Natoli explores the various causes of capping and the methods to prevent it
The term “capping” in tablet manufacturing refers to a tablet failure break across the horizontal plane and occurs immediately after compression during the breaking force or friability test.
Capping is a common defect that should be identified in the development stage, but it’s common to realise the problem during manufacturing. There are many factors that can contribute to the cause of capping, such as powder blend characteristics, material deformation properties, and the mechanical configuration of the tablet press and tooling.
Capping issues should be addressed at the development stage. The study of powder permeability can give the formulation scientist early insight to a capping issue.
Powder permeability is a fundamental property affecting the tableting process. Entrapped air in the powder blend reduces the inter-particulate bonding as well as the final tablet tensile strength, which leads to tablet failures, such as capping. High permeability indicates less air entrapment during the tableting process and fewer tablet defects. These experiments can be conducted on a lab scale Powder Rheometer, such as the FT4 from Freeman Technologies.
Characterising the material’s deformation properties is another fundamental study that should be performed in the tablet development process. A compaction simulator or emulator is a sophisticated single punch tablet press that measures the applied forces and punch displacement data. These tools allow for a science-based approach for selecting the appropriate excipients to develop a robust formulation that will survive the manufacturing process with minimal tablet defects, such as capping.
In the manufacturing environment, adjusting the formulation requires a process following SUPAC (Scale-Up and Post Approval Changes) guidelines, which can be time consuming and halt manufacturing.
But there are also ways to address capping without changing the formulation. Capping potential can sometimes be reduced by adjusting the tablet press parameters. One example is by adding pre-compression force. Pre-compression, or de-aeration, is a compression stage before the main compression event where the final compact is made. A low amount of pre-compression force will allow the removal of air between the particles, which decreases capping potential. The amount of pre-compression force is critical, considering too much force will increase capping. The objective of pre-compression force is to achieve a consolidation of the particles while remove the air but not creating a bond between particles. The 10% of main compression force “rule of thumb” does not always work.
Another parameter to adjust on the manufacturing press to reduce capping is punch penetration. The punch penetration determines where in the die the tablet is compressed. Since air travels upward during compression and air escapes between the upper punch tip and die, the higher the penetration, the less travel length the air must escape, thus reducing capping potential.
Decreasing main compression force can also reduce capping. During the development process, formulation scientists should understand the failure points of their product.
Once a tablet has reached its maximum compactibility, it won’t increase in strength with compression force. Instead, this action results in capping. In this case, reducing the compression force can fix capping issues.
Decreasing the manufacturing press turret speed is another possible way to reduce capping. An acceptable tablet strength might be achieved on a small R&D press, but when transferred to large scale, the tablet is made at a higher compression rate or lower dwell time. If a material’s robustness is impacted by the speed at which it is compressed, it’s known to be strain-rate sensitive. These materials require slower turret speeds.
When a product is in manufacturing, formulation changes can be challenging, but the tablet press and tooling configurations can be adjusted. There are many features of the tool that can be modified to reduce tablet capping.
Tapered die is a common tool that allows a higher rate of air removal from the die cavity. The top surface of the inner die cavity is tapered, which allows more room for the air to escape. In order to take advantage of this feature, the punch penetration much be adjusted so the tablet is being compressed in the tapered area.
The punch head configuration is another tool that can remediate capping issues in the manufacturing environment. Extended head flats on the punch will increase dwell time and, in some cases, increase the tablet strength. There have also been recent studies published by Natoli Institute/Long Island University showing that a reduced head flat can resolve capping issues at high speed. In either case, head flats can play a major role in tablet robustness and capping potential.