Top surface skin
Top surface skin is the top-most layer of top skin, which can be adjusted separately. This allows to print a preset number of layers at a higher quality which results in a better surface finish.
The parameters which can be adjusted are found in their respective categories and can easily be found by searching for 'top surface' with the settings search bar:
Top surface skin extruder
The extruder can be set for the top surface skin layers. The second extruder may contain a different print core or material.
Top surface skin layers
This setting defines how many layers of all top skin layers will be affected by the setting. Two or three layers are often enough to have an improved surface quality.
Top surface skin pattern and line directions
The pattern can be set separately to get a different visual effect. If set to lines, the angle of the lines can also be set for more precise control.
Top surface print speed
The speed can be changed to enhance the print's surface quality. Print it at the same speed as the outer wall to get a smooth finish.
This setting defines which extruder should print the top and bottom layers. Different colors or nozzle sizes can be used for the second extruder.
With the top/bottom thickness you can set the thickness of the solidly printed top and bottom layers of the print. A higher value ensures all gaps on the top and bottom layers are closed completely. However, this can also increase the print time and amount of filament used.
It is advised to always use a multiple of the layer height for the thickness of the top and bottom. This means, for example, that with a layer height of 0.15 mm, it’s better to set the top/bottom thickness to 0.6 mm rather than 0.7 mm.
The model on the left has a top/bottom thickness of 1.4mm, the one on the right is just 0.7mm.
Separate top or bottom thickness
You can also set the thickness of the top and bottom layers separately. This is especially useful for the top as you may need a few layers to close it properly and prevent “pillowing”. By using fewer layers for the bottom you can save material and print time.
Number of top/bottom layers
Instead of setting a height in millimeters for the top or bottom layers, you can set a specific number of layers. The resulting height in millimeters will be calculated automatically depending on the layer height set. Example: Number of top layers 12 * 0.1mm layer height = 1.2mm top layer thickness.
Initial bottom layers
The number of bottom layers printed directly on the build plate can be adjusted separately. Reduce this value to improve print speed and reduce material cost, as the initial bottom layer is always fully supported and quickly watertight due to the surface it is printed on.
Ultimaker Cura allows you to choose from different printing patterns for the top and bottom layers. These are the available patterns:
- Concentric: The pattern is printed from the outside to the center of the print.
- Lines: A diagonally printed pattern with travel moves on the shell of the model.
- Zig-zag: A diagonally printed pattern with connections on the shell of the model.
Lines, zig-zag, concentric, and 'line direction' with custom value 
Bottom pattern initial layer
This refers to the pattern of the layer that is printed directly onto the build plate. It can be changed separately. Use this feature to get models with specific bottom visuals.
Monotonic top/bottom order
Print top/bottom lines in an ordering that causes them to always overlap with adjacent lines in a single direction. This takes slightly more time to print, but makes flat surfaces look more consistent.
Top/bottom line direction
This setting allows you to change the direction in which the top and bottom skin lines are printed. Normally, they print in a diagonal direction. This is the fastest and uses both the X- and Y-motors. Changing the direction can have a visual effect. Multiple numbers can be entered to change the line direction each layer, for example: [90,0] this creates a horizontal-vertical direction.
The model on the left has a line direction of , the model on the right  (vertical)
No skin in Z gaps
If your model has a small horizontal slit, smaller than the top/bottom skin thickness, this setting doesn't bother filling the top and bottom above/below that completely with skin. This saves on slicing and printing time, but might expose infill to the air. If the model has no horizontal gaps smaller than the skin thickness, the only effect will be reduced slicing time. If your model slices correctly you may simply leave this setting disabled.
Extra skin wall count
When printing the top and bottom with the lines pattern, the outer rim of the skin areas can still be printed with a few concentric lines. This is different than printing an extra wall, because the extra skin wall also gets put between the skin and the infill.
The model on the left has no extra skin walls, the model on the right has 2 extra skin walls.
Ironing is a technique where the nozzle travels over the top-most layer after printing it, to iron the top layers to a smooth surface. The settings for ironing can be adjusted to get the desired surface finish:
- Only highest layer: This setting will apply ironing only to the very last printed layer
- Pattern: A pattern can be chosen to iron
- Monotonic ironing order. This will apply the monotonic effect on the ironing layer
- Line spacing: Determines the space between every line
- Flow: While ironing, this percentage of material is extruded on top of the last layer
- Inset: Ironing is applied to an offset of the outer edge of the model in the X-Y direction
- Speed, acceleration, and jerk: The speed of ironing can be adjusted to fine-tune the process
This setting makes the skin overlap a bit with the adjacent structures. This makes the skin adhere better to those structures. The overlap in this setting is expressed as a percentage or the exact amount of millimeters of the average line width between the skin lines and the innermost wall lines.
Skin removal width
This setting will remove very thin strips of skin, replacing them with infill instead. The skin will only be removed if there is infill next to it, so it won't remove the skin at the very top or bottom of your print, even if that is very thin. This will reduce print time for models that do not prioritize strength.
The model on the left has all skin in place, the model on the right has some skin removed, to print faster.
Skin expand distance
Make the solid layers on the top and bottom of your print wider or thinner in a horizontal direction. Normally the solid layers are only generated in parts where there is air above or beneath them, but with this setting you can expand it horizontally a bit further, improving strength by making the skin attach better to adjacent walls and closing gaps in the skin. This setting can be adjusted for top and bottom skin seperately.
The model on the left has the regular amount of skin, the model on the left has all skin expanded by 1mm.
Maximum skin/width angle for expansion
Since skin is present throughout the model, it is unnecessary to expand all of those areas. Rather, only the areas where the angle between the skin's surface and the horizon is less than this setting will get expanded. In that way, flat surfaces with protruding elements will be strengthened, without needlessly increasing printing time for the rest of the model. Instead of the angle, the amount of millimeters the skin may expand can be set; 0mm expands all skin, 1mm does not expand skin thinner than this.
The model on the left has a skin angle of 90°, all skin is expanded. The model on the right has an angle of 45°, only flat areas are expanded.
Continue reading about Infill settings.