Closed loop extrusion pressure control

Tony Owens

I'm a new poster so I hope my raising this topic isn't tedious - please be gentle!
Plasticisation and extrusion rate control in FFF printing are more or less open loop processes. In FDM printing (Stratasys) - I don't know whether that too is open-loop. Looking at the injection moulding and extrusion analogues, the norm is provide both 'nozzle' pressure and melt temperature control loops. It would seem reasonable to believe that these would be Good Things if only they were possible. I also read a posting from someone recently concerning use of miniature high temperature piezo pressure sensors with integrated thermocouple from Kistler for hotend melt monitoring.

My initial thoughts:

hotend pressure would be modulated about the setpoint by the advancement of the melting filament by the extruder.
melt temperature would be maintained at a fixed setpoint as at present, by hotend heaters, or possibly reduced in direct proportion to extrusion rate to allow for shear heating of the melt as it is ejected through the nozzle
the setpoint pressure would be modulated according to instantaneous ejection rate. This would probably need to become a low bandwidth autotuned PID control loop in time, due to nonlinearities between extruder motor speed and hotend pressure.
if 1 - 3 were possible, it might mean that manual material-specific retraction tuning, 'pressure compensation', monitoring of nozzle bore wear and extruder slip would become much less critical to quality, and might ultimately be relegated to an 'adaptive control' loop via printing of test features at the start of each print. It might also mean that during hotend warmup the rheological properties of the current filament could be inferred, by looking at the relationship between measured melt temperature and pressure.

My questions:

if it were practical to do this, could the required extrusion control system be implemented within the planned suite of Duet tool boards?
How difficult would it be to provision a few reasonably high resolution noise shielded analog inputs to Duet for industrial pressure sensing (4-20 ma current loop or +/- 10Vdc) to talk to a Kistler charge amp?
In the remote event any of this actually worked sufficiently well to improve print reliability and quality, how difficult would it be to develop low cost charge amplifier and signal conditioning circuitry on a tool board that could displace the costly Kistler charge amp and crash the cost of closed loop extrusion?

Danal

@Tony-Owens said in Closed loop extrusion pressure control:

if it were practical to do this, could the required extrusion control system be implemented within the planned suite of Duet tool boards?

In my opinion, fairly straightforward PID loops in the firmware. I see no showstoppers for coding this sort of thing. .

How difficult would it be to provision a few reasonably high resolution noise shielded analog inputs to Duet for industrial pressure sensing (4-20 ma current loop or +/- 10Vdc) to talk to a Kistler charge amp?

0 to 3.3V analog inputs are plentiful. A simple outboard circuit could scale from the charge amp to these input.

In the remote event any of this actually worked sufficiently well to improve print reliability and quality, how difficult would it be to develop low cost charge amplifier and signal conditioning circuitry on a tool board that could displace the costly Kistler charge amp and crash the cost of closed loop extrusion?

The tech piece of such an amp is trivially easy. Therefore, I'd assume that Kistler has various forms of IP protection in place. That could stop Duet (the company) cold. There may be ways forward for individual hobby use. Maybe. I'd also assume that this would be developed as an aux board and run that way for quite a while before it integrated into any other boards.

arhi

@Tony-Owens said in Closed loop extrusion pressure control:

In FDM printing (Stratasys) - I don't know whether that too is open-loop.

Kai Parthy (guy who invented wood filament, ceramic filament, lay* filaments, 1730 hotend... inventor from Germany) showed me once broken Stratasys extruder. I have seen some work on YT too. All of them use a DC motor. If you monitor the current and encoder output you could probably (and they probably do) deduct the pressure inside the filament.

Danal

@arhi said in Closed loop extrusion pressure control:

If you monitor the current and encoder output you could probably (and they probably do) deduct the pressure inside the filament.

I'd be VERY surprised. The relationship of pushing the filament vs. chamber pressure is very non-linear.

Were it possible to deduce pressure solely from how hard, and how much, the filament is pushed, it could be done from existing stepper technology.

arhi

@Danal You are probably right and they are probably just monitoring current for stall detection (clog, block..). They don't have any sensors in any of the nozzles I have had chance to see.

Tony Owens

Thanks chaps. Sounds like a breadboard arrangement using current loop analog out from a Kistler amp would be a place to start. The process development side of things would be time consuming - logging extruder speed, filament sensor-determined true filament speed, temps and pressure, and possibly also looking at intrusion force of the filament into and out from the hotend. And correlating these things across a range of print speeds and materials. Sounds like a interesting year long project.
Armed with heuristics from this, the pressure control loop and perhaps some basic adaptation tests could be designed. And in an ideal world it might be possible to just use filament intrusion force and hot and coldend temperatures for popular materials like PETG to estimate melt pressure/mass flowrate. Removing the costly pressure sensor.

bot

One question I would have would be exactly what absolute pressure value are we trying to maintain?

Is there a theory on this yet? Without a theory about how to control the pressure, I don't know if controlling the pressure would help.

IE: constant pressure of 20 psi while extruding, 0 or negative PSI when not extruding? Do we need to ramp pressure with acceleration?

Danal

It should be possible to 'know' the viscosity of the exiting fluid based on material and temp. Given pressure and viscosity, it should be possible to control flow rate. At least control it somewhat more exactly than just stuffing filament at the hot end via controlling the step rate of the extruder motor.

Several "should be" up there... but it sounds like pressure tells us more than just extruder pulses.

Tony Owens

@bot For order of magnitude estimation, a max push force of 40N (9 lbs) on a 1.75 mm dia filament should theoretically generate up to 97 Bar (1,450 psi) pressure in the hotend (plasticiser barrel). The pressure generated by simple gripwheel extruders is more than many peaople realise!
In reality, some of this filament push force is diverted to tractoring-in the fresh filament to the extruder gripwheels, and notably to overcoming friction between the semi-solid melt and the inner walls of the hotend. But it would appear that the extruder pressure in the hotend can get up to 10-15% of those commonly maintained in the barrel of a plastic injection moulding machine (10k psi at the nozzle of the moulding machine barrel).

Tony Owens

@Danal I recall a discussion involving @deckingman some time ago which touched on the elasticity of the 'shot' of melted (hopefully) material in the hotend, and the effect of this on retraction requirements. There was also theorising about the effect of extrusion rate on the average viscisity of hotend melt, down in the vicinity of the nozzle. The thread was contentious, and I remember thinking that one reason for this was the existence of many variables and influences which were not acknowledged fully. I see this in Moldflow analysis of mould cavities which I occasionally do in my day job, where resin melt heating occurs in regions of high shearing rate (e.g. constrictions), even a long way from the gates where the flow front has cooled.
So, for sure, knowledge of average pressure and melt temperature determines a lot about the rheology of the melted material being extruded from the nozzle, and thus instantaneous extrusion rate. This should help with improving extrusion reliability.