Stop for a minute and think about the geometry of the internals of the nozzle up through to the heat break:
The heat from the heat block is conducted to the neighboring metal parts: the nozzle below and the heat break above. The nozzle is typically brass or other thermally conductive metal so the molten filament does not cool and solidify before being extruded. That makes sense. The heat break is designed to minimize heat conduction upwards and which minimizes the length of the melt transition zone - where filament transitions from solid to fluid plastic. The heatsink's job is to prevent heat creep all the way up the hot end, which would result in a very long melt transition zone. The reason this is not good is described below.
Now look at what happens during retract - and this is conceptual. The entire melt transition zone is lifted. The molten filament is viscous enough that it follows along and is pulled up out of the orifice. In this diagram I am attempting to show a hypothetical 1mm retract. Since the nozzle is about 1mm long, in theory, a 1mm retract should pull all of the plastic out of the orifice and create a cavity about 1mm long in the nozzle. In practice, it isn't as simple and clean as this. Several factors influence the retract behavior. Here are a few of the larger contributors but there are others like capillary action, etc.
- Temperature is a key player since the hotter the molten filament, the more fluid it becomes. If the fluid becomes too fluid, it could simply drip out the orifice rather than be pulled back up with the retract. This is why printing at too high temperature than required is not a good idea. Ideally, you want to find the minimum reliable melt temperature to maximize the viscosity of the molten filament.
- The length of the orifice also plays a big part. In fact, my experimentations and modifications a few years ago with the E3D V5 hot end nozzle geometry resulted in a redesign that is now the V6 hot end. If the nozzle bore is too long, materials like PLA tend to not be cleanly retracted and can ooze. Many people then compensate for this oozing my increasing the retract length - which see next.
- Retract length has a very large impact. The longer the retract, the further up into the heartbreak the melt transition zone is pulled. It also tends to elongate as shown in the diagram. When the retract gets to a certain point, the top part of the melt transition will actually cool and solidify in the cold zone (heatsink). This results in a plug or jamming. Depending on many factors, sometimes this plug will be pushed ahead and remelted when the filament is advanced. However, if these factors are aligned against you, a jam occurs and results in filament starving and your print will show gaps. Many times this starvation might self-correct if another retract happens soon afterwards and frees things up. But if you are really unlucky, the plug continues to solidify and your print is completely ruined.
- Retract AND advance speed also play a big role. If retract is too fast, the molten plastic just "snaps off" at the end of the small diameter orifice and the nozzle bore is not cleared. If too slow, you waste a lot of time. On the other side, advance, a different situation exists. If you advance too quickly with PLA, the molten material appears to increase in viscosity to the point where it will not flow. This is called non-Newtonian fluid behavior. Most of the time, this might not cause a problem but in some cases, it can result in a plug and filament starving. With PLA, you can actually retract rather quickly (50mm/s) and advance slower (20mm/s) with excellent results. At this writing, only the KISSlicer provides this capability (and it was added at my request). It makes a difference. I've print 100s of parts a month in PLA and have not had a single jamming/plugging issue in several years.
If you don't optimize your retract and temperature settings, one day you'll come across a part, sliced a certain way, that just doesn't print reliably. Unfortunately, many slicers provide profiles or printer manufacturers or owners advocate profiles that have very high retracts, very fast retract/advances, and high melt temperatures either due to lack of understanding or to compensate for one or more sub-optimal parameters or issues. Temperature and retract length, in particular, are often abused - the "more is better" syndrome seems to be the recommended solution to many printing problems.
A Little MathThe volume of a cylinder can be calculate using the formula: π x r^2 x length (Pi times radius squared times length). So let's take a look at how much PLA is in a nozzle office .4mm diameter by 1mm long:
π x (.2)^2 x 1 = 0.13 cubic mm of PLA
and just for comparison, the stock SeeMeCNC nozzles have a .5mm diameter:
π x (.25)^2 x 1 = 0.20 cubic mm of PLA
Now, let's calculate how much volume a 1 mm retract of 1.75mm diameter filament moves:
π x (0.875)^2 x 1 = 2.40 cubic mm of PLA
This shows us that a 1mm retract would move over 18 times more filament than required to clear a .4mm nozzle bore - in a perfect (Newtonian) world. In practice, with optimized temperatures, a reasonable nozzle and hotend design and geometry and reasonable retract and advance speeds, this 1mm retract is usually more than enough to get clean transitions and minimize oozing problems. Hot ends with longer melt zones, larger diameter orifices, less efficient heartbreak cooling, and other factors typically require additional retraction to compensate.
- First and foremost, determine the minimal melt temperature for your filament. See my Strategies for Successful (and Great) Prints point #9 on how to do this.
- Don't over retract. The E3D V6, SeeMeCNC HE280 and other similar all-metal hotends should perform very well with 1mm retract.
- PLA is more sensitive to retract/advance speeds than ABS and other materials. My recommendation for PLA is to retract relatively fast (40-50m/s) (and short as per the previous guideline) and advance slower (20-25mm/s). At this time, only KISSlicer has this feature. In other slicers, a reasonable compromise is to retract and advance at 20-25mm/s.
- Keep your nozzle tip clean and polished.
Hopefully this post will help you understand the complex process that allows us to print plastic objects with pretty darned good results. I've left quite a bit of details out (and I may be dead wrong on others) but I can say that if you pay attention to the above 4 Guidelines, your reliability and quality should go up markedly.