GBX Update Sept 21 2020

Hi everyone! Here are the GBX R&D updates from the past month:


rPET Water Bottle Flake

Last month I discussed how I was able to extrude rPET water bottle flake and print tensile bars using the Crammer. Before the Crammer, I couldn’t achieve enough flow into the extruder and subsequent extrusion to get a print at all, so using the Crammer with rPET flake is a huge improvement. However, there is still a way to go to achieve reliable printing from rPET water bottle flake. After optimizing print settings such as extrusion rate, speed, and temperatures, there was still inconsistent extrusion across prints with the same settings.

I suspect this is in part due to inconsistency in the amount of rPET flake packing into the Crammer and extruder at any one time. This results in both under- and over-extrusion in the same print.

One possible next step to achieve more consistent extrusion is to optimize the Crammer geometry for more consistent flow. We are also working on a larger extruder with a 0.75” compression screw diameter that should be able to accept more flake at any one time.

The impact of the inconsistent extrusion is macro voids in the tensile bars where there was under-extrusion. This resulted in a much lower than expected Ultimate Tensile Strength (UTS) for the tensile bars. Another possible reason for a low UTS may be crystallization-- I didn’t flash cool any of the prints, which could affect the amount of crystallinity in the prints. Future work can experiment with post-processing temperature procedures and their effect on UTS.

re:3D has been writing a paper in collaboration with MTU with this experiment, and the paper has been submitted for publication.


Feed Throat Rev10

We’ve had intermittent but recurring issues with pellet flow through the feed throat that connects the feed tube to the extruder, resulting in failed prints as pellets stop flowing into the extruder and eventually stop extruding. There have been many feed throat designs in the past:

  • Rev8: Introduces a “trap door” to simplify the material change process
  • Rev9: Keeps the internal geometry as large as possible before reducing right at the extruder inlet. This ended up being a worse design than rev8. rPC flake would consistently have issues with rev9, and upon switching back to rev8, it no longer had any problems
  • Rev10: Based on rev8, with minor improvements drawn from Rev9. It was also brought closer to the extruder so the pellets flow through a more vertical path from the feed tube to the extruder. Although this version was an improvement, pellets would still get intermittently stuck in the feed throat.
  • Rev10 without the trapdoor: The trapdoor was removed to completely smooth out the internal geometry of the rev10 design and eliminate it as a variable. However, This version still had flow issues

The next troubleshooting test involves a version of rev10 without the trapdoor printed from natural PETG so it’s ~clear~. I’m printing a test print that has consistently failed with the feed throat so I can shine a light through the feed throat and see exactly where the pellets are stuck in the rev10 geometry. 


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