This is the second in our series “The Science of…” where we ask Polymer Technology lecturer Colin Hindle questions related to 3D printing. Colin has over thirty years experience in the plastics and polymer industries and has been wonderful to work with as we develop new materials together.
So what about warping, particularly with polymers? Why does the material warp when it is being printed, as opposed to sticking to the bed?
It might stick to the bed but of course it then cools and shrinks. The thermal expansion and hence contraction of polymers is very high. It’s about 10 times higher than metal for example, which is nearly 10 times as high itself as a ceramic. So you’ve got a big differential in expansion and contraction between the glass bed that you are making the model on and the model material itself. The differential shrinkage tends to lead to warping – when plastic products are fairly thin walled, they warp fairly easily.
Would you say that if you reduce the differential between the nozzle temperature and the TG temperature, that would reduce warping and shrinking (or not)?
When you say reduce TG temperature do you mean the bed temperature?
Well, you keep the bed temperature at whatever it needs to be, but maybe you choose a material or design a material where the nozzle temperature at printing is close to the TG. That way the differential between these two figures is small – would that help with shrinkage?
Well it should reduce the shrinkage but might cause you other problems, I don’t think it would flow quite so well.
So, why is there a big range of temperatures amongst plastics? Why are they not all the same, like water boils at 100 degrees, for example?
Well, water boils at 100 degrees but it doesn’t when you put salt in it, it boils at over 100, plastics are full of lots of things. Not least of which the molecules themselves are not all the same length, so the longer molecules have a higher melting point than the shorter molecules. And polymer melting points are not single points, they are quite broad anyway. It could be over a 10 or more degree range that it actually melts. If you take something like water, or metal, or whatever it melts at a very sharp temperature, one degree below and it’s solid, one degree above it and it’s liquid. Polymers have broader melting points and they are not all the same, because obviously the structures of polymers are not all the same.
Why do you have to adjust your setting in a printer?
Well, to get the optimum performance you need to adjust the settings to get the correct temperature of the bed and the heated nozzle. Again because these materials have different softening temperatures and different solidification temperatures for example of crystallisation, then you’ve got to optimise that. It’s normal practice for any material really to optimise temperature, you don’t cook pizza at the same temperature as you bake a cake.
Thanks again Colin. Keep an eye out for the next in the series!