TPU is a capricious material that comes in different shore hardnesses.
- low shore hardness = greater softness and fluidity = difficult to control the flow and manipulate the product
- high shore hardness = faster crystallization = sensitivity to wrong temperatures
The shore hardness 75A-98A window seems to give the best results.
Our blogpost puts this topic in perspective and shows nice results.
1. Extrusion problems
The main extrusion issues are :
- If the temperatures are excessive, TPU can be very liquid and full of bubbles
- If the temperatures are insufficient (especially the nozzle temperature controlled partly by H1), TPU can crystallize very fast and start clogging the nozzle, or building up at its tip.
The challenge is that the window between these two issues is often extremely small (typically 5°C). Temperatures have to be finetuned extremely precisely.
2. Before extruding
We highly recommend you have a look at this guide first.
3. Experimental starting point
It is wiser to start at slightly excessive temperatures.
For TPU, this normally means 210°C (all heaters).
5RPM is always a good average screw speed.
The fancooling percentage is hard to predict, for it depends on the room temperature. Excessive fancooling will freeze the TPU at the tip of the nozzle or even clog it fully, which is why it is wiser to set a low percentage first. For example, you can set 10%, see how it goes for 10min, and then increase to 15, 20,…. If you see that the material freezes at the tip of the nozzle, it means that the nozzle is too cold.
4. Experimental next steps
Once TPU starts flowing, you will probably need to lower the temperatures.
It is important to make very small adjustments (typically 3°C), and wait 30min after each adjustment before changing again.
After each T adjustment, you might need to tweak the fancooling as well.
If TPU starts freezing at the tip of the nozzle, it means that the fancooling is excessive and/or that the temperature (especially H1) is too low.
Usually, once this phenomenon start, it can be very hard to continue testing. You might have to purge the machine with Devoclean MidTemp EZ and start the test again with new parameters.