Saturday, January 18, 2014

Preventing Extruder and Hot End Jams

One of the most frustrating aspects of 3D printing from my perspective is dealing with clogged hot ends. I hate starting a print, walking away, and coming back to a stringy soupy mess with a hot end that won't spit out any resin. I hate disassembling my hot end (seriously, we need a quick disconnect design) while keeping it plugged in so that I can keep the heater on to scrape out plastic.

Fortunately I have found a small trick that works great for me in PLA. At the end of every print I retract my filament by 50 mm then I home the axes and kill the heat sources. Since I have implemented this custom code I have not had a single PLA jam in my hot end (results with ABS or Nylon may vary). The custom code to be added to the end of every G-code file is listed below:

End G-code

G28 X0  ; home X axis
G92 E0; reset extrude coordinates
G1 F250; set feed rate 250 mm/min
G1 E-50; reverse extruder 50mm
M104 S0; turn off temperature
M140 S0; Turn off bed
M84     ; disable motors

Please note that this g-code was created for and is running on the following printer configuration:

* Prusa i2 (makerfarm)
* slic3r for slicing
* Marlin for firmware
* Pronterface for comms
* Typically printing in PLA

As is often the case in the world of RepRap, you may need to adjust the code for your specific system. On my system, this end code retracts the filament from the hot portion of the hot and parks it in the cold portion of the hot end while keeping the filament engaged with the hobbed bolt. I do not know why this helps prevent filament jams. My guess would be that latent heat from the hot end is causing the plastic to cool at a very slow rate. Depending on the resin and additives (colorants, etc.) cooling rate can cause significant changes in the bonding structure of the polymer.

One thing to keep in mind when using this method is that you have to "prime" the hot end before every print by extruding 50 mm after the hot end is up to temp.