Transition material after Devoclean
We are trying to extrude a pharmaceutical polymer (copovidone) which is generally extruded at 100-120°C and starts degrading from 280°C.
So after devoclean, I put directly our polymer and started lowering the temperature while the remaining devoclean was extruded.
The problem is that our polymer melted then resolidifed in the zone under the hopper blocking further extrusion.
So, we had to push it with devoclean. And back to the departure case.
So now, i don't know if i have to put PLA before putting our polymer? Will I have a better control of the excessive melting of the polymer?
Thanking you in advance.
At which temperature did you make the transition from Devoclean to the copovidone?
I would always recommend though using another transitioning material when switching from Devoclean to a lower temperature material.
This is because Devoclean has quite a high viscosity and additive content, which generally makes it difficult to completely remove from the machine with a lower temperature material.
Usually we use HDPE for this, to purge out all Devoclean around 220 degrees C.
HDPE should look completely transparent in molten state (upon cooling/crystallizing it becomes opaque), so that makes it easy to see if the machine is clean or not. The HDPE we supplied with the machine has a minimum processing temperature of 160 degrees, so you could immediately switch to your copovidone at these lower temperatures.
Of course the same applies to PLA (but the transition from Devoclean to PLA takes quite long, then you would have to use HDPE in between).
- Something you could also do is decreasing the temperature of H4 significantly during the transition of the materials. This will prevent the feed zone from becoming too hot. It does not matter if H4 is set too low for the material to melt.
- You could also temporarily increase the RPM so the residence time is shorter.
- You could point an extra cooling fan at the feeding section of the barrel. For this you would have to unscrew the back panel to gain access here.
I hope this information helps, if not, please do not hesitate to contact us again! We would be glad to help you achieve good results.
I used as recommended HDPE. It was easier like this to do the transition from HDPE to a low melting material.
However, when switching from a granulate product (HDPE) to a powder it seems tricky.
The extrusion slows down or stops completely. It seems like the powder can't push the granules off the extruder. Am I right? Do you have any experience with switching from granules to powder?
Thanking you in advance.
Generally speaking, HDPE should be easier to remove than Devoclean, which is why I am surprised to see that the flow deacreases or stops.
At what temperature are you performing the transition ?
Preferably, it needs to be done at 180deg C at least, to ensure good HDPE melting.
Do you think that ratholing is happening? See picture attached.
Whether it is ratholing, or bad feeding in general (which is quite common with powders), this could be the cause of weak flow.
Another method could be to perform a quick transition from HDPE to PLA at 15RPM (this is quite fast), and then put your material after PLA.
Overall, I have never seen a case where powder could not push pellets.
Of course, if the barrel is partially clogged, or if the settings are not appropriate at all, then the material does not feed in ; but that does not apply to powders only.
If you feel like I am not understanding your issue, or if I am not being helpful, we can setup a call and really go through this.
Please try to experiment a bit more to see what is causing the disappearing flow : settings or feeding.
Looking forward to hearing from you,