Cold Pulls with PLA

3D Printers are amazing tools, and while they've come a long way, they're still a pretty new technology and so need attentive maintenance to keep them working (as opposed to a 2D printer, which pretty much takes care of itself these days).  One key maintenance technique is the cold pull.  In short, a cold pull is when you pull your filament back out of your extruder while it's mostly cold.  Hopefully, the cold filament has solidified around any junk in your nozzle, so when you pull it out, you also pull out anything that's been compromising your prints!

I print exclusively with PLA and a .25 mm nozzle, and while I'm not sure how that context might color my experiences, I figured that I should state that upfront.  Why?  Well, some of my experiences go contrary to the conventional cold pull wisdom.  Primarily, it's that I do my cold pulls with PLA instead of switching over to nylon.  I did one round of cold pull maintenance with nylon filament... and it didn't go so well.  The nylon did a fantastic job of pulling out the crud from my nozzle, but I found it very difficult to clear the nylon out so that I could go back to printing PLA!  In fact, after doing 4 cold pulls with nylon (to clean out my nozzle of the burnt crap), I then had to do another 5 or 6 cold pulls with PLA to get the last of the nylon out of my nozzle!  So, these days I just do PLA cold pulls, and it seems to work just fine.

First, how do I know that I need to do some nozzle maintenance?  There are two main signs that I've noticed: extruder clicking or fuzzy top layers.

If you're in the room while your printer is running, you quickly get accustomed to the normal sounds of the machine doing its thing.  It's a soothing background noise that is, depending on the patterns being printed, almost musical.  And then there's an offbeat click.  That click might mean a few things, was it the sound of the nozzle colliding with a curled up part of the print?  Or was it an extruder click?

Extruder clicks happen when the gears on your extruder motor slip because the filament isn't moving.  You've basically got an unstoppable force/immovable object situation where that extruder motor's going to turn and that filament isn't going to move... and the resolution to that stress is for the gears to slip so that that extruder movement doesn't require filament movement.

If you hear just one click in the middle of your print, it probably doesn't really matter (for that print).  I mean, it's not good, because there's some amount of filament that your printer thinks it's deposited that it didn't manage to actually spit out, but your overall print job will probably be fine (unless it's a vase or something).  If you're hearing regular clicks (like one every 10 seconds), you've got an immediate problem and need to pause your print (or even cancel it, depending on how long it's been going on).

What happens if you just ride it out, ignoring those clicks (like if it happens while you're at work)?  That's when I've seen a fuzzy layer.  I wish that I'd taken a picture of it back when I was getting frequent nozzle clogs, but a nozzle clog doesn't always happen all at once.  It doesn't go from 100% flow rate down to 0% instantly.  Instead, it degrades slowly and, as it approaches a full clog, filament starts to extrude in little spurts (as the pressure builds up enough to overcome the clog temporarily).  This results in the top layer of your print looking fuzzy, as little wisps of filament are being deposited intermittently instead of the smooth stream that the machine needs.

So, how do I do a cold pull to clear out the nozzle?  I basically use the same process that Joel discusses in that video that I linked.  Here's the process as text:

1. Disable the filament sensor (this is only applicable on some printers, obviously)
2. Unscrew the extruder idler screws (this relieves the tension on the filament so that it moves without the extruder motor also moving)
3. Preheat the nozzle to PLA temperature
4. Physically push the filament down; ideally some will come out the nozzle, which lets you know that the whole channel is full of molten plastic
5. Un-preheat the nozzle (in the Prusa firmware, turning off the preheat is called "cooldown" in the preheat menu)
6. Wait until the nozzle is about 95 degrees
7. Use my pliers to grip the filament and, with slow and constant force, pull the filament up and out of the printer
8. Snip off the end of the filament just above where I gripped it with the pliers
9. Preheat the nozzle and re-insert the filament
10. Evaluate the clipped bit of filament - if it's clean, move on to 11, otherwise repeat steps 4-9
11. Reattach the extruder idler screws and replace the little bit of plastic that keeps the dust out (basically, reassemble anything that was taken off earlier)
12. Turn back on the filament sensor

Step 10 was a bit confusing for me at first, since I hadn't a clear idea about what dirty filament might look like.  So, here's a photo from the last regular maintenance cold pull that I did.  These pulls show the crud that accumulated over the course of about 5 weeks of printing, which was about 1 KG of filament:

Those pulls went from left to right.  As you can see, the first pull removed all sorts of junk from the machine, taking it out in chunks.  The second pull removed some more chunks, but was down to mostly char.  The third pull was entirely char, and the fourth pull the same (although significantly less).  I probably should have done a fifth pull, but I was eager to resume printing and the fourth wasn't all that messy...

And, there you have it.  The art of the cold pull!