IDEAS Top Layer Smoothing

Discussion in 'Guides, Mods, and Upgrades' started by Spoon Unit, Apr 29, 2016.

  1. Spoon Unit

    Spoon Unit Well-Known Member

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    Just wanted to pass along an idea I had and a method for doing this.

    The bottom layer of the print is typically very smooth. The top layer not so much. I'm mainly using S3D, and a feature it lacks (for me) is the ability to tell the top layer to print the infill first and the outline last. Of course, this wouldn't always make sense, as the infill often uses the outline as something to hang on to, but for many models, the top 3 layers are solid, and so that final layer could almost always swap the order of drawing in order to get a slightly nicer top finish.

    The other thing about the top finish is that you may well have a spot of over-extrusion just to make sure you don't have under extrusion. You're always on a knife edge there. So you're top layer is probably a touch rougher than maybe you'd like. If so, perhaps this process is for you.

    So, my idea is to basically go back over the top layer, movement for movement, with just a hot end, and no actual extrusion. This isn't an option in S3D (maybe it is in other extruders?). And so, it's time to open notepad++ or whatever you want to use.

    The first job is to isolate that final print layer. I copy that and move it to a new page so I can operate on it in isolation from the rest of the file. The first step is to cut out all extrusion commands. The operation of the printer is typically just to move and extrude at the same time. So a typical draw command look like this

    G0 X122.45 Y100.4 E30

    To turn this from a draw to a smooth, we just need the head at the same level and turn off the extrusion. So for that I use Notepad++'s search and replace facility with a regexp to replace

    E[.0-9]+

    And then I replace it with nothing at all. So that snips out all extrusion commands. We still have a few lines that were retraction moves so I find those and remove them be replacing

    G0 F.*

    Again replace this with blank. That should remove all the retractions.

    Now, I've got a set of GCODES that will repeat that final layer but not extract any more filament. For an extra test, I've also re-ordered the GCODES (nicely commented by S3D) to switch around infill and outline moves, so it should smooth the infill first, and then finally the outline.

    We're not quite done here. You'll also want to get that nozzle purged so, at the beginning of the file, dock, purge, and undock.

    That's it.

    Well, I just wanted to share the idea and the process. I'm currently just printing out another twirl with this method inside to go over the final layer and smooth it off. I'd certainly be interested to hear if anyone else has tried to do this or has some fancy, easier way, to get to a similar end result.

    Ideally S3D should handle this for all exposed top layers. My method above is only going to work for the final top layer, not all top layers in a more complex part.
     
  2. orcinus

    orcinus Well-Known Member

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    It won't end up any more smooth than the top layer you're getting from the print normally.

    The mechanism is exactly the same - during printing, the nozzle is flattening the extrusion against the last layer. The roughness is due to changes in direction and the little furrows the nozzle impresses on the layer. You'll get exactly the same result by going over it following the same path.

    If you *do* end up with a better result, that just means your calibration was off and you should actually fix that, rather than fudge it with a "smoothing" pass :)

    My 2c at least.

    By the way - if you want extremely smooth top layers, just reduce the extrusion width as far as you can (without compromising wall quality and extrusion evenness). You'll get very very smooth top layers with, say, 0.25 mm extrusion widths.
     
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  3. Spoon Unit

    Spoon Unit Well-Known Member

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    Not sure how calibration comes into this?
    No matter how small you make the nozzle, you still have that point where it turns and comes back in the other direction, which seems to be the point where you get build-up.
    How about if I go over it using the layer movement from the penultimate layer. For models where the dimensions of the top two layers is identical, this should mean that you move across the direction of printing of the previous layer.

    So ... your idea about the top layer sounds interesting. Let me see if i understand. You're saying use a thinner extrusion width than your nozzle? Won't this have the effect of overextruding? If so that's going to lead to another need to re-smooth.

    How about if I drop the head a few microns for the smoothing pass? Heat it further?

    Thanks for the input @orcinus.
     
  4. orcinus

    orcinus Well-Known Member

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    Roughness is very dependent on Z height, extrusion rate and infill-perimeter overlap.
    You can optimize it (to a point) fiddling with those settings, or make it worse (while still having an otherwise decent print).

    Also, it depends on the geometry of the nozzle a lot, as well as its alignment - a nozzle that's not perfectly perpendicular will dig in with the leading or trailing edge. Back when i had the time, i've experimented with sanding and polishing nozzle tips - that can help too, if you don't go overboard, but it's way too finicky and not very repeatable.

    It won't. You can extrude thinner than your nozzle size.
    Think of extrusion as a string of chewing gum, or toothpaste. You extrude material, and drag the nozzle away - you can stretch it thinner than the nozzle diameter. Up to a point - at which extrusion becomes too irregular, of course, but there's quite a bit of margin (if everything else is tuned in fine). With a 0.35 mm nozzle, about 0.25-0.30 is doable. With a 0.25 nozzle, you can stretch it as thin as 0.15. That's the point at which the extrusion starts becoming too turbulent and uneven.

    You'd end up digging trenches in the top layer then.
    The molten plastic tends to curl around the edges of the nozzle - that includes the front (or, rather, leading edge). But also around the sides as well, if the extrusion width is wide enough. This is why extruding with a very thin extrusion width can help (the effect depends on the nozzle geometry, though).
     
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  5. mike01hu

    mike01hu Well-Known Member

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    It's a technique that I have used with my old printer to fairly good effect. It is particularly important for finish to have a very flat nozzle that is in line with the X-Y motion. I have achieved this by sticking a piece of very fine wet and dry emery paper on the bed then moving the head (in my case) by hand down to touch the paper, then moving the bed around under the head to polish the nozzle end. Of course this wears the nozzle so it has to be done carefully but if you do not change heads or nozzles then this task is a rare one. I inspect the nozzle face regularly with a magnifier to check for uniformity across the surface but, as @orcinus says, it can be a pain and I still get some failures in finish!!!
     

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