NSF Phase II: NIST Artifacts

This week on GBX testing...

To assess print quality from the pellet extruder, NIST test artifacts were printed on Gigabot X and compared to NIST artifacts printed on Gigabot 3, a traditional FFF printer. NIST artifacts are a standardized 3D model with many features that test printer resolution, bridging, and stringing.

Figure 1: PLA NIST Test Artifacts. (A) Top view of Gigabot X artifact, (B) side view of Gigabot X artifact with stringing circled, (C) top view of Gigabot 3 (FFF) artifact, and (D) side view of Gigabot 3 (FFF) artifact. Stringing is circled in red.

Slicing settings for Gigabot X prints were chosen to match the FFF slice settings as closely as possible. One notable difference is the nozzle size: Gigabot X has a 0.8 mm nozzle orifice, and Gigabot 3 (FFF) has a 0.4 mm nozzle. Although the layer heights were the same (0.3175 mm), the different nozzle sizes resulted in a large difference in layer width: 0.96 mm for the Gigabot X and 0.48 mm for the FFF NIST Artifact. However, the larger layer width and the resulting lower resolution did not have a significant effect on Gigabot X’s ability to print the NIST Artifact’s small features. It was able to achieve the same number of small pegs and holes as the FFF NIST Artifact.

The biggest difference is the amount of stringing in the Gigabot X artifact (Figure 1-B). Lowering extrusion temperatures can decrease stringing, but the temperatures have already been optimized, and decreasing them further results in inconsistent extrusion. Besides temperature, the stringing may be caused by these factors:

  • Retraction: FFF printers can prevent stringing with retraction, which involves reversing the extruder motor direction to pull filament up from the nozzle and higher into the hot end. Reversing the motor in the pellet extruder will turn the compression screw in the opposite direction, which may not actually result in a retraction of material. Additional testing with various materials is needed to confirm the retraction capabilities of Gigabot X.
  • Empty extrusion barrel space: The current version of Gigabot X has about a ½” tall empty space inside the extrusion barrel between the bottom of the compression screw and the nozzle. Melted plastic accumulates here during printing before it extrudes out the nozzle, increasing oozing and stringing. To eliminate that space, updated extrusion barrels have been designed and ordered for manufacture.

Next Steps:

The newer extrusion barrels have arrived, and I've been working on putting one on the current inhouse GBX. Hopefully I can rerun some of my test prints with the new barrel next week.

Next Week's Post:

How consistent is the extrusion rate of the pellet extruder? I ran some tests with PLA regrind to find out.


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