First Bridge Layer Has ultra-thin line start, then thickens

[Dana1960 1]

I'm trying to set the best bridge settings. Results are close to the images found in the ultamaker cura bridge article of bad versus expected.

 

I'm working on a part cooling duct and bridges need good cooling. Material is PLA. Settings in the attached 3mf file. This is a monoprice printer with all the upgrades. Prints pretty well now.

 

But, I noticed an odd thing on the first bridge layer. See image. The line starts out and has a very thin start then quickly thickens. I was wondering what settings to tinker with to improve this oddity.

 

Thanks,

Dana

MU_Test - Bridge 2 Layers.3mf

[GregValiant 1,412]

I think there are a couple of things going on.

With your line width at .48 and the Skin Density at 100% the two outboard bridge extrusions are getting welded to the inner wall extrusion making it 3 extrusions wide.  

You could drop the Bridge Skin Density to 60% or so to spread the lines out.  That would eliminate the triple line on the outside.  The Flow rate could be adjusted as well but too low and the printer won't like it.

 

[Dana1960 1]

Thanks for your response...

 

I just discovered, via a YouTube video, the Market Place "Settings Guide" plugin. It explains each setting verbosely.

 

Interactions: Normal wall print speed and bridge wall speed. The greater the difference between them the harder it becomes to control filament flow. The same is true when adjusting bridge flow rates, the greater the difference between the preceding flow rate and bridge flow rate the harder it becomes for the extruder to succeed.  

 

It would seem reducing flow rate differences is a key factor. Therefor would it not make sense to compensate while approaching a bridge segment by slowing down thereby reducing the flow rate change. It looks like bridge coasting intends to do this. But why wait until one is about to bridge a gap. It would make more sense to gradually change both speed and flow rate before approaching a gap. Thereby keeping flow rate within extruder capability.

 

If you have not already done so, consider adding the Market Place "Settings Guide" plugin.

Edited December 21, 2022 by Dana1960

[GregValiant 1,412]

I used the Settings Guide for a while.  The larger help balloons take up a lot more screen real estate and so after a while I disabled it.

 

You will notice that the "Bridge Settings" are in the "Experimental" section.  They are still evolving.

 

One problem you have described has it's basis in what goes on inside the nozzle.  The restriction of the nozzle opening plays a big part in how fast the "nozzle pressure" falls off and how fast it builds back up to extrusion pressure for a given print speed.  Some people are prone to having different speeds for every different feature.  Outer walls at one speed, infill at another, inner walls at a third, and so on.  It's better to have consistent speeds as that allows the nozzle pressure to be consistent and that allows for more consistent extrusions.  Large prints don't really care as the long extrusions allow the pressure to normalize.  Small models with lots of starts and stops and constantly changing from one line type to another are a different story.  If your firmware supports "Linear Advance" then it might be worth looking into as it can predict pressure changes in the nozzle and make adjustments.

[Dana1960 1]

I'm not sure my printer controller can handle Linear Advance. It is an 8bit board with limited memory. But something to look at.

 

The next attempt at improving bridges will be trying to tune for less flow rate change. 

 

I noticed one bridge line, see image, is not controllable within bridge settings. Is this an oversight? The only way to tune this particular bridge line is using the Top/Bottom Print Speed. Seems like it ought to be added to bridge settings. 

 

Nor is there a setting to control the flow rate for that line in bridge settings.

Edited December 21, 2022 by Dana1960

[Dana1960 1]

Question, roughly speaking what is the minimum reliable 0.4mm nozzle flow rate?

 

Keep in mind this is a low-end hobby 3D printer with all the upgrades and up to date marlin firmware. This includes better extruder gears and better spring to exert more pressure.

[GregValiant 1,412]

That's a very large subject and there are so many variables that coming up with a number gets unwieldy.  Consequently the "Roughly Speaking" thing is a must.  It will come down to your printer's extruder, whether it is a bowden setup or direct drive, the material, the hot end temperature,the mechanical properties of the material at a particular temperature, etc..  There are lots of variables of the variables..

For reference:  If you are using a 0.4mm line width then at 10mm/sec and "100% Flow" the Flow Rate would be 0.8mm³/sec through the nozzle.  In Cura - the default "Minimum Line Width" is 0.34 for a 0.4 regular line width setting and if Cura adjusts the line width downward then the flow could drop to .7mm³/sec.  That comes into play when printing things that come to a point (like a spire on a church or a pyramid) or when doing bridging tests.

 

I think the first variable is "what material" and the second might be "what temperature".  Those directly affect the viscosity of the material passing through the nozzle.  A thin material will flow much easier than a thick material.  How the material adheres to the sides of the Inside Diameter of the nozzle also comes into play.  If there is a lot of "stickiness" at the walls of the ID then only the center of the "flow" moves at any kind of speed while the material in contact with the walls would move much slower.  As the print speed increases that will result in turbulence in the flow.

 

My "low end" Ender 3 Pro will print PLA at 205° and at "100% Flow" down to that 0.7mm³/sec number.  I've left my "Minimum Speed" at 10mm/sec because of that.  Could I print slower or lower the flow%?  Maybe.  I've never tested for the "reliable minimum" flow rate.

 

All the same variables affect "Maximum Print Speed".  I did do a test for that and I can print large flat skins up to 175mm/sec which comes out to 14mm³/sec with 0.4 Line Width at 0.2 Layer Height.  If I go faster my extruder doesn't keep up at the beginning of an extrusion.  I might be able to change that by increasing the Retract and Prime speeds (but I ran out of interest as I'm not likely to print anything at 200mm/sec anyway.  Knowing that I can print large things (that don't have curves) at up to 150mm/sec is what I was looking for.

 

There are two sayings I'm fond of:

"Everything affects Everything."

and

"A lot goes on in that little space below the nozzle."

Edited December 22, 2022 by GregValiant