Processability data at real processing rates

A Magna Projects and Instruments product story

Edited by the Engineeringtalk editorial team Mar 8, 2001

Magna's capillary extrusion rheometers generate processability data at real processing rates.

Magna Projects' range of high-performance, automated capillary extrusion rheometers include the benchtop MiniCER instruments, the general-purpose high-shear Eta 2100 and the special-purpose BFR 2100 instruments.

Additionally, Magna can supply instruments with piston speeds of 3 metres/minute if required; indeed, the National Physical Laboratory have been using a Magna-designed high-speed capillary rheometer for many years.

So, what is a capillary extrusion rheometer? It is essentially a highly instrument and controlled vertical ram press.

The sample to be tested, which can be plastics, rubber, food, pharmaceuticals, ceramic paste, bitumen - virtually anything which flows during processing or forming - is loaded into a heated cylindrical chamber fitted at the bottom end with a die or orifice (usually circular) of accurately known dimensions.

After the sample temperature has equilibrated, the material is forced out of the die by driving a piston down the barrel at a series of known speeds.

The pressure required to extrude the material at each speed is measured with a melt pressure transducer located in the barrel wall.

The basic result is a graph of pressure against speed.

If the specimen was an ideal of Newtonian material, then this graph would be a straight line and the slope, by definition, would be the viscosity at that temperature.

Luckily for us instrument builders, there are very few Newtonian materials - water being the prime example.

Many industrial materials, such as polymer melts, are shear-thinning or "pseudoplastic", which means that as speed increases, the pressure increases more slowly, ie the faster the material is extruded, the less energy per unit weight would have to be expended.

For some pastes and other highly filled materials, or for some materials which crystallize under pressure, the materials can be shear-thickening or "dilatant"; the sudden onset of such behaviour can be extremely dangerous.

These effects are separate from the time-dependent phenomena of thixotropy and rheopexy.

For many processes, shear viscosity is not the only important flow mechanism, as extensional flow parameters can be very significant, ie those obtained when the tensile aspects as opposed to the shearing aspects of the flow are considered.

Extensional properties may be estimated from capillary flow experiments, or measured directly with the NPL Extensional Rheometer, for which Magna Projects has the sole licence.

Other flow properties which may be studied using a capillary extrusion rheometer include "die swell", an elastic effect resulting in the extruding material swelling to greater than the die size; "wall slip", where for various reasons the material does not adhere to the die (or extruder) wall; "shear heating" which is caused by friction as the material passes through the die; "no-flow temperatures"; fibre strength; surface texture, and others.

The instrument is an invaluable tool for developing new formulations, for optimising processing parameter and process design, for batch-to-batch quality control and for troubleshooting.