Curling overhangs and dragging hot ends

So far my first experiences with my RepRap have gone better than planned and I have for the most part been able to get pretty spot on prints as long as the speeds are slow. Eventually I started tweaking the acceleration settings in Marlin and have been able to improve the speed and smooth the movements of my heavy print bed.

Still, there are a couple flaws I have yet to hammer out; when printing an object with small over hangs, like a screw hole, the overhanging bits tend to curl upwards and can be as high as a whole mm above the current print layer. For small lips this ends up getting self corrected when the extruder keeps remelting it when it runs into it on each successive layer, and in the end it's hard to tell there were any lips in the first place, but sometimes if the plastic is hardened the head hits it pretty hard and can detatch it from the bed.

Similarly, sometimes when printing a sharp corner perimeter the corners of my upper layers will start to curl upwards.


I would say it probably has to do with my hot end dragging through. It's most apparent when doing solid infills. When a print is finished and has a solid top, it normally is pretty rough to the touch. I have a feeling too much plastic is probably coming out, though I feel any less plastic and my lines might not touch as well. My hot end normally leaves a little trail in the layer when it is traveling; definitely a sign that something is up. I've heard different things like adjusting the layer height, but would adjusting the layer height just make slic3r recalculate the volume to make up for this?

I haven't spent much time adjusting firmware or software settings yet, but first thing I'm going to try is adjusting my steps per unit for the extruder to reduce the volume. I also think I need to find the sweetspot temperature for the PLA I am printing with, being so new to this I am not completely sure what indicates if the plastic is being extruded at a good temperature or not. I'm currently printing with black PLA from Ultimachine. I measure the filament at a couple points before each print because the diameter varies between 2.7 - 2.9 >:-(

Then there's the whole extruder acceleration, E_jerk, retraction length, retraction speed, extrusion multiplier, slic3r advanced parameters, and other parameters I can't think of right now that I will have to play with. Sounds like you can spend an eternity trying every different combination.

I'm sure some of these symptoms probably stand out to some of you more experienced RepRappers, help a newbie out?


Just as a note, I'm printing with a J-head MK-4b and have yet to get my fan working. Don't know if its an issue to be set up in the firmware or if it has to do with me frying most of my FETs the first week... just gotta make an external fan driver and eliminate the point of failure I guess. I know that will help with the overhangs and other small prints, but just from watching the first couple layers of an overhang deposit I know its not a cooling issue because the overhangs will curl up from the get go.

I'm coming into some of the same issues. I get too much plastic at the corners of some of the calibration cudes and stuff. I haven't had too much luck with overhangs above 45D. As you describe, the ABS curls up and sometimes a collision with the nozzle can dislodge the whole part from the bed.

Have you made any progress?

> I know
> that will help with the overhangs and other small
> prints, but just from watching the first couple
> layers of an overhang deposit I know its not a
> cooling issue because the overhangs will curl up
> from the get go.

I was having this issue when I started printing with another color of PLA. I had overhangs curling up instantly when the temperature was too low (even if the plastic is still coming out of the nozzle nicely). Try to increase the extrusion temperature a little bit and you'll be fine.