Heat Deflection vs Cost

While many test reports and normally only single out continuous service temperatures, the Heat Deflection Temperature (HDT) is equally important especially when considering precision components. The HDT is where components start to lose critical dimensions and possibly fatigue and deform and is therefore worth looking into when choosing a polymer for a high-temperature application.

We charted the HDT as against the average price of various polymers because we normally find that the two do seem to go hand in hand. Although this is somewhat true the chart also throws up some interesting caveats with regards to how much more expensive it becomes to have a high HDT.

What is obvious from the chart is that polyamides which are known for being extremely durable under high temperatures do in fact sit at the right-hand side of the chart. However, they do come at a tremendous price with many polyamides costing many multiples of polymers like PEEK or PEI.
 

PEEK, surprisingly, does not have as high an HDT as it should considering how expensive it is. PEEK is normally chosen because of its chemical resistance, extreme wear resistance, steam resistance, fatigue strength, and dimension stability. It is an extremely heat-resistant polymer but for the extent to which it may lose tolerance under high temperatures you do end up paying quite a high premium.

PPS, which is normally filled with 40% glass for stability, appears to offer the best value considering the temperatures that it can withstand. With an HDT of around 260°C it is obvious why PPS is so popular in the semiconductor, automotive, and electrical industries.

The most useful middle ground appears to be PEI and PESU FG30, both of which are priced somewhere in between the more expensive polymers and the much cheaper ones but offer for that a fairly high HDT often exceeding 200°C. The issue with both these polymers is that they are rarely available in stock shape form (rods or sheets) and usually need to be injection moulded.

Finally, PTFE, which often rates itself among the most heat-resistant polymers with a service temperature of up to 260°C, finds itself at the low end of this chart. This is not to say that PTFE is not effective under higher temperatures, but as we have also written about before, PTFE tends to deform at temperatures above 200°C and although it will not melt it is unlikely to hold an extremely tight tolerance at elevated temperatures. It is for this reason that PTFE is usually employed in areas such as lining and gasketing, where tolerances are less important. For precision components, PTFE works best between temperatures of 40-150°C.