What's the Purpose of Extra Shells?

I've had a few prints stuff up on me, the cause of which is Extra Shells. This can be see in the photo below. These can be fixed by turning the extra shells off, but I was wondering, what's the purpose of Extra Shells? What benefit are they meant to provide? It is extra strength? If so, surely a few more percentage points of infill would do a better job.



Edited 1 time(s). Last edit at 11/26/2011 12:37AM by Shibboleth.

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systematictechnology.net

We work with layers. Say we have a hole on a box.

For example setting base layer 1 and alternate layers 3, would make on solid layers, one layer with +1 shell and the next alternate with +3 shells. Then the layers will be intermitent 1 and next one 3, hence it gets better grip on the 100% infill lines which contact 2 extra shells above and below. So should get better adhesion to each other - in theory.

And the setting of extra walls for sparse layers, for example we are happy with 1 extra on solid layers, but to get more structure on sparse layers we set that higher, again, more rigid structure because in that area layers are otherwise kinda empty and we dont want that to affect our "hole walls".

P.S. your issue could also be regarded as a width over thickens adjustment. But thats rather hard to tweak each time and ppls use so different settings. So for a different W/T settings, SF will slice that piece differently, and at some point that gap there will be gone. Or you can indeed set all extra layers to 1, which seems to works faster and reliable indeed. But thats not where the problem originally comes from, this is more like a hotfix - sort of speaking. Also cant have a generic W/T that would fit perfectly on all small object sizes and direction, so the problem is bound to pop up at some point or another. Unless you use a very small nozzle and layer heights and narrow line widths - stuff like that would automatically avoid many such cases.

Edited 3 time(s). Last edit at 11/28/2011 01:54PM by NoobMan.

Width over thickness? Thats where SFACT comes in handy. There you can specify how wide your extrusion should be in mm and dont have to tweak anything else..
The extra shells come in handy if you need a rigid shell and the body is tooo fat to make the fill more dense..
Also one very important use is: If you need really precise holes (or other dimensions) and you are willing to rework with a drill, a tap or a file, you can make the walls thick so that there is material that can be removed without "hitting" the infill.. (The few points more fill wont help you there ..)

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I find that extra shells make a part more rigid on it's surface. There is less "creak" (maybe just PLA?) when squeezing a part between my fingers with extra shells, and the parts just feel more solid.

the second shell will print first, and the perimeter last and slower, so you have a nice, consistent exterior. the second shell also gives another surface for the perimeter to adhere to, making it more solid. the second layer also acts as a buffer between the infill and perimeter, which also helps with exterior finish.

I often use 2 extra shells on all layers, but if it interferes with the slicing or i want a faster build, i use less. I've also found that if you vary the number of shells throughout the print, you get artifacts on the surface, probably caused by differing amounts of shrinking between layers with more/fewer shells.

I also add extra solid layers below surfaces, so that i have 3 layers on all sided.

It may just be PLA, but i use a constant second shell to reduce shrinkage as well as making the walls stronger.