There is an issue with the heatbreak design of the Prusa MK3S printers. To understand what causes this issue (and its possible solutions), please read on.
Unlike the regular heatbreaks used for the standard E3D V6 hot end, the heatbreaks used in Prusa MK3S printers have a small modification: their cooling section diameter is slightly wider (2.2mm) than their neck diameter (2.0mm). See figure on the left.
Apparently, this modification is made to allow the Prusa optional MMU unit to be attached onto every Prusa MK3S printer.
While the feature might be very useful for feeding the different filaments through the MMU unit, it might also create problems with filaments made from some semi-crystalline plastics. Specifically, filament might get jammed into the heatbreak hole (that’s when that dreadful clicking and filament-grinding occurs). This might happen when using our filament, which uses the base raw material Ingeo 3D850.
See figure on the right for an explanation of what happens inside the heatbreak:
- In the cooling section of the heat sink, the temperature increases gradually from the environmental temperature, and as it approaches the end of this zone, it heats rapidly due to the heat propagating up through the neck of the heatbreak. The temperature ranges from 50°C to 140°C.
- In the neck section of the heatbreak the temperature transitions between the cooling block and heat block; it is between 140°C and 180°C.
- In the heating block section of the heatbreak, the temperatures are between 180°C and 210°C and the plastic is in a liquid state, flowing through the nozzle to create the 3D print layers.
Semicrystalline materials like our PLA 3D850 contain crystalline regions, called crystallites, within an amorphous matrix. The crystalline regions give the material some very desirable properties: improved strength and toughness and higher temperature resistance. Also, the material is annealable - that is, it can be heat-treated to improve these properties even further. The amorphous portion of the material has a glass transition point of 60°C. Once that temperature is reached, the material starts to gradually transition from a solid state into an increasingly viscous state, until at some point it is completely liquid. The crystalline portion of the material stays solid until its melting temperature (which is around 170°C) and then suddenly transitions to a liquid state. As Ingeo 3D850 is specially engineered to have a much higher percentage of crystallites than other brands of PLA, its mechanical and thermal properties are greatly improved.
However, a side effect is that—in the particular case of the Prusa MK3S printer—when the filament approaches the neck portion of the heatbreak, the material is amorphous enough to get milder and, due to the pressure created by the extruder feed, it increases its diameter from 1.75mm (initial filament diameter) to 2.2mm (the diameter of the guiding hole in that region). If the material remains viscous enough, it will get pushed into the smaller 2.0mm channel and continue to become more liquid, successfully completing the print. However, if the print takes a longer time and/or there are many retractions, the temperature in the diameter transition step area will get to a value (110°C to 130°C) at which the already enlarged portion of the filament anneals - meaning that more crystallites are formed and the enlarged portion remains solid - and can no longer be pushed through the 2.0mm diameter. At that point the process jams and the print is likely ruined.
We know of 3 methods to resolve this situation:
1. 
Some people suggest increasing the printing temperature to 230°-240°C. This will likely increase the temperature in the step zone of the heatbreak above the melting temperature and will prevent the enlarged portion from solidifying. While this method might work in some cases—and PLA 3D850 can be successfully printed at that temperature—the quality of the printed part might be negatively affected because of oozing and solidification issues. Also, in some cases the heat might creep deeper into the heat sink zone and create jams there.
2. If there is no intention of using Prusa's MMU unit, the best solution is to change the heatbreak part with a standard E3D V6 one, which does not have the 2.0mm to 2.2mm diameter step in the guiding hole. We used this solution on a few of our MK3S printers and never experienced any jamming problems since.
3. If using a MMU unit—or have the intention to use it in the future—another solution is to use a 2.2mm drill and to extend the length of that diameter hole into the heat block area of the heatbreak. We also used this method on a few of our MK3S printers and never had any jamming issues since (see figure on the right).
Have fun printing and
Embrace the layers!
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