3.1. Troubleshooting Prusa i3 Mk3 extruder and hotend problems

Extruder and hotend problems are the most intimidating problems most users eventually encounter. They can be caused by a variety of problems, and fixes may provide only partial relief. My goal on these pages is to explain why problems occur and how you can resolve them.

Note

These pages are under heavy editing and reorganization.

3.1.1. Common symptoms

Extruder and hotend problems usually show up as one or more of the following:

Extruder clicks during printing.
Extruder motor skips or bucking during printing.
Filament jams and clogs.
Erratic under-extrusion.
Excessive extruder motor temperatures (too hot to touch).

We often see these reported in a surge of “heat creep” problems on the Prusa forums during summer months. Users who have printed successfully for months start to experience problems as temperatures rise. These problems have other causes that generate heat and other problems on the printer itself.

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.

3.1.2. Overview of the Prusa i3 Mk3 extruder and hotend

It is helpful to have a good overview of how the Prusa i3 Mk3 hotend functions before trying to troubleshoot something as complicated as a 3D printer. Multiple problems can cause common symptoms. If you haven’t isolated the problem, you can waste a lot of time trying to fix the wrong thing.

Here’s a simplified cutaway showning the major components that make up the Mk3 extruder and hotend (not to scale).

Fig. 3.1 Prusa i3 Mk3 extruder & hotend cutaway

Todo

detailed breakdown and summary

The extruder assembly is responsible for moving filament from the spool into the hotend. It consists of the following major parts:

The extruder motor is a stepper motor that advances or retracts filament in precise increments based on gcode commands.
The extruder motor shaft extends from the extruder stepper motor and into the extruder housing.
The hobbed extruder gear is a precisely machined gear (manufactured by Bondtech) attached to the motor shaft. This gear is fixed to the extruder motor. A matching idler gear attached to the extruder door is held against the hobbed gear using manual tension screws on the extuder housing.

The hotend assembly is responsible for heating filament from room temperature to the desired printing temperature. The hotend consists of the following major parts:

The PTFE guide tube is a short length of tubing that provides a low-friction path from the extruder gears into the top of the heat sink.
The heat sink is a finned assembly that attaches the hotend to the extruder housing with a friction fit collar.
The heatbreak is a precision-machined tube of varying external dimensions. It has a precisely machined 2mm center bore that is used to guide filament from the heatsink into the heater block.
The heater block is an aluminum (or brass if upgraded) block. The heater block contains the heater cartridge and thermistors (not shown).
The nozzle screws into the heater block and forms filament from the 2mm feed path into the extrusion dimension.
A cooling fan (not shown) blows cooling air over the heatsink fins.

3.1.3. Common extruder and hotend problems

Extruder and hotend problems tend to fall into three categories:

Problems caused by hardware or mechanical problems
Problems caused by slicer settings
Problems caused by heat

3.1.4. Hardware and mechanical extruder and hotend problems

Anything that interferes with the filament feed path can cause sporadic problems. Often two or more of these problems will be the root cause of printing problems:

Badly formed filament can have bulges that drag or jam in the extruder gears or PTFE tubing.
Filament spool friction can impede smooth rotation, causing irregular feed.
Filament cleaner attachments can be misaligned, increasing friction.
A stuck, loose, or poorly lubricated Bondtech extruder hobbed or idler gear can cause feed problems leading to under extrusion, filament shredding and feed problems.
Deformed PTFE guide tubing can cause feed problems.
Build-up of filament fragments around the Bondtech gears in the extruder can lead to intermittent jams and feed problems.
Over- or under-tightened extruder gears can cause feed problems and filament shredding in the extruder.
The collet retaining the PTFE tube in the heatsink can be missing or not locked properly, allowing the tubing to slide and cause problems.
The filament feed path from the top of the extruder housing into the Bondtech gears is not optimal in some extruder part revisions. It is offset by roughly 1mm, causing some feed problems for some users. Slight part shifting can also distort the feed path, leading to under-extrusion and shredded filament.
Many of those nice looking fan covers off Thingiverse can wind up killing airflow.
If fans are poorly connected or reversed, airflow is greatly reduced.
Excessive cooling, particularly if the fan duct is cooling the heater block, can cause problems.
Octoprint or other server software can cause problems if the hosting server (Raspberry Pi) is overloaded. This is particularly a problem with the Raspberry Pi Zero.
Prusa ships a modified heatbreak with a wider throat at the top to rectify feed issues with the MMU2. While this works well in most cases, it can aggravate filament feed problems, particularly when coupled with excessive retractions and elevated ambient temperatures. The non-Newtonian behavior of molten or soft filament no doubt contributes to this problem.
An improperly installed nozzle can cause a filament leak and associated jamming in the hotend.
Premature powering off can cause heat creep problems between prints.
Partially blocked or poorly machined hotend or nozzles (usually low cost clone parts) can increase back pressure and reduce flow.

Before changing settings or making hardware modifications, check the basic mechanical components of the filament feed path.

Inspect the Bondtech extruder gears and housing. With the right extruder cover open, inspect with a flashlight. Is the extruder area clear of debris? A rusty-looking smattering of dust is normal, but loose filament and other material is not. Blow it out, clear it with a brush if necessary.
Is the white PTFE tubing protruding from below the gears damaged? Replacement or reshaping of the PTFE tube may be required.
Verify fan airflow.
Inspect the extruder idler. Open the extruder right cover and verify. The idler on the right cover should spin freely without friction. If it can’t, feed problems and jams can occur. Verify the gear is properly inserted into the guides, and that the geared assembly spins freely.
Inspect the extruder tension screws. Is the filament visibly distorted or shredded? Too tight and the filament can be distorted and snag or jam, causing the extruder to do extra work and heat up. Too loose and filament doesn’t feed, causing the extruder gears to slip, resulting in clicks and jams. The screws normally protrude < 1mm from the left side extruder housing cover.
Check for partial nozzle blockage. Raise Z, heat the nozzle to print temp and extrude some filament. Inspect the filament as it is extruded. If the filament does not flow cleanly, or pulls to one side for more than a few seconds, do some cold pulls to remove any crud build-up in the nozzle and hot end.
Check for a clear filament feed path. Raise Z to max and open the extruder. Poke a 1.5mm rod down through the filament path, past the open Bondtech extruder gears and into the PTFE tube. If you remove the nozzle, it should poke through cleanly and without obstruction. See clearing filament jams for more details.
Raise the Z axis and inspect the top of the heater block. You may need to remove the part cooling fan duct for a clear view. Look for filament pooling on top of the block or dripping down the sides. If using a silicone sock, look for filament pudding inside the sock. This can indicate a filament leak due to an improperly installed nozzle.
If you have added increased cooling, be sure you’re cooling the print and not the heater block. Check fan duct alignment and consider reducing cooling. A silicone sock can help reduce thermal shock when cooling kicks in.

If these basic measures are inadequate, more drastic steps may be required:

Examine the PTFE tube for damage. Make sure there are no snags. Replacement may be required. Be sure the collet clip is applied properly to retain the PTFE tubing in the heatbreak.
If you are not using a MMU2, consider replacing the Prusa stepped heatbreak with a standard E3D V6 heatbreak without the 2.2-2.0mm step.
Investigate replacement extruder designs with corrected/improved filament feed paths.

3.1.5. Slicer settings

Slicer settings can also contribute to extruder and hotend problems:

Excessive rapid retractions can contribute to extruder motor heat.
Excessive retraction lengths can pull molten filament up out of the hot zone and into the cold zone, increasing friction.
Insufficient or excessive temperatures can cause filament feed problems.
Excessive extruder motor current can contribute to extruder motor Heat.
Low layer heights can cause problems due to back pressure in the filament path created by trying to move too much filament through too small of a space.

A few slicer setting adjustments can often resolve extruder or hotend problems.

3.1.5.1. Heat and cooling settings

Print temperatures can also be adjusted to fix some problems:

Increasing temps slightly can increase flow, which can eliminate extruder clicking. This can also contribute to stringing and oozing.
Decreasing cooling fan speeds can increase flow without increasing temperatures.
Avoid dueling heat and fan settings. Avoid increasing heat when reducing cooling will do.

3.1.5.2. Retraction settings

Retractions pull the filament up from the hotend to relieve nozzle pressure. This can help with oozing and stringing, but can also contribute to heat-related problems, particularly if printing detailed prints with lots of small details.

Are you using overly-aggressive retraction settings?
Does the point at which your print fails consist of lots of small areas (e.g., fingers) and retractions?

Preview the sliced model in your slicer software to see if you have lots of retractions occurring in small areas.

Increase the minimum travel required to trigger retractions. If using PrusaSlicer, Prusa sets the minimum move to 1mm. PrusaSlicer defaults to 2mm. Try 5mm. You may get increased stringing, but can deal with that separately.
Are you using excessive retraction distances? The Prusa i3 Mk3 uses a direct feed mechanism. Many guides recommend retraction settings of 5mm or more which is appropriate for a Bowden setup, but not the direct feed mechanism of the Prusa i3 Mk3. If your retraction length is above 2mm, you are pulling melted material back up past the heat break. Keep retraction distances around 0.8mm.

3.1.5.3. Maximum volumetric throughput rate

The E3D V6 hotend used on the Prusa i3 Mk3 has a maximum rate at which it can melt and move a particular type of filament through a particular filament. This is rate is referred to as Maximum volumetric speed (MVS) in PrusaSlicer. Any attempts to move filament any faster through the hotend can result in increasing back-pressure, leading to extruder skips or nozzle jams. The stock hotend can process roughly 11.5 mm³/s of PLA through a 0.4mm brass nozzle. Calibrating the maximum volumetric rate for a specific filament and nozzle combination is not difficult, but there are a few quick steps you can try to determine if this is your problem:

Try slowing the print down to 50% or lower from the front panel. Reducing speeds reduces MVS. If skips and jams stop, there’s a good chance you can resolve the problem with slicer settings.
If reducing speeds eliminates skipping and jams, reduce MVS in your slicer settings.
The default PrusaSlicer print presets include extremely aggressive settings, particularly for infill. The flow rate for PLA to 15 mm³/s , which can exceed the E3D V6 capacity. Other materials are set to 1-10 mm³/s , which may explain why PLA fails and others work. Slice your part and view the maximum volumetric rate in preview mode. Reduce max volumetric speeds if using PrusaSlicer. You may be stuck just reducing speeds with other slicers.
Reducing deretraction (filament advance after retraction) speed can reduce pressure due to the non-Newtonian characteristics of filament. PLA in particular will only flow at relatively low speeds.

3.1.5.4. Avoiding problem prints

You can do some analysis before printing to avoid many problems and judge the effectiveness of these measures yourself.

Slice your model in PrusaSlicer.
Click on the Preview button at the bottom.
Select Volumetric flow rate in the drop-down box at left. Do you see it hitting or exceeding the 11.5 mm³/s mark at the places it commonly fails?
Select Feature types on the drop-down and enable showing Retractions with the checkbox. Do you see a lot of retractions at the failure points?
Select Speed in the drop-down and go through the layers and view the failure point. Are speeds high at these points?

This analysis might help pinpoint why you’re having problems. A few slicer setting adjustments are far easier to adjust than replacing hardware (which may or may not fix the problem).

3.1.5.5. Filament stuck in extruder or hotend

Filament blobs can form inside the extruder mechanism when hot filament deforms in the extruder or hotend and cools. These can sometimes be large enough to prevent removal of the filament through the top cover, or pulling it up past the Bondtech extruder gears, requiring opening the extruder cover and snipping the filament. One tip for avoiding blobs is to heat up the hotend and extrude a small amount (1 inch/25mm) of filament before unloading.

Ideally, you will be able to eliminate blobs from occurring using the steps listed above, but there are printable covers on Thingiverse that unscrew to allow removing the blobs without cutting.

3.1.6. Heat problems

I think there needs to be a general “heat awareness” in the community. It’s one of those things that’s obvious once you’re aware of it, but that a new user might not immediately realize. Every summer, we’ve seen a spate of “heat creep” and “extruder clicking” posts on the Prusa forums as many parts of the world moved into ever-hotter summers. There was a huge spike after the Prusa Lack enclosure blog entry was published in April and happy new owners tried using them in hotter months. The causes of excessive heat are varied, and many people have declared victory once they found one solution that worked in their specific circumstance, but it may take fixing several factors to actually correct the problem.

Not surprisingly, heat is a big part of the FFF 3D printing process. If temperatures are too cold, your filament won’t feed. Too hot and you’ll get stringing, jams, and other problems. Heat can cause problems directly, or contribute to other problems.

Heat problems usually fall into one of two categories:

Problems due to heat in the extruder.
Problems due to heat in the hotend.

Heat-related problems don’t have any one cause. A 3D printer works best when a variety of mechanisms work in harmony. If one is off, print quality suffers and attempts made to fix one problem can quickly aggravate others.

Ambient temps matter. Particularly with the onset of summer, temps become a real issue. A room that is suddenly hot for you is hot for the printer as well.
Enclosures are designed to provide stable ambient temps. Prusa published a very popular blog post on constructing an inexpensive enclosure using Ikea Lack stands in April. After the initial burst of enthusiasm, many users ran into problems as spring temperatures turned into summer heat.
Cooling air path obstructions can be a real problem. Thingiverse is full of fun prints that put grills and other goodies on the printer. Unfortunately, some of these seriously impede airflow.
Powering off the printer before the hotend has cooled can cause jams. Be sure to leave the printer powered on until the hotend cooling fan shuts down after a print. Turning the printer off prematurely removes the cooling necessary to prevent filament from deforming in the hotend.
Fans can be disconnected or reversed during kit assembly or extruder maintenance.
Excessive software retractions. This is really a software issue, but excessive retractions can contribute to heat as the extruder motor works overtime.
Over-zealous print speeds exceeding the maximum volumetric rate of E3D V6 hotend. If filament is pushed through faster than the hotend can process it, the result is back pressure up the filament path and ultimately to the extruder motor, causing it to work harder and thus generate more heat.
Motor current settings. Early firmware releases applied excessive current to the extruder motor, contributing to heat. Recent firmware updates (> 3.2.1) address altered motor currents. Recent PrusaSlicer releases have included printer presets that include startup gcode to adjust extruder motor current based on print speeds (quality settings).

3.1.6.1. PLA: The Canary of heat creep problems

Heat problems are usually first encountered when printing with PLA. This makes sense: PLA has a glass transition temperature much closer to ambient temperatures. Add in some a bit more heat and PLA can soften in prematurely. This can create feed jams and back pressure into the extruder. Once the filament softens, you get into a feedback loop as increased filament friction adds to friction the extruder motor must overcome, which further contributes to heat. Turning up nozzle temps can increase nozzle flow to a point, but this adds yet-more heat into the system.

Unfortunately, heat only accumulates over time. These problems often don’t manifest until well into a long or complex print. Small diagnostic prints commonly used for testing don’t generate enough heat to trigger problems, further complicating troubleshooting.

3.1.6.2. Extruder heat problems

The extruder is a relatively simple device. The extruder motor rotates to either advance or retract filament. The hobbed extruder gear grips filament and guides it into the PTFE guide tube. No fan or other active cooling is normally necessary for cooling the extruder. Unfortunately, it is possible heat to build up in the extruder. Common causes are:

Excessive ambient temperatures in excess of 40C (104F).
Excessive enclosure temperatures in excess of 40C (104F).

The same mechanical and slicer settings problems that cause intermittent print problems can also contribute to heat problems, or become more prevalent with increased heat:

Excessive, rapid retractions.
Excessive extruder motor current.
Friction in the filament feed path creating excessive load on the extruder motor.
- Badly formed filament with bulges that jam in the extruder gears or PTFE tubing.
- Filament spool friction preventing smooth rotation.
- Filament cleaner attachments increasing friction.
- Jammed extruder idler gear.
- Poor lubrication of extruder hobbed or idler gears.
- Deformed PTFE guide tubing.
- Debris in extruder assembly.
- Poorly machined hotend or nozzles (usually low cost clone parts).

Over time, heat may accumulate either due to friction in the extruder, or transmitted from a hot extruder motor along the shaft into the extruder assembly. As heat creates friction, filament may soften as internal temperatures reach the glass transition point, causing feed problems.

Filament may be shredded by the hobbed gear, eventually wearing away until it cannot be gripped and fed.
Filament may deform above the hobbed hear, preventing easy unloading.
Filament may break off below the hobbed gear, remaining stuck in the PTFE tube.
Softened filament may deform and form a plug at the top of the PTFE tubing.

Here’s a simplified picture of the Prusa i3 Mk3 extruder (not to scale) showing heat creep.

Fig. 3.2 Prusa i3 Mk3 extruder heat creep

3.1.6.3. Hotend heat problems

The hotend is where all the magic happens with 3D printing. A solid strand of filament is melted and formed, and pushed through a fine nozzle to create a precisely sized extrusion. Filament undergoes multiple state transitions as it is fed through the hotend.

Filament is warmed until it hits the glass transition temperature (Tg) at which it begins to soften. In this state, filament is not fully fluid, but can easly be formed.
Filament continues to be heated until it hits the melt temperature (Tm) at which it flows freely.

The job of the hotend is to allow these transitions to be controlled, softening filament when appropriate to allow it to be fed into the hotend, and heating it when appropriate in order to extrude it through the nozzle. If anything is off in this process – particularly cooling – problems result.

Here’s a simplified view of the Prusa i3 Mk3 hotend (not to scale) showing the cooling process.

The hotend is divided up into 3 zones:

The cold zone is the upper end of the hotend, consisting of the finned heat sink and top of the heatbreak.
The transition zone is where the filament is heated to the glass transition temperature (Tg).
The hot, or melt, zone is where filament is heated to the melt temperature (Tm).

The heatbreak provides a small gap between the cold heatsink and heated block. This is accomplished by a small machined gap between the upper (cold) and lower (hot) heatbreak sections. The stock E3D heatbreak is aluminum, but can be upgraded to Titanium or other metals with lower thermal efficiency, slowing the tranfer of heat between the hot lower and cool upper ends.

Under normal conditions, the heatbreak is sufficient to isolate the hot and cold zones. The heatsink cooling fins can usually be touched without burning thanks to the efficiency of the E3D V6 hotend design. This is an air cooled design. The flow of cooling air over the hotsink fans pulls away heat. Any heat that moves past the heatbreak is quickly dissapated.

The E3D V6 design can cool efficiently at up to 40C ambient temps. If the ambient temperatures exceed this level, cooling efficiency is rapidly reduced and heat can “creep” up into the cold zone. If filament reaches the glass transition point prematurely, filament can deform in the hotend, creating feed problems.

If the flow of cooling air is impeded, efficiency drops rapidly. If sufficient heat accumulates above the heatbreak, filament can soften up in the cold zone creating a variety of problems. Here’s a simplified picture of the Prusa i3 Mk3 hotend (not to scale) showing heat creep.

Fig. 3.4 Prusa i3 Mk3 hotend heat creep

Excessive cooling can also be a problem. If filament is being pushed to quickly, or the hotend is not able to sufficiently heat filament, feed problems can also occur.

Common causes of hotend heat problems are:

Excessive ambient temperatures in excess of 40C (104F).
Excessive enclosure temperatures in excess of 40C (104F).
Excessive, rapid retractions.
Excessive printing speeds exceeding the hotend heating capacity.
Ornamental fan covers reducing hotend cooling efficiency.
Poorly oriented part cooling fan shroud directing cool air against the heater block.

As heat builds up, intermittent problems can begin to show:

Hot filament pulled up into the cold zone, contributing to feed problems (particularly when excessive retraction settings are used).
Extruder skips or jams as filament softens prematurely in the cold zone, possibly jamming in the hotend or PTFE guide tube.
Underextrusion due to poor filament flow.

This problem will usually first be noticed as distinctive clicking noises during printing, usually during rapid extruding movement. If the flow of filament is impeded, back pressure can be created that can create problems higher in the extruder. In short: Anything that contributes to heat that can’t be vented away from the hotend (rated to 40C ambient) impedes air cooling efficiency and can contribute to problems, often in unpredictable ways.

3.1.6.4. Remediating heat problems

There are several things that can be done to reduce heat build-up in the extruder and around the hotend heat sink.

Improving airflow worked for some users. A simple table fan blowing around the hotend was sufficient to allow PLA printing during hot months.
Simply opening printer enclosures made the difference for many users.
Several users reported that early Mk3 kits shipped without the thermal paste between the heatbreak and heat sink recommended by E3D. Adding thermal paste improved cooling efficiency and reduced problems for many users.
Prusa released the R3 extruder parts intended to improve air cooling by opening up the extruder housing around the hotend heat sink. (Note that some users have complained of softening PINDA mounts and other problems with these parts.)
A quick fix may be to switch over to printing with PETG if you can’t do anything about temps right away.

By rectifying one or more of these contributing factors, it’s very likely you can get below the threshold at which problems occur. If you’ve got an enclosure, open it up for PLA. Improve airflow around the heatsink. Just keep this list in mind if you encounter the problem again as months get hotter. You may have to tweak several things.