The Manufacturing Process — Part 6
Common Tablet Defects — Making tablets batch after batch without an occasional defect would be unusual. Some products start up with problems and end with them. Tablet to tablet weight variations create tablet defects. Consistent tablet weight is essential to making a good tablet. Without good and consistent weight control, solving other defects will be difficult (if not impossible) because of how a tablet press operates. Some of the most common tablet defects are:
- Weight variation
- Friability variation
- Picking and Sticking
- Double Pressing
Often problems with compression can be associated with many root causes. One very common issue is machine start up. The tablet press is cold. the steel is cleaned and bare metal can allow granules to stick to these metal surfaces. The reason a lubricant is in the product is to prevent granules from sticking. Many companies are very accustomed to a double start up.
The first one distributes powders which begin to stick, especially to the punches and to the die table. The operator will often remove the stuck granules and then at second start up no sticking occurs because the working surfaces are now protected by the lubricant.
Tablet weight is the key to controlling hardness and friability. Controlling tablet weights within a tight range will contribute to better tablet hardness and friability.
Many variables can influence weight fluctuations. The key weight control factors are product uniformity in particle size & density, proper tablet press set-up, and control of flow rates into the die cavity.
However, the importance of weight control cannot be over emphasized. Weights must be uniform in order to trouble shoot most other tablet defects. Friability testing is done by tumbling tablets to see how well they will withstand the tumbling action which replicates typical handling situations. This test is done to make certain that the tablet does not fracture of break apart. Too much friability means that the tablet chips or fractures break away from the rest of the tablet.
Picking and Sticking
Picking & Sticking occurs when granules stick to the punch faces during compression. Sometimes the punch face design and debossing can be modified to eliminate the problem. Other times granules are not dried properly. They become case hardened during the drying process, which means that the granules are wet on the inside.
During compression these granules break open and the wet product sticks to the punch faces. If this occurs, the drying process must be improved. To overcome sticking on the press, increase hardness by making the tablet thinner and increase dwell time to make the wet granules adhere to other granules rather than the punch face.
Also, if a blend is incomplete this could mean that the lubricant in the formula is not protecting the granule from sticking to the punch cup surface. If all else fails polish the punch cup surface.
Capping and Laminating
Capping is often referred to as air entrapment. During compression, air is evacuated from between the granules to allow the granules to lock to one another. If the “air” does not escape during the compression process the top of the tablet (the tablet cap) wants to come off. The tooling (punches & dies) are designed to allow air to escape during compression along the upper punch tip and die wall. This is why capping occurs on the top “cap” of the tablet.
Capping is not just air entrapment. During compression air evacuation pushes the very fine dry granules out with the air. It is these dry & light particles that do not want to lock together, resulting in tablet “caps” wanting to come off the tablet.
Lamination is when the tablet splits apart anywhere except at the upper cap. Lamination is often blamed on over compressing. Too much compression force flattens out the granules and they no longer lock together.
Lamination can also occur when groups of fine and light particles do not lock together. These groups of fine and light particles simply will not compress well. Reducing thickness and increasing dwell time will give these particles more of a chance.
Dwell time can be increased by adding pre-compression or slowing the machine speed down. Machining a taper into the die will help eliminate capping and lamination.
Many tablets are sensitive to chipping after compression. First make certain that the punch tip edges are not damaged. Some punch tip designs are more sensitive to damage from handling than others. Once confirmed that the chips are not being created by damaged punches then make certain that the “take off blade” is set correctly for proper ejection off the machine. If the blade is too high it will allow the tablet to wedge under the blade causing chipping. If the tablet is friable the tablet can become chipped as the tablet travels off the press, down the tablet chute, through the tablet metal detector, tablet deduster and finally into the collection bin.
Transferring finished tablets must be done carefully. Many times investigations into chipped tablets discover poor handling and transfer of tablet bins from compression to storage and then onto the packaging floor. Packaging machinery can also cause chipping.
Double Impressions will happen on a tablet press when the punches are allowed to twist or jump. Round punch tips want to twist naturally due to the rotation of the press. Double impressions usually occur on the bottom of the tablet from the lower punches. It usually means that the lower punch retainers are loose and the punches are jumping during compression.
Make certain the lower punch retainers are clean and not worn. They do need to be replaced often. When a machine starts up it is cold. As it warms up, lower punch retainers can become loose and may need to be tightened to prevent double impressions. Therefore, it is important to check them often at start-up.
Also, many newer machines now use punch seals. As seals become worn they will allow the punches to bounce or twist during compression.
-Michael Tousey, Techceuticals