Tuesday, 28 April 2020

Design Properties for Engineers: Coefficient of Linear Thermal Expansion (CLTE) of High Performance Polymers

In this post we discuss the thermal expansion of different high performance polymers. The coefficient of linear thermal expansion (CLTE) is a performance indicator for the dimensional stability of materials when they are exposed to temperature. In general, plastics expand under the influence of temperature. The expansion is big compared to other materials. Length changes of millimeters at a temperature difference of 10 Kelvin are not unusual.

Furthermore, the effect of thermal expansion is different depending which polymer processing technique is used (injection moulding vs. extrusion). Different values are obtained in polymer flow direction and perpendicular to the flow direction. Thermal expansion is lower in flow direction compared to perpendicular to it.

Influencing the thermal expansion of injection moulded parts during the design phase is limited. This gets even more difficult when semi-finished plastics parts are used. The final part geometry will be cut out of the semi-finished block.

In addtion, the thermal expansion in length direction increases with temperature. Therefore I have listed the CLTE values up to 150°C and above 150°C in the chart below. Also, it can be seen that fluoropolymers react to temperature changes with a higher dimensional change compared to the other listed polymers. Important to know is that PTFE has a phase change at 21°C. The phase change results in a 1% volume change combined with a non-linear relative length change. This dimensional changes need to be considered when designing parts with PTFE.

CLTE values up and above 150°C of different high performance polymers

Reduction of CLTE

A good way to reduce the CLTE of all high performance plastics is with filling materials and reinforcements. The use of carbon fibers lowers the thermal expansion in the best way. Additionally, reinforced Polyaryletherketones and Polyamidimides show similar thermal expansion as Aluminum. Some of them have even a lower thermal expansion than Aluminum. Therefore, such materials are especially suitable for overmoulding Aluminum based bushings and other parts.

I hope you found this information on CLTE of high performance polymers useful and can apply it for your next part design.
Thank you for reading and till next time!
Herwig Juster

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