Thursday, 19 September 2019

Plastics Part Design: Coefficient of Linear Thermal Expansion (CLTE) of 136 Polymers

Topic of this blog post is the coefficient of linear thermal expansion (CLTE).
CLTE is often presented with the letter “α” and is calculated using the following equation:

α = ΔL / (L0 * ΔT).

L0 is the length of the part at room temperature; ΔL is the length variation of the specimen when it is heated up, and ΔT is the temperature difference between start and end. More details can be found in the standard ASTM D696.

Polymers in applications such as bus bars, which are used in traction motors, battery modules and power electronics, need to pass thermal shock tests. In such tests, metal bars are overmoulded. Thermal cycles between -40°C (1 hour) and +150°C (1 hour) of the overmoulded bars are done. The cycles are counted until cracking of the polymer layer occurs. The similar the CTLE value of both materials and the better the elongation at break of the overmoulded polymer are, the easier the selected material will pass such tests.

In the table below you can find the maximum CLTE of 136 polymers. Furthermore, I added a factor which shows how similar the polymer is to copper in terms of thermal expansion. This is useful for overmoulding of copper elements.

You can add this table to your part design library. Here, you can find some more part design related data: continuous use temperature and thermal conductivity.

Thanks for reading & till next time!
Herwig Juster


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