This thread is in direct reference to the Snapmaker 2.0 single extruder print head, but its concepts may also apply to other SM machines and modules.
There are two primary reasons for hot end clogs…
- The heat break becomes clogged by filament that melted and re-solidified
- The heat break becomes blocked by the degradation of the PTFE liner
As you can see, both have to do with the heat break part of the hot end.
The hot end is made up of 3 main parts…
- The heat break (also known as the “throat”)
- The heat block (this also contains the heater cartridge and temp sensor)
- The nozzle
The nozzle, heat block, and the lower portion of the heat break that is actually inside the heat block all make up the “hot zone” or “melt zone.” The thin portion of the heat break directly above the heat block is the actual heat break zone, responsible for keeping the heat isolated and contained to the hot zone as much as possible. The wider portion of the heat break above that is what inserts into a fan cooled heat sink further inside the print head and is the “cold zone.”
Issue #1 - Retraction
The default retraction distance setting in Luban is 5mm. As you can see in this photo, the section of the heat break that is inside the hot end is barely 4mm. This means that any retraction distance greater than 4mm will actually be pulling melted filament back out of the nozzle and into the heat break zone, where it is prone to cooling and will solidify, causing blockage. Retractions over 6.5mm will have the hot filament traveling all the way back into the cold zone. Generally, not enough heat gets to either of these areas to keep the filament melted, or to re-melt it, so setting smaller retraction distances and as few retractions as possible will help prevent this from happening by keeping melted filament inside the hot zone.
Issue #2 - Heat Creep
This one is CLOSELY related to the retraction issue. There are numerous articles across the internet that do a great job of explaining the few various specific forms of heat creep, but they basically boil down to the same thing. Heat creep is when the heat in the hot end manages to radiate its way past where it should be, into the heat break zone and cold zone, and causes the filament to soften or melt where it is not supposed to. This leads to the same result as with excess retraction: clogging. The further the heat creep extends outside the hot zone, smaller and smaller retractions are needed to avoid having melted filament pulled into an area where it will cool and solidify. In worst case scenarios, the heat creep will extend far enough that it will happen even if retraction is turned off. The hot end cooling fan is supposed to help prevent this by blowing cooler air around the cold zone to keep it cool enough that the heat stays in the hot zone. If the fan fails, heat creep can become an issue very quickly. The PTFE liner is also a component to assist with the heat isolation due to its low thermal conductivity, but…
Issue #3 - PTFE Liner
The stock heat break on the SM2 single extruder print head’s hot end is made of stainless steel, and uses a PTFE liner along its entire length, including extending down into the hot zone. The PTFE liner is a very thin 3mm outside diameter, and 2mm inside diameter. The heat break and the removed liner are shown here.
As the heater cartridge activates and the heat block temps begin to rise, at temps around 240c and above, the PTFE liner gets soft and starts to deteriorate; with higher temps degrading it even faster. Eventually, the PTFE degrades to the point that either it melts all on its own, or a filament retraction will pull against the liner, in both cases blocking the filament path. With the path blocked, the filament won’t move, and it is often mis-diagnosed as an issue with the extruder gear grinding on the filament. A heat break with a good liner is shown here on the left, next to a heat break where the liner has melted and is stuck inside.
At this point, the liner is beyond use or repair. If the blockage is not too terrible, it may be possible to remove the old liner and install a new one, but often times it will be bad enough that the whole heat break must be thrown away and replaced.
Solutions
The best things you can do right away are to make sure that the temps you are printing at are not any higher than they need to be, check that the hot end cooling fan is operating properly, and adjust your retraction settings.
Some people have also replaced the stock hot end fan with an aftermarket one that provides better airflow and keeps the cold zone cooler to prevent the heat creep from expanding too far.
Another option that is growing in popularity is modifying the hot end by replacing the stock heat break with an “all-metal” or “bi-metal” heat break, like the one shown here on the right next to the stock one.
An all-metal one is designed to work with no PTFE liner at all. A bi-metal heat break can come in a similar design, or can be designed to work with a liner, though usually a MUCH smaller one that does not extend into the hot zone or the heat break zone. Bi-metals are also usually made of different metals such as copper, brass, and titanium, which is intended to help both isolate heat to the hot zone, and cool the cold zone more easily. Shown here is the stock liner next to the liner for one of the bi-metal heat breaks, and how far that liner extends in its heat break.
The stock liner is approximately 30mm long, while this bi-metal liner is only 13mm. It is also thicker with a 4mm OD x 2mm ID, which does a better job of heat resistance.
These heat breaks can be great at reducing or eliminating certain issues, but can come with their own set of characteristics that must be accounted and adjusted for. This mod is commonly accompanied with the better hot end fan as a result. Be sure to research the heat break mod carefully before attempting it.