Why are my strands thinning?

Hello first time post.

On straights and infill my strands start off the right thickness then they get thinner and then get thicker again.
I have the same problem with both my 0.3 and 0.5 hotends.
I am using repiteir host and firmware with slic3r.
I have tried slowing the prints down but i get the same results.
I have turned down my extruder power incase it was missing steps.
My extruder has plenty of power and bite so i dont think it is struggling at all.
It is almost as if the extruder acceleration isnt following the x and y acceleration correctly.
I have tried adjusting a number of settings such as extrusion width, flowrate, filiment extrusion multiplier and I can increase the thickness of the strands but they are still not uniform they still thin in the middle.

Has anyone else experienced this?
Does anyone have any suggestions on what to examine or try?
Many thanks.

Have you checked the teeth of the hobbed bolt? A small section of filled hobbing might cause that symptom. Just gotta floss. Another thing to check might be clogged nozzle. A bit of grit might be partially blocking, gets out of the way after a retraction then works its way back into the nozzle.

Besides checking the teeth in your hobbed bolt, you could also make sure your extruder idler screws/springs are tight, but not too tight. You could take the hotend off, and the extrude some filament. Check how the teeth marks look in the filament that comes out. You should see clean, good teeth marks. If you don't see any, try tightening the idler block screws. But, if the filament looks squished (not round), then your block is too tight. One more thing you could do is go ahead and print some, then loosen the idler block and pull the filament out. Look at the teeth marks and see if they look good/clean, or if it's stripping the filament, in which case your screws are too loose, or you're not printing at a high enough temp, or you e-steps/mm are too high. I've found you can tell quite a bit by looking at the teeth marks in the filament.

---

Anita
3dbotshack.com

I have had the exact symptom... it was faulty drivers

Hi everyone thanks for the suggestions.

The clog in the nozzle sounds like a good idea but I think it is unlikely as I have the same symptom with two hotends and also because it gets thicker again before retraction.

I must admit I do tend to have my extruder springs pretty tight definatley something to investigate.

I do have spare drivers so trying new ones isnt a problem.

I am just thinking, I wonder if it could be power?
I have a 15a 12v PSU which should be plenty of power, but I do have pretty burly stepper motors plus the heated bed working hard to maintain 110C, obviously overtightening the extruder springs means the extruder needs to work harder, also my printer is still fitted with its original hand etched ramps board which might have bottle necks.
...
Ok I am really starting to suspect power, I think I have been underestimating the power draw of my heated bed at 10a.

15A is quite a low rating for the PSU. Many heated beds take about 10A and hot ends may take up to 4A. Add the microcontroller, stepper motor and fan current and you may well be exceeding 15A. For typical 3D printers, 20A is the minimum I would advise. I use a 12V 25A PSU which cost me about GBP20 on eBay.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].