Odd behavior of plastic while bridging


This is done with nGen plastic. Bridging is done at 5mm/sec. layer height is 0.4mm, and the bridge is 20mm.
This was sliced with Slic3r

Things to watch for:
There appears to be a bug in Slic3r where it extrudes while moving the print head to the next level. This is the cause of the strings on the left pillar.
Note the shape of the extruded line on the 1st layer of the bridge, how it seems to bend up above the height it is being extruded at.
Note that the shape of the extruded line is not stable, sometimes the ark of the filament collapses causing lumps in the layer.
Note that the 1st layer is nearly level until the 2nd layer of the bridge is put down, then it starts to sag.

I noticed when the fans are on during the bridging, the temperature of the print head drops about 5 degrees. I’m thinking of insulating the print head to protect it from the cooling effect of the fans. It looks to me like the primary direction of the stock fan vents aims a bit too low. I intend to redesign the fan ducts to direct the air directly at the tip of the extruder.

These parameters are not ideal for bridging, but they show the odd behavior of the plastic. I originally thought that the plastic was adhering to the outside of the print nozzle, but the video clearly shows it is curving up without touching the outside of the nozzle.
I think part of the problem with the sagging is that Slic3r shuts off the cooling fans for the second and later layers. I think this may allow radiant heat from the head to soften the previously printed bridge.

I apologize for the really shallow depth of field in this video. Next time I’ll try and get more light on the subject, and I may try a telephoto lens with the camera further away to see if I can get more of the object in focus.

That’s a rather good bridge for that first layer. How does that print compare to one driven with Cura LE-generated gcode?

That is good bridging. I would expect a 20mm span requiring supports.

Your expectations for printing in mid-air may be a little high… :slight_smile: Each time the nozzle or new extrusion touches the bridged layer is a chance for deformation. A slight decrease in extrusion temp could help, just be careful of poor adhesion to the previous layer. As always, stay within the filament manufacturer’s suggested temp ranges.

Check if Slic3r has a setting for “wipe” at the end of a layer or retract when crossing spaces. This may help the stringing at the left post. Lowering extrusion temps could also help.