Top and bottom layers: the solid ceiling and floor of a part

What they are for

The inside of a part is only a sparse infill, so above it you need a solid ceiling to cover it, and below it a solid floor. According to the OrcaSlicer wiki, the number of these layers is set separately for the top and the bottom. At a layer height of 0.2 mm, 5 layers correspond to a thickness of roughly 1 mm.

The walls surround the part, and the top and bottom layers close it. Together they form the shell with the infill inside.

What happens when there are too few

When there are too few top layers, the infill shows through the surface and holes or sunken spots appear (pillowing). The ceiling above the infill has to bridge a sparse gap. With too few bottom layers the underside of the part is weaker and may have holes. With a thin shell, water or air may seep through the gaps, which matters for containers.

Conversely, too many solid layers lengthen the print and use more material with no meaningful gain in strength.

The underside of a part and the first layer

The bottom layers lie on the bed, so their appearance is affected mainly by the first layer. The side of the part that touches the bed is always the smoothest and the flattest. So choose for the bottom the surface that has to be accurate or good-looking.

For your order

You do not set the number of these layers. You choose strength with three levels of infill, and the rest is handled by print preparation, from which the price and delivery date are worked out. In the model give ceilings and floors at least 1 to 1.5 mm, otherwise the solid layers will not fit. A very thin sheet of 0.4 mm prints with just one line.

Layers and the look of top surfaces

The top surface is made of layers that gradually overlap. The lower the layer, the finer the steps on sloped surfaces. The steps are most visible on a gentle slope, which is why sloped surfaces have their typical ribbed look.

If the top surface matters, turn the model so the important surface is flat and horizontal. If you want a smooth underside, leave it on the bed. There is more in the model orientation entry.