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Air gap requirement for custom supports

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I've been trying to print an object that has a horizontal bridge piece over the buildplate, for which I've designed a custom support (with a sort of dovetail-shaped surface making contact with the underside of the bridge). However, so far I haven't been able to print this piece with an air gap large enough that the support doesn't stick to the model. It also seems like the underside of the bridge is being printed as infill rather than a bottom surface. The layer height is 0.12 mm, and the largest air gap I've attempted is 0.25 mm. Is there a rule of thumb for how big this can be, or needs to be? Thanks for any responses.

Edited by Guest

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What gap did you leave between the custom support and model? That is the gap you will likely want to use in Cura.

I just did something similar recently and built a slightly slanted plane from bed to bridge bottom and left a 0.2mm gap. It spanned 160mm and was quite thin.

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What gap did you leave between the custom support and model? That is the gap you will likely want to use in Cura.

I designed the supports in Sketchup along with my model, and I left a gap of 0.25 mm. Is there something that I'm supposed to change in Cura to get this to work properly?

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The settings you used should work. Is there a gap in the layer view? Does it actually print the gap? Maybe there's another reason it's sticking.

I model critical sections with 0.1mm or 0.2mm z-axis resolution from the base depending on the layer height I plan to print at. If you use 0.2mm resolution that allows for 0.1 or 0.2mm layers with proper alignment. So, I personally would not leave a 0.25mm gap, but rather 0.2mm.

Imagine a feature in your model starting at 1.0mm. If you use a first layer height of 0.3 and a layer height of 0.2 that feature will not begin where you modeled it. I keep this in mind while modeling and changing settings in Cura.

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I haven't had a chance to retry with xisle's suggestion of keeping all heights to a multiple of the layer height, but I'll report back when I do. Even if it's difficult to detach, I'm still not sure why the underside of the bridge looks like infill rather than surface. Labern mentions that it might be because the gap is too big, but why would that be the case?

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I'm still not sure why the underside of the bridge looks like infill rather than surface.

 

The first layer or two of the bottom of a bridge won't have a smooth connection between each x/y pass since they droop and won't stick together. I often cut a few of the strings off to clean up the bottom if the span is great enough to warrant it.

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If there Is a gap of 2 horizontal flat surfaces in cura then it will do a top layer then infill. Its a bug and not sure when it will get fixed. If the gap is small then it wont be effected but the bigger the gap the worse it gets.

 

I see, yeah that sounds like what's going on, because there was more solid surface above the gap infill.

Edited by Guest

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Just an update, I retried the print (0.12 mm layer height) with a few different gaps, none of which worked:

- 0.12 mm gap, layers are totally fused

- 0.24 mm gap, layers are fused but seam is visible

- 0.36 mm gap, layers are separated enough to break apart, but then I get the infill pattern on the bottom surface of the bridge

So yeah, I haven't been able to get this to work yet. For the time being I've switched to a very large gap between the bridge and the support structure and then I let Cura fill supports in that gap.

Edited by Guest

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