Terminology of plastics technology - humorous and easy to understand
Did you know that in many cases, comparisons with nature and humans can be used to explain facts in plastics technology?
Today's term: Creeper behaviour
„Anyone who knows plastic chooses cement.“
A professor of plastics engineering at what was then Darmstadt University of Applied Sciences is said to have once uttered this sentence. Of course, it was meant tongue-in-cheek. Because anyone who really knows plastics knows that they are fantastic materials – you just have to understand their peculiarities. And one of them is called Creep.
What does ‘creep’ actually mean?
Imagine you sit down on a new sofa. At first, it feels wonderful. After an hour, you realise that you’ve sunk in a few centimetres. This is normal for you, as it’s more comfortable than sitting on a stiff board, and when you stand up, the foam seems to return to its original shape. But if that were really the case, why do sofas look worn out after a few years’ use?
In technical circles, this is also known as ‘compression set’. If a plastic is subjected to a load over a prolonged period, it continues to deform slowly, even though the force applied does not increase. This time-dependent deformation is known as Creep.
Why does a plastic do that?
The difference lies within the material itself.
Metals have a very rigid crystal structure. Plastics, on the other hand, consist of long molecular chains that can slowly slide past one another when under stress.
Don’t jerk it.
Not all of a sudden.
But at a leisurely pace.
The higher the temperature and the greater the load, the faster this happens.
Why is this important?
For design engineers, creep is not a fault, but a property that must be taken into account.
Under continuous load, a seal may change shape and will not return to its original position once the load is removed.
A snap hook that holds perfectly today may, after prolonged use, eventually fail to „snap“ quite as effectively.
That is why, in the case of plastic components, it is not enough simply to calculate whether they today hold.
The key question is:
Will they still be around in five or ten years’ time, and what can be done about it?
Design engineers have a number of options:
- select suitable materials,
- Reduce stress,
- Keep temperatures low,
- Design components to be stiffer,
- Use glass fibres or other reinforcing materials,
- or distribute loads as efficiently as possible.
After all, plastics are not bad materials – they simply require a little more long-term thinking.
Conclusion
Creep behaviour is one of the most important properties of plastics. Anyone who ignores it may sometimes be in for some unpleasant surprises.
Anyone familiar with them can develop durable, reliable and cost-effective components.
And that brings us back to the famous quote from the professor:
„Anyone who knows plastic chooses cement.“
Perhaps we should add the following today:
„… or he chooses the right plastic – and designs it in such a way that any creep only occurs where it doesn’t bother anyone.“