Issues regarding model thickening and hollowing

[StoneAICoder]

Hello,
I applied a "hollowing and thickening" operation to a cubic STL model.
The original box dimensions were [10, 10, 5], and I applied a 2mm thickness. However, the resulting model dimensions were [12.346, 12.346, 7.234]. This did not yield the expected result; I anticipated dimensions of [14, 14, 9], as a 2mm shell thickness should logically add 2mm to each side (i.e., +2 × 2mm) along the length, width, and height.
Additionally, analysis using third-party software revealed that the internal wall thickness (left and right sides) was approximately 1.17mm, rather than the intended 2mm.

Below are images showing the operation and the analysis results.

[StoneAICoder]

Additionally, I took a standard STL model and thickened it by 4mm. Upon inspecting the cross-section (Z-slice), I noticed that at certain heights, the outer wall was embedding itself into the inner wall (where the inner wall represents the original model).

See the images below:
The arrow on the left points to the problematic area, whereas the result on the right—generated using Magic RP with the same 4mm outward thickening—is correct.

Do you have any suggestions or strategies for resolving this embedding issue?

[Grantim]

Hello, neither images nor archive are opening, it seems that that they link to this issue instead of actual data.

[StoneAICoder]

I've re-uploaded the images and the zip file.

[Grantim]

I see images now, but still cannot see archive. As for ties - it is expected for highly curved areas, you might try tuning "Normal Trust" parameter to achieve better result, but at the end it is not guaranteed with vertex-based approach. You can try voxel-based thickening, which produces self-intersections free result

[StoneAICoder]

hou-div4-repair.zip
OK，The file has been uploaded. It is a standard yet complex model included with Blender.

[Grantim]

Hello!

This tool applies thickness to vertices, along their normals. So It is expected that in case of cube you have 2*sqrt(3)/3 ~ 1.15mm face offset (while distance between corresponding vertices should be 2mm)

[StoneAICoder]

I understand what you mean about the algorithm—something similar to the logic used for triangle edges—but:
How do you achieve a uniform 2mm thickening based on the faces?

[Grantim]

Use voxel-based algorithm, in Offset tool, it also guarantees to output non self-intersecting mesh

[StoneAICoder]

Alright, I'll give it a try and see how it performs—though the standard model seems to yield pretty good results already.