What is the extruder fan supposed to be cooling?

As I said in another forum, I've actually had my Kickstarter Robo3D for quite a while now, but due to a lack of time, I haven't had a chance to actually set it up and use it until just recently, and the first order of business has been to upgrade all the software and then start printing hardware upgrades and mods for the printer itself..

I'm about to print a fan duct for the extruder fan, but here's my question- what is this fan supposed to be cooling? Perhaps I don't understand the concept, and if so, please correct me..

The duct design I'm looking at points the airstream at the nozzle end, but thinking about the design of hot ends and their limitations and why people have been going with all-metal hot-ends seems to be the issue of the interface with the "cold" part of the extruder and heat migration softening the "cold" parts and causing leaks, etc. Now I suppose by cooling the tip of the nozzle, you do bring down the overall temperature, but I've had a couple of lamination failures in printed objects, which led me to turn the temperature up vs. the default print settings, and my gut says that I really do not want to cool the product that's coming out of the nozzle, but instead aim the airstream higher up to try to create a nice sharp hot/cold transition just *above* the heater block.. During "break-in" and tuning, I had the hot-end running as high as 240C without the fan and just with the dissipation to ambient air and the PWM control, the temperature never varied more than a degree, so other than trying to keep the temperature of the overall extruder under control so that the "cold" parts don't start melting, I'm not clear on what the fan is actually doing and where the airflow should point, and why?

 

fan with a duct pointed at the nozzle is to cool your filament being extruded.. mostly used for PLA prints and other prints that benefit from this. It's not there to cool the tip of the nozzle.

the heatsink fan is used to take away heat from the heatsink and keep the temperature within the working range of the filament.. especially pla. because pla will tend to suffer from heat creep (it starts to soften in the tube causing jamming) the fan minimizes this problem.

For standard style jheads (made of peek) a heatsink fan is required to keep the peek temperature in check, while also doing the above mentioned..... if not the peek could eventually melt and damage this type of hotend.

fyi..240c is somewhere in the range of peeks melting point (although having the fan on the peek will keep that temp down. it's better to stick to lower temp for wiggle room. So you don't inadvertently melt your hotend.. and of course this is not a problem with all metal hotends. Just peek style hotends.

 

Also that fan allows for longer bridges. There are Slic3r settings that turn the fan on maximum for bridging moves. Also it is for areas of an object that have a low area per slice, i.e. a short time per layer. Without the fan it may extrude over an area that has not completely cooled yet. This can result in melted looking sections and sections that can lift up and be hit by the extruder on the next layer (and knock the print off the bed).

I suggest you closely look at the auto cooling settings. Keep it on Auto or all the time for PLA. Use the fan sparingly or not at all for ABS.