3D Printer Quality Evaluation Methods

About: Engineer, designer, and artist who develops and applies novel 3D printing technologies.

Traditionally, desktop 3D printer manufacturers and users have discussed print quality and precision in terms of layer height. While layer height does play a part in perceived print quality, it is just one of several measurable contributors to overall functional (dimensional) and perceptual (surface) quality. Print quality is a combination of visual perceptions and functional characteristics, such as dimensional accuracy, surface finish, overhang capabilities, deposition control, motion mechanics, motion control, material properties, and slicing algorithms. Many of these factors are inter-related and adjusting one often affects others, making it somewhat difficult to identify each’s contribution to overall perceptions of print quality. However, it is possible to create test geometries that probe specific components of print quality and to individually evaluate those geometries while holding all other variables constant. This allows for a more parametric and quantitative assessment of print quality than could be achieved by comparing any number of more traditional printed models.

Presented here is a methodology for systematically evaluating specific performance parameters in a controlled and quantifiable manner. Make Magazine's third annual 3D Printing Shootout was conducted using these files to benchmark performance of desktop 3D printers. You can read all 26 machine reviews in Make:'s Annual Guide to 3D Printing 2015 (Volume 42) and you can download a 20 page summary of the top ten machines here.

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Step 1: What Is Being Evaluated?

These test geometries can be used to evaluate the following:

  • Changes to materials, software, electronics, hardware, or slicing parameters on the same machine
  • Performance differences between machines, holding material, software, and slicing parameters constant

At the 2015 Make: Magazine 3D printer shoot out, these test geometries and this evaluation methodology were used to compare two dozen 3D printers using manufacturer-recommended machine settings while holding material constant. The testing team used Ultimachine orange PLA for all PLA machines (the majority).

Step 2: Dimensional Accuracy

Dimensional Accuracy: Measure the second from the bottom tier of the print (with a target diameter of 20mm) across the X and Y directions, following the guide on the bottom of the print. Differences between X and Y measurements indicate the magnitude of backlash present in the system. Take the worst (largest deviation) of the X and Y values and use if for the following scoring rubric.

  • Assign the print a "1" if deviations in X or Y are greater than 0.4mm.
  • Assign the print a "2" if deviations in X or Y are between 0.4 and 0.3mm.
  • Assign the print a "3" if deviations in X or Y are between 0.2 and 0.3mm.
  • Assign the print a "4" if deviations in X or Y are between 0.1 and 0.2mm.
  • Assign the print a "5" if deviations in X or Y are between 0 and 0.1mm.

Additional Information Generated:

The final two tiers of the print, with target diameters of 5mm and 2.5mm, serve as a basic fine positive space feature test on printers with heat extraction and mass deposition problems. Additionally, the top surfaces of each tier can reveal problems in flow calibration (under- or over-extrusion). The most useful defect that this geometry exposes is backlash, which is common in the Y-axis of Replicator 2s and many machines that use a bracket-mounted Y-axis drive motor, coupled to the gantry by a short timing belt. Backlash can be caused by loose timing pulleys, poor quality belts or pulleys, and loose belts. Mechanical changes to the gantry can be evaluated for lash using this geometry.

Notes:

The other tiers of the print can be measured to increase sample size.

Step 3: Bridging Performance

Bridging Performance: inspect the five bridges for dropped perimeters and infill.

  • Assign a "1" if any bridge has dropped infill.
  • Assign a "2"if only the longest two bridges have dropped infill.
  • Assign a "3" if none of the bridges have dropped infill, but all have dropped perimeters.
  • Assign a "4" if the shortest two bridges compiled without any dropped perimeters.
  • Assign a "5" if all bridges compiled without any dropped perimeters (drooping of less that 2mm is acceptable).

Additional Information Generated:

Often this test can reveal whether the slicer has a module to handle bridge features. Slicers with such a module use different speeds toolpaths when spanning a bridge. Slicers that do no have such a module will often apply the usual "perimeters plus infill" paths to a bridge, which usually results in infill hemorrhaging out of the bridge.

Notes:

Print orientation plays a big role in bridging performance with some machines, particularly when there is an active cooling fan. Experiment with different orientations to find the optimal orientations for future parts with features requiring bridging.

Step 4: Overhang Performance

Overhang Performance: Inspect the 30, 45, 60, and 70 degree overhangs, looking for drooping perimeters, wobbling extrusions, and infill hemorrhaging.

  • Assign a "1" if the printer did not compile any of the individual overhangs.
  • Assign a "2" if the printer compiled the geometry but dropped loops and infill on the 60 and 70 degree overhangs.
  • Assign a "3" if the printer only dropped loops on the 70 degree overhang.
  • Assign a "4" if the printer didn't drop any perimeters and the surface of the 60 and 70 degree is only slightly different from the surface of the 30 and 45 degree overhangs.
  • Assign a "5" if there is little distinguishable difference in surface structure between the four overhang angles.

Additional Information Generated:

--

Notes:

Print orientation can affect overhang performance, especially when there is an active cooling fan mounted on the extruder. Experiment with different orientations to find the best options for

Step 5: Negative Space Tolerances Performances

Negative Space Tolerances: Remove the captive pins by hand without using tools.

  • Assign the print a "1" if no pins can be removed.
  • Assign the print a "2" if the 0.6mm radial tolerance pin can be removed.
  • Assign the print a "3" if the 0.6mm and 0.5mm radial tolerance pins can be removed.
  • Assign the print a "4" if the 0.6, 0.5, and 0.4mm radial tolerance pins can be removed.
  • Assign the print a "5" if all pins can be removed.

Additional Information Generated:

--

Notes:

If the machine being tested exhibits backlash, this test often is affected.

Step 6: Fine Positive Space Features (Retraction Performance)

Fine Positive Space Features Performance: Evaluate based on the quality of deposition composing the spires:

  • Assign the print a "1" if the spires did not compile due to extruder jam/lack of material flow.
  • Assign the print a "2" if the spires compiled but are often connected by strands of material.
  • Assign the print a "3" if the spires compiled and there are only a handful of connecting strands but the main deviation from target geometry is due to mass flow issues (under- or over-extrusion).
  • Assign the print a "4" if the spires compiled, there are no connecting strands, but there are mass flow issues.
  • Assign the print a "5" if the spires compiled and there are no connecting strands and no stepping or ridging due to volume low issues.

Additional Information Generated:

This test exposes over-extrusion in both the top surface of the print base and in the fabrication of the spires.

Notes:

This geometry is greatly impacted by retraction settings and can be used to tune them.

Step 7: XY Resonance

XY Resonance: This test evaluates both resonance in the XY gantry, deposition control during linear extrusions, and deposition control at layer changes. As resonance is difficult to measure quantitatively, this is a binary test. If there is any rippling at the corners or at the midpoint of the print wall with the inset, assign the print a "fail" value of "0". If there is no rippling, assign the print a "pass" value of "2.5". While expressly designed for evaluating resonance, the print can also be used to evaluate deposition control in a more qualitative manner.

Additional Information Generated:

Loose bearings in the Z stage or other forms of incomplete constraint of the Z stage may manifest as a ripple that has the same period as the leadscrew's thread pitch. This print will also expose many problems with proper deposition rate and retraction settings.

Notes:

Be careful to only evaluate the print based on the presence or lack of resonance effects near the corners. Disregard other print errors such as blobbing, gaps, layer registration, or seaming.

Step 8: Z Resonance

Z Resonance: This test exposes resonance in the Z axis if present and is subject to a binary evaluation. When illuminated from above (see image), if there is a noticeable loss of layer registration in the top half of the print, manifesting as horizontal ridging, assign the print a "fail" value of "0". If there is no loss of layer registration with increasing Z height, assign the print a "pass" value of "2.5". In addition to evaluating Z resonance and layer registration, this print can expose misalignment in the Z-axis if there are consistent ridges of the same pitch as the leadscrew.

Additional Information Generated:

Loose bearings in the Z stage or other forms of incomplete constraint often manifest as a ripple that has the same period as the leadscrew's thread pitch.

Notes:

While we printed this in the middle of the print bed at the 2015 Shoot Out, you can print at the location furthest from you printer's lead screw (assuming a leadscrew-driven Z-stage) to amplify any problems. Be careful to only evaluate based on differences in layer registration between the top and bottom half of the print and to ignore seaming or deposition control issues as these are not part of this evaluation.

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