Help Needed: Extruding blobs on layer start/finish

Hi there,

I have had my Prusa Mendel printer up and running reliably for some 6 months now. I have been steadily improving, upgrading and recalibrating my printer with corresponding increases in print quality and accuracy, however I am still having problems with blobbing on layer start/finish. This is getting troublesome as I am trying to print interfacing parts, and I end up with a vertical line up the outside of a shaft, or the inside of a hole, or along a mating surface.

As you can see from the attached picture of the single wall thickness cats, the ridge created from the blobbing at point where the head starts/finishes its layer is quite noticeable. Mostly on the Orange cat, and somewhat smaller on the dark blue (apologies for the poor photo..) so there is an aspect that depends on the filament, however it is still present in every filament colour and type that I have printed with. The circles-towers photo shows that the blob ridge runs up the inside of holes and the outside of the towers.

My Setup:
Makergear Prusa Mendel with 40mm fan blowing over extruder nozzle.
Marlin Firmware
Slic3r (blobbing is present on all versions I've used so far from 0.7.2b onwards)
Pronterface

What I have done:
- Upgraded from Sprinter to Marlin. This improved the quality of the curves, and really drew attention to the layer start/finish issue.
- Tested different filaments. - See pic
- Calibrated my extruder E-Steps for each filament type I use before I use it. My linear dimensions are fine, so Slic3r knows how much plastic to extrude.
- Changed retraction settings (currently using Length: 1mm, Speed: 14mm/s, 0 extra length on restart) - It doesn't seem to be a retraction issue, as it's the start or finish of each layer
- Tested different layer heights (from 0.15 to 0.25) - No change
- Different Print Speeds - The start/finish blob ridge is still present when I vary print speeds

I suspect that it has something to do with a dwell time at the start/end of each layer, and I have tried changing my firmware acceleration settings, however I do now know what a suitable value is. My Marlin acceleration settings for Default_Max_Acceleration of (900,900,50,10000) were copied from the Sprinter firmware that came with the printer.

If anyone has any suggestions I would be very grateful, as I would like to print clean parts which I can use without having to drill out every hole, or file down every mating surface.

Cheers

Edited 1 time(s). Last edit at 11/20/2012 08:44AM by Stamps.

You can try increasing all of the following: Z jerk to 5.0, Z max speed to 5.0, Z acceleration to 100. (Z jerk at 5.0 is a bit high, but I was just using that on one printer.) If these work on your printer, you should see pretty fast layer changes.

Other than the Z speed, I found that the extruders with high gearing, resulting in 700-800 E-steps-per-mm parameter, limit the retraction speed too much. It's just too sluggish to prevent blobbing no matter how you set the retraction speed in the slicer. (I think it's the max step rate being hit.)

I fixed this by dropping the E microstepping from 16x to 8x (with the jumper), reducing the E-steps-per-mm to 350 or so, and this made the retractions significantly sharper, reducing blobbing.

Thanks ttsalo, I didn't know about the z-jerk speeds, so I'll try modifying my firmware to your suggestions.

For comparison, my current settings are:
Z MAX_FEEDRATE: 7 mm/s
Z MAX_ACCELERATION: 50 mm/s
Z JERK: 0.4 mm/s

Good suggestion on the extruder gearing issue too. I was reading Brazen Artifice's blog post on the issue, and had some success with lower retraction speeds, however I will work through your suggestions too and test their effects. My current extruder E-steps value is 1249, so that's much higher than the values you have said were problematic!

I'll report back - Now I have test cats to print!