Manufacturing
Dwell Time and its Influence on Tablet Production
Manufacturers of pharmaceutical tablets are continuously under pressure to make production more efficient. It needs to be quicker, more cost-effective and able to keep in line with the competition from fellow oral solid dose producers and developing markets. Let’s not forget, on top of all these challenges quality has to be a top priority. So what is the answer? What production process can meet all of these requirements? There is no all-encompassing solution to efficient tablet production. It involves many elements and considerations from the design of the tablet, through to what tablet tooling material and coating is used. However, a very important consideration when looking for solutions to optimise manufacturing processes is the dwell time. Dwell time is defined as the amount of time that the surface of the tablet punch head stays in contact with the compression roller in a standard tabletting press. This can only be counted as dwell time when the compression force applied is above 90% of its peak value. Compaction, or dwell, plays a significant part in the quality of the final tablet. In order to produce tablets from granule or powder, it is necessary to use the correct compression force. Too much, or too little of either, can result in tabletting problems like sticking.
particles returning to their original shape immediately when the applied stress is alleviated. Those ingredients displaying plastic properties are permanently deformed when stress is applied above their elastic limit. Any stress will permanently change the shape of the particle. The force employed and the length of time in compression can affect the way the formulation reacts, where the behaviour of a particle under compression can either stay deformed or return to its original shape. In cases of formulations with more timedependent consolidation behaviour, a long dwell time is important to create strong bonds between the particles. ‘Punch displacement velocity (i.e., strain rate) and dwell time are two factors that can significantly affect the compression behaviour of powders. As a rule of thumb, slower compression and decompression speeds and longer dwell times will improve the mechanical properties of a tablet. When certain elastic particles are subjected to a compression force for a longer period, further plastic behaviour is demonstrated; less “spring back” happens, which results in a more stable compacted tablet.’1 Finding a Solution to Friability Friability, or the tendency to crack, chip,
crumble or break during compression can be hugely challenging and negatively impact production. The problem is in part due to the formulation. It is important to get the compression force right – too high and it can adversely affect the tablet, but if the formulation is not cohesive and does not bind together sufficiently, then friability will occur. Many tablet formulations are dwellsensitive and require more time under compression to guarantee that they come out of the press without any faults. Some granules are difficult to compress effectively and require extended time under peak compression to ensure they receive the required hardness to shape into the fully formed tablet. Let the Air Out At pre-compression, a long dwell time at low to medium compression force is essential to expel air from the powder bed and for uniform distribution of granules in the die bore prior to final compaction under the main compression. Air must be expelled in order for the particles to stick together and form the tablet. Air in the formulation can cause severe problems during manufacture. If the air is insufficiently squeezed out and/or density
Understanding a Formula’s Features Many issues can be traced to the characteristics of certain ingredients in a formulation that display different particle deformation. When compression force is applied to the formulation, the particles will react in different ways depending on whether they have plastic or elastic properties. These deformation characteristics can lead to tabletting issues such as picking and capping. Particles that exhibit elastic deformation will change shape during applied stress. This effect, however, is reversible, with the 56 INTERNATIONAL PHARMACEUTICAL INDUSTRY
Summer 2021 Volume 13 Issue 2
