Consistent blobbiness on first n layers

]I seeing blobs on the internal and external boundaries of my ABS prints, but only for the first 10 or so layers and I'm at a loss to explain it. I've calibrated the extruder with the 100mm extrude method. Extrusion temp is 240C, bed temp is 95C (actual) in a heated chamber at 45C, printing on PET tape.

The model is sliced with Skeinforge and the filament diameter is set correctly. Anyone any suggestions?
Thanks,
Tony

Could be due to printing filament with a cross section that is bigger than the cross section when extruded into free air. If the filament isn't stretched it can squirm when being laid down. Not sure why it would be worse lower down though, other than the first layer where it is extruded more slowly and hence has less die swell. Is it repeatable? If it is a one off it could be the plastic viscosity changing. Or the nozzle could have been partially blocked and then became free.

What is your layer height and width? What diameter is an extrusion into mid air?

---

[www.hydraraptor.blogspot.com]

Hi Chris,
Definitely consistent..on every part. Nozzle diameter is 0.5mm and set to 0,4mm in skeinforge. Will check extruded diameter tonight.
Tony

If it comes out smaller than expected you might want to try reaming the nozzle, it can become smaller over time due to a build up of carbonised plastic. A give away would be if it no longer spans bridges well but I can't see any on your parts.

---

[www.hydraraptor.blogspot.com]

I looked at the gcode I was using on my other printer and found the extruder temp was 253C. After changing the mendel 90 gcode to match this, the blobbiness has gone.

Was it higher or lower before?

---

[www.hydraraptor.blogspot.com]

It was lower at 240C

Hmm, very odd as ABS should print fine at 240C, just be a bit weaker.

I can't explain why the lower temperature would make it blobby or why it would only affect the lower layers, since if anything it would be hotter closer to the bed. Also hotter reduces die swell and makes it come out thinner and hence more likely to not be stretched. So both effects would make me think it was too hot, not too cool.

I have also known too hot cause bubbles in dark colours due to something volatile in the pigment boiling off, again the opposite.

Maybe there is something in the filament with a higher melting point that makes it lumpy when cooler. That still doesn't explain why it only affects the lower layers though.

---

[www.hydraraptor.blogspot.com]

My thoughts too. I really can't explain it either. I though at first it might be somehow down to the chamfer at the bottom of the part, but all vertical features seem to inherit it.
At least I managed to find a solution.