Summary

This is a 3-dimensional representation of the Klein bottle, a non-orientable surface without a boundary. Like the Möbius strip this object only has one side, so you can go from one point to its opposite without crossing a boundary.

UPDATE: File "Klein_bottle_without_barriers_Thingiverse.stl" does not have a wall through the tube, so that you can fill in a e. g. a fluid.

Print Settings

Printer:

CTC Bizer

                                            <p class="detail-setting rafts"><strong>Rafts: </strong>                     <div><p>Doesn't Matter</p></div></p>                                                                                <p class="detail-setting supports"><strong>Supports: </strong>                     <div><p>No</p></div></p>                                                                                <p class="detail-setting resolution"><strong>Resolution: </strong>                     <div><p>0.25 mm</p></div></p>                                                                                <p class="detail-setting infill"><strong>Infill: </strong>                     <div><p>100 %</p></div></p>                                                        <br>                        <p class="detail-setting notes"><strong>Notes: </strong>                    </p><div><p>When printing with a transparent material, do not use support since it will be visible and impossible to remove. The model is only the surface and not a solid object, so you should print with 100 % infill to completely fill in the wall.</p></div>

How I Designed This

Constructed using Blender. Since you cannot determine the normal vector, I had to make a cut for a consistent normal vector on the surface.
I have added the Blender file. There you can change resolution or wall thickness.