Question about auto mode nozzle width

[ashleywebster]

RichWebb, the effect you refer to is die swell and it varies with material and extrusion speed. But you don't have to lie to the slicer about anything nor should you need to change flow rate multiplier when changing extrusion width. Extrusion width is the width of each thread laid down.

If you cut a print in half vertically and look at a single cross section of filament, it would have a height equal to your layer height and a width equal to your extrusion width. That's called the extrusion profile. The width should be at least 1.5 times the layer height and greater than the nozzle diameter to ensure that the filament is sufficiently squished to the layer below. If your extruder steps / flow rate multiplier and filament diameter are calibrated well then the correct volume of plastic should be delivered to match your desired extrusion profile, no matter what width / layer height combination you choose. I frequently print extrusion width as high as 0.75 mm on a 0.4 mm nozzle and always have very smooth surfaces. If you have to set the extrusion width as low as 0.4 on a 0.4 mm nozzle to get a smooth top surface, then your flow rate is too high and you need to recalibrate e-steps or measure the filament diameter again because the printer is delivering more plastic than the slicer expected. Lowering the extrusion width helps a bit because it reduces overall flow rate (flow rate = layer height * extrusion width * print speed) but it's just masking the problem of over extrusion.

Dennis, this is why I don't think that extrusion width is the solution to your support material challenge - circumstantial as you say. But you are correct, the tool path will change accordingly to pack thinner or thicker lines, and yes, increasing the extrusion width to 0.6 will cause more plastic to come out. A 0.4 mm wall gets ignored by the slicer if you set extrusion width to 0.4 mm or higher because it can't meet your demands. It's like trying to build a 1/4" wall with 1/2" plywood. 0.39 mm filament works because it's narrower than the wall but keep in mind that layer bonding will suffer as the extrusion width approaches the nozzle diameter. This is why I am somewhat disappointed that auto width doesn't do much of anything - the slicer could be smart enough to detect a 0.4 mm wall and reduce the extrusion width to accommodate. It detects bridges and other features, why not thin walls?

One other thing to note is that a perfectly calibrated extruder will print a 0.4 mm wall closer to 0.45 mm because the filament bulges a little bit at the sides. As far as I can tell, S3D, like all the other slicers, assumes that the extruded filament is square, which is incorrect. See the following links for a more detailed explanation of this.

http://hydraraptor.blogspot.co.uk/2011/ ... -rate.html
http://hydraraptor.blogspot.ca/2014/06/ ... wrong.html

[ashleywebster]

Here are my calculations to confirm that auto width doesn't have any relation to layer height. A single model sliced with 2 processes, one at 0.3 mm layer height and one at 0.1 mm layer height. All other settings are the same. Note that the final width (W) is the same for both processes.

PROCESS 1

filament d: 2.75 mm
layer height: 0.3 mm
width: auto

G1 X-1.504 Y24.406 E0.0402
G1 X-1.504 Y25.085 E0.0168
Length of thread: 0.679 mm

cross sectional area of raw filament
p*r^2
p*1.375^2
= 5.9395736106932028414684351465128 mm2

volume extruded: 0.09978483665964580773666971046142 mm3 (area * E)

cross sectional area of extruded filament
= 0.09978483665964580773666971046142 mm3 / 0.679 mm travelled
= 0.14695852232642976102602313764568 mm2

assume square extrusion profile
A = W * H
0.14695852232642976102602313764568 = W * 0.3
W = 0.4898617410880992034200771254856


PROCESS 2

filament d: 2.75 mm
layer height: 0.1 mm
width: auto

G1 X-9.352 Y4.725 E0.0396
G1 X-9.352 Y4.046 E0.0056
Length of thread: 0.679 mm

volume extruded: 0.03326161221988193591222323682047 mm3 (area * E)

cross sectional area of extruded filament
= 0.03326161221988193591222323682047 mm3 / 0.679 mm travelled
= 0.04898617410880992034200771254856 mm2

assume a square extrusion profile
A = W * H
0.04898617410880992034200771254856 = W * 0.1
W = 0.48986174108809920342007712548559

[RichWebb]

Thanks, Ashley. Good info.

Also, "die swell." I was so close!

[dennisjm]

Great info Ashely. This makes me rethink my calibration process. I'm thinking of going back to the basics and doing a few more calibration cubes/walls. Perhaps I'm extruding just barely enough material and it's causing a little bit of stretch around the corners.

[dennisjm]

So if I wanted to shoot for the 1.5:1 ratio w/h, and I set layer height to .3 and extrusion width to .45. Then I print a single wall, should I expect to get the wall to come out around .45 or like you said in practice should it be coming out around .5? I know there is some increase in width due to the layers not lining up perfectly but I'm not sure if you're suggesting that we tune the extrusion multiplier until the width matches exactly the intended extrusion width or if you're intending that it's slightly higher.

[dennisjm]

I went back to the default profiles for the flashforge. Noticed that all the preconfigured profiles use manual width of .40. Thought it interesting that they aren't using auto for the preconfigured default settings.

[ashleywebster]

Yes, a 0.45mm wall will come out closer to 0.5mm. The formula for determining the exact width of a real world rounded filament profile are linked in my last post. But I don't think that printing thin walls is a very effective way to calibrate the extruder. A 1-2% change in the flow rate multiplier can make a big difference to the print quality but the corresponding change in wall thickness is difficult to detect. Assuming you have done the very basics of running 100mm through the filament drive and then scaling esteps in the firmware accordingly, I prefer to fine tune by slicing something at 90% infill and observing as it prints. 90% infill should be nearly solid but with very thin gaps, and the surface should be nice and smooth. If you have a USB microscope, inspect your prints to make sure the infill is well bonded to the perimeter and the whole thing looks reasonably water tight without being rough. Design your own calibration piece that tests a few things at once, 90% infill, thin walls, bridging, overhangs, inner holes, etc, and then run it for every new material you use.

http://reprap.org/wiki/Triffid_Hunter's ... tion_Guide

[dennisjm]

More good info, thanks.

I was reading through that article and checking my own machine's settings for cross reference. I'm using the Flashforge Creator X (Makerbot clone). I didn't see any way to adjust e steps in the firmware. Seems like that guide is mostly applicable to reprep firmware. Has good info though. I've thought about creating an object to test multiple things all at once. There is probably already several on Thingiverse.

[dennisjm]

So I've been testing out extrusion width's and multipliers and looking a little more closely at exactly what happens. One thing I came across, the nozzle width doesn't always line up with the object your printing in a way that covers the surface right. Notice the gap on the bottom layer at the bottom and the 2nd layer on the left.

This was a 20mm calibration cube with extrusion width set to .42. If I set it back to .40 it prints as expected. Printed at .42 you can see the gap.

On a side note, I tried changing the dimension of the cube to 21mm so a .42 extrusion width would fit perfectly but it didn't fix the issue. Nor did changing the nozzle width. Oh well, not sure what the issue is there but thought it related a bit as I was wanting to print calibration cubes with differing extrusion widths but the defect in the layers caused me a bit of confusion at first as to whether it was a problem with the amount of filament I'm extruding or some other parameter.

[jimc]

well one thing is odd or different and that is you have your infill set at 0 and 90 deg instead of 45 and -45. you would not have that issue when your infill runs diagonally. your parts will be stronger as well. keep random infill placement turned off as well. that bottom gap quite simply doesnt put a line there because it cant fit one.