Stubborn PLA bridge artefact
I am working on learning to create my own print profiles. This bridge test shows a consistent artefact that I don't seem to be able to get rid of, any ideas on what to try next is appreciated. The printer is a fairly standard Core One.
This test piece is intended to be harder that the others I have found in that it is just one single layer bridge of 0.2mm thickness.
Settings
Filament: Prusament PLA signal white, dried.
Nozzle: 0.4mm brass standard flow
Temperature: Tried 225-230. 230 seems a fraction better but not significant
Thick bridges: Seems like a total failure, keep off
Bridge speed: 18-27 mms/s, no speed completely remove artefacts.
Bridge flow ratio: 1.75-1.85, tried a lot of combinations of speed and lower/higher bridge flow ratio but higher values lead to more artefacts while lower lead to under extruded bridges not bonding.
Once tuned to above the test models all generate the same type of artefact just at the first strands of the bridge bulging. This artefact prints the same regardless of bridge shape, after the artefact the print settles to a good solid bridge. There are signs of under extrusion in these bulges but these disappear as the bridge becomes more stable later in the print.
The first things tried are bridge speed and bridge flow ratio. A higher bridge flow ratio does not remove the bulge, going higher than 1.9 the bulge increases. Bridge speed between 18 and 27 seems best, increasing speed does not remove the bulge, lowering speed does not remove the bulge.
I have tested a few different shapes and print orientations. The span of the bridge does not seem to affect the artefact, it always occur on all shapes at the first part of the bridge, then the print clears up. I have tried rectangular, square and round shapes, the artefact is the same on all of them. I have tried different print orientations and bridge angles, they make no difference.
Initially it seemed as if a 100% fan would move bridge extrusions and bond them into artefacts but once bridge flow ratio and speed was homed in changing fan speed between 70% and 100% does nothing.
One theory was that the plate temperature affected the bridge. However moving the bridge higher does nothing.
The next theory was that this is related to changes in volumetric flow. I aligned speed ton all slicer features to achieve the same actual volumetric flow in the whole test piece. This does not affect the artefact. A bridge speed of 18mm/s matches with a prints speed of 30mm/s for other slicer features.
Best so far
Nozzle: 0.4mm brass standard flow
Temperature: 230
Thick bridges: keep off
Bridge speed: 18mms/s
Bridge flow ratio: 1.85
Matching speed for perimeters and infill 30mm/s, this matches volumetric flow rate across the print to remove influences from abrupt changes in flow.
The print starts the bridge from the side where the artefact appears, this is consistent. There is a slight bulge that then collapse to a dip with some under extrusion. After this the print recovers to a good bridge with good bonding and no bulges or sags.
/Anders
What happens with clear or dark coloured filaments?
White contains so much pigment that it can become non Newtonian and inconsistent in behavior when pushed to extremes.
Cheerio,
RE: Stubborn PLA bridge artefact
Good point, I will try different colors and see if that makes a difference.
/Anders
RE: Stubborn PLA bridge artefact
tried same gcode in prusament PLA orange and galaxy black. It is a bit hard to see in the picture but side by side comparison they are more or less identifikation, same shape and same gaps in the sag part. The orange has not been used for a while and was at 30% moisture before hitting the dryer, the galaxy black is never and more recently used. The white is most fresh and most recently used, none of that seems to matter though all 3 print fine and replicate the same artifacts.
/Anders
RE: Stubborn PLA bridge artefact
Can you post the stl file? I typically have awful results with bridges and would like to give it a try.
RE: Stubborn PLA bridge artefact
My best guess is the geometry. The shape of the object is directing air around itself on that side and so you get different airflow/cooling. Once the fan shroud moves over a bit its blowing on a 'level' area and the airflow goes where you would expect.
RE: Stubborn PLA bridge artefact
Can you post the stl file? I typically have awful results with bridges and would like to give it a try.
I tried to attach the 3mf but that seems to have failed, I will upload and add a link tomorrow. It would be great to see if you see the same result.
/Anders
RE: Stubborn PLA bridge artefact
My best guess is the geometry. The shape of the object is directing air around itself on that side and so you get different airflow/cooling. Once the fan shroud moves over a bit its blowing on a 'level' area and the airflow goes where you would expect.
I have tried a few different shapes and also bridge angles, the angle of the airflow with respect to the bridge direction does not alter the artifact, it comes out the same on rectangular shapes and if the bridge prints in x or y direction. Fan speed does no difference. 
/Anders