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I making a beer mesh:

1. After selecting the desired faces of the mesh

Selected desired item

Using the hotkeys Shift + D to duplicate my selection.

2. And using P to separate the selection enter image description here

(Looks completely normal in EDIT MODE) enter image description here

The selection is succesfully duplicated and added in a separate layer.

3. Then I exit from EDIT MODE, the new mesh changed to this! enter image description here

I tried to make the scale of my actual bottle to 1 and doesnt work i have no idea what i made to look the new mesh like this.
Better view of the mesh out of Wireframe mode enter image description here Is my first post so i hope i being enoughly self explanatory about my problem

beer mesh file

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    $\begingroup$ I don't see major differences in size, but anyway original object has Subsurf modifier on, so does the duplicated piece of it. While smoothing the object modifier slightly changes its size $\endgroup$ – Mr Zak Jun 16 '18 at 23:14
  • $\begingroup$ Thanks! so should i desactivate the sub and duplicate the disire piece? $\endgroup$ – Pablo Larreátegui Jun 16 '18 at 23:18
  • $\begingroup$ If you mean dealing with Subsurf modifier then generally 2 options - remove / hide modifier from original object, then duplicate or duplicate, separate and remove modifier only from the copy. Modifier works on object level, so it will be present on the copied geometry when the latter is separated into different object $\endgroup$ – Mr Zak Jun 16 '18 at 23:37
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The subsurf modifier works by interpolating vertices between the existing geometry to create smooth curves. So the the shape of the object depends then on not only on the placement of the vertices, but also on all the surrounding vertices connected to them.

If you copy and detach a just portion of the object, the Subsurf modifier will deform the object differently, as the surrounding vertices are different, there are less vertices connected to it. And even though the vertices are in the same place as in the original object, the way the faces deform in the original bottle are pushing the faces to a different place...

So what can you do?

First of all, it is really not a good idea to have objects with faces that occupy the same space. That will result in a problem called Z-fighting. Just like in real life, two surfaces cannot be in the exact same space, one will be on top of the other inevitably, and the other behind... But with 3d even when surfaces are very close together you might still experience Z-fighting.

(by the way you will have the same problem with the liquid inside the bottle, as it is overlapping completely with the bottle)

(read: Moire Like Blinking when preview rendering problem)

"pero qué tío más pesado, menos rollo..." I know...

How do you fix this then?

It all depends on what you are trying to do with that new object that you want to create. Maybe instead of adding more geometry you can assign those selected faces a new material. A lot of the times with 3d you are faking things, unseen geometry still uses computation resources and being true to reality doesn't bring any benefits...

enter image description here

An alternative is to select the faces in the original object and extrude them slightly so that they remain connected to the bottle and they subdivide in a way that follows the overall shape.

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