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ABS Issue

Posted by simono71 
ABS Issue
February 17, 2016 12:51PM
I just built my second 3d printer, a prusa i3 clone based on the prusa3d.com github files, but with 10mm screws instead of 8mm to replace a 3 year old mendel max that was always a pain to setup and tune. I am using a generic ramps 1.4 board and arduino clone along with an e3d clone.

I have been getting great results with pla, and have managed to get almost perfect layer alignment. However when I print abs I get definite layer alignment issues. I think I can rule out mechanical issues as the pla prints great. I have no warping issues using a heated bed with gluestick, and have managed to print some big prints in abs such as the Dr Doom mask at 7-8 hrs a piece with no warping at all. I just can't get the layers aligning as well as the pla

I am just frustrated that I am not getting the same precision out of the abs. In the attached image, the gold is pla, and the white abs. I don't understand. In my mind there are a few possibilities:

1) Mechanical, but as I print slower with abs and get great results with pla I think I can rule this out
2) Temp issue. I have noticed that despite doing a ramps autotune for the hotend at 240, the temp does seem to fluctuate a fair bit. I have gotten the best results extruding around 230. I am using both 1.75mm hatchbox abs and pla.
The temp seems to bounce around from 225 to 235 a fair bit. Could it really cause the layers to appear like this? It seems to hold more steady at pla temps.
3) Extruder issue, somehow the abs is not extruding as consistently as it should. However it seems to be working fine as far as I can appear to measure. I also have tried constantly un-raveling the spool to make sure it is not getting excessive tension from that.
4) Possibly the heat bed? I have read about heat bed issues slowly raising and lowering the height causing ripples. I print on 3mm borasilicate glass on top of the heat bed. The heat bed temp seems to hold pretty steady, but I haven't tried putting a cork mat or other substance under the bed yet

Anyway it is frustrating as I like abs better for what I print, and would like to be getting as good a print with the abs as I can with the pla.

Any help appreciated.
open | download - pla-abs.jpg (562.8 KB)
Re: ABS Issue
February 17, 2016 01:01PM
Have you fiddled with the layer height? Perhaps higher layer height with ABS than PLA helps ? 10°C change on hot end sounds quite a lot.

Prusa i3, Ramps 1.4. Catnozzle 0.4mm
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How To Draw 3D Printable Snap Fit Ball Joint In SketchUp: [youtu.be]
Re: ABS Issue
February 17, 2016 01:19PM
What are you using for filament diameters? Your prints may be varying because the actual diameters are almost never equal to the nominal diameter. A thinner filament will under extrude and a thicker one will over extrude. Measure the filament diameters in 20-30 places and calculate an average value. Use that when you slice. It will vary from one spool to the next, so do this for every spool you use.

The hot end should not vary so much in temperature- you really shouldn't see more than a 1 degree variation. The variation you're seeing is something I'd expect if you had a print cooling fan running while printing the ABS and the air was blowing against the heater block. Don't run a print cooling fan when printing ABS.

The actual temperature variation in the hot end may be greater than what you're seeing, and I wouldn't be surprised if the extrusion is "uneven". When the block is hot the plastic flows and when it's cooler it doesn't flow so easily. Try this- fan off, set extruder to 240C, run PID autotune, then try printing in ABS again.

If the bed temperature varies by much you may see this effect, too. As the bed heats and cools the center portion (where your print is sitting) will rise and fall with temperature. That will cause layers to get squished out flat when the temperature is high, and to be thicker and narrower when the temperature is low. I run PID on the bed temperature and it doesn't vary by more than a degree or two after an initial overshoot of 5C settles out.

Edited 1 time(s). Last edit at 02/17/2016 01:25PM by the_digital_dentist.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: ABS Issue
February 17, 2016 01:49PM
I don't have a fan running (except for the e3d fan, but the fins are pointed up and away from the surface. It is this design prusa extruder. I like it a lot so far over the greg/wades extruder on my old mendel max.

My gut was telling me that the temp was probably the issue. I have tried running the M303 auto tune at different temp settings, but it still seems to bounce around +/- 5 degrees or so. The pla temp range stays solid (maybe +/1 a degree). The heater gets the temp up there pretty quickly so I don't think the canister heater has an issue, and I checked my temp sensors (both on the bed and extruder) against my known working ones from my old printer. Any ideas how to get the temp more stable if the autotune is not working?
Re: ABS Issue
February 17, 2016 06:32PM
Make sure both the temp sensor and the heater cartridge are in solid contact with the heater block. Check the wire connections from the heater all the way back to the controller.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
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