The Case for Extended Dwell Flat Tooling

PAVEL CHAGOCHKIN/SHUTTERSTOCK.COM

The requirement for quality tablets to be produced quickly and cheaply have led to the advancements in tooling. To achieve problem-free tablet production, many processes must be investigated, and one of the main considerations to examine is the dwell time.

Dwell time is defined as the amount of time each individual punch head flat is in contact with the compression roller of a rotary tablet press when the compression force applied to form the tablet is above 90% of its peak value.

How formulation characteristics can have an impact on dwell time

Many issues can be traced to the characteristics of certain ingredients in a formulation that display differing plastic or elastic properties. These plastic or elastic properties can lead to tableting issues such as sticking and capping. Particles that exhibit elastic form will change shape during applied stress; however, this effect is not completely permanent, with the particle returning to its original shape when the applied stress is alleviated. Those ingredients displaying plastic properties are permanently deformed when stress is applied above their elastic limit. The force employed and the length of time in compression can affect the way the formulation reacts, where the behavior of a particle under compression can either stay deformed or “spring back” to its original shape. In cases of formulations with more time dependent consolidation behavior, 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 behavior of powders (1). As a rule of thumb, slower compression and decompression speeds and longer dwell times will improve the mechanical properties of a tablet (1). When certain elastic particles are subjected to a compression force for a longer period, further plastic behavior is demonstrated; less “spring back” happens, which results in a more stable compacted tablet.

The effect of dwell time on friability

There may also be the problem of tablet friability with the tendency to crack, chip, or break during compression because of 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 dwell sensitive 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.

Trapped air and its negative effect

At pre-compression, a long dwell time at low to medium compression force is essential to expel air from the powderbed and for uniform distribution ofgranules in the die bore prior to finalcompaction under the main compression.Air must be expelled in order forthe particles to stick together and formthe tablet.

Air in the formulation 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 (when the top of tablet separates horizontally when ejected from the press) or delamination (when the tablet splits apart) increases.

Other techniques

The crucial importance of extended dwell time can be illustrated by the frequent application of other 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 capping or insufficient hardness 

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

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

These options are not always viable with the strict time and monetary constraints put on tablet manufacturers. This dwell time issue and customer demand led I Holland to investigate a method to increase the dwell time without slowing the press so that production could run sufficiently. 

I Holland designed an elliptical head form, the eXtended dwell flat (XDF), to increase dwell time on existing presses without the need for expensive modifications. XDF can run on standard cams, giving users higher press speeds with challenging products and formulations. It also enhances tablet compaction/cohesion and can increase dwell time by up to 50% over a standard punch head, allowing more dwell than a D-type punch on a B-type tool. This increase helps to solve compression problems without upsizing punches or investing in a new press. 

The following case study illustrates the benefits of XDF tooling in a production environment when tested by a leading pharmaceutical manufacturer. 

Case study 

The challenge. A specialty pharmaceutical company agreed to assess the XDF tooling. The goal was to create an operating environment where tablet quality was increased and waste reduction was improved on the production of a cold and flu tablet. The formulation would regularly stick to upper and lower punch faces. To minimize the problem, several methods were used to increase the compaction force, such as run the press to rejects to clear the sticking and manually scrape the tooling or remove the tooling for a polish, which resulted in downtime during manufacture.

Equipment. A Fette 2090i high-speed compressing machine with industry standard B type tooling was used in the trial during the manufacture of a tablet measuring 10.5 mm round and weighing 3.46–3.66 g. I Holland provided a full set of punches including the new 
XDF tooling.

Results. The maximum press output of 150,000 tablet per hour (tph) was regularly reduced to prevent sticking as the formulation was found to have low hardness and friability issues. XDF tooling stopped the problem of sticking, while increasing output from 150,000 to 225,000 tph. The use of XDF improved compaction force dwell time by 44% and an output of 225,000 tph was achieved, an improvement of 50%. 

Overall it has been demonstrated through rigorous trials that the use of XDF tooling helps to prevent sticking, friability, capping, and tablet hardness. XDF also enhances tablet compaction and cohesion and can increase dwell time on a standard punch type. This increase helps to solve compression problems without upsizing punches or investing in expensive modifications or new presses.

Reference

1. Pharmaceutical Dosage Forms-Tablets, Larry L. Augsburger, Stephen W. Hoag, Eds. (CRC Press, Boca Raton, 3rd ed., 2008). 

Article Details

Pharmaceutical Technology
Vol. 42, No. 8
August 2018
Pages: 26, 28, 47

Citation

When referring to this article, please cite it as R. Blanchard, “The Case for Extended Dwell Flat Tooling," Pharmaceutical Technology (8) 2018.