Ask an Expert – How Can I Increase Dwell Time Without Impacting Tablet Production?

Dwell time is the amount of time a tablet compression punch head flat is in contact with the compression roller of a rotary tablet press. The compression force applied when forming the tablet must be 90% above its peak value.  Usually, the longer a product remains under peak compression, the greater the chances of compressing it to the required hardness.

Dwell time plays an important part in determining if a tablet can be produced successfully, especially in those incorporating formulations that are difficult to compress. Using the correct amount of force is important, as excessive compression or over-pressuring of the punch can cause tabletting problems, tool failure and damage to the press. 

Some formulations are extremely challenging to compress effectively, for example, those with plastic or elastic properties. If not compressed correctly, it can lead to tableting problems like capping, where the top or bottom of the tablet separates horizontally from the main body.  

Particles with elastic form will change shape during applied stress but will return to their original condition when compression has stopped. Conversely, those displaying plastic properties will be permanently deformed when stress is applied above their elastic limit.

Their behaviors can change depending on the compression force used and the length of time beneath it. In formulations that have more time-dependent consolidation behavior a long dwell time is essential to create strong bonds between the particles and form a quality tablet. Particles subjected to extended compression force will demonstrate further plastic behaviour. Less ‘spring back’ happens, resulting in a stable compacted tablet.

In addition to the properties of the formulation, tablet manufacturers should also consider the additional science behind the formulation and the potential production problems that could occur.

A very common problem that tablet manufacturers face is sticking. Sticking occurs when the adhesive forces between the product being compressed and the punch tip faces are higher than the cohesive forces binding the solid dose together. When this phenomenon occurs, the product will build up on the tablet punch faces. This build-up of product on the punch tip face causes tablet press downtime and reduced tablet output. To counteract the problem, the press speed is often reduced, or a higher compression force often applied. Although this may effectively resolve the issue, it will increase tablet hardness (and therefore also increase dissolution rates) and can also affect productivity and the lifetime of tooling.

 Air entrapment is another major problem when manufacturing tablets, as it can lead to capping. At pre-compression, an extended dwell time at low to medium compression force is essential to expel air from the powder bed and also for even distribution of product in the die bore before final compaction under the main compression. Without the removal of air, density variations occur in the tablet volume, tensile strength is reduced and the risk of tablet capping or delamination increases.

An obvious answer to dwell-sensitive formulations is to increase the compression time and slow the press down. This, however, leads to a reduction in tablet output and time and has cost consequences.

Alternatively, dwell time can be increased with larger diameter compression rollers. Although this will increase the total compression time, it is expensive or simply unfeasible on some presses. The most practical answer is through the use of extended dwell tooling. This tooling is designed specifically with an elliptical head form with an extended head flat. The diameter of the flat area on the top of the punch head is larger, allowing the time it is in contact with the compression roller to be longer.

It increases dwell time on an existing press without any expensive modifications. It enhances tablet compaction and cohesion and can increase dwell time by up to 50% over a standard punch head,  allowing more than D-type punch dwell on a B-type punch. This increase helps solve compression problems without upsizing punches or investing in a new press.

The primary goal of tablet manufacturers is to produce quality tablets quickly and efficiently. This requires production process improvements. Dwell time is one area that can immediately make a difference.

When formulations are challenging, increasing the dwell time may be the only answer, but it does not need to come at the detriment of efficient production. Through the use of innovative tooling like an elliptical head form, tablet manufacturers can cost-effectively achieve higher press speeds, increased output, and a better-quality product.