Tablet compaction simulator uses Festo pneumatics

When Huxley Bertram Engineering developed its latest ESH compaction simulator, the engineers chose to completely update the electronics and control software and implement a number of innovative design features, including a new three-hopper feed system, to facilitate production of multi-layer tablets. The hoppers are driven into position by pneumatic cylinders controlled by Festo valve terminals and are fitted with vacuum ports to suck away any excess pharmaceutical powder and keep the die-plate clean.

ESH compaction simulators enable formulation scientists to model the mechanical behaviour of pharmaceutical powders during the compaction process. Such simulators are used extensively during the development of new commercial drugs and can also be employed as test stations to investigate production issues. Throughput is rarely an issue - most machines have a typical output rate of just one or two tablets per minute - but the important criteria are ease of use, ease of parameter adjustment, flexibility to accommodate a wide variety of tablet types and formulations, and, above all, high-accuracy repeatability.

Huxley Bertram's new ESH compaction simulator incorporates two high-speed closed-loop servo-hydraulic actuators mounted on crossheads and controlled by a dual-channel digital system linked to a computer-based HMI (human machine interface). The menu-driven software complies with the latest requirements of GAMP4 and the USA Food and Drug Administration's 21 CFR part 11 guidance, so includes full data traceability and operator identification, and facilitates fully-automated tablet production, data collection and analysis of test results. The software enables a wide variety of production parameters to be measured - including humidity, ambient temperature, die temperature, punch temperature and die wall pressure - and can present all compaction analysis results in an easy-to-interpret graphical manner.

Hopper design

The simulator is constructed extensively from stainless steel to allow easy cleaning between different trial batches. The new powder hoppers can be gravity fed or fitted with stirrers, are expressly designed for easy removal and cleaning, and can operate with as little as 5g of powder. For multi-layer tablet production, the simulator can be equipped with up to three hoppers, each of which is driven into place above the system's die-plate by a pair of pneumatic cylinders. The cylinders' vacuum control valve sub-system and associated air preparation equipment was developed specifically for the application by Festo.

William Bertram, Managing Director of Huxley Bertram, says: "Our original ESH compaction simulator was a four-pillar system, whereas the new one is a two-pillar design that has a much smaller footprint on the laboratory floor. Using compact pneumatic cylinders for powder hopper positioning has enabled us to minimise the system's work envelope and, with Festo's help, we have implemented a very flexible control approach. Most tabletting simulators on the market use cam-driven compression tooling, requiring the system to be mechanically reset whenever there is a change of tablet type. Our new design provides a much more flexible solution - the entire compaction cycle is programmable, allowing very precise control and fast changeover to different tablet formulations. And the ability to bring any of three powder hoppers into play at the drop of a hat adds considerable value to the process."

Simulator operation

In operation, the powder used to make the tablets is loaded into one or more hoppers, which are then sequentially driven into and out of position above the die-plate in the centre of the machine. The powder falls into the die and the hopper is retracted, while a vacuum nozzle is energised to ensure that any loose powder is removed from the die-plate. At this stage, the powder sits flush with the die table, and its level is checked by a laser sensor with a resolution of better than 0.1mm. The shaped die and upper and lower punches define the finished dimensions of the tablet.

Servo-controlled hydraulic actuators located above and below the die are capable of providing up to 50kN of controllable compression forces, and are equipped with axially-mounted precision load cells and two LVDT (linear variable differential transformer) position sensors to provide feedback on distance travelled. After compaction, the tablet is ejected into an output carousel that is sized according to the type of tablets being produced - a typical unit would have 36 pockets. The carousel is driven by a stepper motor with an integral encoder, enabling individual pockets to be identified and tracked for quality control purposes.

Huxley Bertram believes that its new ESH compaction simulator sets a new benchmark for flexible, easy-to-use pharmaceutical laboratory equipment.

About Huxley Bertram Engineering

Huxley Bertram designs and builds specialist equipment for industrial processes and scientific research. Founded some 20 years ago, the company has successfully completed more than 700 projects in the areas of automation, research and product development, including micromanipulators, advanced vision systems and precision optical manufacturing equipment. The company is based near Cambridge. For more information, please visit www.huxleybertram.com.