Extrusion flow at slow and high speeds

[jcosmo 5]

I has on of my first failed prints last night and am looking for a few hints on what might be wrong, not so much with this print which I will just reattempt, but more with extrusion in general.

A (poor) photo of it can be found

http://www.flickr.com/photos/jasoncosmo ... hotostream

Its bluebots fan shroud, printed at 50mm/s with a .2 layer height, in PLA.

The print began well but suddenly, around layer 10, it developed a jagged "top" to the print which made it impossible to properly lay down the subsequent layers and the stringy effect appeared. I aborted at this point. I'm wondering if it is a rate of extrusion problem, as at this point the print began to speed up. I've no other problems with fast prints, and have done 100mm/s prints before, and successfully printed the 20mm hollow cube very shortly after this print in an attempt to confirm nothing was blocked. Perhaps a partial blockage?

Slow prints seem to give me more problems with extrusion, especially if it is a small item, where the flow can drop to almost nothing and the print becomes patchy.

I've not taken the hot end apart to check for a plug, as I'm loathe to touch it more than I must, and it's printing well for normal situations.

So, does anyone have hints for printing small items with a steady flow rate?

Does the apparent drop in rate when the printer sped up imply also a problem with flow rate, or is this something that just happens sometimes?

Also worth noting on the picture is a small string of lines that cross the print to the support Id chosen to include. Is it normal to get this kind of effect when crossing an open space? I'm presuming it is because it happens pretty much all the time for me.

[jcosmo 5]

Some investigation has sorted most of this out.

Primary problem with the fan shroud was in the slicing - I'd sliced with 0.5mm walls and portions of it are so thin that Cura/SF skips them. Slicing at 0.32 fixed it.

Flow issues were almost certainly due to a partial blockage/plug.