5.1. Hotend and nozzle maintenance using cold pulls

Nozzle clogs and hotend jams are an inevitable part of printing with current consumer-grade FFF printers. The process of feeding thin, flexible filament through complex feed paths, melting it, then pushing it out a narrow orifice if fraught with opportunities to go wrong. Eventually you’re likely to run into a clogged or partially-clogged nozzle that blocks or distorts extrusion. Fortunately, some basic preventative maintenance can help avoid or at least reduce the severity of such problems.

Note

These notes are based on my experiences with the Prusa i3 Mk3 and Artillery/Evnovo Sidewinder X1 printers. If you are using a different printer, please verify the hardware details are similar.

This page describes the “cold pull” method of clearing the nozzle and hotend of filament build-up and gunk that can accumulate over the course of printing. This is a very simple procedure that can help prevent a lot of headaches with jams, clogged nozzles and sunder-extrusion. Cold pulls are particularly important if you are switching from a high-temperature filament (e.g. PETG) to a lower-temp filament (e.g. PLA). Filament residue that will melt out at higher temps remains solid, cooks and eventually carbonizes in the hotend and nozzle.

5.1.1. Diagnosing partial clogs

A full clog is easy enough to recognize. Filament simply will not extrude from the nozzle. This will usually be accompanied by extruder clicks as the gears vainly attempt to push filament through the hotend and nozzle. The extruder gears will eventually shred the filament to the point that the gears cannot push filament at all.

A partial clog can be much more insidious. You may notice:

Occasional extruder clicks and skips. If you have mounted an extruder visualizer (recommended), you’ll see the gears kick back as they encounter insurmountable pressure.
Filament debris build-up around the extruder gears caused by grinding of the gears against immobile filament.
Under-extrusion, irregular extrusions and gaps in printed parts.

Diagnosing a partial clog is straightforward:

Raise the extruder to maximum Z height to allow an unobstructed view of the nozzle.
Heat the nozzle to a temperature suitable for printing the currently-loaded filament.
Extrude a length of filament and observe the flow from the nozzle.

If the feed is erratic or pulls to one side, you most likely have a partial clog. With a clear feed path, filament should flow smoothly and evenly and form a gentle uniform coil below the nozzle. A cold pull is recommended when this occurs.

Fig. 5.1 Uneven extrusion with a partial clog

5.1.2. Clearing blockages with acupuncture needles and bits

You probably received an acupuncture needle with your printer. A variety of very low cost nozzle cleaning bits are sold. These instruments are intended to be shoved into the nozzle opening to poke at and clear clogs. While this is a simple enough solution for the manufacturer since no disassembly is required, it can create as many problems as it fixes for the user.

The needle is fragile and can easily be snapped off inside the nozzle, creating further jams that might require an extensive extruder tear-down to clear.
The needle may be too large for smaller nozzle sizes.
Finally, wiggling a steel needle about may damage the soft brass around the nozzle aperture, leading to problems with stringing and irregular extrusion.

I recommend avoiding the use of these needles and similar bits for clearing clogs.

5.1.3. Using cold pulls

Using cold pulls regularly, I’m able to swap freely between PLA, PETG, NGEN, XT and a variety of carbon, wood, copper and other filled materials with no issues. Example extremes would be switching from 3DXTech PETG that prints at 265C to PLA at 200C. Rather than simply printing and hoping to avoid a jam, I prefer to do a precautionary cold pull.

I recommend cold pulls in the following situations:

When switching between material types, particularly from high-temp (e.g. ABS) to low-temp (e.g. PLA). A precautionary pull after removing the higher temp material can remove build-up in the nozzle that might ruin later prints due to jams and under-extrusion.
Before removing a nozzle for storage to avoid having to worry about what material was last printed.
As part of regular maintenance keep the nozzle and hotend clear.
If you begin to experience uneven extrusion using settings that worked previously.

5.1.4. Recommended tools

2.5mm hex wrench (included with printer) or ball hex wrench (recommended)
Soft brass wire brush (recommended)
Cleaning filament (ABS, nylon or optional specialty cleaning filament – eSun cleaning filament recommended). The filament used for cleaning should ideally melt at higher temperatures than the filament you printed with previously. Nylon and specialty cleaning filament are flexible and are less likely to snap prematurely during the cleaning procedure. Specialty cleaning filament can be used over a wide temperature range, ensuring that you can do the pulls at high temperature but know that any left-overs in the hotend or nozzle will still melt and extrude even if printing at lower temperatures next time.

5.1.5. Preparing for the cold pull

It’s a good idea to prepare things for doing the cold pull to avoid needlessly repeating steps.

Heat hotend to whatever temp the currently-loaded material prints at.
Eject the currently-loaded material. Put a clip on the filament and spool to avoid tangles and store it away.
Raise Z to max. Press and hold the front knob until it chirps, then rotate it to lift the Z axis. Push the knob again when done.
Heat the hotend to a temp higher than normal printing temps (e.g. 265C). The goal is to melt any material you might have used previously, so be sure to use a sufficiently high setting.
As you wait for things to heat up, take the opportunity to clean the nozzle and hotend. You want to remove any built-up filament and other gunk that has built up before it starts dropping into your prints. Using a soft brass wire brush to give the hotend a light scrubbing, being careful to avoid the thermistor and heater cartridge wires that protrude from the left side as viewed from the front of the printer. A toothpick or other (non-plastic!) tool can be used to clean around delicate wires.

Caution

If you are using a coated nozzle, avoid using the wire brush on it. A small 1in/25mm strip of corrugated cardboard can be used to rub away any accumulated filament.

Load either ABS, nylon (white recommended) or cleaning filament. I personally use eSun cleaning filament and really like the results.
Feed sufficient material to clean the hotend out. I usually answer the “is the filament loaded?” question “No” twice to make sure the hotend and nozzle are completely full of cleaning filament. As the molten cleaning material flows through the hotend it will melt and pull away burnt filament and crud from from the hotend and nozzle internal surfaces and trap it in the filament if it’s too large to extrude out the nozzle.
Reset the printer with the button below the knob on front to reset the temperature.
Let the hotend cool with your cleaning material still loaded into the hotend and nozzle. As the material cools, any accumulated crud will be trapped in the cleaning filament.
While it’s cooling, loosen the extruder tensioning screws on the left of the extruder housing to avoid putting stress on the extruder motor in the following steps.

5.1.6. Performing the cold pull

Now we’re ready to do the actual cold pull. The hotend has cooled with the cleaning material still filling the hotend. With any luck, any left-over filament and build-up has been trapped in the cleaning material. We now want to remove the material to pull out the debris.

Let the hotend cool to below 90C. The exact temperature is unimportant so long as it is low enough for your cleaning filament to fully harden and trap any accumulations.

Note

If you’re in a hurry, you can do a warm pull before the filament has completely hardened. This may be effective in removing blockages, but also increases the risk of leaving behind some of your cleaning filament.

Set the temp to a high temperature (260C).
Watch the hotend temp. When it gets up near 90C, grab the cleaning filament with pliers and brace the extruder with your other hand.
When the temperature reaches 90C, pull firmly and steadily straight up. Don’t jerk it, and avoid twisting the extruder housing. You want the filament to stretch and pull out without breaking. The material should be soft but not molten.
As you pull it up, the material should warm enough to become slightly elastic and pop out.

Inspect the pulled filament and you should see the shape of the hotend and nozzle interior. If you performed the pull at a low enough temperature, it should not be distorted. If the pulled strand is stretched, try starting to pull at a cooler temperature. If it appears to have snapped off, let it warm a bit longer.

Note

Not every pull will produce a perfect looking result. The key thing is to pull stuff out of the hotend and nozzle.

You may see loose bits of other filament carbonized build-up. If so, reload cleaning filament, reset to let the hotend cool and repeat this process. You may have to push cleaning filament through manually if extruder tension screws are still loose.

5.1.7. Finishing up

Once the pulls come out clean, you can continue on as normal.

Re-tighten the extruder tension screw(s) to < 1mm above the cover. (I always forget this step.)
Heat the nozzle up to your next desired filament temp.
Load your next filament.