How to Disable Shell Thickness when 3D Model Has Non-Zero Thick Walls?

[mbmast 0]

I am unclear on the Shell Thickness setting. This setting seems to make sense only when printing a 3D model that is not solid, but whose wall's have zero thickness.

If, instead, the 3D model started as a solid and then has been "hollowed out" producing walls that are 2mm thick in some areas and 3mm thick in other areas (for example) then what should be specified for the "Shell Thickness" setting?

My question also applies to the Top/Bottom thickness setting.

Thanks.

[gr5 2,170]

Cura will *only* let you print solid objects - so if you use sketchup (which lets you create infinitely thin walls) it might refuse to print your walls if they aren't connected.

It's silly to print the solid parts of the model solid as PLA is very very strong. So usually I print hollow with "shell" set to .8mm (2 passes) or I'll do maybe 20% infill.

You can certainly do zero shell and 100% infill but all the passes will be diagonal. Or you can do all shell and no infill (set shell to 4000mm - it works!). Does this help?

[mbmast 0]

I'm still confused by this. My model has vertical walls that rise perpendicular to the build platform (think of an upside down dixie cup). The walls are meshes taken from a solid and are 2mm thick. So why do I have to specify a non-zero shell thickness in Cura?

[peggyb 170]

notice there is a difference in wall thickness in a cad drawing: this defines what the model looks like in measurements.

Then you will export the model as a stl, which is only a description of the outside of the model, like a balloon is the skin to keep the air in.

In Cura you decide how to build up your model with strands of molten plastic, size 0.4mm for walls, and lots of other settings like layer height, speed temperature..

So in Cura wall thickness means, how many strands of 0.4 mm do I need to get my necessary wall thickness (a multiple of 0.4). Thinnest wall is 0.4 and from there on 0.8-1.2-1.6-etc., with 0.8 being the standard thickness.  When printing a 2 mm object, means you will end up with 2 strands of 0.4 mm (0.8 total) next to each other all around and an empty space of 0.4 in the middle.

It is not necessary to print models totally solid, a wall thickness of 0.8 is really sturdy.

Take a look at the https://ultimaker.com/en/tips-tricks to get more information about printing.

Edited February 12, 2016 by Guest

[avogra 68]

Hey @mbmast,

I tried to paint something up Maybe it helps to understand how slicing works. Another wording of what @peggyb said: You give Cura a solid model and it will hollow it out to save material. Wallthickness is how much material is left vertically, bottom-/top-thickness is how much material is left horizontally.

Edited February 12, 2016 by Guest

[gr5 2,170]

If your part is shaped like a dixie cup upside down - and if your cad software specifies "wall width" - this has absolutely no relation to cura "shell".  Cura doesn't think of those as "walls".  An STL file contains triangles only.  Cura slices each layer with a horizontal layer and turns the triangles into line segments.  These line segments have NO RELATIONSHIP to each other - they are not sorted.  Cura tries to connect them together - figure out which lines are going to the same vertex.  For your "cup" it will have two polygons - one for the outer surface of the wall and a separate polygon for the inner surface.  The spacing between these 2 polygons needs "fill" or "plastic".  If the cura shell is .8mm and the distance between the polygons is < .4mm it won't print anything (no walls at all).  If the distance between the polygons is 5mm (5mm thick walls) then cura will do .8mm solid on the inner and outer surface of these walls but then will do an infill pattern in between (criss cross).

Usually you want your CAD walls to be at least 1mm wide if you have a .4mm nozzle (actually .8mm is usually the minimum but there is floating point errors and curves converted to lines and so it might get thinner so 1mm is usually safe). Sometimes Cura will print walls down to .4mm - I don't know why. But usually not.

Edited February 12, 2016 by Guest

[mbmast 0]

Then you will export the model as a stl, which is only a description of the outside of the model, like a balloon is the skin to keep the air in.

 

I don't believe this is true. My model has internal structures and those are preserved during the print. If STL were only a description of the outside, none of the internal structures would be preserved. Also, the "thickness" of the walls that I put there would also not be preserved, and they are.

[mbmast 0]

Here's what I've observed. If I extrude my walls to 2mm, then Cura for some reason still wants to know the shell thickness. I get an undesirable print result, but the print takes a reasonable amount of time, consumes a reasonable amount of filament and weighs a reasonable amount, as my model is mostly hollow (except for a few internal structures and the thickness of the walls).

If I give Cura the same model, only I have not extruded the walls (i.e. the walls have zero thickness), then Cura tells me the print time will be 68 hours, it will consume 161 meters of filament and it will weigh 587 grams. Clearly, if I were to print this, it would be solid.

So no setting in Cura works. If I give it a model with wall thickness, then I must specify a shell thickness that makes no sense. If I give it a model with zero wall thickness, then Cura generates gcode to build a solid model. I can't seem to win here. I want Cura to print my walls, as specified in the STL (i.e. my extruded meshes) solid and not add any of it's own "shell thickness." How do I do this?

[gr5 2,170]

I don't think you read the posts above very well. Sure there was a misunderstanding of how your model looked but read them again. The answer is above.

Another important detail. After it slices always. ALWAYS. look at your model in slice view. This will explain how cura intends to print your model.

It could be that it thinks your model is completely solid. If so maybe your cad software is creating a bad model (check in xray view - anything red is a problem) or more likely you have one of the "fix horrible" boxes checked. Try unchecking all the "fix horrible" boxes in the expert tab. They can fill in areas that are supposed to be hollow.

[peggyb 170]

can you show us a a picture of your model, that would help in explaining.

[mbmast 0]

I've spent the last few days experimenting with actual prints and with viewing layers in Cura.

I think what I've been told in this thread is the following:

1.  If the walls of the model have zero thickness, then you must specify something for the shell thickness (and it should be a multiple of the nozzle diameter).  And I find that this works, so long as the model has no internal structures.   It does not work if the model has internal structures (perhaps because the model is a blend of surfaces and solids).

2.  If I extrude my walls so they have thickness (within the model itself), then I can get a print by specifying zero shell thickness and 100% fill.  My model prints reasonably well and is completely empty inside, except for the internal structures I've placed there (solid tubes) and these internal structures print fine.  So this is the closest thing to what I'm trying to accomplish (the texture is very rough though, see photo below).

If I extrude my walls and specify a shell thickness, the results are bad.

One side effect of extruding the walls and printing with a shell thickness of zero is that the texture of the print is very rough, as the extruder has moved in a zig zag pattern.

In the below image, on the left, the wall had zero thickness and the shell thickness in Cura was set to 1.2mm (and the model contained no internal structures).  On the right, the wall was extruded to 1.2mm and the shell thickness in Cura was set to zero (and the model does contain internal structures).  Note the difference in texture.  Why was the image on the right printed with a zig zag pattern while the image on the left was printed linearly and is there any way to get a better print?

The X-Ray view does show some red, but very little and only near the very top of the model.  I have not fiddled around with the horrible settings yet.

[tinkergnome 927]

Hi @mbmast,

"a blend of surfaces and solids" does not work at all for any kind of manufacturing. Consider that things like "walls with zero thickness" does not exist in a physical world...

First:

You have to design your model solid, or "watertight" (or "2-manifold" if you like).

There are several articles that explain it further, here is an example.

If your stl-model is not solid, the results are not predictable (i.e.: the results are depending on the "auto-repair" capabilities of the slicer).

Second:

Usually the slicer lays a "shell" all around your "walls" (with the defined "shell thickness") and fills the gap (inside of the shell) with "infill" (you can define the fill percentage).

If you omit the shell, only the infill is printed (the "zigzag"). Ususally that's not what you want... (you have seen the results)

 

If I extrude my walls and specify a shell thickness, the results are bad.

 

To be honest: that's the way all other people are designing (and printing) their models...

Can you explain the "bad results" a bit further?

As a rule of thumb for slicing:

Looking at your model - each "wall" begins at the first surface and ends at the second surface (and so on). The space between two surfaces defines a wall, everything else is hollow. That means: if you count the surfaces (from any perspective): it has to be an even number.

Another idea:

How thick are your walls? For example: you can not print a shell if the wall thickness is smaller than 2*nozzle diameter.

I'm also not sure about your definition of "internal structures". Certainly you can print a cube with several cylinders inside (e.g.) - but again: your model has to be solid (all faces closed - no matter where the faces are).

Edited February 15, 2016 by Guest

[avogra 68]

@mbmast, you haven't yet understood the concepts of slicing.

It would be helpful if you give us some more information: Which program do you use to create your model? Give us some screenshots of your settings in Cura, as well as screenshots of the model in standard view and in sliced view. Show us a screenshot of those inner structures so that we know what you are talking about.

 

I think what I've been told in this thread is the following:

1.  If the walls of the model have zero thickness, then you must specify something for the shell thickness (and it should be a multiple of the nozzle diameter).  And I find that this works, so long as the model has no internal structures.   It does not work if the model has internal structures (perhaps because the model is a blend of surfaces and solids).

 

No, that is not right. I guess in the software you use to create models you can define a wall thickness, but Cura's parameter shell thickness has nothing to do with it. Furthermore in the STL file format there is no information stored about any thickness or walls or such, it only describes surfaces enclosing your model volume. That is also one of the problems of the format for 3D-Printing: It is possible to create STL-files which have no clear inside and outside. That is what X-Ray-Mode is for, there you will see such errors.

Maybe the word surfaces is better to explain how slicing works. For Cura, it doesn't matter if those surfaces are inside like a drilled hole or if they make the external boundary. (There is an exception to that, as Cura by default removes all surfaces that are entirely enclosed inside the model).

From the surfaces inwards, Cura lays the shell lines, so the "shell" can easily be in your model.

[avogra 68]

I just looked over my post with the drawing again. Unfortunately I used the word "wall"-thickness there. That is not correct and might confuse. Actually it should be shell-thickness.

[mbmast 0]

Ok... I appreciate all the good input.  Give me a day or two and I'll post back here, with photos.  Is there a way to post the STL file here (or a zip of the STL file)?  The 3D Upload appears to only want a photo of the printed model and doesn't accept STL files.

Edited February 15, 2016 by Guest

[yellowshark 153]

Hi @mbmast, try this simple object. You have a 20mm cube; within the cube you have, centrally located, a 15mm cube of space. So your object has, around the space, an object which measure 5mm in thickness. Cura will print the 5mm thick part leaving a 15mm cube of space.

Now centrally, in the cube of space you have 4mm rectangle which extends from the bottom part to the top part, so that will be 10mm in length/height. Cura will then print the 5mm thick part as before, along with the 4mm*4mm*10mm central piece leaving a 5.5mm thick area of space around the sides of the central piece; not around the top and bottom of the central piece as it is attached to the 5mmm thick object.

Now what you have to decide, and tell Cura, is how to build the solid parts which are 5mm thick and 4mm thick. This has nothing to do with your 3d model. In this instance it has built a 5mm thick piece around the void and a 4mm thick piece inside the void. How you build them is split into 2 parts, walls and infill. The reason for this is that it is not always efficient to build your parts of 100% solid plastic. You can build them hollow, partially filled or solid. To print something solid will take you more time normally than printing them hollow or partially filled and will certainly use more material.

Whatever you choose the first step is to define the size of the "walls", i.e. that thickness of plastic that goes around the outside of the part. This is normally set to 0.8mm; if you are going to print hollow then it may be better to specify 1.6 to give the part more strength.

Normally you will not print hollow and will often print partially filled. So in this example you may define 0.8mm for the wall and 50% infill. So you will get a solid outer wall around each piece of 0.8 and in the middle of the walls you will get 3.4mm thickness of 50% plastic. If you want to print solid then you can either change the 50% infill to 100% infill, or set the wall thickness to 2.4mm - or maybe 2.5mm or 2.8mm, I am not sure which one would be best. As someone said  earlier, you are telling the 3D printer how to manufacture the 3D model;

Edited February 15, 2016 by Guest

[gr5 2,170]

yellowshark - tldr - sorry. Pictures please.

[gr5 2,170]

mbmast - you still have a basic misunderstanding of the terms like "wall" and "shell" and "solid". I can't quite put my finger on the misunderstanding but there is a major communication flaw between you and the rest of us. Eventually a light bulb will go off in your head and you will see your very simple but incredibly important misunderstanding.

All the posts above from the other people are trying to fix this misunderstanding - maybe if you read them again more carefully.

I don't understand what the hell you are saying with "extrusion" as this is not a normal thing in my cad software, nor cura. When I think of extruding walls -- well never mind.

Let's split the CAD step and the SLICE step up a bit.

The CAD step must never define walls. It must define solids. If you are designing a cube that must be hollow or a cup, then you can define the "walls" which are really not walls - but solids. Don't call the "walls" "walls". Call them cubes or cuboids. A cuboid is a box - a rectangular solid that may have different lengths. Boxes have 3 lengths. Don't call them walls because it seems to be confusing you.

Again, the CAD step defines *only* solids. Never hollow things. It defines what areas in 3d space are solid and which are air.

In the SLICE step you can make it so that solids are hollow inside PURELY to save material and time. People won't know it is hollow because they can't see inside.

In the SLICE step you can set how thick the "walls" or "shell" of these solids are and/or give it a diagonal infill pattern with varying density.

If you are printing a "house" with "walls" don't think of them as walls. Think of them as solid cuboids for the sake of the CAD step and the SLICE step. Cura will not slice cuboids with a horizontal dimension less than 2 times the nozzle width so make your cuboids at least .8mm thick.

[yellowshark 153]

George - tldr? . Lol the picture is in my mind. A cube, with a hollow middle shaped as a cube, with a square column in the middle, running from "floor" to "roof".

[gr5 2,170]

Like a cube with an empty air space inside shaped like a donut?  But squared off?

[gr5 2,170]

20mm cube minus 15mm cube leaves only 2.5mm thick, um, walls. Let's not use the word wall in this topic as it is a source of confusion for the original poster. Anyway the point is your dimensions don't add up yellowshark.

[yellowshark 153]

Lol yes my mental maths were incorrect :DThat would of course be 2.5 mm thickness all around the inner cube of space.

Well if you are relating the object to be a donut and the donut ring to be the space inside then yes. Except the donut ring is not visible from the outside because the top and bottom of the ring both have 5mm of material at the perimeter, so probably not the right analogy. And in the centre of the ring is a 4mm square column 10mm high.

Edit the rest of it which was equally inaccurate/confusing.

Put it another way - in your 3D software you draw a 20mm cube. The cube is hollow. The thickness of the cube all round is 5mm, which gives you a 10mm cube of space in the middle. Running vertically, through the middle of the void is a 4mm square column; by definition the height of the column is 10mm. On the four sides of the column you have 3mm of space.

... my original post from para 2 follows on

it is a 20mm cube with 5mm walls and zero infill and 5mm thick top and bottom layers. Then running vertically though the cube, positioned on the x,y axes at dead centre, there is a 4mm square column. Hope that clarifies.

Edited February 16, 2016 by Guest