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

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.

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

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 V/GND wires. But it came to my intension that things are way more complicated.

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?

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?

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    $\begingroup$ Depends: do you want it to work well, or at all? $\endgroup$ – Carl Witthoft Sep 19 '16 at 11:28
  • $\begingroup$ I think @fred_dot_u's answer is excellent. Despite being a simple concept, the print head is a fairly complex piece of design that juggles several conflicting requirements: lightweight, able to apply reasonable filament pressure, rigid, hot in one place not in another, quite tight temperature control, able to operate consistently for tens of hours flawlessly, and more. Any working system like this is the child of hundreds of improvements. One of the joys of owning a 3d printer is you start really appreciating things like this! $\endgroup$ – SusanW Sep 22 '16 at 8:01
  • $\begingroup$ In all honesty, the more I read about 3d printers, the more I understand how delicate everything is. I guess the following proverb applies here: "don't touch it if it works". :) $\endgroup$ – bvdb Sep 23 '16 at 14:10
  • $\begingroup$ Keep in mind -- one thing you will have trouble with as your print area grows is warping of parts -- because they will cool as you print them. -- For larger cartesian robots, CNC (i.e. subtractive manufacturing) is a much more attractive option. And it's way easier to control, because you just use a relay to turn a well mounted dremel on and off and you're good to go. $\endgroup$ – BrainSlugs83 Oct 8 '16 at 7:16
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You will certainly find that the print functionality of a 3d printer is a bit more complex than you suggest. The mechanical portions include a means to push the filament into a heated nozzle as well as the software portion to regulate the speed of the filament movement. You haven't referenced the heater cartridge and temperature sensor, but you will discover that aspect soon enough.

The "air tube" you think you've seen is likely called a bowden tube. Such designs permit lighter weight print heads, which is beneficial for speed, acceleration and precision, but has complications with respect to compression of the filament as well as retraction considerations. Non-bowden print heads will have the extruder motor as part of the moving assembly, with the drive wheels very close to the nozzle opening. This allows for flexible filament and more precise control of the filament feed.

Either design has compromises, so one must determine priorities for the design.

Cooling is also a factor. The heater cartridge is designed to heat the nozzle to a specific temperature for the type of filament used, but also requires a means to keep the heat from traveling to the portion of filament not in the nozzle. You'll discover terms such as heat break, referring to narrow threaded portion connecting the nozzle assembly to the heat sink. There will also be a cooling fan to blow air over the heat sink and very often a cooling fan to cool the filament as it exits the nozzle and attaches to the model being printed.

You suggest to ignore the mechanics and software, but it's important to be aware of both when considering the principles of the print head assembly.

Simplified, filament enters bowden tube then into heat sink, pushed by extruder motor (or) filament is pushed into heat sink by extruder motor. Filament travels through heat break, gets melted in heater block and exits nozzle. Sheesh, that's way too simple.

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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.

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    $\begingroup$ This is effectively a link-only answer. If you think a question is too much effort to answer ("because of its sheer mass") then don't answer and vote to close as "too broad". $\endgroup$ – Tom van der Zanden Sep 18 '16 at 7:17

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