This might sound dumb, but what “exactly” are the causes of ooze? Specifically, what causes filament to continue to flow after the filament drive stops?
The back pressure in the nozzle. This is why you need to retract the filament to drop the back pressure to zero
Both of the above plus thermal expansion of the plastic to some degree.
And the underlying cause of the back pressure is thermal expansion of the plastic as it transitions to less than fully solid, along with filament compression.
Compression is more of an issue in bowden extruders, but is still present (to a much smaller degree) in direct drive systems too.
The plastic sitting in the nozzle will continue to heat, expanding and becoming less viscous. As the plastic becomes less viscous, it will resist the existing back-pressure less, while thermal expansion adds to this pressure.
Gravity probably has the least effect, except in cases where the nozzle sits hot for about a minute or more, after pressure and temperature have reached equilibrium and air starts to work its way up into the nozzle, forming a bubble behind the plastic that is oozing out.
Thanks for the info, I’m interested in exploring this more.
So what extruder/hot-end traits lead to some nozzles oozing more or less than others?
Length of the hot zone is the biggest one. A hot end with a short, tightly-controlled hot zone will have less molten plastic in it, so the pressure will equalize with less plastic coming out.
@Jason_Gullickson according to the knowledgeable @Sanjay_Mortimer , the length of the melt zone makes a big difference, smaller the better.
Man, I have to raise my question answering game - keep getting shown up by @Whosa_whatsis
In my defence, I am wrangling a high energy toddler at the same time.
I have some PLA that oozes no mater what, then i have some ABS that pretty much doesn’t. Not sure if its from the different kinds of filament and or the stock Solidoodle hotend.
Based on this, I wonder if my oozing problems stem from gradual changes to the thermal properties of my hot end. Perhaps the cement I used to fix the heater has cured more or otherwise changed some since to originally PID tuned it, etc.
I’m going to re-tune and see if that changes anything.
Thanks for all the info, I love digging into this stuff here and cross-referencing all of the experience.
PLA oozes much more than ABS from my experience, but I’ll add a disclaimer that I use the two on two separate machines
PLA is always more likely to ooze (especially the gravity-powered type of oozing) because it’s more liquid at extrusion temperature.