Stubborn PLA bridge artefact  

RE: Stubborn PLA bridge artefact

You're a bad influence! I've now spent several days fooling with settings and have come full circle to about the same settings you were using, with similar results. I'm starting to think that the bridge strategy used by the slicer could have a few more settings. It's possible that the flow takes a bit to stabilize when the bridge starts at a new speed. Possibly there should be an option to ramp the speed at the beginning of a bridge. I could see having a delay and even a dedicated nozzle temperature just for bridges. I'm using a high flow nozzle, but I don't think that makes much difference. Oddly, many things I read online don't hold true here. Cooler isn't better and the flow has to be up around 1.75 to 1.85 to avoid gaps. I find 25 mm/s is as good as slower. What I thought was uneven spacing (pairing) seems to be a slight vertical height difference that's mostly visible from the top.

What bothers me most about the bulge is what will happen with the next layer if the nozzle hits, so for most things I'd probably use setting that keep the bulge down, even if that means some gaps. Hey, they'll get paved over on the next pass!

I know I won't be able to leave this alone, so let me know if you learn anything else.

RE: Stubborn PLA bridge artefact

Can't leave this alone but have come around to about the same setting you're using. I'm thinking the slicer may need a couple more bridge-specific tuning settings. Let me know if you learn anything more.

RE: Stubborn PLA bridge artefact

Sorry about two posts. This site can have a several minute response time, tell me it timed out, then add my "lost" post much later!

RE: Stubborn PLA bridge artefact

Sorry about the bad influence. I am also leaning towards that this is similar to the benchy hull line, there is something here that challenges the slicer and not enough settings or adjustments to prevent it. I did find a strategy to avoid it but that involves changing geometry, if the initial bridge is very short, starting in a corner the problem does not seem to occur. For now I think the best strategy  is to configure the angle such that it starts the bridge in a corner, of course with this round test piece that is not possible, which is why I made it round in the first place. 

RE:

Hello, could you please test again with the print fan set to 100% from the very beginning? So that the fan doesn't have to reach full speed at the start of the print bridge, but runs at full speed right away.

This applies to the entire part, including the perimeter.

RE: Stubborn PLA bridge artefact

I was thinking something similar so ran the test with 100% fan and no automatic anything. The result was unchanged. Then, thinking too much is just enough, I aimed my heat shrink gun at the part (with the heat off) and blasted it with cool air from before the bridge started, to the end. It wasn't a revelation, but there was some improvement in how far in the defective area went, 8 vs. 9.5 mm.

I have to wonder if the stepover needs to be reduced for bridges. I don't think it's an option. We increase the flow substantially, so the bridge fills in, but with a closer spacing, we wouldn't have to do that. With lower flows, the bulge doesn't happen, but the bridge is more of a soup strainer than a solid layer.

RE: Stubborn PLA bridge artefact

I have experimented with fan settings between 70 and 100 and direction by altering bridge angle, neither will remove this thing. Also, the defect is still there in the multi segmented bridges printing later when fan speed definitely is up.

RE: Stubborn PLA bridge artefact

Thank you very much for the tests. This is a very interesting topic.

RE:

Have you tried it on another printer? For example, an MK4S with the same extruder or an MK3S with a different extruder / hotend?

RE: Stubborn PLA bridge artefact

I only have access to a core one, so no, but interesting idea to isolate printer vs slicer dependencies.