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Blender 3.6.4 on Linux.

I've made 2 surfaces with the "Mesh/Math Function/Z Math Surface" add-on (from "Add Mesh: Extra Objects")

They have the same size (x-y), and are on top of each other.

I selected a first border of the first surface, and the corresponding from the second, and hit 'f', to make a face between two 2. I repeated this for the 3 other borders, and got what seems to be an enclosed volume.

enter image description here

If I try to substract a cube from it, I can see that it is hollow (no surface is made where the cube is substracted)

enter image description here

If I export it in stl and load it in Cura, the gap in the first function are partially "filled", floored at the level of the 0 of the lower function (see image)

enter image description here enter image description here

How can I "fill" between the two surfaces, so that I can later substract volumes,... like I can do with "regular" meshes (sphere, cylinders,...) ?

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  • $\begingroup$ Hi. Please use a title that matches the content of the question. It should be descriptive but succinct, unique and identifying, summarizing the issue in such way that anyone searching for a similar problem is likely to find it. Use the edit link above, remove anything superfluous, avoid words like "this", "help", "issue with" or "question about". Remember, your title is the first thing potential visitors see, answers you get depend heavily on how insightful it is. See What is the problem of asking “How do I do this?" $\endgroup$ Oct 6, 2023 at 20:38
  • $\begingroup$ Are the normals of your combined surface all facing outward? (Perhaps the bottom half is inside-out with respect to the enclosed volume).. (Alt-N) $\endgroup$
    – Robin Betts
    Oct 7, 2023 at 7:43
  • $\begingroup$ The "fills" are obviously created in Cura to make the model stable enough for printing. What Cura does, Blender cannot prevent. ``` How can I "fill" between the two surfaces, so that I can later substract volumes,.... like I can do with "regular" meshes (sphere, cylinders,...) ? ``` What do you mean by this? When or how do you want to do later something with it? In Cura? This is Blender Stack exchange. We don't know and can't answer questions about Cura. $\endgroup$
    – maddes8cht
    Oct 7, 2023 at 9:39
  • $\begingroup$ Let me try to clarify. If I make a sphere and substract a cylinder from it (using a boolean modifier), I can see that blender created surfaces where the cylinder surface intersected the sphere. The resulting partial sphere is closed. But with my volume here, no such surfaces where made when I substracted a cube (you can see the inside of the shape in the screenshot). It seems like Blender considers my object hollow, when the sphere is considered full, but maybe that just my impression and not the correct explanation. $\endgroup$
    – Vin100
    Oct 7, 2023 at 17:09
  • $\begingroup$ I suspect that my model has something wrong, but I don't know why. I have two reasons to suspect that: (1) when substracting a cube, it doesn't do what I expected (see above), and (2) Cura doesn't handle the resulting shape as it should. With (2°only, I would suspect Cura, but (1) is fully in blender, so I think that it's a problem with my model. Sorry for any confusion. $\endgroup$
    – Vin100
    Oct 7, 2023 at 17:13

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