Bridge not sticking, loops of bridge falling down  

RE: Bridge not sticking, loops of bridge falling down

Insufficient cooling applied during the bridge extrusion; usually with bridge detection disabled. That, and printing too fast.

And yes, infill can be oriented, but not if it happens at alternate layers (even odd are always 90 degrees).

RE: Bridge not sticking, loops of bridge falling down

@tim-m30 - thanks, OK, basically speed and cooling.

For the photos I attached, I was using the standard PrusaSlicer setting profile "0.2mm QUALITY MK3" in which I enabled Extra perimeters if needed, Avoid crossing perimeters, and Detect bridging perimeters in Print Settings > Layers and perimeters. I always enable those. I don't see anything else related to bridge detection. PrusaSlicer did identify the bridges in my model with the proper colors in the slices.

I don't see how I can increase the cooling. Under Filament Settings > Cooling, the fan speed is set to 100% except for the first layer. This is the profile default.

Under Print Settings > Speed, Bridges is set to 30 mm/s by default. Is that too fast? Is there guidance for the best setting? I thought these preset profiles that came with PrusaSlicer were already based on extensive testing and optimizing.

I know infill can be oriented, but this doesn't appear to determine bridge orientation. If the layer position determines that, I can experiment with that, although it's weird to be spending a lot of extra time in CAD just to compensate for slicer behavior.

RE: Bridge not sticking, loops of bridge falling down

What nozzle and temps are you using?

Those layers are awfully pronounced for a .4 mm nozzle...

Your first layer looks good. Maybe it is just the magnification... 6 mm is not wide at all for a bridge. My 1" dice bridge beautifully...

RE: Bridge not sticking, loops of bridge falling down

Filament and temps can have an effect, and some filaments appear more sensitive to bridging artifacts than others.  Reducing print temp might help some bridging situations, but where layer adhesion is a problem, more heat might be needed.

A closer look at the bad section of filament, it looks as if it is a lot more material than normal.  If you've handled the spool and hand fingers touching the filament, oils can cause the extrusion of foam and swell, and your print sort of looks like that might have happened.  Could be an optical photo effect, but something to consider.  If the diameter of those drooping extrusions is 0.4 mm, and not the 0.6 mm they look like, then it isn't spool contamination.   

As for the defaults working well?  They really do a great job.  But there are so many different printing situations, one setting may not work in 100% of the cases.  I use predominately the defaults, but I have tweaked a few settings to match the filament I use.  Cooling primarily.  And even printing Prusament, I find 215c too hot for good prints. So I use 210 after layer 1. And sometimes will drop the nozzle to 200 or speed to 50% during the print if I see issues forming (usually curling infill).

There is a whistle in the sample folders.  It bridges about 20 mm, and is a good test since it is a gcode file.  Try printing that model and report back.  There was some discussions a while back where folk were having issues with it, but I don't recall the exact solutions. 

Whistle

And here's some filament extruded that shows what contamination looks like: note the changing diameters.

The issue here was I have handled the filament tip quite a bit while loading a different color: and the result was plenty of oil was boiling off in the nozzle, foaming the filament.

ps: bottom line is that you should not be having any problems with your part.  If you post a 3mf (export project) or the stl zipped up, we have have a look at what your settings are or try slicing the part to look for anomalies.

RE: Bridge not sticking, loops of bridge falling down

@tim-m30 and @robert-rmm200 -

I am using the standard 0.4mm nozzle that came with the printer. The photos are highly magnified. Those layers are 0.2mm thick, and the fallen bridge strand is 0.4mm.

I'm using the standard silver PLA Prusament that came with the printer. I have not had much reason to load and unload it because it's the only filament I have, so I haven't been handling it except when a loop of filament comes off the spool occasionally. That might explain an occasional problem with a dropped bridge strand, but I'm getting this pretty frequently for this model.

By the way, the photo is for part of my own parametric design for a distress whistle. I modeled the whistle by modeling the interior airspaces in OpenSCAD, using that to cut the interior out of a larger body. For saving time during testing, I am not printing the body enclosure, but instead I put a Minkowski skin around the interior airspace and then cut it hollow again with a copy of that same airspace with extensions for air ingress/egress. Even with the failed bridge problems, the whistle performs better than anything else I've found on Thingiverse.

For some reason I can't attach stl files to this post. So here it is on my Google drive. It's the same as shown in the picture. The resonant cavity has some built-in ridges at the top to give support to the bridge there, which frustratingly wants to go along the length of that cavity instead of the width. I observe bridge strands falling off in the mouthpiece opening (in some prints) as well as the whistle window (between the windway exit and the blade). It takes about 20 minutes to print.

RE: Bridge not sticking, loops of bridge falling down

Not exactly what you're asking, but when experimenting with some PETG that likes to sag on long bridges, I noticed that the 1st pair of initial strands were straight, but subsequent strands between the cause enough heat to cause them to droop. Tinkering with the bridging infill pattern helped to a point. There was still some sag, but not the long stringy strands. This part is oriented upside-down.

Side-by-side comparison of a few tests:

KISSlicer is a quirky slicer, but allows setting separate cooling for infill and bridges along with some other interesting features with checking out.

RE: Bridge not sticking, loops of bridge falling down

Posts must be in a ZIP file.  Raw STL or GCODE are not permitted here.

RE: Bridge not sticking, loops of bridge falling down

The whistle stl looks fine, though at 0.20 mm Quality is it printing pretty fast.  Increasing cooling fan to 100% may help.

RE: Bridge not sticking, loops of bridge falling down

If you rotate the part does the bridging change? Can't test myself ATM.

RE: Bridge not sticking, loops of bridge falling down

@tim-m30 - the cooling fan is already at 100%. That's the default setting. Perhaps slowing down the bridges so that the cooling fan has more time to cool them might work.

Rotating the part would make it non-functional, as the shape of the windway orifice and blade edge are critical, and those features would become bridges with an unpredictable shape. I'd have to use dissolvable support, but I designed the part to be printed without supports. As I said earlier, the bridge failures don't cause functional problems; it's a pretty loud whistle. I started this discussion because I'm curious about what's causing bridge strands to fall sometimes.

What appears to be happening is this: The nozzle anchors the bridge on one side and pulls the strand to the other side, and it should stick there, but then the nozzle turns around to extrude a new strand in the opposite direction and pulls the previous strand off its anchor, as it if didn't stick, and I end up with a small loop of filament instead of a bridge. So the problem seems to be poor adhesion of the filament when the bridge is being printed.

I thought at first the nozzle wasn't moving far enough before turning around. If it moved further, the bridge strand anchor points would stick better. The only other thing I can think to do is slow down the bridge speed (half speed? I have no idea) to cause the nozzle to spend more time at the anchor points before turning.

Mind you, I don't see this problem all the time, and not always in the same place on this part. But it has happened often enough to ask about it here. Now that I'm home from work, I see that the photo and stl I linked aren't exactly the same (the stl has a thicker skin than the part in the photo) but the bridge is still the same.

@bobstro - that's interesting, I'm still new to this, and I never heard of an adjustable bridge infill setting. I don't see such a thing in PrusaSlicer though.

RE: Bridge not sticking, loops of bridge falling down

The angle of bridging looks okay on my slicer; and you photo of the print.  You want the bridge to be the shortest path, and it is.  On the STL, the overlap was more than adequate to my eye - but you might try increasing it from 25% to 50% to make sure the ends tack properly.  There will be other effects, but the bridge seems like the important factor for now.

RE: Bridge not sticking, loops of bridge falling down

@tim-m30 - The bridge direction is OK only because I forced it by creating ridges in my CAD model. If I didn't, the bridges would traverse the length of the resonant cavity.

How do you tell if the overlap looks good? If I change it to 50% (the only setting I can find is in Print Settings > Advanced) the slicer output doesn't look different.

Before you replied, I tried again with bridge speed reduced from 30 to 20, and temperature of non-1st layers reduced from 215  210, and I still got bridge drops - several.

I observe that the bridge perimeters never collapse, but the bridge infill is problematic. I could build 1-layer ridges close together in the CAD model to force all bridges to be perimeters but I'd rather not have to.

I believe I figured out why I am consistently getting bridge separation in the whistle window. It's because the slicer generates a bent bridge perimeter:

Naturally that perimeter cannot bend exactly that way when extruded into empty space. The bridge infill doesn't reach it. Although this doesn't explain the separation in the pictures in my first post, I can see why the bridge would fail in the area shown here.

I went ahead and reset my speed and temperature to defaults, and printed this with 50% overlap anyway, to see what would happen. Amazingly, it worked! The bridge infill reached that bent bridge perimeter and tacked onto it.

This won't be a problem when I surround this whistle with the actual body enclosure, because that will allow for straight bridge perimeters.

50% overlap solved the problem though. I don't see any ill effects from it other than the brim being more difficult to remove.

Thanks so much for your help in zeroing in on the problem. Along the way it let me print whistles with slight adjustments to cavity size and window width, and the last print produced something that makes a very loud sound with little effort.

RE: Bridge not sticking, loops of bridge falling down

Those of us with grandchildren who like loud whistles (and get sent home to blow them) might appreciate your .3mf file posted here...

RE: Bridge not sticking, loops of bridge falling down

@robert-rmm200 - I plan to put it on Thingiverse. I was inspired by another loud whistle there, but it requires too much breath for a child (and the model has bridge and overhang problems). The one I've come up with seems just as loud for less effort. And I'll include the parametric OpenSCAD file so anyone can make their own.

RE: Bridge not sticking, loops of bridge falling down

@anachronist I finally had some time to experiment with your part and Print Settings->Infill->Advanced->Bridging angle.

With the default angle of 0 (calculated by the slicer), I get the undesirable behavior you described:

I'm able to change it to a different angle, 135 degrees in this case:

The pop-up tool tip mentioned entering 180 degrees to get 0:

RE: Bridge not sticking, loops of bridge falling down

Ah, the ol' editing feature bites again. Pretend that 1st pic is this one instead:

Interestingly, rotating the part 90 degrees on the Z axis does this at 0:

RE: Bridge not sticking, loops of bridge falling down

@bobstro - That's weird. I don't see that setting in Print settings > Infill > Advanced. The only thing there about bridges is "bridge flow ratio."

The little bridges I created in my CAD model force the bridging angle to change in the slicer, otherwise the bridges would run along the length of the part.

I'm using PrusaSlicer 2.1.0+linux64. What are you using?

RE: Bridge not sticking, loops of bridge falling down

@bobstro - oops, I see it now. I was looking in Printer settings > Advanced. I wonder why I hadn't found it before. Thanks! With the setting of zero, I adjusted my CAD model to force the infill to the direction I wanted. Now I see I didn't have to do that. That's OK, the CAD model is now more robust as a result.

RE: Bridge not sticking, loops of bridge falling down

Posted by: @anachronist

@bobstro - oops, I see it now. I was looking in Printer settings > Advanced. I wonder why I hadn't found it before. Thanks! With the setting of zero, I adjusted my CAD model to force the infill to the direction I wanted. Now I see I didn't have to do that. That's OK, the CAD model is now more robust as a result.

I've run into some of the same issues doing my parts, so I sympathize. "Bridge/support awareness" is definitely a good thing if you are designing parts. I'm working with printing parts at various angles (nothing original) to reduce problems. I hope to put out designs that don't depend on specific user slicer settings to get good results. I think that's part of the never-ending balancing act involved in 3D design and printing proficiency. Still, nice to know these tweaks exist, and Team Prusa does seem to be adding new niceties and expanding features.

Your bracing technique has some definite advantages. If the walls are thin, it can help prevent them bending inwards above certain heights. There was a thread on a related topic not long ago. Do the braces interfere with wind flow?