What is ramming?
I’m far away from my partially built MMU2, killing time on a business trip reading about your builds.
One thing I’ve seen references to that I’m not clear on: ramming. Could someone explain what it is, the value it adds, how it works, and how to configure it?
From reading about MMU1 issues, I assume it has something to do with shaping the tip of the filament nicely before retraction, but a pointer to (or explanation of) the theory and physics behind would be helpful.
Re: What is ramming?
Thanks for asking this quesiton. If you conduct a search using just the word ramming at the top of the page and you'll see the posts I have. It seems to be how much time it allows for loading new filament, but I may be wrong.
Re: What is ramming?
From reading about MMU1 issues, I assume it has something to do with shaping the tip of the filament nicely before retraction, but a pointer to (or explanation of) the theory and physics behind would be helpful.
Spot on. Before the filament is unloaded, about 5 linear mm are extruded quickly onto the purge tower and then immediately and quickly retracted. Putting the cooler filament into the hot end and then retracting helps form a tip with a small pointed "plug" on the end and without any stringing.
Peter
Please note: I do not have any affiliation with Prusa Research. Any advices given are offered in good faith. It is your responsibility to ensure that by following my advice you do not suffer or cause injury, damage…
Re: What is ramming?
Here's what I got out of one of the searches.
Ramming settings
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Wed Sep 05, 2018 8:46 pm
The newer versions of Slic3r have a new button under Advanced Filament Settings for "Ramming settings."
Clicking this button brings up a scary-sounding warning dialog saying that you shouldn't mess with these settings unless you really know what you're doing. Ramming is apparently a rapid extrusion the MMU performs just before unloading a filament, in order to ensure the tip of the filament is properly shaped.
Re: What is ramming?
There are multiple meanings for this word, but mods will ban me if I would explain all of them .... 😈
But your understanding is correct. This function try to achieve a perfect shaped filament tip. It highly depends on the filament/brand you're using. The theory is based on fluid mechanics because you're pushing and dragging melted plastic in a curved (nozzle) environment. In the graph you can variable define how much plastic you want to push over specified time.
Often linked posts:
Going small with MMU2
Real Multi Material
My prints on Instagram
Re: What is ramming?
Thanks, that makes sense.
The way I'll picture it: at any given time the temperature of the filament pool in the melt chamber will vary depending on flow rate of filament. The metal of the chamber will have relatively constant temperature, but the average temperature of the melted filament will vary (lower temp with high flow rates and much energy going to melting new filament, higher temp with low flow rates where the pool is basically sitting there ramping up to full hot tend temp).
Because the flow rate at the time of retraction is unpredictable, the shape/string of the retracted tip will not be predictable. There is a preference for a cooler tip (less stringing). Therefore before each retraction, gcode is inserted to rapidly extrude filament, cooling the melt chamber to a consistent temperature with fresh filament just before retraction.
Makes sense. Thanks,
Paul