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In cura one of the options under "build plate adhesion" is "skirt", which seems to simply print a loop around, but not touching, my print. How is this supposed to help my prints stick to the bed?

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The skirt will only prime the nozzle, which is something really important for the first layer adhesion. Ususally the first layer has slightly more material (or a lower height) respect to the other layers, and this is done mainly to push the most material against the plate, and this will give adhesion. Without a proper priming of the nozzle this just won't happen everywhere.


From the Simplify3D guide:

Adjustments to the First Layer Height allow you to vary the amount of pressure the filament exerts against the bed. There are actually two ways to adjust the height and pressure:

Percentages under 100% will reduce the height of the layer being printed (with no change to the extrusion amount). For example, if you entered 75%, your first layer height is reduced while your extrusion remains at 100%. Another way to think of this is that 100% of your extrusion will be forced into a space that is 75% of the layer height. This reduction in height generates extra pressure and more surface area for that layer, which will help the first layer adhere to the print bed.

In other cases, using a percentage above 100% is helpful. For example, if you are printing at very fine layer heights, such as 0.05mm, a tiny variance in your bed leveling can result in poor first layer adhesion. Using a First Layer Height well above 100% can be extremely helpful in these cases. Many machines benefit from 200 or 300% for the first layer height when the layer height is 0.05 mm or 0.1 mm. The increased thickness of the first layer can help absorb small defects in the build plate and provide more surface contact area, which will result in better first layer adhesion.

Imho, the life is too short to print at 0.05mm, and my printer doesn't show bed issues, therefore i'll go with less than 100% first layer height.

I don't have the standard cura settings available, and i also think that with different 3d printers with different filaments it makes only a little sense to discuss about "cura standard settings". In any case, my very standard standard settings are: 0.2mm layer height, 0.15mm first layer height. With these settings and some hairspray on the glass i almost forgot about model pop off in midprint.

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  • $\begingroup$ I disagree with first layer has slightly more material and lower height. Actually the first layer usually is larger (it includes the paper thickness), look at the standard Cura settings where the first layer has even a larger value than all other layers. Furthermore, more material is only given to the first layer when you have requested that, again, that is not standard, and either way not required. $\endgroup$ – 0scar May 31 '18 at 5:30
  • $\begingroup$ What does "is larger (it includes the paper thickness)" means? The footprint of the object won't change, except for a fraction of the nozzle diameter if you squeeze the material. And in "even a larger value than all other layers" which property is larger? Anyway, i'll reply editing the answer because there's not enough space here. $\endgroup$ – theGarz May 31 '18 at 8:19
  • $\begingroup$ True, discussing tool setting is not a good thing to do, I merely added it as an example. I see Simplify3D does it differently. But, your first layer is not 0.15, the actual layer includes the paper thickness, so it is more or less 0.25 or something. Increasing the flow (or decreasing the percentage as you shown in Simplify3D) does help in adhesion, but this is not an advantage of the skirt. The sole purpose of the skirt is priming. $\endgroup$ – 0scar May 31 '18 at 8:40
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These skirts they don't contribute at all to help your product adhere better to the build plate other than priming your nozzle so that it is ready to lay down filament for your product.

A skirt does give a good indication of the adhesion prior to printing your product, if the skirt does not adhere, maybe it is a good time to stop the print and re-slice with different options or fix the bed level.

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  • $\begingroup$ Helps to adjust Z height prior printing. $\endgroup$ – Fernando Baltazar May 31 '18 at 0:16
  • $\begingroup$ @FernandoBaltazar Yes! Hence fix the bed level :) $\endgroup$ – 0scar May 31 '18 at 6:03
  • $\begingroup$ prior? it helps during: you can reach under and twist the knob if one corner of the skirt is too light/dark. Lowering the feed rate helps to avoid any indiana jones style just-in-time hand escapes... $\endgroup$ – dandavis Jun 3 '18 at 10:14
  • $\begingroup$ @dandavis Yes prior, to me the skirt is not considered to be your product. If not adhering at all cancel the print, if uneven fix the level while printing the skirt. The printer types which raise their bed are easy to adjust as there is lots of space to get to the leveling screws. $\endgroup$ – 0scar Jun 3 '18 at 11:34
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Skirts prime the nozzle- getting the filament to first adhere to the build platform is necessary for the entire print. Once the filament starts to stick it usually will continue to.

Skirts can also help block any air drafts that might blow the filament away from adhering in the 1st layer of the print

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To use skirts as a bed adhesion tool

  1. Make the value of Skirt Offset from part to 0
  2. Set the number of Skirt outlines to 10 or 15 ,so that it forms an extended outer perimeter of the first layer

notes:

  • This also doubles as a primer for the nozzle
  • Also can be used to make minor adjustments to the distance between the bed and the nozzle , before the first layer begins printing
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    $\begingroup$ Basically you are describing a Brim... :) And that is an extreme case of a skirt. $\endgroup$ – 0scar Jun 4 '18 at 12:37

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