Object with different print settings and top surface removal?

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I’m trying to create an object with good strength and rigidity, but keep some “give” in the top surface, as this object will be screwed against a rough wall

I tried different approches, but none of those gave me the expected result :

1. I created the object and added a modifier on the last upper 2 mm, to define a lower wall count and density, and remove the upper layer (top surface)
   this give me 2 different areas, as expected, but no solid layer between them both areas

2. I split the object by creating 2 parts with the expected properties (same as using the modifier)
   if I print each part separately, I get the expected structures, bottom part with an upper layer, and upper part with a solid first layer
   but when I print the whole object, I loose any separation layer between the 2 parts, leaving the bottom part infill visible

3. i tried exporting the assembly as one .stl file, but the result is only one set of settings

I seems that merging 2 parts in one object confuses the upper and lower surfaces settings of the parts, or I missed something.

Any one tried this and could share some advice ?
thanks in advance

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here is a picture showing :

• on the left, the individual print of the 2 parts,
• on the right the print of the object made by merging both parts : no intermediate layer, the bottom part infill patern is visible
  
  

I also noticed that the prints from Bambu Studio (bottom of the picture) have a partial top layer, while the same objects printed on the same printer using Orca (upper row) don’t have this unexpected upper layer

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answering myself for reference if some one is looking for the same :

single object with modifier is the way to go, with the followinfill %ng options :

• no top layer (thick =0mm, 0 top layer)
• infill % 25%, patern : giroid (or any other patern you like)
• “insert solid layer” parameter set to the layer(s) you want to create the flat area
• modifer for the upper part of the object , with lower density fill in (and patern if you want)