Air gap requirement for custom supports

[Jarls 0]

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 September 30, 2015 by Guest

[Labern 774]

If the gap is to big then you start to get the infill issue.

I normally set the gap to 0.1mm

Have you tried no support?

[xisle 34]

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.

[Jarls 0]

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?

[xisle 34]

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.

[yellowshark 153]

Hi guys, sorry for my ignorance, but if you have a gap surely the filament printed for the bridge just drops and attaches itself to the support material. Clearly I am missing something in my understanding?

[peggyb 170]

I keep this rule in mind: leave a one layer gap. If you print in 0.1 layers leave a 0.1 gap, so that the print skips one layer before printing the next. The bridge layer will 'drop' onto the support but can be removed easily.

[xisle 34]

The support broke off very easily with a 0.2mm gap..

[Jarls 0]

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?

[xisle 34]

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.

[Labern 774]

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.

[Jarls 0]

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 October 5, 2015 by Guest

[yellowshark 153]

Bump

 

Hi guys, sorry for my ignorance, but if you have a gap surely the filament printed for the bridge just drops and attaches itself to the support material. Clearly I am missing something in my understanding?

 

[Jarls 0]

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 October 7, 2015 by Guest

[Toolman12345 0]

Where the heck IS the air gap setting! I can’t find it, even by searching, lol.

[Labern 774]

This was a very old version of cura. The air gap isn't a setting. It's the amount of gap in the part model between surfaces as this thread is about custom supports that the user is designing.

[Toolman12345 0]

1 hour ago, Labern said:

This was a very old version of cura. The air gap isn't a setting. It's the amount of gap in the part model between surfaces as this thread is about custom supports that the user is designing.

 

Thanks. I found it after posting my question, of course. In the current version of Cura, what some people refer to as "Air Gap" is now "Support Z Distance" with separate settings for Top Distance and Bottom Distance. Like many, I struggle to remove supports, especially when using PLA+, and am always looking for ideas. My last struggle was to get a mere 2mm  worth of supports off the bottom of a delicate piece, about 4 inches by 3 inches with several small sections that wanted to come off with the supports; the supports were too small to grab hold of with pliars even, and I had to use a sharp scraper - I was afraid I was going to slice myself open in the process, it was very difficult to get them off without destroying the piece. One person's idea was to increase the air gap/support top distance to .4 or .6, to ease in support removal after printing, thus my search.

[geert_2 557]

There is a post of me with more tips for single nozzle - single material supports, but I can't immediately find it.

 

Anyway, here a few of my standard pics with tips:

 

Tiny custom support of 10mm wide (dark blue) for the overhang. Ribs on top (0.5mm) allow tighter gaps, but still prevent the support from sticking too hard.

 

Supports as thin layers, not connected. When printing they will sag a bit, but can be peeled off one by one. Usefullness depends on the model.

 

Free hanging support concept for overhangs. The inverted staircase reduces the tendency of triangles to curl up. The tiny distancing pins (1mm gap, pins are ca. 0.5mm wide, 0.2mm high) allow easy cutting off, without much damage to the model. The 1mm gap is to be able to insert a knife or scalpel.

 

Idem. This concept of free hanging supports reduces the amount of support needed, and they do not destroy underlying parts of the design (see my name), contrary to vertical support-columns.

 

The orange and pink parts are supports: they extend from the model, so I can grab them with pliers and wiggle them loose. And they have extra brims to improve bonding, as they are very small, too small to get in there with a knife or scalpel to remove. For reference: the text caps-height is only 3.5mm.

 

A few more concepts: separate chunks of support, so you can wiggle them loose individually. Extended supports, and supports with holes, so you can insert a tool to pull them out, or grab them easily with pliers. Slightly bigger supports than the model (bottom right) to improve the outer edge of the bridge. Make a test pattern with different methods and gaps to see what works for you: it depends on materials, models, size, and printing speed and temp, so you have to try.