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I am consistently getting these strings in the overhangs on models that folks say they are printing without supports. Can anyone tell me what is causing them and what I can do to fix them?

I've attached a picture mid-way during a BB-8 print. I'm using PLA at 75 % speed.

Stringing on overhangs

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  • $\begingroup$ Welcome to 3DPrinting.SE! Can you please edit your question and include the other parameters you are using? $\endgroup$ – Pᴀᴜʟsᴛᴇʀ2 May 4 '19 at 2:13
  • $\begingroup$ Could you please tell me what to add? (I've just started 3d printing recently, not sure what's important to know) $\endgroup$ – oneself May 4 '19 at 14:28
  • $\begingroup$ Take a look at this Meta post.. $\endgroup$ – Pᴀᴜʟsᴛᴇʀ2 May 4 '19 at 14:40
  • $\begingroup$ At which layer did this happen, or at what height? You can look into that layer/height in your slicer. E.g. in Ultimaker Cura you can watch each layer and e.g. show travel paths. This helps you to define where the strings come from, from the part itself or oozing during travel. Please add print temperatures and speed (in mm/s) as absolute minimal data. $\endgroup$ – 0scar May 4 '19 at 19:21
  • $\begingroup$ What's your layer height? Looks like 0.3 or so which is unlikely to work for a print like this. $\endgroup$ – R.. GitHub STOP HELPING ICE Jun 4 '19 at 2:51
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While this looks a lot like stringing, it's not what I would call stringing. I usually reserve that term for material that should never have left the nozzle to begin with, due to insufficient or missing retraction or excessive compression of the filament between the extruder and the nozzle. The "strings" you've shown look like desired wall extrusions that did not adhere to the adjacent walls in the same layer or to the previous layer.

This is common when printing concave perimeters that are overhanging, due to a combination of minimal-to-no contact with previous layer and acceleration of the nozzle away from the previous wall in the current layer. Sometimes under-extrusion can also be a factor.

You can often mitigate this by:

  • using thinner layers (for the whole print, or "adaptive layer height" that will dynamically adjust as needed). For a given wall slope (fixed rise over run), this will reduce the "rise", and thereby place the walls of the next layer such that they overlap more with the previous layer. Sometimes this makes the difference as to whether they overlap at all.

  • increasing hotend temperature. This will improve bonding with adjacent wall and with previous layer, if there's any contact with it.

  • decreasing print speed, especially for outer walls. This will reduce the effect of the nozzle pulling the wall away from the adjacent wall it's supposed to bond to, and will also deliver more heat to the adjacent wall and previous layer wall you want to bond to.

All three of these will also help if under-extrusion is part of the underlying cause.

If you can't get any of this to work, using supports is always an option, but spherical (as opposed to flattened) domes generally "shouldn't" need support to print.

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You did print overhangs without supports. I have printed a Thermal Detonator for StarWars day (May the 4th be with you!), which is pretty much an empty pair of domes. To ensure that the upper layers stay up, I have activated support starting at 40°, and it came out perfectly fine. Without support, the lines would have had nothing to hang on and sag down like in your print.

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  • $\begingroup$ I've downloaded the model and sliced it in Ultimaker Cura, it does not look like it needs supports to be printed. The original maker also suggests to print it without supports. It must be the settings that are incorrect. $\endgroup$ – 0scar May 4 '19 at 20:58
  • $\begingroup$ this needs an overlap to avoid the bridge over hang $\endgroup$ – Fernando Baltazar May 5 '19 at 6:12

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