<p>I've built the mechanics of my 3D printer myself, because I need to print parts that are really huge, (and for budget reasons). <strong>So, I already have the 3D movement functionality.</strong></p>

<p>But what I need now, is the printing mechanism itself. I've been reading a lot, but it became clear to me that things are more complicated than I thought. </p>

<p>Let's skip mechanics and software, I'm just interested in how the print head works. Can somebody explain me that?</p>

<p>To be honest, I was so naive that I thought that I just had to buy one part with one data wire (print/noprint) and the 5&nbsp;V/GND wires. But it came to my intension that things are way more complicated. </p>

<p>For example, these RepRap printers have some kind of air tube attached to the print head. I'm not sure what that's all about, is it cooling?</p>

<p>Perhaps I'm always reading the wrong manuals (i.e. the more advanced ones). Can somebody enlighten me or point me to a good starting point?</p>

The first point to start would be the RepRap wiki entry for
extruders
:
cold end
The "Cold End" is usually the bulk of the extruder. It is
often the actual carriage on one axis and supports the rest of the
parts. In some designs, the "Cold End" is split into two parts; one
part does the driving of the filament that is stationary and connected
to the carriage portion, of a lighter weight design for easier
movement, with a flexible tube. The drive is a motor that rotates a
knurled, hobbed, or toothed pinch wheel against a pressure plate or
bearing with the filament forced between them. Usually, the motor is
geared to the pinch wheel to increase available torque and extrusion
control (smoothness). The gearing can be a 3D printed pinion and gear,
stock worm wheel and gear, or a more expensive integral motor gearbox.
Stepper motors are used almost universally after initial trials with
DC motors did not achieve the required repeatability. Servo motors are
an option, though they are not seen in the literature yet. The final
function, some form of cooling, keeps the "Cold End" cold. With the
close proximity to the "Hot End" and possible heated build platforms
and enclosures, it is sometimes necessary to have additional passive
or active cooling of the cold end parts. Heat sinks and fans are often
used; water and Peltier effect cooling is also discussed. Much of this
bulk is usually made from 3D printed parts and the temperature is
maintained within safe limits.
hot end attachment
The "Cold End" is
connected to the "Hot End" across a thermal break or insulator (the
Bowden tube if used is on the cold side of this thermal break). This
has to be rigid and accurate enough to reliably pass the filament from
one side to the other, but still prevent much of the heat transfer.
The materials of choice are usually PEEK plastic with PTFE liners or
PTFE with stainless steel mechanical supports or a combination of all
three. A Hot End is frequently joined to the Cold End using a Groove
Mount where the thermal break or insulator is part of the Hot End
assembly and the Cold End body is provisioned with a cylindrical
recess. Many cold ends push the filament out a large hole centered
between 2 small holes about 50 mm apart. (Is there a name for this
de-facto standard?) Some people rigidly attach a groove mount hot end
to such a cold end with the mounting plate adapter and two short
bolts. A few people put 2 long bolts through those holes and then put
a spring around those bolts to make a spring extruder.
hot end
The
"Hot End" is the active part of the 3D printer that melts the
filament. It allows the molten plastic to exit from the small nozzle
to form a thin and tacky bead of plastic that will adhere to the
material it is laid on. The first RepRap hot end was made of brass.
Researchers have also made hot ends from glass or aluminium. The hot
end consists of a melting zone or chamber with two holes. The cold end
forces the filament into the hot end -- into the heating chamber of
the hot end -- through one hole. The molten plastic exits the heating
chamber through the other hole at the tip. The hole in the tip
(nozzle) has a diameter of between 0.3mm and 1.0mm with typical size
of 0.5mm with present generation extruders. Outside the tip of the
barrel is a heating means, either a wire element or a standard wire
wound resistor. The heat required is of the order of 20W with typical
temperatures around 150 to 250 degrees Centigrade. For feedback
control of the nozzle temperature, a thermistor is usually attached
close to the nozzle, though a thermocouple may serve with suitable
control hardware. High temperature materials are needed here. These
include metals, cements and glues, glass and mineral fibre materials,
PEEK, PTFE and Kapton tape.
mount to rest of machine
The ways
extruders are mounted on the rest of the machine have evolved over
time into informal mounting standards. These informal standards
include the Vertical X Axis Standard, the Quick-fit extruder mount,
the OpenX mount, etc. Such de-facto standards allows new extruder
designs to be tested on existing printer frames, and new printer frame
designs to use existing extruders. (Does the "greg-adapter.scad"
adapter in the Prusa i3 Build Manual let me mount an OpenX extruder on
a Vertical X Axis machine?)
You can also google for extruder and/or hotend in combination with 3d printing for a first starting point.