# Extruder velocity profile during retract or prime

• I realise that I could get the answer to this question by looking at the code but I hope it's a quick question to answer.

When doing a retract or prime with no axis movement does the extruder velocity ramp up and down at the extruder acceleration rate with possibly a constant velocity segment in the middle depending on whether the extruder velocity is reached? If not, what is the profile? Let's assume the extruder jerk is 0.

Thanks

• If pressure advance is set to zero, it does exactly what you say.

Edit: pressure advance is not applied to retract or prime moves, so it always does exactly what you say.

• Thanks.

• Just as an exercise, I put some numbers to this. The time to reach a given speed is (Vf-Vi)/a where Vf is the final velocity, Vi is the initial velocity (zero in the case) and a is the acceleration. So in my case I have extruder acceleration set to 2,000mm/sec^2 and have firmware retraction set to 2400 mm/min = 40mm/sec. Putting those numbers into the formula reveals that the time to reach 40mm/sec is 0.02 secs. To calculate the distance moved while accelerating s=1/2at^2 = 0.4mm. So if the retraction amount is less than 0.8mm, it'll be either accelerating or decelerating. Hmm, never thought about that before….......

• Just coming at this the other way if you want the extruder to retract at 2000 mm/min set this as your e jerk value. I appreciate this might cause a stall depending on the extruder/gearing and you might have to use a lower value but at least you have control over it. I'm using a cable driven extruder on one machine and have to use low jerk and acceleration to get the motor to reverse and turn at high rpm (40:1 gearing) so I expect it never reaches the intended retract speed. It's irrelevant as it retracts enough to prevent blobs and strings.