Tablet Pro — Part 4
Tooling — A set of punches and dies are often referred to as tooling. A station of tools consists of an upper punch, lower punch and die.
The job of the upper punch is to control compression position within the die. Most modern tablet presses have adjust upper punch entrance which means that the machine can be adjusted to control how far the punch enters the die.
Typically a good place to start is around 3mm upper punch penetration into the die. If the upper punch is set too high the product maybe pushed out of the die before the upper punch enters the die. Many machines allow punch entrance as deep as 6mm. The problem with being too deep in the die is that air may become entrapped contributing to a capping issue. Notice that the upper is smaller in diameter than the lower.
The Lower Punch
The job of the lower punch is more complex than that of the upper. The upper punch only controls penetration depth into the die.
- The lower punch first overfills the die with too much powder.
- Then to achieve final and proper tablet weight the lower punch is moved up to allow the scrapper to sweep the excess powder off the surface of the die table.
- Then the powder in the die is moved downward into the die which is called under fill to prevent the effects of centrifugal force from slinging powder back out of the die. This feature is not found on all rotary presses.
- Pre-compression is a feature found on most modern tablets presses. It’s designed to help tamp the powders together lightly compressing the powder and driving the air out of the die prior to final compression.
- Main compression is where the final tablet is compressed.
- Ejection is achieved by simply pushing the tablet up out of the die.
Working Length: The most important dimension of the tooling is the working length variation within a set of punches. Working length is the distance from the bottom of the cup to the head flat. Working length is controlled to a very tight specification. Variations in this specification will result in weight, thickness and hardness variation. All companies should have the ability to inspect this dimension. When tooling is new the allowable variation is .002”. Using tooling with variations over .005” would be considered extreme. This variation must be maintained within the entire set. All upper punches must be the same length and all lowers must be the same as each other with the set. Uppers and lowers need not be the same.
Overall length: The distance from the Cup edge (land) to the head flat is called the working length. This dimension is not critical to the success of the tablet. It is acceptable for new tooling to have punch variations in the overall length by as much as .006” when new.
In the world of tablet press tooling there was a Standard tool head design and a different design used on European tooling. This meant that the tooling and cams used on a machine in the US was different than the tooling and cams used in Europe and the rest of the world. From this we have learned more about tooling and have been able to compare the benefits of each design.
From that design a third design has been developed called the TSM Domed Head design. This design will not work into a Eurocam machine. It offers a larger more consistent head flat design and a radiused outside bevel for improved transition onto the pressure rolls. This design is becoming a standard and has proven to be a superior design.
The TSM is an excellent guide to understanding tooling and the tooling influence on the compression operation. This manual has been put together but industry professionals that include Tablet press manufacturers, Tooling manufacturers, Tablet manufacturing companies. This group has put together the best reference available for tooling specification, terminology, and machine interchangeability. TSM is an excellent resource for new and experienced professional that work with tablet compression.
Every company should have several copies in key departments and should reference this technology when ordering, inspecting, polishing, cleaning and repairing tooling.
Dwell time is the actual amount of time that the powder is under pressure. The key factors to controlling dwell time are punch head flat diameter, number of compression points and rpm. To increase dwell time, simply slowing the machine down will provide more dwell. Conversely increasing rpm will decrease dwell time.
It is important to recognize that all products are not dwell sensitive. Some products will recompress well at any given speed, others are very sensitive to even the slightest change.
The punch head flat diameter is a contributing factor that is often overlooked. Look at the photo here on the left, you can see the many different diameters of head flats in this one set of tooling. Most up to date machines have pre-compression and maincompression stations which means that the tablet is being compressed twice. Using precompression with a dwell sensitive powder will allow increased speed without sacrificing dwell.
The bottom line is tablet hardness is (for most products — not all) directly effected by dwell.
The head of a punch is designed to match and follow the contour of the cams on the tablet press. It is important to be able to visually monitor wear of the punch head and the corresponding cams.
The difference between a good operator and a great tablet press operator is the ability to look at wear of the cams and punches and be able to take corrective action to prevent further damage.
Most companies use a simple gauge called a Go-No-Go gauge to determine acceptable punch head wear.
Michael D Tousey, Technical Director/CEO