Rarely a day goes by without the mention of the words “high temperature” for someone’s specific application. Its a very popular question but the answers aren’t always as straightforward as they could be, so out came the data sheets and hopefully a bit more plastics fog has been lifted.
What’s the main confusion?
We’re given two sets of data, one says “intermittent or short term” and the other column says “continuous or long term”. If you’re like us, the first question on our minds was, what does this “short term” mean, is it ten minutes? An hour? Or are we talking seconds here?
Short term means:
Believe it or not, short term seems to mean around 2 hours, which is a bit longer than you may have thought. There is a proviso though which is that at these particular top end temperatures, any load on the plastics needs to be very low as deformation will start to occur.
Is it a one-shot deal?
Apparently not, as long as the plastic is allowed to cool sufficiently, the cycle can repeat but every time the plastic is pushed to its limits, there is some permanent degradation, which means its never quite as good as it was before, the more cycles under this stress, the more the plastic starts to deteriorate.
What’s going on?
The molecules and the chains are being damaged, the whole internal structure is starting to unravel and change into a more basic structure. What this means in plain terms is that the “tensile strength” will plummet to around 50% of its original value. The teeth on a nylon gear, if they hadn’t already deformed would be much more likely to shear off at this stage.
What’s the” long term” then?
This has an estimate of between 5,000 and 20,000 hours. The load on the plastic has a bearing on this as you might expect though. The higher the load means the lower of the estimated life span. The lower the load, the nearer to that 20,000 hours it’s going to get.
Lets get some perspective:
The temperatures we’re talking about here need to be pretty substantial though, in reality there aren’t as many applications as we would think that do go over or close to these limits. Once a more realistic or accurate reading of the predicted temperature is assessed, we often find that the operating temperature is actually not as high as we think, a hundred degrees centigrade is a lot hotter than we sometimes imagine.
The big guns:
The high end plastics for extreme temperatures are PTFE and PEEK, these guys are right up at over two hundred and fifty to three hundred degrees before we need to worry, and that’s pretty crazy stuff for an engineering plastic.
The little guns:
The low end plastics for temperature endurance are PVC and HDPE, these guys can only sustain temperatures of sixty and eighty degrees respectively, its not earth shattering but they have other priorities going on and those temperatures are still pretty hot
Things to remember then:
- “short term” means around 2 hours
- “long term” means around 5,000 to 20,000 hours
- The load needs to be low or it will deform
- Excess temperatures will make things brittle
- Try to find the true operating temperature if you can
- Look towards PTFE or PEEK if the temperatures are very high