GBX Update April 27 2021

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


Hopper Gantry Design for Terabot X

In my past two posts I described the hopper gantry design for GBX in the Regular and XLT sizes. This past month, I adapted those designs for a Terabot X sized hopper gantry. The CAD and drawings have been made, and it will be assembled in the next month.

Some notable differences in the Terabot X hopper gantry as compared to the Reg and XLT versions:

  • The X gantry is 1.5x wider to provide more rigidity since it is about 1.5x longer than the Reg and XLT GBX hopper gantries
  • The channels are taller, which allow for taller bellow pleats and maximize hopper movement range

Bellow Calculator

Along with the hopper gantries, I’ve been doing a lot of work around bellows. We’re moving toward using hopper gantries for all build volumes of GBX (Regular, XLT, Terabot, and more), and any printers with an enclosure will also have bellows installed in the hopper gantry system to close off the top of the printer. Calculating bellow dimensions became extremely complicated with the different printer sizes, especially once I began exploring using different channel sizes to optimize the hopper movement range. So… I made a spreadsheet calculator!

The tricky thing about bellows is that as the extension length increases, so does the collapsed length, or in other words, the minimum amount of space the bellows take up. This affects the hopper’s movement range: if there are bellows on each side of the hopper in the X and Y directions, then the hopper can only move in the X and Y directions up until the bellows are fully collapsed.

Since we have bellows on both sides of the hopper in both the X and Y directions, it also creates an interesting optimization problem. If collapsed bellows on one side of the hopper take up D length of space, then the bellows on the other side of the hopper don’t need to extend over that extra D length.

Another consideration is bellow pleat height. Bellows with taller pleats can extend the same distance with fewer pleats, which then translates to a smaller collapsed length. See the formulas below:


Extension length = Pleat Height * Number of Pleats * Constant A

Collapsed Length = Number of Pleats * Constant B



Constant A = ratio of extension length to pleat height

Constant B = the collapsed length of 1 pleat


Bellow pleat height is determined by channel dimensions… so I chose a range of channels types, plugged their dimensions into my calculator, and determined the resulting optimized bellow dimensions, and how they affected the hoppers movement range in the X and Y directions.

In conclusion, I was able to choose channels of 1.75” x 1” for Terabot X’s hopper gantry design. This spreadsheet would also be useful for future build volumes, and calculating the necessary bellow dimensions to provide to our bellows supplier.

rPET Water Bottle Flake Material Testing

Before this past month, the last time I had worked with rPET water bottle flake was the summer of 2020 (see this forum post). For context, as part of re:3D’s NSF Phase II grant, we’re figuring out how to 3D print straight from rPET water bottle flake. Last summer I developed the Active Feeding System (formally known as the Crammer), which is a feed throat with a motorized conveyor screw that conveys material into the extruder. Using this system enabled me to print tensile bars, but there was clearly inconsistent extrusion and a very low extrusion rate

Since then, the inlet hole into the extruder has been widened, improving flow on a variety of materials. In this past month, I updated the feed throat to interface with the new inlet geometry and retested the rPET water bottle flake.

The extruder inlet update improved extrusion consistency and extrusion rate, with the extrusion rate more than doubling since last summer. The results were the Moai trials 1 through 6 in the picture below (top left to bottom right). The mottled appearance is a result of variance in cooling and crystallization across the part. Trial 1 showed a need for part cooling, so part cooling was introduced for subsequent test prints by placing a fan in front of the printer since our part cooling solution is still in progress at MTU. However, even with the external fan setup and the inconsistent cooling, the part cooling greatly improved the print quality of the Moai.

The Moai serve as a proof of concept for printed small parts with a 0.8mm nozzle. The next step is to attempt larger prints to verify the print reliability over a longer period of time, since the Moai shown only took about 10 minutes each to print.

rHDPE Material Testing

This past month, I put our lovely intern David through the ropes and had him do some material testing! His first material was an rHDPE regrind produced from granulating hoses from Home Depot. His material testing results are below:

Initial free extrusion testing with the rHDPE proved to be a success with a max extrusion speed of 4.5 mm/s and GBX extruder temperatures of bottom, middle, and top heater blocks set to 180°C, 170°C, and 150°C.

Although the free extrusion testing results showed some promise, the next phase of testing (Calibration Cylinder Print) gave me some issues regarding bed adhesion and extrusion rate. Using the original extrusion temperatures established in the free extrusion printing phase and the default extrusion rate of 55 steps/mm, there were immediate signs of under extrusion and the first layer did not adhere to the bed at all. After running various test trials that included altering extrusion temperatures, bed temperature, extrusion rate, and testing a variety of 3D printing adhesives, there was slight improvement with bed adhesion, but not enough to result in a viable first layer. Ultimately, bed adhesion was noticed only briefly when using Magigoo for Polypropylene (PP), a 3D printing adhesive, and extrusion was inconsistent during printing which did not help with the bed adhesion issue. 

With some additional testing performed by Helen, the rHDPE material was able to adhere to the bed for most of the first layer, but material shrinkage was observed which eventually led to less bed adhesion. 

A viable first layer of the Calibration Cylinder was not obtained with testing of the rHDPE material, however the following parameters, obtained from various testing trials performed by Helen and I, prove to have the best results.


GBX Prints

Jody has designed and printed some more cool objects on GBX:

Chair frame printed with 1.75mm nozzle and rectilinear infill - using reclaimed wood flooring for the seat and back and remnant scraps for the legs. Experimenting with different leg lengths and angles for ergonomics.


Hexagonal succulent planter printed in vase mode with a wood insert:


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