Preventing tablet capping

Robert Sedlock describes the cause of tablet capping during production and reveals how to prevent it from occurring

The term “capping” in tablet manufacturing refers to a break across the horizontal plane that is found when performing a breaking force or friability test. Many factors can contribute to capping, including a formulation’s blend characteristics, the material’s deformation properties, and the mechanical configuration of the tablet press/tooling. Pharmaceutical manufacturers commonly discover capping during manufacturing. It’s preferable to identify problems in the developmental stage, because changing formulation after a product has moved to full-scale manufacturing can be challenging.

Blend characteristics to prevent tablet capping

Formulators can identify a potential capping problem by characterising a formulation’s permeability, which is a fundamental property affecting the tableting process. Entrapped air in a powder blend can reduce interparticle bonding and the tablet’s final tensile strength, both of which can lead to tablet failures like capping. If a formulation’s permeability is high, less air entrapment will occur, resulting in fewer tablet defects.

Permeability tests can be conducted using a lab-scale powder rheometer, which measures the powder’s resistance to airflow. Using a vented piston under a range of applied normal stresses, air is passed through the powder bed. The relative difference in the air pressure between the bottom and top of the powder column represents the powder’s permeability.

Simulators to prevent defects including tablet capping

A compaction simulator or emulator is a single-punch tablet press that measures applied forces and punch displacement data under conditions similar to that of a production-scale rotary tablet press. The simulator allows an analysis of the force displacement curves (work curves) that show the formulation’s elastic recovery after an applied load is released. These tools allow users to select the appropriate excipients to develop a robust formulation that will survive the manufacturing process with minimal tablet defects, including capping.

Tablet press configuration to address tablet capping

Capping can be addressed without changing the formulation by adjusting tablet press parameters such as precompression force, punch penetration, main compression force, and turret speed.

Precompression, or de-aeration, is a compression stage before the main compression event that forms the final compacted tablet. Precompression force can consolidate the particles and remove entrapped air without creating a bond between particles. Precompression force should be 10% of main compression force, but this doesn’t always work.

A low precompression force allows the removal of air between the particles, decreasing capping potential. The amount of precompression force is critical because too much force can increase capping.

The correct amount of precompression force can be tested by rotating the turret manually while applying only precompression force. After the compact is ejected, it should appear to be an intact tablet, but when you squeeze it with your fingers it should fall apart back into the original particles. This shows that particles have not bonded together but instead are consolidated and closely packed.

The punch penetration determines how high in the die bore the tablet is compressed. Since air travels upward during compression and escapes between the upper punch tip and the die, less punch penetration reduces the distance the air must travel to escape and minimises capping potential.

Decreasing main compression force to prevent tablet capping

A decrease in main compression force can also reduce capping. When a tablet reaches its maximum compactibility, it won’t increase in strength with additional compression force. Instead, the excess compression force can reduce interparticle bonding and cause capping. In that case, reducing the compression force may fix a capping issue.

Turret speed to reduce tablet capping

Decreasing the tablet press’ turret speed may also reduce capping. An acceptable tablet strength might be achieved on a small R&D press, but when moved to production scale, the press may use a higher compression rate or a lower dwell time. If the formulation is strain-rate sensitive, compression speed will impact the tablet’s robustness. Such formulations require slower turret speeds.

Reduce tablet capping with tooling configuration

Features of the tooling can also be modified to reduce tablet capping, including using tapered dies or reconfiguring the punch heads.

Tapered dies

A tapered die allows a higher rate of air removal from the die cavity. The inner die cavity tapers outward toward the top of the die, which allows more room for the air to escape. To take advantage of this feature, adjust the punch penetration so the tablet is being compressed in the tapered area.

Remediate tablet capping with punch head configuration

A new punch head configuration may remediate capping issues. Extended punch head flats can increase dwell time which can increase the tablet strength in some cases. Recent studies have shown that reducing the head flat can resolve some capping issues at high speeds. In either case, head flats can play a major role in tablet robustness and capping potential.
Robert Sedlock is with Natoli

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