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Extrusion before printing and start of skirt run

Posted by animoose 
Extrusion before printing and start of skirt run
June 25, 2015 12:02AM
I have two related questions which affect first layer quality. My printer oozes some filament from the nozzle when it it getting near the target temperature, but before the first printing starts. This means I often end up with a ball of filament loosely stuck to the nozzle. If I'm lucky it get pushed out of the way, if not, it can make a mess of the first layer. So my question here is: is this normal, and if not how can I stop it?

The second question concerns the skirt. The head moves over the path of the skirt for a while before it starts to print, or at least before any filament sticks. Maybe it is extruding but staying stuck to the nozzle. Again, is this normal? It doesn't affect the quality of the print as far as I can see.

I took a short video which I hope shows what I mean (https://youtu.be/CopaYKGuSoI). Sorry for the low quality, but you should get the idea. It starts when the nozzle is at about 175C. This is 1.75mm PLA, with a first layer temperature of 195, 185 thereafter. Bed is 70. Varying these doesn't make much difference. The printer is a Folger 2020, which has a rather basic direct extruder with a 0.4mm nozzle. I think the Z height is good - I can just get a piece of paper between the bed and the nozzle at all points. The first layer thickness also doesn't make a difference. I don't think it is an adhesion problem, as the first layer is fine in every other way. I use Slic3r, and my initial gcode is
G28 ; home all axes
G1 Z5 F5000 ; lift nozzle
G92 E0 ; zero the extruder position

Could this be just a limitation of the extruder? Would retracting some filament in the end Gcode help, so there is less to ooze on the next run? I welcome any sage words.

(Also, please forgive me if I'm asking too many things on these forums. I've posted a lot of questions the last few days. As a noob, it's hard to understand what factors make a difference.)

Edited 1 time(s). Last edit at 06/25/2015 12:04AM by animoose.
Re: Extrusion before printing and start of skirt run
June 25, 2015 01:11AM
Yes, it would help to retract in your end Gcode, but you'll need a minimum length on your skirt to make sure the nozzle is primed when the part starts printing and you'll definitely have a gap in the start of your skirt, so you'll need to figure out how to deal with that if you want to do a tall skirt.

By default, Slic3r sets temperatures and then does your start gcode, so your nozzle is coming up to temperature while the axes home. If you manually move the axes closer to where they'll home ahead of time, this won't take as long and the nozzle won't have as long to ooze ahead of time. Alternatively, you can add temperature setting commands to your start Gcode and specifically heat the nozzle just before the print launches. For oozier extruders and/or oozier filaments, I've also been known to pluck the accumulated ooze off the nozzle just before the head starts moving, but this takes needlenose pliers and some experience with the machine to do it reasonably safely.

I haven't tried it, but I've also heard that the nozzle can be set on the bed to limit the amount of ooze.
Re: Extrusion before printing and start of skirt run
June 25, 2015 10:38PM
Adding a retraction command helped a little, but not as much as I would like. I decided to add a chunk to the start G code which prints a simple one-layer pattern before starting the main print. I think I am seeing fewer cases of a blob of filament coming off the nozzle when it starts the print for real, though this may be wishful thinking. I'll try it like this for a few days and see how it goes. BTW, I wasn't losing prints from this "first layer blob" problem except very occasionally. It's more that it just offends my perfectionist tendencies.
Re: Extrusion before printing and start of skirt run
July 08, 2015 01:17AM
The printers at the local makerspace extrude two little blobs in the corner before printing. The kislicer is setup to not print any skirts. This seems so effective, I was trying to setup a start gcode to do it but couldn't figure it out.
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