Bridge not sticking, loops of bridge falling down  

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Anachronist
(@anachronist)
Trusted Member

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?

I was concerned about this too. There's no airflow in the resonant cavity, just sound waves resonating, but I didn't want the cavity walls absorbing any sound. It turns out that variations significantly smaller than the wavelength don't matter.

The braces are designed to be 1 layer thick (I actually included a layer thickness parameter in my OpenSCAD file). The bridge strands extrude at the full nozzle diameter (0.4mm) rather than the layer thickness (0.2mm), so the bridge surface profile on the interior has in&out variations that exceed the layer thickness, suggesting that my braces wouldn't interfere with airflow any more than the bridges themselves would.

I fixed the problem with the bent bridge perimeter in the wistle window. Then I tried printing this with PETG. Thinking PETG is more sticky, I reset the infill overlap to the default 25% but got the falling bridge loops again. I'm going to add a 1-layer brace across that window opening to see if that helps, otherwise it's back to 50% overlap for this part.

Posted : 08/12/2019 3:09 pm
Tim
(@tim-m30)
Illustrious Member

Just to comment, the first STL you posted a link to printed without incident using essentially a stock profile.  Makes 85dB at 1 meter.

This post was modified 8 months ago by Tim
It is always wise to get more than one opinion......
Posted : 08/12/2019 9:10 pm
Anachronist
(@anachronist)
Trusted Member
Posted by: @tim-m30

Just to comment, the first STL you posted a link to printed without incident using essentially a stock profile.  Makes 85dB at 1 meter.

Looking over my growing collection of test prints, I too have one that printed OK without doing anything special, but most have bridge drops.

I find this doesn't occur often in the mouth end (my initial photos). It occurs most frequently inside the whistle window opening (the gap between the airway exit and the blade-edge). The slicer makes a bent bridge in mid-air and I never get any other bridge to stick to it unless I set overlap to 50% as you suggested. I consistently get one or two fallen bridge loops in that gap.

My fan is already at 100% and I tried slowing down the speed for overhangs, to no avail. The overlap did the job. Now I'm experimenting with internal 1-layer braces in the CAD model to see if I can leave the overlap setting alone.

Hmm, only 85 dB... I was going for >100. That means use a bigger window that requires more breath. And maybe reducing the pitch a bit by increasing the internal volume.

This post was modified 8 months ago 2 times by Anachronist
Posted : 08/12/2019 10:23 pm
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