Wow @GregValiant THANKS for that thorough explanation.
You're saying that the layers printed on the bottom and top of the model, excluding the inner and outer walls are the skin (am I correct ?). I'm still confused though, under the shell category there is no way to set number or thickness of skin (just top surface skin layers), but I can change Top\Bottom thickness. I'm concluding that there are Top\Bottom layers (adjustable thickness ) and skin layers (non adjustable thickness), my question is what is the difference or relationship between Top\Bottom and skin ?
"Skin" generally refers to the top and bottom layers. Sometimes the word "mesh" is used (but a Mesh can also refer to a model in Cura). If you are printing a solid box (no lid or opening on the top) the "brim" or "Skirt" get printed first around the periphery of the model. Brims help hold models down. Skirts just get the plastic flowing (and can give you a look at how level the bed is).
Next the walls get printed. If you have a .4 nozzle and the line width is set to .4 then the wall thickness should be a multiple of the line width. A wall 1.2 wide will take 3 passes - 2 inner-walls and then 1 outer-wall. If the setting for "additional wall" is checked then after the outer-wall the print head will go around the inside one more time and add a 3rd inner wall. The printer will then print the bottom "Skin" by filling in the entire area between the walls. in the case of the box, it's the bottom layer you will be able to see when the print is done and off the build plate. It is the base for the whole print and it's critically important that it goes down right and adheres to the build plate.
The printer keeps adding walls around the outside, and skins on the inside until the Bottom Thickness and/or Bottom Layers is reached. As the print goes up from there, the walls keep going (except for the Additional Wall" which is only for skin) and "Infill" takes the place of "skin". Once the print gets to the top of the box, skin replaces the infill and you want the last skin to look nice so you might even iron your skin. That is the S&M part of 3D printing that nobody ever really talks about.
Do you need skin? No. If you slice the box with top layers set to "0" it will not have a top skin and the infill will go right to the top of the walls. If you then set Infill to 0, you will have an empty box with no top. If you set bottom layers to 0 you will print a rectangular tube - no bottom, no top, no infill.
Time to hit Thingiverse (you can get there from inside Cura) and get a calibration cube to play with.
Always remember that the #1 use of pigskin is not the making of American footballs. The #1 use of pigskin is to hold pigs together.
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