Issues printing widening concentric circles

Hello everyone, I'm just starting to get some really nice results on my Prusa, but only with certain objects. The objects that aren't coming out nicely are things like the Yoda bust, that widen as they get taller. What happens is as my printer lays down the outermost layer of plastic, the outer lip curls up, and is higher than the rest of the object. This means that on the next pass the hot end runs through it, while adding plastic, and makes it even worse.

Im printing ABS on a J-head mkv, 250 degrees C.
Slic3r, Pronterface, sprinter

I have tried several things to fix this problem, and I can't decide what the issue is.
Am I extruding at the wrong temperature?
wrong speed?
not enough/too much plastic?
Wrong bed temp?
some crazy slic3r setting?

any suggestions would be great! Thanks

Hi Quentin,

If you provide more details it will help us to help you. What slic3r settings are you using (especially layer hight, speed, bed temp)?

I suggest the following:

1. Calibrate your extruder feed rate. A good intro/instruction is here: [richrap.blogspot.co.uk]

2. Make sure your filament diameter setting in Slic3r is correct. I use digital calipers to measure mine (I got a pair that has served me well for years for just a tenner on eBay).

3. Try printing at a lower temp. ABS is very variable. I have one batch that needs 245-260 depending on the print, and another that needs 230-240. Overhangs and bridges tend to work better with a lower temperature (until it gets so low that the plastic won't bond or your extruder slips).

4. Try printing slower. 20mm/s seems to work well for tricky prints such as Yoda.

Generally I've found overhangs curling up when I print too fast and/or too hot.

Also it's usually good to get it working well at 0.4 or 0.3 layer before printing versions with thinner layers.

There's a recent article about printing Yoda on the RRK blog: [reprapkit.com]

Please let us know what works for you.

Cheers,

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Leo

leo@RepRapKit.com
[RepRapKit.com]

I calibrated my extruder feedrate quite accurately while setting up my firmware, but as I have been printing some boxes, I realized that my y axis was not set up accurately, which was quite odd to me, considering it could be easily calculated (I calculated 80 steps per mm as most people do). I recalibrated it anyway using measured values, and now it is much more accurate(now I have around 85 steps per mm). does anyone know why this would happen?

I also have been testing a couple more Yoda prints, and I found that I can easily print at 230C, and printing with a lower perimeter speed also helped. Halfway through the last print that I did I pointed a fan at it, and the entire surface evened out and was amazingly flat. I think I'm going to go buy a 40mm fan to mount permanently to my extruder.

Ill still post my settings here in case someone sees an issue. Thank you for the help!

Layer height: 0.15
100% first layer height infill every layer
perimeters: 3
solid layers:3
fill density: 0.5
Rectilinear fill

Retract: 1mm
z lift: 0
Retract speed: 30mm/s
extra length after retraction: 0
minimum travel after retraction: 2mm

nozzle diameter: .35mm
perimeter speed: 25mm/s
small perimeters: 75%
all infills: 50mm/s
bridging: 40mm/s

travel: 80mm/s
first layer speed: 50%

Filament diameter: 2.826
extrusion multiplier: 0.92 (I got this by dividing the nozzle diameter by the diameter of a freely extruded filament which was about .38mm)
extrusion temps 235C
bed temps: 110C

Thanks!
Quentin

oh yes, I've also lowered the max accelerations of my printer because my print bed is quite heavy. here are the settings
also, I have no idea what the point of the xy jerk is, or how I should change it, any recommendations would be great

// X, Y, Z, E maximum start speed for accelerated moves. E default values are good for skeinforge 40+, for older versions raise them a lot.
#define _ACCELERATION 800 // Axis Normal acceleration mm/s^2 default 1000
#define _RETRACT_ACCELERATION 2000 // Extruder Normal acceleration mm/s^2
#define _MAX_XY_JERK 20.0
#define _MAX_Z_JERK 0.4
//#define _MAX_START_SPEED_UNITS_PER_SECOND {25.0,25.0,0.2,10.0}
#define _MAX_ACCELERATION_UNITS_PER_SQ_SECOND {2500,1750,50,5000} // X, Y, Z and E max acceleration in mm/s^2 for printing moves or retracts
// default acceleratioin #define _MAX_ACCELERATION_UNITS_PER_SQ_SECOND {5000,5000,50,5000}

Thanks,
Quentin

Your belt must have the wrong pitch as the distance travelled in one revolution is number of pulley teeth times belt pitch. You should be using timing belts with steel or kevlar wires in it to stop it stretching. If you were to use a rubber belt it could have any pitch depending on how tight you stretch it.

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[www.hydraraptor.blogspot.com]

I think jerk is the speed below which it doesn't use any acceleration.

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[www.hydraraptor.blogspot.com]

I'm using belts with steel cables in them from ultimaker. I just went out and bought some dupont teflon silicone lubricant, and now my y axis moves too far by the same amount that it used to be short, so my problem must have been lubrication.

and about the jerk, I've heard of the jerk in calculus, which is the derivative of the acceleration. If thats what it is then the jerk would be the rate that the acceleration increases, but that just didnt make any sense to me, and I also didnt know a reasonable value for it. Is 20 a good value?

That is what jerk should be mathematically but I think Reprap firmware uses the name inappropriately. If your machine can move at 20mm/s with no acceleration without losing steps then 20 would be a good value.

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[www.hydraraptor.blogspot.com]