased in the opposite direction the roller comes down and hardness is decreased and thickness is increased. The upper roller is fixed and freely rotates it does not move up or down.
4. Ejection Of Tablet
The last step in process of compression is the ejection of the tablet. Tablet formed by compression machine is ejected from the die cavity by lifting the lower punch by ejection cam and tablets are collected in a container through discharging chute after striking with take-off bade. Take off blade is attached to the backside of the feeder.
A rotary compression machine is a complex machine and consist of large no of parts which play an important role for compression of tablets. Following are some important parts of the rotary compression machine.
- Compression Rollers
- Take Off blade
- Discharging chute
- Lubrication System
Hopper is the part of compression machine where we add powders or granules which are required to be compressed. So the first part where powder or granules come in contact with compression machine is hopper of machine. Hoppers are made of Stainless steel because the product comes in direct contact with a hopper that’s why hopper is made of SS 316L.
Powder flow through hopper due to gravity and some hoppers have a vibratory mechanism which forces the powder to flow. In advance, compression machines hoppers are equipped with a vacuum system which transfers the powder or granules from containers to the hopper by the suction of the vacuum system.
Feeder or feeding system feeds the powder or granules to die cavity. Some feeders are equipped with a force-feeding system which has paddles to force the granules or powders in the die cavity. High-speed machines usually require a force-feeding system to control weight variation.
Compression machine has two types of punches called upper punch and lower punch. The physical difference between upper and lower punch is that the neck of the upper punch is small and neck of lower punch is large. The shape and size of tablet depend on punch shape and dimension. Upper and lower punches apply pressure on powder or granules to form tablets. Punches are installed in punch holder of the turret and when turret rotates punches also rotate.
Dies are installed in die table and also rotates with the rotation of the turret. Powder or granules are filled in dies and then compress to form tablets. Different types of dies are available I.e tapered dies and lined dies.
The turret is the main part of the rotary compression machine. The turret is called the heart of a rotary compression machine. The turret contains punch holders in which punches are installed and also have die table in which dies are installed. Due to the rotation of turret punches and dies are moved.
The turret determines how much punches and dies will be installed in a rotary compression machine. Speed of compression machine is also determined by the rotations of the turret. Advance compression machines have exchangeable turrets in which complete turret is removed from compression machine and another turret is installed with new tooling for the next batch. Exchangeable turrets reduce machine set up hours.
Compression machines have different types of cams. These cams are punch tracks which guide the movement of punches. Different types of the cam are upper cam, lower cam, weight adjustment cam and ejection cam.
7. Compression Rollers
Pressure in a compression machine is applied by compression rollers. Some machines have only main compression rollers whereas advance types of compression machines also have pre-compression rollers. Pre-compression rollers apply less pressure and are to remove entrapped air from granules and prevent capping. Main rollers exert more pressure after pre-compression and form tablets of the desired hardness and thickness.
8. Take off the blade
It is attached to the backside of the feeder and when the lower punch is lifted by ejection cam tablet strikes with take-off blade and come into the container through discharging chute.
9. Discharging chute
The discharging chute is attached to the compression machine which guides the tablet to the container after ejection from the die cavity.
10. Lubrication System
Advance compression machines have a built-in lubrication system that lubricates the machine parts automatically after some time intervals. The lubrication time interval can be increased or decreased depending upon the requirements.
HMI is a human-machine interface. Its is a touch screen by which the operator gives instructions to the machine to change the parameters according to requirements.
Compression Machine Tooling
For compression machine generally, two types of tooling are available
- B Type Tooling
- D Type Tooling
1. B Type Tooling
In B type tooling punch barrel diameter is 19 mm and outer die diameter is 30.16 mm. B type tooling machines exert less pressure than D type tooling machines. In B type tooling tablet diameter is also smaller than D type tooling.
2. D Type Tooling
D type tooling punch barrel diameter is 25.4 mm and outer die diameter is 38.1mm.D type tooling machines exert more pressure than B type tooling machines. In D type tooling tablet diameter is larger than B type tooling.