Blobs!

Hi all -
Fully calibrated Prusa Mendel I2, 3mm filament, slic3r.

I have been printing many parts, and most come out okay. I do get occasional blobs that seem to accumulate in certain areas of the print, and cause problems with upper layers. Image attached. Anyone had similar issues? What is the cause? As you can see, most of the print is okay, just certain sections have this seemingly random effect.

Help appreciated! I am so close to having repeatably clean prints!

Thanks in advance,
Rob

And I forgot to mention my retraction is set to 1mm. I am playing around with this parameter now.

I have a working theory that this is a bed-leveling issue. I do check levels regularly (four corner method, etc), but I releveled once more and things seem to be much better (no blobs). The theory goes, if the bed gets "out of level" even by a little bit during the print, then plastic will be squished outwards and cause a blob. My heated bed is suspended above the metal plate by four springs in the corners. These are handy for quick leveling, but the screws do come loose over time. Anyone had to deal with this? Locking screws perhaps? I don't want to use loctite or anything, because I do adjust these screws when leveling.

Rob

I would say you are too close to the bed on the first layer and have poor adhesion to the bed. I would raise the first layer, apply new tape and clean the tape with acetone right before starting the print. Only a quick wipe, you do not want the tape to get wet. Also do not touch the tape after, the oils in your skin will get on the tape.

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Do you change your extruder temperature between the first and following layer ?
On my printer, it waits on the same place until the new temperature has been reached and most of the time, there is still plastic coming slowly off the nozzle creating this kind of blob. (the darker, brownish tone is due to overheating during a too long time)