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How to optimise dwell time during tablet manufacture

Rob Blanchard, I Holland, talks through the effect of dwell time on tablet production

Dwell time can have a huge impact on tablet manufacture, with many formulations sensitive to how this key element of compression is performed. The importance of this process is even more apparent in today’s need for tablet production to be fast, efficient and fault free. But the question is how do you ensure the dwell time in your manufacturing process is optimised to form quality tablets at maximum productivity? 

What is dwell time?

In order to produce tablets from granule/powder it is necessary to use compression force; however, it is vitally important that the correct amount of force is used for just the right amount of time. Too much, or too little of either, can result in tableting issues. Excessive force or over pressuring of the punch can cause catastrophic failure of the tool and damage to the press. Not only can this be very expensive but it carries significant health and safety risks to press operators and the patient, therefore it is vital to understand how to optimise dwell time correctly. 

Dwell time in the context of tablet compression is the amount of time each individual punch head flat is in contact with the compression roller(s) of a rotary tablet press; and that the compression force applied when forming the tablet is above 90% of its peak value. Key factors affecting dwell time are punch head flat size and shape, use of pre-compression and RPM used during production.

Dwell time plays a significant part in determining if a tablet can be produced successfully, especially those incorporating formulations that are challenging to compress. Not all formulations are dwell sensitive as some will compress effectively at any speed, however the majority are very susceptible to even the slightest change. Sometimes the punch head flat diameter is a major factor that is overlooked or misunderstood, and it is this area that tooling manufacturers like I Holland are investigating and producing innovative technologies in order to offer productivity enhancing solutions.

Formulation characteristics

The characteristics of ingredients in a formulation can severely affect tablet manufacture leading to common tabletting problems which we will discuss below.

Plastic or elastic properties

Many issues can be traced to the characteristics of certain ingredients in a formulation which display differing plastic or elastic properties.

‘Punch displacement velocity (ie. strain rate) and dwell time are two factors that can significantly affect the compression behaviour of powders. As a general rule of thumb, slower compression and decompression speeds and longer dwell times will improve the mechanical properties of a tablet. When particles are subjected to a compression force for a longer period of time, further plastic yielding can occur and the degree of elastic recovery is reduced.’

Pharmaceutical Dosage Forms - Tablets, Third Edition - Edited by Larry L. Augsburger, Stephen W. Hoag

Essentially, where the behaviour of a particle under compression can either stay deformed or ‘spring back’ to its original shape, the dwell time applied can be critical. In cases of formulations with more time dependant consolidation behaviour, a long dwell time is important to create strong bonds between the particles. When particles are subjected to a compression force for a longer period of time, further plastic behaviour is demonstrated, less ‘spring back’ happens and this results in a more stable compacted tablet.

Sticking

Tablet sticking is perhaps the most common problem in tablet manufacture. This build-up of granule on the punch tip face causes tablet press downtime and reduced tablet output. It has a negative effect on tablet appearance and often results in the removal of tablet tooling from production for regular cleaning and maintenance.

The first resort for tablet manufacturers is often to apply more compression force or to slow the press down to solve this problem. Either or both of these can often solve sticking, but carry with them trade-offs in productivity and tooling/press lifetime.  What if you could extend dwell time without slowing the press down? The eXtended Dwell Flat from I Holland tooling does exactly that.

Capping

Trapped air pockets in the forming tablet can cause severe problems during manufacture. If the air is insufficiently squeezed out and/or density variations occur in the tablet volume, the tablet tensile strength is negatively affected and the risk of tablet capping or de-lamination increases.

There are several solutions available such as employing tapered dies or using a press with a pre-compression stage, but one of the most effective solutions employed by tablet manufacturers is the method of slowing the press down and extending dwell time in order to allow air to escape.  Once again, today’s requirement for faster and more efficient tablet production makes this an undesirable option and so   increasing dwell time through tooling is highly desirable.

Friability

Friability, or the tendency to crack, chip or break during compression is in part due to the formulation. If the formulation is not cohesive and does not bind together sufficiently then friability will occur.  There are several ways to compensate for friability including looking at dwell time, weight control, expansion and of course tooling condition. All of these should be examined to produce a quality tablet. In selected trials I Holland has found that eXtended Dwell Flat tooling has completely eradicated friability issues.

Moisture

All formulations have very different characteristics, and compressing a tablet containing a variety of differing powders can be difficult. Each powder contains a different moisture content, which is often needed to help the binding, or compaction effect but too much water within the tablet can be one of the causes of a rise in adhesive forces and therefore sticking. Moisture can enter into the process either in wet granulation or due to excess humidity in the compression chamber, formulation preparation and storage areas if they are not temperature and humidity controlled.

Michael D. Tousey explained the effect of formulation moisture in his paper ‘The Granulation Process 101’.

‘Think about the example of making a snowball: If the snowflakes are rather large and wet, then they compact very easily into a snowball. However, if the snowflakes are very light, fluffy, and dry, then compaction is more difficult. Every kid knows that to make a snowball with light, fluffy, and dry snowflakes, they must hold the snowball together for a longer period of time (dwell time) and be careful not to over compress. If the snowball is over compressed, then the flakes no longer lock together but instead laminate (flatten out) and fall apart. The same is true of powders used in pharmaceutical tablets. If the formula has some of both characteristics—large particles with high moisture content and small, dry particles—then the tablet may or may not compress well and probably will have difficulty holding together.’

In these instances, the importance of using the correct dwell time can have an impact on the end product.  The dwell time needs to be optimised to allow for the formulations moisture content.

Conclusion

The pharmaceutical industry’s need for technology and push for operational efficiency that allows them to produce quality tablets quickly and efficiently without major cost implications involved with the purchase of new equipment or through solutions that can reduce productivity are a major influence upon manufacturing decisions. This trend in cost reduction without consequence to tablet quality is a key factor for tablet tooling development.

The crucial importance of extended dwell time can be illustrated by the frequent application of a number of techniques to increase the time that the punch is in contact with the compression roller, for example:

• Reduction of the tablet press speed in case of sticking, capping or insufficient hardness

• Installation of larger compression rollers to increase the total compression time

• Use of up-sized punches with a larger head to increase the size of the dwell flat.

These solutions are not always viable due to strict time constraints and budgeting so other solutions are sought. Making small changes to the size of the compression roll does not extend dwell time, so another method is required. The most feasible answer is through the use of specific tooling that can be used without slowing the press so production runs sufficiently.

Specifically designed tooling for increased dwell can do the following;

The requirement for quality tablets to be produced quickly and cheaply will always lead to innovation and the introduction of newly developed elliptical tooling like I Holland’s patented XDF is helping to make the tablet production process one which is faster and more efficient with tablet manufacturers able to achieve higher press speeds with challenging products and formulations.

The purchase of a modern tablet press with dwell enhancing features will probably improve productivity but obviously involves significant capital expenditure. It should also be noted that the use of elliptical eXtended Dwell Flat tools, (which are currently the only way to achieve more dwell time without slowing down your existing press or upsizing punches both of which are inefficient,) will add cumulative associated benefits in dwell time to a modern press.

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