Strange result slice polygons

[Darwin33]

I am trying to slice volume A, it has vertical faces curved in one axis. 

When I apply slice plane, then the polygons change the curvature and create an strange edge in the original face. You can check this in element , as result we have element C, where clearly you can see an strange geometry, a face with curvature in several axis..

Could you explain this.

[10DSpace]

@Darwin33 

It's hard to tell from your image because you are not showing "edged faces", but it looks like for the object labeled "B" you may not have enough support edges along the curved surface.  So what you may be seeing is a smoothing error because of the new vertices/edges introduced by the slice plane.   If so, to correct this make sure you have regular support edges connecting the upper and lower surfaces and then try slice plane. 

For the object labeled "C" it looks like you have already assigned a material to it, and it may be that when you sliced it you changed the alignment of the UV Mapping and the texturing is off.  I don't know what type of UV mapping you have applied to the object so this is again a guess based on what you show.    But this could also just be the same lack of support edges as mentioned above for "B".  

[10DSpace]

@Darwin33 

Upon further reflection, I think the problem is most likely lack of support edges for both "C" and "B". 

[Darwin33]

If I made the slice in the center of the element then it works properly,  but when I make the slice in another section different than the center, then the anomalous surface arise.

Pls check the pic attached.

[BenBisares]

I would try adding a "Edit Patch" modifier which will triangulate all polygons, then add an "Edit Poly" modifier to continue doing the Slice.

Generally you'll get less surprises with triangular polygons when doing arbitrary modifications like with the Slice tool.

Probably a more elegant way to do this but I been using this method ages ago when I model objects for 3D printing and never looked for alternatives.

[10DSpace]

@Darwin33 

To explain a bit more about what is meant by "support edges" see image below.  In your object you seem to have only vertices without any support edges.   In the object shown below I can slice anywhere along the object without getting the anomalies you are seeing.  

You can fairly easily add support edges by the method that @BenBisares suggested (adding an Editable Patch modifier).  This will create triangular edges in addition to the ones that I show above, but the slice will work fine with these as well.  If you want all quads (4 sided polys) you can always add an edit poly modifier and click on "Quadrify All" in the Edit Poly modeling ribbon.   

@BenBisares 

Thanks for the tip re Edit Patch automatically adding edges to geo like this.  I was not aware of this.  Combining this with Edit poly modifier and "quadrify all" works nicely to get quad geo where there were missing or no support edges.  It may not rise to the level of "elegant", but it works for me.