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Strange bands in first print layer

Posted: Mon Apr 13, 2015 3:13 pm
by 3dwim
Hi,

In the attached image you can see some strange bands in the first layer of these prints. After a certain distance the printer misses out a line and makes a small local mess. I tried both loosening and tensioning of the filament feed. But it persists. As the bands are at very nice regular intervals I checked the filament for irregular thickness or similar, but it looks and feels good.When I let the extruder extrude a long line of filament it is also nice and regular.

Shown here are all first layers. After it fills up the result is better but when I look at the lines they look a little bit sparse as if not enough filament is deposited. I suspect a mechanical problem, but I'm not completly sure of that.

Any guess what might cause this problem? And how to best get rid of it?

Regards,
Wim.

Re: Strange bands in first print layer

Posted: Mon Apr 13, 2015 10:40 pm
by satoer
Try experimenting with the calibrated distance to the bed. Just print another test-piece and while printing push the Z-rod trough the steps and watch what happens.

Re: Strange bands in first print layer

Posted: Tue Apr 14, 2015 8:54 am
by 3dwim
I searched in another direction. Next test-prints (photo attached) shows TWO layers printed, then I stopped. The white filament on the right side is the same filament as the first picture but because two layers are printed you see a nice recurring pattern in two directions with holes (!) on the crossing points.

After letting the machine cool-down to give me time to think, the only thing I did was changing the filament and printing the exact same gcode-file for two layers. Same extruders, same file, same temperature, same everything. That is the silver piece on the left.

So now I ONLY changed the filament and I got a different result. As this white filament (RRW) shows the problem and the other does not, it sure must be something with it. Maybe the chemical composition sucks or might it contain water or something? Manufacturing problem? As it recurs at a given interval I tried to find something by inspecting the thread, but I cannot find it.

Regards,
Wim.

Re: Strange bands in first print layer

Posted: Wed Apr 15, 2015 8:51 pm
by Dreide
I think it is something with the drive gear. Some possible causes:
1. The set screw of the drive gear is loose. The set screw should be in a right angle with the flat part of the motor axis. But the drive gear might sit too tight to be pushed in the right position just by tightening the set screw. So even if the set screw seems to be tight, check the position of the set screw with respect to the flattened side of the motor axis.
2. The drive gear has some "weak spot" (damaged dent, or plastic that has been grinded off the filament and now fills some troughs, ...).
3. The drive gear gave in (or gives in) where the motor axis is flattened and, thereby providing less support for the drive gear against radial forces as compared to other angular positions. Regarding this problem, too high idler tension is bad because it might bend the drive gear irreversibly; too low is bad, because it might not compensate for even small (and reversible) radial giving-in of the drive gear. Besides that, it is IMHO best to mount the drive gear so that its thick ring (which hosts the set screw) is located as close as possible by the idler.

All filaments are a bit different (diameter, friction with the drive gear/idler/extruder, extrusion temp, whatever), which might explain why the problem surfaces with one filament but does not with others. What happens if you vary the idler tension?