GBX Update October 22 2020

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

Academic Paper for 3D Printing with rPET Water Bottle Flake

As a result of all our experiments with 3D printing with rPET water bottle flake, re:3D published an academic paper in collaboration with Michigan Tech University (MTU). The paper, titled Towards Distributed Recycling with Additive Manufacturing of PET Flake Feedstocks, can be found for free here: https://www.mdpi.com/1996-1944/13/19/4273

Crammer Model Files Available on OSF

The turning point for printing with rPET water bottle flake was the development of our Crammer, which is a 3D printed motorized auger screw that compacts and conveys irregularly-sized flake into the Gigabot X extruder. More information about the Crammer can be found in previous forum posts (August and July) and in our paper. We have also published the Crammer files on the Open Source site OSF, where they can be downloaded for free for those who want to experiment with the Crammer.

Feed Throat Updates with Vibration Motor

In addition to the Crammer, we’ve been experimenting with new feed throats to improve feeding into the GBX extruder, especially for printing materials with irregularly-sized particles, such as regrind or flake. One of our experimental feed throats has an attachment for a vibration motor that runs throughout a print and helps material flow through the feed throat. This is especially important for longer and larger prints, where you need reliable material flow throughout the print. Initial results with the vibration motor are promising, but we’ve run into issues with the motor overheating partway through the print and breaking. This has led to the most current iteration below, which holds the motor securely while still allowing airflow around it to prevent overheating.

Purge Testing

As part of re:3D’s various R&D efforts with GBX, we feed a huge variety of materials into our GBX printers: materials with a range of compositions, melting points, viscosities, and other properties. Basically, we treat it as an all-in-one machine. Because of this, we’ve encountered some issues when we change the material loaded in the extruder:

• Huge differences in printing temperatures between the material in the extruder and the material that’s replacing it can make it difficult to purge. For example, when trying to switch from Material A with a print temperature of 250C to Material B with a print temperature of 130C or vice versa, the extruder must be hot enough for the Material A to flow, which means that Material B is subject to temperatures far above its print temperature. This can result in degradation or offgassing in Material B. Also, the extra heat often makes the viscosity of Material B very low, making it unable to push the higher viscosity Material A out of the extruder
• Even if materials have similar operating temperatures and viscosities, contaminants can build up on the inside surfaces of the extruder over time. This can cause issues ranging from weakening final printed parts to causing failed prints due to nozzles clogged with contaminants.

In the injection molding industry, material changes and contaminants are addressed by using purge materials, which are specially formulated to clear plastic from machinery, gently scrub contaminants from surfaces, and/or be easily removed for cleaning when an extrusion screw is pulled from an extruder.

We’ve been testing a few purge materials with GBX to mitigate our struggles with material changing. One complication is that injection molding equipment usually switches between similar plastics that may only be different in color, and purge compounds are often formulated to operate in a specific temperature range. Then injection molding machine operators purchase the purge compounds with the operating temperature that matches their specific plastic. In contrast, GBX extrudes a huge range of plastics, and a purge compound with a limited operating temperature won’t help with changing between plastics with 100 plus degree differences in print temperatures.

We’ve had a lot of initial success with Dyna-Purge’s F2 purge material, which has a reported operating temperature of 160C to 329C. So far I have tested it from 160C to 225C, and it maintained a good extrusion viscosity throughout the temperature range. I used the Dyna-Purge F2 to purge TPU (printing temperature 135C) from the GBX extruder at 160C without visible degradation or offgassing from the TPU like I’ve seen during previous purge attempts. 

Once the TPU was purged, I heated the extruder up to 225C and switched to ASA Clean’s NF purge material, which is glass-filled and designed for scrubbing off carbonized deposits in extruders. When switching from the Dyna-Purge F2 to the ASA Clean NF, the F2 maintained a good viscosity the entire time, and transitioned smoothly to the ASA Clean NF.

I knew this particular GBX had black contaminants in it, so I ran a bunch of the ASA Clean NF through the extruder in an effort to scrub it out. I ended up with extrusions that varied in color, which is presumably the contaminants mixing with the lightly-colored ASA Clean NF.

The next step is to confirm that the purge worked is to resume normal printing and check that the contaminants are gone!

In conclusion, the Dyna-Purge F2 purge material is extremely promising for “bridging” between materials with vastly different print temperatures, and it also allows me to use intense purge materials like the glass-filled ASA Clean NF even for materials well outside of its operating range. This combination technique has tons of applications, and I’m excited to continue testing it.