Elevated service temperature and plastic products, how to avoid failures.

In practice, all plastic materials come with a datasheet that states their mechanical properties and service temperature area. Usually, the values are measured and given at room temperature only. It is important to bear in mind that elevated temperature has a significant effect on the strength and modulus of plastic materials.

Amorphous plastics gradually soften as temperature increases. For example, an acrylic PMMA product has totally different characteristics when used at +70°C than at room temperature – even if both temperatures are within the given service temperature area according to the material datasheet.

Semi-crystalline plastics turn gradually softer as well, but they may also change their behaviour more rapidly when they cross their material-specific glass transition temperature, Tg. They change from glassy state (below Tg) to rubbery state (above Tg). In the glassy state semi-crystalline plastics have higher strength and modulus, but they are more brittle. PBT, for instance, has Tg of app. 55°C. If you set a Plasticprop sample made of PBT into an oven, you can probably feel the difference between 45°C and 60°C.

Short-term use: how to avoid problems with design

  • Establish the maximum temperature your product is going to be used in.
  • Take this into account when choosing the material. The maximum service temperature should be clearly within the given service temperature area.
  • Explore how the material strength and modulus are changed by the elevated temperature. Stress-strain curves are a good tool for this. It is also informative to set some reference samples into an oven and study how their properties change in different temperatures.
  • Make sure your design is still strong and stiff enough when the mechanical material properties have decreased below the usual service temperature. Take the Tg into account if it is within the service temperature area.
  • Do not exaggerate. The combination of maximum load in maximum temperature is often very unlikely. If your design is based on this, it might be too heavy and too expensive for the great majority of the users.

How does long-term use differ from short-term use when it comes to elevated temperature?

It is important to understand that in plastic product design short-term and long-term service temperatures require different approaches.

In long-term use, we have to keep stress-cracking, fatigue and creeping in mind. The most efficient way to avoid their unwanted effects is to make the product significantly stronger than short-term loading would require.

For example, if a lever must carry a continuous load of 10 units, you should design it in a way that it can easily carry a load of 50 or even 100 units. But in the case of continuous elevated temperature, this ratio must be based on the material properties at the expected temperature, NOT at room temperature.

It is also worth knowing that long-time exposure to high temperatures might degrade a plastic product. This phenomenon is called thermal degradation.

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