Keeping extruder fan on.

Is there any way either through a setting I'm missing or something in code for a marlin setup that I can change to get the fan on D9, I'm using it as the extruder fan for a direct drive extruder, to stay on as long as the extruder is above a specific temperature? Right now after a print is finished and the printer parks and everything gets turned off including the fan but the extruder is still very hot. I've been manually turning the fan back on at the end of a print but longer prints would not have me around to do this.

I'm new to 3d printing and really new to Marlin so if I'm missing an obvious solution just beat me over the head with it until it sinks in.

How about custom ending gcode? Keep the fan running for x number of seconds after turning the extruder heater off?

I would try a hardware approach

use something like [www.aliexpress.com] near the hot end, and don't tell it to turn off in software. Not sure it would survive being right on the hot end, but it doesn’t need to be right on it, just close enough. Of course it also means that fan on isn’t instant, unless over the temp threshold

This is whats in those 12v power supplies to turn the fan on and off as needed

That would be acceptable, how do I do it? I'm using Cura so does that mean I can add something to the default profile that will add this to any opbject I run through it?

I usually see the temp drop off about a minute or so with the fan on after the extruder is turned off so for the sake of overkill having the fan on 2 minutes after the end of the print should be fine.

I'm actually surprised there isn't something more like an auto fan setup in Marlin that you could set a temp point for the extruder that would keep the fan on as long as the temp was above that point. My main concern is that since the extruder is all metal that if I don't keep the fan on after a very long print it might be possible that the temp on the plastic parts might get up enough even with the fan running during the print that once the fan stops that there might be a window where carry over heat might get to the melting point and with no fan on to help wick away the heat I'd wind up with a slumping extruder

While not as sophisticated as an automated command, why not just turn off the hot end, run the fan at 100%, and send the command G4 Sx where x is the number of seconds, say 600 for a 10 minute cool down? It's not failsafe, but neither is any other solution other than not running it unattended.

There is a similar command for cooling down the bed that is more automated. It's detailed here. You might also look into M143 if you want to set a max temp just in case the fan dies or you have thermal runaway.

That would work, is there any way to automate the code insertion in Cura? Can I add the line somewhere in a config file so that every print it slices will have that code in it?

It appears so, however I don't use Cura so I can't tell you what the steps are to get to that particular configuration screen. It might be under an advanced menu option/dialog box.