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NSF Phase II: Recycled Polypropylene

Background

For the past couple weeks I've been running tests to print recycled polypropylene on GBX. Spoiler alert: I had some success!

Polypropylene was sourced from the healthcare company Roche, which sent us test tube holders that we granulated at the re:3D Houston office with our onsite SHINI granulator.

I conducted initial testing a few months ago and didn't make much progress because every test print suffered from two main problems: bed adhesion and warping. I tried heating the bed to 60C and using polypropylene tape to improve bed adhesion, but the warping was so severe that it peeled the tape up from the bed.

 

Testing with Calibration Cylinders

This time, I tested multiple methods to tackle bed adhesion. For a test print, I used a calibration cylinder with no top or bottom and 1 perimeter printed in vase mode. The following methods did not improve adhesion, and caused print fails due to the print unsticking from the bed and moving in the middle of printing:

  • Printing on a polypropylene cutting board clamped on top of the 3D printer bed.
  • Sanding the polypropylene cutting board
  • Spraying 3DLAC on the polypropylene cutting board

The board warped heavily when the bed beneath was heated (temperatures from 60C-100C were tested). To combat the warping, I affixed the board to the print bed with three clamps, which heavily limited where the extruder could travel. Even when dialing in the z gap distance and reducing the size of the test print, adhesion wasn't improved.

I moved away from using the board and began testing with the regular PrintnZ bed. I applied Magigoo's bed adhesion solution for polypropylene to the PrintnZ. and printed more calibration cylinders starting with the bed temperature at 60C and incrementing up to 80C, 90C, and 100C. Poor bed adhesion caused print fails at all bed temperatures until 100C, at which point I printed 3 calibration cylinders without any bed adhesion failures. Therefore, Magigoo's PP product appears to need a bed temperature of at least 100C.

 

NIST Artifacts

After succeeding with calibration cylinders, I moved on to printing NIST test artifacts. By inspecting the first layer, I optimized the extrusion rate to E300 steps/mm (set by the M92 gcode command in the scripts tab in Simplify3D). However, warping caused sections of the first layer to peel off from the bed and caused eventual print failures when the extruder ran into the raised sections

 

Address Warping with an Enclosure

To help with the warping, I put an enclosure on the GBX. Unfortunately we ran out of certain parts and I couldn't complete it, so I used shipping blankets to drape over the uncovered portions of the enclosure frame. I then placed a thermometer about 4" above the bed to measure the internal temperature of the enclosure. With the extruders heated to polypropylene temperatures (225/220/175C) and the bed set to 100C, the temperature inside the enclosure reached a maximum of 115F or 46C, right at the lower bound of recommended enclosure temperature range of 45 - 60C for printing polypropylene.

With the enclosure, I was able to achieve a first layer that completely adhered to the bed without warping up in the corners. However, warping always occurred on the second layer and overcame the bed adhesion, pulling the part up from the bed. Increasing the bed temperature to 110C did not prevent the extreme warping on the second layer.

 

Next Steps

This testing established:

  • Printing temperatures
  • Extrusion rate
  • Recycled polypropylene granulate extrudes consistently
  • Using Magigoo on PrintnZ at 100C is a viable solution for bed adhesion

I've decided to hold off on further polypropylene testing until we develop more temperature control features for GBX. As part of a supplement to our NSF grant for GBX, we are designing a high temperature chamber that can reach much high temperatures than the enclosure, as well as have temperature control. We are also developing a part heating/cooling fan system that can deliver air of a specific temperature directly on the printed part during printing. Both of these features should improve warping.

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