Slicers that work with valve-based / fixed speed extruders

I was playing around with a cnc glue dispenser and thought about using it as a 3d printer for silicone and similar paste-like materials.
The extruder isn't really an actual extruder with a stepper motor, but rather a valve that can be controlled via g-code to be open or closed, but there is no way to adjust the flow rate in software.

Does anyone know if there are slicers that support extruders like that where they can't control the extrusion rate or where it can be set to some fixed value?

A workaround might be to define a minimum and maximum flow rate or extruder feed and then post process the g code to replace extruder commands by the m codes that open and closes the valve. I looked at a few different slicers, but haven't really found a setting that would cause the extruder to extrude only a a fixed rate.
Slic3r supports a maximum extrusion volume per second, but there doesn't seem to be a minimum setting, so it occasionally still tries to extrude less than that.

Edited 1 time(s). Last edit at 06/12/2022 08:47PM by QuantumKittens.

... to avoid acceleration/decceleration sequences you have to move pretty slow and adjust acceleration+jerk, so it would switch "instantly" from stop to constant speed and back.

By the way, I'm using "drop on demand" piezo-jetters and adjust the dispensing rates with the E-value of G1-moves ... but this jetters are pretty pricey (up to 10k€ per set)

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This actually used to be common on early RepRap-based printers that used DC motors for the extruder. The first printer I ever built used a DC geared motor for the extruder, which was only on/off, rather than acting like another axis. X-Y movement of the printer ran at a fixed speed with no acceleration, which required the printer to run quite slow (I don't think I was able to run it faster than 25 mm/s). Because the extruder could not retract, I had to use a "coast" setting that would stop extruding a bit before the end of the extrusion move to prevent oozing all over the place.
That was using a software called Skeinforge, which has not been updated in about 10 years.

I'm not sure if modern slicers have a similar option for an on/off extruder. Maybe one that can generate gcode for pen plotters, lasers, milling, or similar?

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[reprap.org] this shows reprap gcode. M108 Sxxx was used in the old days to set extrusion speed. i don't know if it is used that much anymore, or if modern firmware does testing on it. it was used to set the PWM duty cycle of the pulses to the DC motor to average out power under load to motor. the motor windings would average out the power of the DC pulses. So your system might need to be able to do something similar.

with valves it depends on speed of response, and on how analog it would be. if it has an analog open close setting then this might work with some integration of electronic components (driver circuit, relay... ect..)

often these settings are done at start of print, and layer change and at end of print. so these are easy to add to custom machine start and end gcode of machine settings in cura.

for example add to start gcode something like M108 S255 , and to end gcode M108 S0

maybe see if further answers come along also.

Edited 2 time(s). Last edit at 06/21/2022 03:17PM by jamesdanielv.

I would have thought, with a fixed rate of flow, your printing move speed would depend on what layer height you set.