That really takes the biscuit!
Our sofa used to be really lovely. The seat was firm, the cushions were high and completely wrinkle-free.
Then, after sitting there for a few minutes, we talked about the Creeper behaviour I gave some thought to plastics. After that, the sofa looked just as it did in the catalogue.
After a few years, our sofa had a pretty good idea of where our favourite spot was. The seat had sagged permanently – even though our own weight hadn’t increased by any significant amount.
What we can observe here is a Compression set. It describes the proportion of a previously induced deformation that remains permanently after the load has been removed and a defined recovery period has elapsed.
The three heights of our sofa
And that is precisely why we need three height measurements:
- which original height H0: the seat height of the new sofa, which is stated as 45 cm in the catalogue,
- which Height under load H1: the seat height, which is still 35 cm when we are sitting on it,
- which Height H2 after relaxation: the seat height, which is still 42 cm after the load has been removed and a sufficient recovery period has elapsed.
So, when the sofa was in use, the seat was compressed by 10 cm. After getting up, 3 cm of this compression remained permanently.
The residual compression deformation can be calculated as follows:
(H0–H2)/(H0–H1) × 100
In this example, our sofa has a Compression set of 30 %.
The higher this value, the greater the proportion of the original deformation that remains after the load is removed. But bigger isn’t always better – that’s exactly the point!
The technical background
If an injection-moulded seal is made from a thermoplastic elastomer If subjected to stress over a prolonged period, it may, depending on the material, temperature and load, to become increasingly permanently deformed. Some of the deformation reverts to its original state once the load is removed, whilst the rest remains.
What the figure indicates
The compression set is an important parameter, particularly for soft, elastic materials. The lower the value, the better the material’s resilience.
The magnitude of the compression set depends, amongst other things, on Material, temperature, duration of loading, degree of compression and recovery time . A percentage figure must therefore always be considered in conjunction with the underlying test conditions.
Our sofa merely illustrates the principle – it is, of course, not a test carried out in accordance with the relevant standards.
For a technical plastic component, however, such a degree of permanent deformation can be crucial: if a component does not sufficiently regain its original shape after prolonged loading, its function may also be impaired.