Now that the mechanics of FFF 3D printing are clear, it's time to take a look at how a print is built. If an FFF print is stopped partway through, or observed during printing, the following can be seen:
Layers: FFF prints in layers, with each layer sitting on the one below it. Prints can be made with thicker layers so that they print faster or thinner layers so that they look better.
Outlines: When starting a layer, the outline of that layer will usually be printed first. FFF prints often have two or more outlines so that the outside of the print is strong.
Infill: once the outline is done, the rest of the layer is filled in. If an area of the print will not be seen from the outside when the print is done, a loose infill is used to save material and give layers above something to sit on. Top layers are filled in completely. Most FFF prints are largely hollow.