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occasional irregular segment

Posted by mfinn 
occasional irregular segment
May 10, 2012 05:51PM
I keep having problems where suddenly my plastic doesn't stay where it should. I thought I had it narrowed down to inconsistent extrusion, but I just completely replaced the extruder (same makergear hotend) and am getting the same result.

It seems like sometimes, for up to 20 seconds, I don't get enough plastic, to the point that it doesn't exactly stick to the previous layer correctly. After a short (vaiable) amount of time or distance, it's fine again.

This is causing some pretty bad prints at times when several of the perimeters fail due to this problem. They don't stick, and get dragged out of position as the head rounds a corner. Infill is the easiest place to see it though, and I've included a picture of a test to show an example.

In the picture, you can see about 10 back-and-forths where the problem popped up. The ridge running top to bottom in the lower left is where this same problem happened in the next layer down, and was covered up.

I'm using slic3r, marlin, and a mostly stock makergear mendel prusa, although I am printing with a gregs wade reloaded [www.thingiverse.com] at the moment, and am having the same problem. Layer size has been .2 and .25, nozzle is .35, PLA, tried temps from 185-195 degrees. Hot end temp is stable. I've had this problem ever since I built the machine.

I don't think the filament is slipping, Since it is pretty uniform, and i don't get plastic shavings in the extruder.

I'm starting to get very frustrated. Help!

Edit: I added another picture of the print I'm making right now. The middle of the image should be solid infil, and isn't. Also, you can see some spots where the perimeter failed on the left side.

Edited 1 time(s). Last edit at 05/10/2012 06:07PM by mfinn.
open | download - extruder_test_small.jpg (457.2 KB)
open | download - print.jpg (338.1 KB)
I've had similar problems and found it was to do with filament slipping. I would try slowing the print speed right down to see if the problem reduces or disappears. Filament can be easier to extrude at a slower speed, reducing slippage.

Re: occasional irregular segment
May 10, 2012 11:43PM
I now tried switching my hotend and nozzle to a 1.75mm set I had, and I get the same results.

Z axis / bed height?
Re: occasional irregular segment
May 10, 2012 11:45PM
Airtripper, I just don't see how it could slip so uniformly for a while, then go back to normal.

I will try a very slow print tomorrow.
Re: occasional irregular segment
May 11, 2012 12:31PM
Is that where the infill starts?
There is clearly less plastic there, there is a small amount of hysteresis after retraction that can cause a delay in the hot-end extruding at the same rate. Backlash in the hotend gearing can make it worse.
If it is the start of the fill and you are using retraction, I'd try turning retraction off and seeing if it still does it.
Re: occasional irregular segment
September 21, 2012 06:38PM
Did you ever figure out what was happening? I have been having similar problems on more than one machine. I noticed that the slow down of extruded material happened in about the same rotation of the gear. I have changed several hobbed bolts and large & small Wade's gears. I have even changed to a herringbone design. Same problem. Any suggestions?
Re: occasional irregular segment
December 12, 2012 02:47PM

Sorry for digging up the past but this has not been addressed and I just solved this issue for myself:

Its mechanical. For me, my belts in both the x and y were rubbing up and catching on the guides.

This caused a distortion of speed/location of the bed vs the tip. There was one special spot where both belts liked to jump up on the side of the guide causing a distortion of the straight line (and part in general) and a thinner line where the tip was pulled more quickly than the software intended.

Hope this helps someone.
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