Change gcode to stop oozing at startup
April 23, 2018 03:36PM
Hi,

I use Cura to slice and I modified the start-up code a bit but in basis it is as it wass when I downloaded Cura. Now I want to change it but I'd like some advise on that. The object of the excersize is to avoid PLA oozing out on start-up. The code does this:
G1 Z5.0 F1000 ; Move the platform down 5mm
G92 E0
G1 F220 E3
G92 E0
G32 ; Home XY and auto bed leveling
G92 Z5.2 ; Correct Z value
And then printing starts

The first line is pretty clear as the explanation is right there.
The G92 E0 is reset extruder, I googled that, but what does that mean? Does it set the length of PLA fed to zero?
What does G2 F220 E3 do? It must be what heats the hot-end before it is allowed to move, but I cannot find it googling.
And then another reset extruder, why I don't know.
After these lines there are some more which I wrote myself, homing and such, I understand those.

I would like to understand the G1 F220 E3 line, and if it is indeed the heating, move it around. My idea is to keep the hotend cold during homing and only heat it once the printing starts. So my guess would be:
G1 Z5.0 F1000 ; Move the platform down 5mm
G32 ; Home XY and auto bed leveling
G92 Z5.2 ; Correct Z value
G92 E0
G1 F220 E3
G92 E0
And then printing starts

Does that make sense? I always use ether a brim or a skirt so missing a bit of material in the beginning is no problem.
Please advise,
Cheers,

Hugo
Re: Change gcode to stop oozing at startup
April 23, 2018 08:00PM
I have a different Ideal, though is would have the same effect.

When Printing the 3D Printer homes, High on the X0,Y0, Z-Axis about 10.5 then slowly drops while moving to the location the print begins reaching the bed to begin the print.
Is there a way to drop to the bed then move to the location the print begins, thereby leaving the oozing in the wake (Leading to the model).
Re: Change gcode to stop oozing at startup
April 24, 2018 02:04AM
I figured out the G1 F220 E3 has of course nothing to do with heating the hotend. Obviously Cura sets that to the print-specific value before running the start-code we can program. The code just primes the extruder with 3 mm. Doing this before homing and leveling makes no sense, on the contrary. So here's my new idea:


First Cura heats the hotend, then:
G92 E3 ; set extruder's current value to 3 mm
G1 F220 E0 ; retract extruder 3 mm
G1 Z5.0 F1000 ; Move the platform down 5mm
G32 ; Home XY and auto bed leveling
G92 Z5.2 ; Correct Z value
G1 F220 E3 ; undo retraction
G92 E0 ; set extruder's current value to 0 mm
And then printing starts

This way when the hotend is hot, the fillament is retracted before moving about for homing and leveling, and only comes back when all that is done and the printing starts.

Hugo
Re: Change gcode to stop oozing at startup
April 24, 2018 02:52AM
You have to see the start code in context with the end code ( of the former printed part ).
In the end code the filament is retracted by 3mm to avoid oozing, while the hotend cools down.
The start code only brings the filament back to "zero" position.

The problem with this start code shows, when the former print wasn't succesful and you stopped it, before the end code was executed. In that case you have to send G1 F222 E-3 manually, while the hoten is still hot.
Re: Change gcode to stop oozing at startup
April 27, 2018 09:30AM
There is no 100% useful start or end gcode, but I advise you to fully customise it to your needs. Delete the slicer's default start gcode and use your own. One thing to remember most slicers will add heat bed and heat hotend unless you specify them in your custom start gcode.

Mine sets printing speed, pressure advance, retraction. Preheats the bed, but the nozzle only to 130. Then I probe, then move the nozzle outside of the bed, heat to printing temp, prime and cut the excess off by wiping on the side of the bed, then print.

Edited 1 time(s). Last edit at 04/27/2018 09:31AM by DjDemonD.


Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions
Re: Change gcode to stop oozing at startup
April 27, 2018 09:38AM
If you want no ooze the filament needs to be retracted before you heat the nozzle, otherwise the thermal expansion/offgassing will push a bit out.

I retract 2mm at the end of every program. At the start of every program I extrude 1.8mm and do a long skirt to prime the nozzle.

It works as long as you make sure the retraction actually happens. If you cancel a print you need to manually G91 G1 E-2 F200 or the next program will smoosh a blob onto the build plate.
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