Magna to supply NPL viscosity apparatus

A Magna Projects and Instruments product story

Edited by the Engineeringtalk editorial team Aug 8, 2000

Magna Projects and Instruments Limited has been awarded an exclusive licence by the UK's National Physical Laboratory to manufacture and market NPL's PCV Pressure Coefficient of Viscosity apparatus

Magna Projects and Instruments Limited has been awarded an exclusive licence by the UK's National Physical Laboratory to manufacture and market NPL's PCV Pressure Coefficient of Viscosity apparatus for use with capillary extrusion rheometers such as Magna's Eta 2100, BFR 2100 and MiniCER instruments.

The PCV is an easy-to-use instrument that allows a second die to be fitted below the normal capillary die of the rheometer, creating an intermediate chamber in which the specimen material is compressed during flow to build a small zone of high hydrostatic pressure, which is measured by a second pressure transducer.

The device may be built into the rheometer barrel or be fitted as an adaptor.

Adaptors may be supplied to fit capillary rheometers other than those supplied by Magna.

By running a series of tests with upper and lower dies of different geometries, a matrix of results is compiled over a range of shear rates and hydrostatic pressures, and thus the pressure dependency of viscosity may be determined.

Data from capillary extrusion rheometers are used widely with computer- aided design and development (CADD) packages to describe how a material will flow, for example, in an injection moulding tool or extrusion die, or in a mixer.

However, the data are generated usually by extruding into ambient pressure.

In processing, the materials are often required to flow under conditions of high hydrostatic pressure.

In such circumstances, the flow parameters (eg viscosity) for the material may be significantly different from those measured under ambient conditions.

If this change is not taken into account, then expensive errors in tooling design and processing conditions may result.

Additionally, the resulting product may have inferior mechanical properties.

Typical problems that have been encountered are "short mouldings", ie incompletely filled mould cavities, and "scorch", ie premature cure caused by shear-induced overheating.

A graph (available from Magna) shows the apparent viscosity of a polystyrene melt at 200C increases with hydrostatic pressure over a range of shear rates (ie extrusion rates).

Given that this is a logarithmic plot, it can be seen that the actual value for the viscosity of the melt at the working pressure can be over twice the ambient- pressure viscosity.

This can have a profound effect on mass transfer rates, specific energy input, and a whole host of other processing parameters.

Factors that influence the magnitude of the viscosity change with pressure include the amount and type of filler and plasticiser used, the nature of the polymer or other binder.

Magna's PCV adaptor allows this important yet much-overlooked parameter to be determined with ease.

It is available with an instrumentation package for temperature control and pressure monitoring, and, for Magna's own rheometers, with a module to allow data logging and analysis with the RheoTenax software suite.

Other products available from Magna include the ETA 2100 high-speed, high-pressure capillary extrusion rheometer, the BFR 2100 "special" capillary extrusion rheometer for sensitive materials, the MiniCER benchtop capillary extrusion rheometer, and the NPL Extensional Rheometer.

Magna is always happy to look at special and one-off projects, and offers a consultancy service.