Odd blob on print and extruder skipping step right after filament change
 
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Odd blob on print and extruder skipping step right after filament change  

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Jan_P
(@jan_p)
Active Member
Odd blob on print and extruder skipping step right after filament change

I'm using Prusa Slicer 2.5.0, and have configured a printer profile to have 2 extruders through a single nozzle.
I have disabled the wipe tower, and added M600 to tool change custom GCode.
It's works fine except for one thing.
After the filament change is complete, the printhead moves back on to the part, lowers down to the print layer in progress, and then attempts to rapidly extrude a large amount of plastic. Since the print head is just at the print height, it can't squeeze that much filament out with that speed.
The result is the extruder is skipping steps, and it it leaves a large blob on the print.

I looked at the GCode, but it looks fine to me.
It looks like it does a retract, then move to the part, and extrudes the retracted amount.
In fact, after the filament change is complete, and I confirm the change is correct on the LCD/knob, it then extrudes about 5cm of filament right before moving the print head back over to the part.
So the filament is fully loaded in the nozzle, and the nozzle is still oozing from the filament change while moving.
Anyone know why it does this, and how I can prevent it?

Here's the change specific GCode:

 Filament-specific end gcode
M600
T1
M104 S220 ; set temperature
M900 K30 ; Filament gcode
G1 E-1 F2100
G1 X121.404 Y106.426 F15000
G1 Z.2
G1 E1 F2100
Posted : 26/11/2022 6:54 pm
fuchsr
(@fuchsr)
Famed Member
RE: Odd blob on print and extruder skipping step right after filament change

What printer?

Posted : 26/11/2022 8:46 pm
Jan_P
(@jan_p)
Active Member
Topic starter answered:
RE:

Oh, Prusa MK3S+, running firmware 3.11.0-4955

This post was modified 2 months ago by Jan_P
Posted : 26/11/2022 9:23 pm
fuchsr
(@fuchsr)
Famed Member
RE: Odd blob on print and extruder skipping step right after filament change

In fact, after the filament change is complete, and I confirm the change is correct on the LCD/knob, it then extrudes about 5cm of filament right before moving the print head back over to the part.

That's normal behavior to re-pressurize the nozzle, and you want to be ready to grab the filament and pull it back. I don't know of a way to prevent it short of modifying the firmware as it's part of the printer's M600 ritual.

lowers down to the print layer in progress, and then attempts to rapidly extrude a large amount of plastic.

Now that part I don't understand. Yes, it does a de-retraction but that shouldn't squirt out large amounts of plastic. Are you on the latest firmware?

 

Posted : 26/11/2022 9:46 pm
Jan_P
(@jan_p)
Active Member
Topic starter answered:
RE: Odd blob on print and extruder skipping step right after filament change

Yeah, it's the last part I don't understand either.
Why it would try to push out a large amount of filament, after placing the nozzle on the part is puzzling to me.
I'm on 3.11.0, which as per the web side is the latest release.

Posted : 26/11/2022 10:15 pm
Jan_P
(@jan_p)
Active Member
Topic starter answered:
RE: Odd blob on print and extruder skipping step right after filament change

I think I've found the offending line(s) in the GCode. It does not appear on the first layer, but on the subsequent layers, the filament change looks like this.
Note the G1 E4 after the M600/T1 sequence, and after moving back over the part.

I'm guessing it comes from the wipe from prior to the filament change. It looks like it remembers that it retracted 3.5 mm prior to the tool change, but doesn't account for the filament change, and that the nozzle is already primed.

;WIPE_END
G1 E-3.57851 F2100
G1 Z1 F15000
; Filament-specific end gcode
M600
T0
M104 S215 ; set temperature
M900 K30 ; Filament gcode
G1 X174.837 Y63.887
G1 Z.4
G1 E4 F2100
M204 S800
Posted : 27/11/2022 12:13 am
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