First layer issues (and fix)  

RE: First layer issues (and fix)

Odd way to fix an adhesion problem. But the bottom line is you are laying down the exact same surface area of plastic. Just thicker.  So you really aren't changing the "tack" of the PLA to the bed.

RE: First layer issues (and fix)

In theory, yes, the cross-section ought to be the same. But in practice the thicker and fatter first layer lines stick much better to the print bed. That's presumably why many slicers have a thicker/wider first layer extrusion by default (e.g. S3D, Slic3r, MakerBot, etc.) so I was actually a bit surprised that Prusa's copy of Slic3r didn't.

https://manual.slic3r.org/first-print/first-layer for reference.

YMMV, of course. Do what works for you - just passing along that using the recommended Slic3r settings for the first layer worked great for me.

RE: First layer issues (and fix)

Well, two extrusions - both 0.42 mm wide - is the same as a single extrusion 0.84 mm wide. Same surface area in contact with the bed.  When you can explain why it is better, I'm all ears.

As for the Slic3r manual - they are considering the user to have a printer that has no smooth bed - masking tape anyone?, no bed heat, no mesh leveling, etc. Hence the propensity for thick layer ones.  

RE: First layer issues (and fix)

The fluid dynamics are different - the 0.42mm wide extrusion isn't perfectly pressed into the bed much as an 0.82 wide extrusion is, because the first extrusion is closer to round while the second one is extrudes twice the material so the plastic flows wider, so there's more actual surface contact between the bed and the plastic. And a thicker first layer means more heat flowing from the nozzle to the bed (i.e. twice the flow rate), bonding the first layer to the bed. It's similar to how, despite it theoretically not mattering, in practice a slower first layer sticks better, because there's more time for the plastic to be pressed down into the bed (and to bond with adjacent plastic) before the nozzle moves on, less stress on the molten plastic due to a rapid nozzle movement, etc.

it's odd that Prusa's default behavior is to make the first layer _thinner_ than the rest of the print. I'm not sure why that would be, unless perhaps they're assuming that users configure the nozzle to be too close to the print bed for some reason?

RE: First layer issues (and fix)

Posted by: laird.p

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it's odd that Prusa's default behavior is to make the first layer _thinner_ than the rest of the print. I'm not sure why that would be, unless perhaps they're assuming that users configure the nozzle to be too close to the print bed for some reason?

Really?  My first layer is 0.20 mm, and my second and subsequent layers are 0.15 or less.  And once in a while I do use the same height of 0.2 mm, even less often I have printed 0.3 mm (once as far as I can recall, and mostly just to see how it worked). 

So why do you say the first layer is thinner than the rest of the print?

Regardless, as you have said. ymmv.  I am glad you've found a solution to your printing problem.

RE: First layer issues (and fix)

I was speaking of layer width, not height. For 0.15mm Quality profile, for example, Prusa uses 0.42mm first layer width, 0.45mm layer width on the other layers. I have much better bed adhesion with first layer 150% (which works out to 0.675mm. For first layer, they use height 0.2mm for the first layer, 0.15 for the rest, which is fine.

RE: First layer issues (and fix)

With a clean bed PLA sticks very well at 0.42 mm. No need to reduce print resolution to get better adhesion. Remember that the best radius a 0.8 trace can do is 0.4 mm. But a 0.42 can produce a 0.21 radius.  So using a fat trace decreases print quality.

RE: First layer issues (and fix)

Posted by: laird.p

[...]

- Under Configuration / Mode select "Advanced" mode to see these settings to change.
-Under print settings / Advanced set First Layer to 200%. This will make the extrusion of the first layer of the print 2x the standard width, making it nice and "fat" so that it sticks well. The slicer will use half as many lines of extrusion, so the resulting print will look fine.

Are you referring to the settings under Print Settings->Advanced->Extrusion Width ? If so and you enter a percentage into an extrusion width box, that percentage is relative to layer height, not nozzle size. Hover over the entry box and an excessively verbose and confusing tool tip will pop up explaining this. You'll notice if you enter 0 it will use... 200%.

- Under print settings / Layers and Perimeters set First layer height to 0.35 (for an 0.4mm nozzle). This makes the first layer taller, so it's more forgiving of any issues with bed leveling, and also works nicely with the fatter first layer extrusion.

That would explain your findings. You've set the 1st layer height at 87.5% of your nozzle size (assuming you've got a 0.40mm nozzle mounted), and by setting the extrusion width to 120% of that, you've basically set your extrusion width to 0.45mm. That's the Prusa default for that setting. Simplify 3D goes with 120% of nozzle size, so 0.48mm. Cura goes with 100% of nozzle size, so 0.40mm for 1st layer extrusion widths.

Recommendations for extrusion widths are up to 120% of nozzle size, and layer heights up to 80% of nozzle size. 

As to the 0.20mm 1st layer height convention: First layer heights are intended to compensate for uneven print bed surfaces. The less even the bed, the thicker the first layer needs to be. It's an ugly work-around. The Prusa bed is smooth enough that you can use a 0.20mm 1st layer height consistently for good 1st layer results. Thicker layers may not cool evenly, and you want an even surface for good bed adhesion throughout the print.

I always use 0.20mm 1st layer heights until I get down to a 0.20mm or finer nozzle, at which point I go with the 80% convention.