Diffusion - or why a plastic bucket full of water doesn't just get bigger because of water pressure Merkle-CAE-Solutions-GmbH-getriebeölung-3.0
This process is called diffusion. Diffusion also takes place in solids, for example, the water molecules penetrate our plastic bucket and the plastic swells until it is saturated. It is saturated when it no longer likes any molecules.
I know this from eating schnitzel, a certain amount goes, but then it's good.
When a plastic component falls out of the injection molding machine, the plastic is still hungry for water. In this dry state, the material is more brittle, stiffer and stronger. If the plastic component now lies around for a while, it absorbs water from the environment, it swells. Just like we do when we eat too much shredded meat.
But what happens beyond that: the E-modulus of the plastic becomes smaller and the strength values decrease. And stresses are created!
Nature does not like concentration differences in a liquid or a gas. Since the atoms all move at temperatures above absolute zero, the concentration differences are balanced out without external intervention until everything is nicely evenly distributed.
These aspects should therefore be taken into account when designing plastic components, or at least they should be known.
In addition to mechanical stresses, plastic components are also exposed to environmental influences. These environmental influences can be caused by chemical interaction with the media in contact (fuel in the tank), by radiation (e.g. UV radiation), as well as temperature. The advantage of plastics is that they can be designed by additives according to the stress.
We can therefore determine how many shreds will fit in.
But how can such a thing be calculated?
The universe is sometimes complex and sometimes simple. Diffusion behaves like heat. The driving factor is now the concentration instead of the temperature, the heat capacity corresponds to the saturation value and the thermal conductivity to the diffusion coefficient.
And already we can calculate what our bucket does even with a standard FE solver. The necessary values can be found in the literature. It is nice when a test shows that we have calculated correctly.
If you want to know it technically more exactly, you are welcome to contact my Dr. Brehm.
An already planned lecture on this topic was unfortunately cancelled by the organizer due to Corona. He will therefore be happy to pass on his knowledge to you.
You can also find more on the subject of plastics here.
I am only responsible for the rough stuff here.
Your Stefan Merkle
