BS ISO 20965-2021 pdf free download – Plastics — Determination of the transient extensional viscosity of polymer melts.
5.3 Temperature measurement and control The test temperature should preferably be measured using a device that is mounted close to the specimen. Contact of the device with the specimen is not permitted. It is essential to mount temperature sensors in at least two positions to monitor temperature uniformity along the length of the specimen. NOTE The uniformity of the temperature along the specimen length is critical to the measurement of the transient extensional flow properties of polymer melts. Localized hot spots result in excessive strain in those regions. This can lead to premature failure, particularly for materials that do not exhibit a high degree of strain hardening. The spatial temperature variation shall be within ±0,75 K. The temporal temperature variation shall be within ±1,0 K of the set temperature. The temperature-measuring device shall have a resolution of 0,1 K and shall be calibrated using a device accurate to within ±0,1 K.
5.4 Strain and strain rate measurement The strain and strain rate of the specimen shall be determined either from measurement of the displacements and/or speeds of the clamp or clamps, or measured directly from the dimensions and/or local velocities of the specimen. NOTE The diameter of the specimen can be measured during the test by use of optical or cutting methods to derive strains and strain rates and to assess the uniformity of deformation. The cutting method results in the test being terminated once the cuts have been made and thus prevents data to failure from being obtained. Local velocities can be measured using optical methods. Corrections for slippage of the specimen at the clamp or clamps may be applied, obtained through independent measurement of the strain of the specimen during testing through measurement of its diameter or local velocities by other methods. The apparatus shall have an accuracy of strain determination or measurement to within ±3 % of the absolute value. The apparatus shall have an accuracy of strain rate determination or measurement to within ±3 % of the absolute value.
5.5 Force measurement The force on the specimen shall be measured during the test by an appropriate means, for example a leaf spring arrangement (see NOTE). The resolution of the force-measuring device should preferably be no greater than 0,1 % of the full- scale value. The apparatus shall have an accuracy of force measurement to within ±2 % of the full-scale value. NOTE Typical peak forces measured in testing of polyethylenes are estimated to be up to approximately 1 N for specimens approximately 3 mm in diameter. 5.6 Calibration The force, displacement, rate of displacement and temperature functions of the rheometer shall be calibrated at least once in 6 months. It is preferable that calibration be carried out at the test temperature as measurement of these functions, in particular that of force, can be temperature sensitive. No traceable standard reference materials are known to exist for checking the calibration of such instruments. Where a reference material is used for checking the instrument, it is preferable that the transient extensional viscosity of the reference material, and the dimensions of the specimen produced using it, have values that are similar to those encountered or used during normal operation of the instrument.BS ISO 20965 pdf download.