The encasement in the middle is printed in Stereolithography (STL) technology. In comparison the blue and red case is printed in polished Selective Laser Sintering (SLS).
The STL case is very smooth.
The STL encasement has a very smooth and soft surface and is even more solid than the polished SLS version.The STL looks rather matt compared to the shiny surface of the polished SLS print.
The red and white encasements are identical. They have threads for M3 screws to fasten a cap. It is easy to turn the screw in the SLS case but it is much more difficult to turn the screw in the STL case.
body interaction 
The STL prints looks very smooth. Which printer did you use?
The STL print was done by a 3d printing service. Unfortunately I don’t know which printer they use.