Another ABS Help Post

[colin]

I am very new to using ABS. I was able to get one very good print with it. I printed a file that includes two parts side by side (photoed). On the right you can see the first time i printed it and everything came out great. The second is the exact same file printed again, but I get a shift in the plastic and more aggressive zits. Any idea what causes this and what i can do to prevent this. I'm using the settings from - https://groups.google.com/forum/#!msg/m ... EId-LqyBsJ

Again just to clarify, two pieces on the right were my first good print, pieces on the left are my second problem prints.

I really appreciate the help from this group!

[jimc]

well assuming your using the exact same settings and the same plastic then there are a couple things. #1 the step you got there is where the x or y stepper skipped a step. they dont happen often but they do happen on every brand of printer. if it happens constantly then you have an issue. as for the bumps, with the same settings that can happen if the print is curling more. the abs warps upward toward the nozzle basically lowering the layer height. all that excess plastic has to go somewhere so a blob forms. when you have a print that warps you usually see odd irregularities on corners like that. perhaps this print didnt stick to the bed as well as it did the first time around

[Tim]

It's also worth mentioning that an offset, if it's in the X direction only, can also be caused by the extruder motor turning inside the motor mount bracket. This can be a sign of a cracked bracket, or just not having the clamp tight enough. I just finished printing a replacement motor mount to address that problem, which was happening more and more to me recently, and found a second crack in my motor mount (I had already glued the first crack). Plus, I should add that the motor was most likely to shift if the print was warped, because the top layer of the print was pressing hard against the nozzle. So, warping may be the underlying cause, but it might be a good idea to check how firmly the motor is seated in the motor mount.

[colin]

jim, I am printing with the exact same gcode and material. I actually had the same issue previously near the top of a print yesterday where a few layers were shifted. But for the second print today I cheated and simply removed the previous print and put a fresh layer of hair spray on and started the second. I'm not sure if this would cause my sticking and/or warping issues but this time around I cleaned it off entirely and am waiting to see how it goes.

Tim, thanks for your advice. I'll look into the motor mount. I have never had this issue before trying out ABS. With PLA I have never seen a similar shifting issue occurring, but something may have coincidentally happened as I made the transition.

[ednisley]

more aggressive zits.

Comparing the "new" column with the "old" column, the zits below the layer shift appear in exactly the same locations and seem to be only slightly larger in the new one, so they're baked into the G-Code. That, plus the blobs on the skirt extrusion, suggests the retraction settings aren't quite right for ABS; slightly increasing the retraction distance or speed should help. Also check the tiny screw that attaches the filament drive gear to the extruder motor (actually, the gearbox) shaft, as a tiny bit of mechanical play can cause a remarkable amount of confusion.

The layer shift affected both the X and Y axes equally, because the upper part of the "new" columns is offset equally toward both Y+ and X-: inset on the front and overhang on the left. A nozzle-on-object mechanical snag typically won't blow both axes, because the X and Y stages have vastly different mechanical properties. A specific set of G-Code instructions can trigger that failure by exceeding the acceleration limits for both axes at the same time, but it won't be dependable; the firmware limits must be close to the actual mechanical capability, so the failure depends on imponderables like lubrication, temperature, and power supply voltage.

All in all, stuff like that will take your mind off all your other troubles ... [grin]