Volcan perimeter gap seam

Discussion in 'Volcano' started by lancej, Mar 22, 2018.

  1. lancej

    lancej Member

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    I am using a Volcano with a 1.0mm nozzle and a 0.7mm layer height. I get a very noticeable "seam" or gap in the outer shell. I realize that some of this is normal, but this one is particularly bad, maybe because of the nozzle size.

    I have uploaded a photo of a section of a 25mm cylinder printed with PLA. Everything looks great except the seam/gap where the perimeters start/stop. You can see the z-step (horizontal) and a significant gap on every second layer. The cylinder is actually a concentric pair with no infill and only one shell width on each cylinder.

    Its connection to Volcano may be indirect -- the larger nozzle/layer/width may just be more noticeable.

    Has anyone seen this and, if so, is there a way to improve on it?

    I have looked at the g-code and watched the print and I see that there are two differences between good and bad perimeters. One is the direction of the shell (clockwise or counter) and the other is whether the outer perimeter starts immediately above the outer perimeter finish on the previous z-step, or if it starts after completing the inner cylinder on the same z-step. Of course, how it starts also affects how it ends. If it started on the outer perimeter, it will end on the inner.

    I have tried many of the usual things (extrusion multiplier, print speeds, temperature/fan settings) but the issue persists. I even hand-edited the G-code, adding delays overlaps, etc., but nothing has worked yet.

    I mainly use Cura (in Repetier host) but I have also tried Slic3r for some of it's extra settings. I do not own Simplify3d, but I'm willing to buy it if there's a solution to be had.

    Thanks,
    Lance
     

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  2. Antoine

    Antoine Well-Known Member
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    Volcano does make such artefacts more visible. I'm not quite sure I understand the reason for this particular issue. If this appears on restarts, the small gap may be due to some of the plastic oozing out as the nozzle travels and then not extruding for the first mm or so. This can be countered by either reducing the print temperature or adding some "extra restart distance" (S3D) or equivalent in Cura.
    If this gap is caused at the end of the line, it may be due to your slicer having a "wipe" or "coast" setting which is too aggressive if your system does not ooze much. Reducing this can potentially reduce this gap, but this doesn't look like it is the case here.
    You could make the seam less visible by randomising the start point, but this does not really fix the issue more than it hides it.
     
  3. lancej

    lancej Member

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    Antoine, thanks for the quick and thoughtful reply. But there is no retraction (I disabled it with no net effect) and I have no settings for wipe or coast (I'm using Cura). The seam is essentially unaffected by speed and temperature, which I have varied by over 2:1 (speed) and 20 degrees with and without fan.

    It is hard to say for sure which end (start or stop) is the problem. I might have better luck determining this using a hollow cube instead of a cylinder.

    I have experimented with the g-code files and I have made the seam change in appearance, but the gaps remain. My experiments included adding dwells at the start of a perimeter, extending the path at the end (to overlap the start), and extra extrusion at the start. But these were done earlier when I had less insight into what was happening (not that I have much now). I will need to repeat some experiments, but my tools are crude (editor and spreadsheet). I'll have to look for something better (suggestions welcome).
     
  4. lancej

    lancej Member

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    I think I may have stumbled on a solution, which may well be obvious to more experienced 3d printers. I increased the shell width. In Cura, the shell is specified in width rather than count. (Note that the version of Cura in Repetier host does not have all the modes/settings as Cura from Ultimaker, so I am somewhat limited.)

    Originally, I had a 0.9mm shell thickness for 1.0/0.7 (nozzle/height) setup. I only wanted a single shell perimeter. I found that the slicer retained a single shell up to 1.5mm after which it would do two shells. So I printed at 1.5mm and the results, while not perfect, are much improved (see picture).

    I wanted to repeat the experiment with two perimeter widths. I couldn't figure out how it determine the extrusion based on the nozzle/height and shell thickness parameters, but I found the thickest wall I could choose before it went to 3 shells was 2.9mm. So I printed at 1.51 and 2.9 to see if could discern a difference. (These correspond to the min/max thickness to generate 2 perimeters.)

    There was a clear difference (see the photo). The 1.51mm wall print (on the right) had gaps about like the original single wall print. The 2.9mm wall had smaller gaps and even bulged slightly on one side of the seam. Another thing I found interesting is that the 2.9mm wall print was much slower -- close to a factor of two. I'm not sure why. Maybe it's limited by the extrusion rate, although I could speed it up with the feed rate control.

    I learned something with these experiments and I hope it can help others.
     

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  5. Redemptioner

    Redemptioner Member

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    Your shell/extrusion width should be somewhere between 1 and 1.5 times the nozzle size (generally 1.2 times), so a 1mm nozzle should have a 1.2mm extrusion width. When you increase the width your extruder will have to increase how much it extrudes, if you have not given the extruder much head room in your driver settings for the extruder stepper motor speed then the print will slow down to the speed the extruder can run at. By default most firmware on the control boards have this set much lower than you want in their config (due to legacy of direct extruder drives) so just up the speed for the extruder and you will see the speed come back on the print.
     
  6. lancej

    lancej Member

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    Redemptioner, thanks for the tips and the information. I will try the 1.2mm width and see how it looks. I will eventually increase the max extrusion step rate, but I can live with slow speeds for now. By the way, my setup has a direct drive extruder.

    With my 1.0mm Volcano setup, I usually use 0.7mm layer height, but I sometimes use 0.25mm when I want smoother z-layers, but don't feel like changing the nozzle. Is the shell width a function of layer height as well? Should I be using a bigger width with the smaller height? Are 0.25 to 0.7 layer heights within "usable" range for a 1mm nozzle?
     
  7. Redemptioner

    Redemptioner Member

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    no, the layer/shell width will remain the same no matter the layer height, although if the layer height is reduced and the extruder speed is the limiting factor the print speed will increase although you might not notice due to the 3 times longer print time due to the increase number of layers needed.
     
  8. lancej

    lancej Member

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    Redemptioner, I'm starting to understand a little better with your help and more googling.

    As I understand it, the nozzle diameter sets "guidelines" for what is practical, but the actual cross-sectional area is controlled by the amount of material extruded (and the distance the nozzle travels) and is not necessarily the same as the nozzle diameter. If the layer is higher for a given width, then more material will be extruded. The nozzle diameter affects how much material it is practical to extrude.

    Cura apparently computes the extrusion width based on the nozzle diameter. It does a computed number of perimeters with a single extrusion width, which it normally picks to between 1x and 1.5x the nozzle diameter based on the shell thickness. So, if want a certain number of perimeters at a certain extrusion width, I have to back-calculate what shell thickness would give me that.

    I mounted a camera to the print head and watched some other builds where there were gaps (I haven't watched the cylinder build with it yet.) It seems that the problems are at the start of a line when it is has traveled a sufficient distance to cause a retract. But this may be a different mechanism than the cylinder because I had disabled retract on the cylinder with no effect.

    Antoine had suggested some retract-related setting (extra restart, wipe, coast...) that I should consider. I believe that they can help with the gaps in other builds and maybe even with the cylinder if I figure out which ones to tweak. Of course, Cura doesn't give me many knobs in that regard, so I may need to start using Simplify3D.
     

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