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jonnybischof

Cura 14.05 RC2 - bridging

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I've noticed that there some new bridging behaviour made it's way into the newer Cura versions.

I want to point something out:

I'm printing a printhead at the moment. There is a bridging part when the "tunnels" where the linear bearings sit in are printed. This is supposed to be a very easy bridge, it's only about 2mm in length, but 35mm wide:

Bad bridging 1

1 layer before bridging

 

Bad bridging 2

bridging layer

Now, I've watched that layer print and noticed that instead of doing the usual 45° infill pattern (I'm doing 100% infill), Cura prints the infill at a 90° angle (or almost 90°) in order to make the lines across the bridge as short as possible.

Now, this is a problem here, because the printhead moves at a 90° angle to the edges of the bridge which makes it crash into the edge and bends it upwards.

It made a huge mess out of this bridge and was not able to close it until like 5 or 6 layers later. Sadly I didn't take any pictures. Maybe I can take some when it gets to the second linear bearing tunnel.

If the printer would do that bridge at a usual 45° angle, there wouldn't be any problem, because that way the nozzle doesn't crash into the edge that hard and wouldn't bend it up.

This can very well be seen at the layer after the bridging, which does a much better job than the actual bridging layer. But at that time the whole edge is already bent up and destroyed - the bridge gets pretty ugly in the end.

Now, my suggestion would be to only use 90° bridging, if the bridging length exceeds a certain limit. Something like 10mm, where it would actually matter to make the bridging strings shorter.

Any bridges shorter than 10mm should just be done with the regular 45° angle.

Some settings:

I'm printing Colorfabb XT @ 230°C, 30mm/s, 55°C bed temperature. Fan is not too strong, but running.

Layer height is 0.1mm, 100% infill. No speed changes for infill or outer shells.

 

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Why do you mean? I'll try again with less than 100% infill, but I'll have to make sure my part is strong enough. 100% would be ideal...

Missed the second part, so no video yet -.-

But of course it made a mess again. This time it was even worse than the lower part.

I'll try printing the part with 30% or something infill and report back. If that solves the issue without ruining part strength, then I'm ok with it :)

 

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Does the bridging code increase the extrusion rate for the first layer? Someone at Maker Faire last week pointed out to me that when bridging over empty space, there's nothing squishing the filament into its normal layer height, and instead it tries to form more of a 0.4mm diameter tube, but it seemed to him that it was still only providing enough material for half or one quarter of that height (whatever layer height is being printed), assuming that there's material underneath that will spread it out. As a result, the filament gets stretched out much thinner than it should be, causing gaps, and making it more likely to break. He claimed that by increasing the flow a lot to compensate on that layer, he was getting much better results.

Seemed like a plausible explanation, and opportunity for improvement, so I thought I'd mention it.

 

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100% infill causes that there is no difference between sparse and full infill, which in turn causes problems with detecting bridges.

 

Wait.. The briding is detected. That's why it prints 90° infill istead instead of the usual 45°.

My problem is, that 90° bridging is in this case bad. "Not detecting" the bridging, but instead just printing a normal layer as every other one would get me a better result in this case (short, but wide bridging).

That's what I wanted to say, and suggest to be investigated an improved if possible. Short, wide bridging is probably better off not being detected as a bridge.

/edit:

Haven't yet been able to test with less than 100% infill.

 

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Why is the bridge bad that way?

(As for the other bridge comments. Bridges work better at 0.2 layers in my experience. So the proper improvement would be to print bridges at a different layer height. Can most certainly be done, just printing bridges at different layer heights. But does needs some work)

 

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I also like the recent idea of always overextruding bridging such that there is enough plastic to create a .4mm diameter string (equivalent of .4mm layer while in the air but not when over other layers).

 

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bridging

I tried to visualize the problem (top view).

If you "hit" the "edge" with the "bridge line" at a 90° angle, that's like a frontal collision -> "crash". At that point, the printhead picks up the edge and folds it right over.

This is assuming that the edge is slightly raised which is usually the case, especially with round, overhanging edges which are very thin at that point.

But, if you hit the edge at 45°, that's not a "crash" but rather a "cut". This way, you don't hit the edge as hard and have a better chance not to mess things up.

Maybe printing the bridge at 0.2mm layer height along with other improvements will solve the problem too. But I'm sure this approach would improve things even more.

As for overextruding: I doubt that's a good idea if you're just bridging over a 2mm gap. Surely, that's a good idea for longer bridging lines, but not very short ones..

I'm trying to bridge over a horizontal "tunnel". If I cut off the top section of the circle and make it flat, I can improve the printing because the edge won't be as thin. But this tunnel must be a very tight fit for a linear bearing so I want to avoid that if possible.

Now, just for recap:

I'm only talking about short, wide bridges. Not "real" bridging where you try to cross a few cm of air gap.

The 90° approach is certainly a big improvement for longer bridges!

 

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But, if you hit the edge at 45°, that's not a "crash" but rather a "cut". This way, you don't hit the edge as hard and have a better chance not to mess things up.

 

I would say this is about the same situation as for 0 degree, just with a speed reduced by the square root of two.

To avoid hitting the edge, one would have to use z movements in addition.

 

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But, if you hit the edge at 45°, that's not a "crash" but rather a "cut". This way, you don't hit the edge as hard and have a better chance not to mess things up.

 

I understood you the first time but I'm not convinced 45 is better. It just means you approach it slower. You could instead slow down by 70% and get the same effect. Actually better because if you slow down you are less likely to get underextrusion as well.

The root of the problem has more to do with overhangs and how the lip of an overhang (raised edge) gets too high and the head hits this and sometimes the part comes off the bed. There's some discussion of this in another thread but there was no complete "cura". You can partly help by doing less overhangs, and also doing more fan helps quite a bit and printing cooler. But these don't eliminate it completely. Also there is some theories that thicker layers would help but no one tried it. I think ABS doesn't have this problem as badly also because ABS cools so damn fast.

 

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A friend has a new 3D printer. When I played with an Ultimaker a long time ago, the PLA was "easier" and I didn't see this problem. However, with the ABS shrinking a bit after solidifying, the overhangs tend to bend up. My friend doesn't have anything but ABS yet, so he's seeing this problem: The head hits the overhang, and potentially bumps the object from the bed.

Solutions: I don't know.

I'd think that doing the overhang "too high" when there isn't anything below, could work. Then approaching the material on the other side from the top might help. But difficult to parametrize: how much up is enough? How slow do you need to make "XY" movement for the Z to "catch up". And even if you get it right, maybe you're creating a bigger bump, causing even more problems on the next layer.... Hard!

 

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I tried to visualize the problem (top view).

If you "hit" the "edge" with the "bridge line" at a 90° angle, that's like a frontal collision -> "crash". At that point, the printhead picks up the edge and folds it right over.

This is assuming that the edge is slightly raised which is usually the case, especially with round, overhanging edges which are very thin at that point.

But, if you hit the edge at 45°, that's not a "crash" but rather a "cut". This way, you don't hit the edge as hard and have a better chance not to mess things up.

 

Interesting. Never thought of it that way. I'll let this one sink in for a while.

 

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(As for the other bridge comments. Bridges work better at 0.2 layers in my experience. So the proper improvement would be to print bridges at a different layer height. Can most certainly be done, just printing bridges at different layer heights. But does needs some work)

 

What about even thicker? Isn't what makes 0.2mm layers better the fact that they are thicker, and therefore start out that much closer to the 0.4mm diameter that the unsupported/unsqueezed plastic is going to try to shrink into anyway?

 

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