Capping happens when a fracture occurs at the top of the tablet and the top, or cap, separates itself from the body of the solid tablet. It is often caused by air trapped in the powder material during the compression stage. It can also arise because the press fails to compress the formulation due to the collection of powder fines.
When a tablet is compressed air is expelled from the powder granules allowing each of the particles to stick together. The press is designed to allow the air to dissipate during the compression process. As the air is released from the granules it can also push very fine dry granules fines outwards. These particles generally do not stick together and when the particles are pushed into the line of air being released near the cup and the tablet band, the fines prevent the granules from being compressed resulting in the tablet becoming fractured.
As the air is released when the upper punch die tip is raised, capping only occurs at the top of the tablet. If a fracture occurs in the lower part of the tablet, it is referred to as lamination, and is discussed below.
The faster the press speed, the more likely it is that capping will occur and simply reducing the speed of the press will often solve the problem. That said, there are other ways that this problem may be solved.
Poor formulation as well as bad processing practice can often cause capping. Dry blends can cause capping, as a low moisture content of the formulation tends to prevent the particles from blending with each another. The binder, which is added to a formulation to help bind the particles together, may not be adequate for the particular materials. In the event that capping does occur, it is therefore worthwhile reviewing the choice of binder. Improper mixing or blending can add air to the formulation, and inadequate mixing can also cause powders to segregate, leading to tablet capping. Over blending can itself cause problems so careful optimization of the blending process is required to ensure consistent quality of tablet production.
Capping is usually easy to fix either by increasing the dwell time (i.e., slowing down the production) or by careful analysis of the formulation and production process.