What is ISO 20965 about?  
ISO 20965 specifies the general principles of a method for determining the transient extensional viscosity of polymer melts. The procedure details the measurement of polymer melt specimens stretched uniaxially under constant strain rate and constant temperature conditions.
In ISO 20965, Hencky strains and strain rates are used. Values of strain, strain rate and the temperature outside these limiting values can be attained.
Note: The operating limit of an instrument, in terms of the lowest transient extensional viscosity values that can be measured, is due to a combination of factors, including the ability of the specimen to maintain its shape during testing and the resolution of the instrument.
Who is ISO 20965 for? 
ISO 20965 on transient extensional viscosity of polymer melts is relevant to: 
- Plastic industry
- Laboratories
- Chemical engineers
- Product engineering
Why should you use ISO 20965?
For fluids such as polymer melts, the extensional velocity is measured using rheometers that apply extensional stress. This extensional velocity is determined by both, the rate of stretching and strain imposed. In describing and modelling plastics processing, the use of Hencky strain is preferred.
The general principles of a method for evaluating the transient extensional viscosity of polymer melts are laid out in ISO 20965. This will help you identify and avoid large deformations caused by the straining process thereby ensuring durability of end-products.
 What’s changed since the last update?
BS ISO 20965:2021 supersedes BS ISO 20965:2005, which is withdrawn. BS ISO 20965:2021 includes some technical changes concerning BS ISO 20965:2005. These include: 
- Figures have been updated and figure keys have been introduced
- Calibration period in 5.6 has been clarified
