The drawing shows a mock-up of the extruder and a spool of filament. As you can see, the filament is
pulled into the extruder (cold end) and then pushed into the nozzle (hot end) via a special PTFE tube (also
called a Bowden tube). Once heated, the filament is extruded onto a build plate (a very flat surface used as
the base for the object). Typically, the outer edges of an object are printed first, and then the interior edges
are printed, and finally, the interior of the layer is printed as either a solid layer (for outermost layers) or as a
fill-in matrix for inner layers.
Notice that the filament from the spool is much larger than the heated extrusion. This is because most
nozzles (the small part where the filament exits the heater block) have a very small opening ranging from
0.2 millimeters (mm) to 0.5mm or larger. Notice in the drawing that I’ve exaggerated how the layer is built
from multiple lines of heated filament. Although grossly simplified, this is effectively how a 3D printer takes
filament and builds a layer of the object.