Flow of plastic stopping for a while after 2-3 layers.

[rewolff 0]

We're trying to print some stuff, but after two or three layers the flow of plastic stops. It prints 2-4 layers of "mostly air" and then starts putting out the proper amount of plastic again.

I've added G-code to the "start G-code" window to start by extruding 20mm of plastic (at Z=20). Then move to the platform and deposit a nice "wall" of a few layers of plastic.

The wall looks mostly solid every time.

All this has made very little difference: it still stops extruding for a while during the actual build.

Oh. It is not something mechanical with the material feed mechanism. The stepper turns and the PLA moves into the machine just fine (at that point in the machine).

Suggestions?

[rooiejoris 5]

Did you also put a G92 E0 after your extrusion code...?

Your E values are absolute, does your extruder wheel runs reverse after your startup code...?

[technicality 0]

Have you tried raising the temperature? Could be that the first two layers are being printed with slightly hotter PLA and then it cools down, to recover later. This could be (but hopefully isn't) a classic bowden plug.

[rewolff 0]

@technicality: I was informed I probably have a "bad" bowden tube, so I'm getting a new one. The prints are reasonable, and I wasn't informed how "bad" a "bad" bowden tube is. (I'm not expecting it to turn zombie on me....)

(what's a classic bowden plug? Googling for that phrase gives hits like:

)

Yes, I've nulled the Extrusion position after extruding the "wall".

I've found that I can prevent having to edit G-code manually by enabeling the skirt. So now I have 8 loops of plastic on the bottom layer around my object and I've eleminated the "possibly faulty" G-code that I manually wrote.

I've tried getting skeinforge generated G-code to work. That was a big disaster. Absolutely nothing worked. I've given up and I'm back to slic3r.

At what temp do you guys extrude PLA? First layer same temp?

[update] Some more experimenting... I've written a program to generate the G-code for a "cup". Doing the bottom layers slow, speeding up for the higher layers. To get the flow going I draw a 160mm line around my object. This works reliable. I think the retractions influence the extruded material. I've written my own "write-the-G-code" program because I was anoyed at slic3r who generates blobs in my objects because he stops for changing layers. The stopping with extrusion means the solid PLA is pulled back, but the molten PLA keeps coming out. The result is at least blobs of extra material near the place where layers are changed. My program simply increments the layer and gets really nice results. (Just that the sides of the cup don't attach strongly enough to the base. :-( )

[technicality 0]

The classic bowden plug is when there is still a gap between the PEEK/brass and the bowden tube. Molten plastic will come up (because of backflow:

 

) and stick to the PEEK. Because the PEEK is much cooler than the brass, the PLA will also solidify. EVen more plastic comes up, sticking to the other cooled plastic and thereby creating a plug.

With ABS there is no backflow.

Edit: there are some other options for slicing. You can use Cura, which is a nice front-end for a (modified) skeinforge. It uses printrun to communicate with the machine. And some people are using KISSlicer which seems to generate top-notch results so far.

[technicality 0]

@technicality: I was informed I probably have a "bad" bowden tube, so I'm getting a new one. The prints are reasonable, and I wasn't informed how "bad" a "bad" bowden tube is. (I'm not expecting it to turn zombie on me....)

(what's a classic bowden plug? Googling for that phrase gives hits like:

)

 

I just made this animation that explains the 'classic' bowden plug:

http://wiki.ultimaker.com/images/Bowden_plug.gif

 

[concreteturtle 0]

Is your bowden tube is one of the newer, wider ID PFA tubes?

I'm having similar issues, and I wonder if it's to do with having the filament a little loose in the bowden tube. My kit came with one of the defective tubes and this replacement is pretty loose fitting.

[rewolff 0]

Aaargh... We've fixed it. I don't remember what we changed to get it this way, but this hasn't happened for a few days now. Grrr. Sorry guys.

I think switching to cura and printing a skirt has helped....