Compared with other plastics, hardness is a more important properties for TPU granule. TPU hardness is generally tested by the Shore hardness tester. Shore hardness divided into Shore A and Shore D, which represent different hardness ranges. TPU hardness below 90A test by Shore A hardness tester, TPU hardness above 90A test by Shore D hardness tester. Although the hardness test is relatively simple, different test conditions and methods can cause deviations in hardness values. For general products deviation must be controlled within ±3~5 degrees (A), and some products such as headphone cables that with high requirements for hardness, even require the hardness deviation of TPU materials to be within ±2 degrees (A).

Test factors that influence the deviation of TPU hardness

1. The Flat of test sample

Shore A hardness value is measured by the depth of the needle pressed into the sample.  The thickness of the TPU hardness test piece is 6mm. uneven surface of the TPU hardness test piece will result in deviation towards high hardness value or deviation towards low hardness value.

2. The length of the needle on the Shore hardness tester

The standard test method required tester needle exposed on the pressing surface 2.5mm. The pointer should point to the zero point in the free state, correspondingly the pointer of the instrument should point to 100 degrees when the pressure needle is on a smooth metal plate or glass. if instrument point to greater than or less than 100 degrees, it means that the exposed height of the pressure needle is greater than or less than 2.5mm. The measured hardness value will be higher than actual hardness if the exposed height of the needle is greater than 2.5mm. In this case, please adjust the instrument.

3.The effect of temperature 

TPU is a thermoplastic polymer material, its hardness value influence by the environment temperature. The hardness value decreases when the temperature is high. For thermoplastic elastomers, the effect of temperature is significant. The hardness of the same grade in summer is about 5 degrees (A) softer than in winter.

4. The influence of reading time

The reading time of the Shore A hardness tester has a great influence on the test results. Immediately reading instantly after the needle press on the sample and reading after the pointer is stable, the results are very different and the difference between these two can reach about 5 to 7 degrees. Therefore, please reading the data immediately after the sample is compressed.