[R] Attempts with PET Pellets

A project log for Filament Furnace [gd0042]

A low cost filament extruder to try and create low cost GF PBT filament to replace my PLA usage.

kelvinakelvinA 07/10/2022 at 23:440 Comments

I probably should've dried it but I didn't so that could be a reason to retry the experiment. Long story short, it ended in failure.

These are the pellets I have:

These are medical grade apparently, but I can only imagine that this specific formulation is for something that needs low viscosity because that's exactly what this PET is when heated. There's no middle ground either. After a few hours, I was able to hone it down to probably 248C, with 245/6C at the nozzle eventually leading to it coming out "frozen fresh out of the nozzle" (I assume it crystalized) and 249C being a hot liquidy goop of uncoolable plastic. When molten, the PET doesn't have enough strength to hold up more than about 20cm of it's own weight.

I started at 269C as recommended by 3Devo but that was wayyy too hot so I went down to 239C and nothing extruded. The pellets I have don't have a datasheet, so I looked around the internet and most that did melted in the 245-250C range with only 2 being higher. 259/259/259/249 was near liquid so I tried 245 at the nozzle and eventually nothing would come out; the crystalized PET would slowly close the nozzle. My last test was at 252/252/252/249 and I feel like I'm right on the threshold between crystalisation before or after the nozzle, with the extrusion being closer to the latter. 

The PET is pretty flexible when amorphous while being transparent, and a nice white when semicrystaline. I want the latter due to the higher melt temperature and other bonuses, as I found out in this paper:

I can't exactly read the graph, but it looks like I was right on with my experimentation that PET melts at 248 degrees C.
When I was extruding, I didn't think water had much effect, but the paper claims otherwise: