This post is an input for discussion.
I have done some measurements in an instrument I built. It will measure the force on a 1.75mm filament entering an E3Dv6 with a 0.4mm nozzle. I use a Bondtech to feed the filament. The filament used is ColorFab Ngen. To make it easier to understand the result (for me) I use the ratio between 1.75 an 0.4 to express the extrusion speed. The plastic is extruded into open air, so no back pressure.
The graph below shows the force for different temperatures (210C to 260C). Note that the force doesn't converge on origo. For higher extrusion speeds the curves would probably be less linear.
When the extrusion stops slowly there is a remaining force that almost always is around 1N. It is possible that this force is caused by the friction between the hot, semi melted filament and the nozzle and lower part of the heat break. I the force is subtracted from the earlier graph it looks like the graph below. For the higher temperatures the curves now can be extrapolated to origo.
It is also possible to to measure the enthalpy for melting the plastic as shown below.
And the total power used, graph below.
The first graph can also be plotted in an other way and will then show, for a given print speed, how the force decreases with increasing temperature.
I have some thoughts regarding Pressure Advance
- If there is a remaining force of 1N the trapped volume between the nozzle tip and the non sliding section of the filament would heat up, expand and ooze out.
- Is Pressure Advance (M572) and non-linear extrusion(M592) overlapping.
- These measurements are steady state measurements. Do they confirm the theory behind Pressure Advance?
Any other comments?