Mike Nicol highlights the importance of instrumentation for bi-layer R&D and production tablet presses.
When producing bi-layer tablets, a force instrumentation system is a critical feature in the development and manufacturing process.
Without a high quality data acquisition system the running condition of the press is unknown. It takes additional time and product material to test a sample of tablets just to find out that adjustments need to be made. This process can take hours to optimise.
Data acquisition
There are many aspects that make up a complete high quality data acquisition system, one of these is the importance of an accurate first layer compression transducer.
Most bi-layer tablet presses are capable of exerting 80 to 100kN of compression force on both layers.
An 80kN force transducer with a typical full scale error of 0.25 per cent would result in an error range of +/-200 Newtons. Target first layer tamping forces are frequently in the 1 to 2kN range. A 200 Newton error represents a measurement error of 20 per cent for a 1kN force.
Some of the improvements to reduce this error include:
* Layer one force transducer designed for a full scale of 25kN instead of 80kN;
* Transducers made from special material with a reduction in the Modulus of Elasticity resulting in greater electrical output;
* Platinum tungsten or semi-conductor strain gauges in place of constantan producing greater electrical output;
* Tighter restraints to the software filter resulting in an improved signal to noise ratio;
* Machine fast stop features when force exceeds 25kN of compression to protect the transducer;
These improvements allow the measurement accuracy needed for layer one compression force.
Set-up times
The use of real-time force displays is an excellent tool to shorten set-up times and quickly identify weight variations.
Additionally, complex analysis of the tableting parameters data results is critical in limiting material losses and set-up time. The following are just two parameters of the many available:
Compaction profiles are a very effective tool in formulation development and understanding the physical properties when compressed at different levels on a tablet press.
Conventionally the data is presented showing the relationship between the applied compression force and the resulting breaking force (also know as hardness).
This relationship changes with a change in tablet geometry and is a more meaningful approach if the applied compression is converted to compression pressure and tablet breaking force into tensile strength.
The strain rate study provides an understanding of compaction properties when compressed at different loading rates or dwell times.
Some excipients and actives are acceptable with good mechanical strength when made on a smaller research press, but fail when produced on a larger high-speed manufacturing press.
Such materials are said to be strain rate sensitive. This study can therefore predict potential problems during the scale-up phase.
For more information at www.scientistlive.com/eurolab
Mike Nicol is with Riva Europe Ltd, Aldershot, Hampshire, UK.
