RE: [CAD_CAM_EDM_DRO] reprap build started

The pullback is more complex that you might first expect. The drool is not just a matter of surface tension siphoning molten plastic out of the nozzle.

At any given extrusion rate, there is some part of the filament in the melt chamber that has not melted yet; as the extrusion rate drops so does the amount of unmelted filament. These plastics tend to have high thermal expansion coeffecients (compared to metals, for instance), so when the extruder stops, the expansion of this section of filament as it melts causes some of the drool.

It should in theory be possible to develop a model of this phenomenon, giving an estimate of the mass (rather than volume) of plastic in the melt chamber from a profile of recent extrusion rates, and based on that, tweak the actual feed stepper position so the cooled volume of filament coming out of the nozzle is actually the amount the GCode has called for.

-Kevin Martin
the Papertrail Handmade Paper & Book Arts
New Dundee, Ontario
518-884-7123
www.papertrail.ca



-----Original Message-----
From: CAD_CAM_EDM_DRO@yahoogroups.com [mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of KM6VV

[...]
I just heard about pull-back, that sounds like it might possibly be
accomplished with some sort of back-lash comp algorithm (?).