Turn, Turn, Turn

Probably the single most important choice one can make when putting together a 3D printer is the choice of extruder and hotend. The extruder drives the filament into the hotend, which melts the filament, and forces the filament out the other end through the nozzle into a smaller thread, which is then laid down on the build plate. If this component has issues, you will never get a decent print.

For the extruder the most common types are Direct Drive and Bowden. In Direct Drive, the extruder motor sits directly above the hotend (and often the hotend is inserted into the extruder body) so the filament only has to travel a short distance into the hotend.

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FrankenPrusa Mk1 used a Direct Drive style extruder as seen above. The biggest drawback is weight. With the weight of the motor on the carriage, the max speed it can travel is reduced. To get around this the Bowden extruder was developed. In this, the motor and gear is moved off the carriage to a fixed spot usually on the frame of the printer. The filament is usually guided by a tube into the hotend that must remain mounted on the carriage. This reduces the weight on the carriage and allows the print head to move at faster speeds. I recently converted FrankenPrusa Mk2 to Bowden style, as seen below:

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The extruder motor is at the top of the wooden frame. The filament feeds into it from the rear – the black reel sitting in the Home Depot bucket – and feeds down into the hotend. One of the biggest issues with Bowden style is that with the long length of filament from the gear to the hotend there are issues with retraction of the filament. Retraction is necessary to help keep the molten plastic from oozing out of the nozzle. If the retraction is set incorrectly, it can result in the filament being stripped as the gear teeth chew up the filament.

For FrankenCore, I have chosen to use a dual-extruder setup with direct drive motors:

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I can print more complex items in one extruder and print water- or chemical-soluble support structures in the other, print two colors, or even use the same color in both extruders.

As far as the hotend, there are also several type. Most fall in 3 categories, J-head, all-metal and hybrid. The J-Head style usually have a plastic filament guide path. This can cause problems if the heat from the hotend creeps up to the extruder body, which often is printed from ABS, which has a melting point in the 240 – 250deg Celsius range. There is also a limit to the temperature of the hotend, since too hot and the PTFE lining these have will melt inside the hotend. As an alternate, there is the all-metal hotend. Because the entire hotend body is made of metal, there is usually a fan constantly blowing on the “fins” of the body to keep it cool. This prevents heat creep and allows the hotend to run at much higher temperatures some exotic filaments require. Then there is a hybrid. While the body is all-metal, it still has a PTFE liner on the inside. It tends to be lighter than the J-head, but can be cooled easily by a fan.

FrankenCore will use the hybrid – 2 E3D Lite6s for the reduced cost – it’s almost half as much as the all-metal. While I will use the ability to print at higher temperatures, I generally wouldn’t be doing that with this printer. This printer is going to be used to make larger parts, so I’ll be sticking to PLA and ABS. FrankenPrusa has an all-metal hotend, so I will be using that for the exotic filaments and any testing. The FrankenCore will run nozzle sizes nearly twice the diameter of standard hotends. This will allow me to print the large parts faster. I may lose some detail, but that usually won’t matter on these large size prints.

Build Log

Well, Misumi came through and got my new extrusion faster than I expected. After cutting it to the CORRECT length, I assembled the basic frame – see the Featured Image at the top of the post. I have placed a sheet of PEI that will be placed on the build surface that is 12″x12″ to give a size comparison. I have also temporarily assembled the extruder motors and mounts (see previous picture in the article). Looking good.

Next time: Filament – what do all these acronyms mean? And work on the printer continues, with the Z-Axis assembly starting.