Capillary rheometry has its origins in polymer melt processing, but is also directly relevant to many other material processes such as high speed coating and printing applications. Based on controlled extrusion of a test material, capillary rheometry enables material flow and deformation properties to be characterized under conditions of high force (or pressure), high shear rate and at elevated temperature.
The basics of the capillary rheometry technique are as follows:
- The sample under test is loaded into a bore in the temperature-controlled barrel of the capillary rheometer.
- A capillary die of known dimensions (diameter and length) is mounted at the bottom of the barrel bore.
- A piston is used to extrude the sample through the capillary die, and the resultant pressure is measured at the die entrance.
- Shear viscosity is calculated from knowledge of the capillary die dimensions, piston speed and pressure.
- The shear rate of the test can be varied to produce a flow curve (viscosity versus shear rate).
- Using a twin bore barrel and a ‘zero length’ die allows simultaneous determination of shear and extensional viscosity.
Capillary rheometry also enables other rheological and process characteristics to be evaluated, including die swell, melt fracture, wall slip, flow/no-flow stress, melt strength, haul off and pVT relationships.