I wanted to bump this thread in case anyone searches for it and could benefit from finding out what my solution was.
I used two solenoids to control air pressure from my compressor to my syringe. One solenoid is used to control air pressure going to the syringe, the other solenoid is used to vent the syringe when the extrusion is finished. I used a nordson efd syringe. Kinda pricey but it’s good quality. In order to control the extrusion rate, I installed an air regulator upstream of both solenoids.
I used the enable pin logic to control my solenoids by making the following changes to the firmware:
In the appropriate “pins” h file I set my E0_ENABLE_PIN to pin 12 which is wired to my extrusion solenoid. My venting solenoid is wired to pin 13.
In Configuration.h, I defined E_ENABLE_ON as 1, DISABLE_E as true, and DISABLE_INACTIVE_EXTRUDER as true. This makes pin 12 high when the extruder should be running, and makes pin 12 low when the extruder should be off.
In order to make pin 13 vent at the appropriate time, I set up an if statement at the beginning of the main marlin loop that sets pin 13 to the opposite value of pin 12.
So far this has worked for me and I hope this helps someone else at some point.
