Or it could simply be a case of bed leveling being a bit off. Re-check that to make sure that the bed isn't tilted down towards the front. If the bed is too close to the nozzle towards the back of the printer the filament wont be able to squeeze out of the nozzle.
Also it would be helpful to know if you have an original Ultimaker or an Ultimaker2
Thanks for your replies guys.
Its an ultimaker 2. I already leveled the bed several times to be sure.
It makes no difference, so i am more for the first idea, to put some oil in the tube.
Won't that influence the quality of the prints ?
Marc
I don't think it is bed leveling as that usually should be compensated a few layers up in the print.
And I assumed we were talking about overall print quality?
Again, pictures are helpful! 
Or it could also be the teflon coupler (white piece between hot and cold. depends on your version of um also) that has some scratches, carved out inside (I had a very similar issue).
I don't think any issue with the teflon pieces shows at the back of the bed while printing and not at the front.
It either shows or not..
Yes Sander, it does, at least for me it did. But I was a bit 2 quick as it is an UM2 (and I have ONLY an UM1 .
I could re-produce, that there were more issues (underextrusion) at the back of the print, especially with retraction ON. I even made pictures :mrgreen: . I can not say it was at the back of the printer but at the back of the print. Also, sometimes it would not extrude well at all. At the end, after replacing the teflon thingy, all works well.
Back (orientation was backside of the printer, nogood):
Front (good):
EDIT:
just saying, does not mean it's the problem described, but might give a clue. Picture might help!
AND, picture from the carved out teflon piece (I guess, as it was carved out quite a bit at the end it somehow bent over a bit with the force of the bowden tube?
@suminvent - please post pictures. What you see as underextrusion could be something else. It could be a fan/temp issue. It could be a bad nozzle (deformed nozzle due to damage). It could be the bent tube. It could be a lot of things. I doubt it's leveling unless it only happens on the bottom layer.
Oil will not make any noticable difference to your print other than allow the material through the tube better. Plastics are made from oil by the way.
To post pictures:
http://umforum.ultimaker.com/index.php?/topic/4525-how-to-upload-an-image-to-the-forum/
I read that a bit too quick, in my mind it was happening in the beginning of the print. Sorry about the confusion.
I'll make some pictures tomorrow.
The pla stops coming out of the nozzle (brand new, just replaced) and after that i have to retract the filament, cut a piece of it, and do some manual feeding.
After that i can print again.
Marc
I wonder if it's due to the seating of the Bowden tube in the teflon coupler above the print head. You might check that the bowden is fully seated down into the teflon (the end of the tube should press a couple of mm into the white part), and that it doesn't move around during printing.
So sumnivent, take a picture of the print head also so we can see where the bowden tube ends.
I posted the pictures this morning but that post is not appearing. No idea why.
Marc
Again.
Marc
Appearantly size was to big for two pictures.
I tried again but they are not appearing. Is there a limit on filesize ?
Marc
How to upload an image:
http://umforum.ultimaker.com/index.php?/topic/4525-how-to-upload-an-image-to-the-forum/
Robert, thanks for the explanation.
Here are the pictures.
Marc
Damn it. This changes everything.
You didn't say it was only the first layer. That's a leveling issue. I can't tell if the head is too high off the bed or too low but I strongly suspect too high.
It could be you have bent rods or something. In the gantry. Hopefully not.
I know you said you leveled a few times. Here's what I recommend.
1) Assume nothing is wrong with the printer - level one more time but this is 3 point levelling and you don't level for the left rear directly - instead the rear screw and the front right screw control the back rear the most. If the front right screw is too high (too close, too tight on the paper) then the back left will be to loose. So make sure you have the *same amount* of friction on the paper as you slide it in and out. Try printing one more time. Also in Cura set your bottom layer to be .3mm thick (this is the default). There is a homing bug on some machines (will be fixed with software soon). To avoid the homing bug make sure the bed is not all the way down before levelling and again before printing. It needs to be at least 5mm off the bottom position.
2) If that doesn't work - assume (temporarily) that the rods are bent (or the glass?) and test for flatness. Look visually at the printer - does it sit flat on the table or rock on two corners? Look at rods and glass - does it seemed warped or bent? Probably not. To test for flatness more accurately install pronterface:
http://koti.kapsi.fi/~kliment/printrun/
Connect computer with pronterface to the UM2. Tell the printer to go to Z=0 position. I think you can do it with the buttons or you can type command:
G0 Z0
Even better go to .1mm up
G0 Z0.1
Then push the head around with your hand to the 4 corners and check leveling with paper. See if there is a "low spot" in the back left corner but *not* a high spot in front right. If there is a low spot *and* a high spot you just didn't level it well enough so rotate those thumbscrews some more and get it perfect.
OK, thanks. I'll try this next week and report back.
Marc
OK, i leveled it again, and now with a thicker paper.
As you can see, the printing in the back goes better, but you can still see that the density of the printing is getting less, and when it continues to print in the front, the problem continues to get worse.
I don't think the problem is competely due to leveling, but of course i can be wrong.
Testing with pronterface now.
Marc
Maybe install the latest firmware? (if it's not done already)
OK, here are my findings with pronterface:
I put z at 0.2, so the paper could just pass under the nozzle (paper = 160 Gr)
Back
No friction OK No friction
No friction Heavy friction No friction
OK Heavy friction OK
Front
What i can decuce here is that the bed is not ok but slightly curved !
If its ok on one position, its not on another position.
I rotated the glass plate on its back, and now its better.
Going to try again. Nozzle is blocked :-(
Cleaning first.
Marc
Each time the nozzle gets blocked the filament looks like this, as if it uses to much force which -in combination with high temperatures- makes the filament wider which prevents it from continuing.
Marc
ArunC posted a topic in UltiMaker Cura,
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Dustin posted a topic in Firmware,
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Hi Marc,
Thank you for your post.
It may be because when the printer is printing at the back of the plate the curvature in the bowden tube is stronger compared to the front of the bed.
You could try to reduce the friction to add 1 or 2 drops (not more) of sewing machine oil in your bowden tube.
It always helps to include some images as well
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