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Rolling diaphragm seals single-use mixing system

06 October 2010

An injection moulded elastomeric diaphragm that constantly rolls and unrolls over the drive shaft of a mixer is a critical component in a patented single-use mixing system that enables pharmaceutical manufacturers to prevent cross-contamination while eliminating the need for cleaning and sterilising traditional stainless steel equipment between batches.

HyNetics Corporation introduced the first system of this type in 2002, but for the diaphragm it recently switched from a conventional thermoplastic elastomer (TPE) to the Medalist MD140 medical grade elastomer from Teknor Apex. The HyNetics system is used to mix powder/liquid or liquid/liquid ingredient combinations, hydrating powders in five minutes or less, and efficiently mixing materials that are either difficult to process or shear-sensitive. The system includes permanent stand-alone hardware in six sizes from 30 to 5,000 litres, with corresponding disposable assemblies. Mixing is achieved by a reciprocal, rather than a rotating, action.

The bell-shaped diaphragm is welded to a polyethylene bag that serves as a mixing chamber. It provides a seal between the bag and a shaft affixed to a mechanical drive that is part of the permanent hardware for the HyNetics system. Inside the bag, the shaft connects to a slotted disk, which moves up and down to provide the mixing action. To maintain a seal during each up-and-down cycle, the diaphragm rolls and unrolls over a mandrel located below  the diaphragm. HyNetics supplies the bag, disk, diaphragm and other components as a disposable assembly that is used once for a single pharmaceutical batch. 

Medalist medical elastomer welds readily to polyethylene, is free of animal-derived materials such as stearate lubricants, and offers superior mechanical properties. The compound has also proved highly resistant to stress cracking, a fact supported by tests involving rolling/unrolling cycles of two per second. The standard TPE failed after 50,000 cycles, while the Medalist part continued working through a million cycles.

The Medalist programme for medical manufacturers now includes 28 standard high-purity elastomer compounds, along with a comprehensive body of test data and resources for designers and processors. Compounds are available with Shore A hardness from 5 to 87 and include clear, translucent, and opaque formulations.

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