Error on my printout

Hi,

On all my printouts I'm obtaining always the same error - some kind of material blobs (you can see this phenomenon on the attached photo). I've tried to manipulate with the material volume during slicing - result NOK. I've also tried to print test model from Pronterface and then the same model from SD card - also the result was NOK.
The same phenomenon occurs on PLA & ABS.

Can somebody tell me, what causes this phenomenon & how to eliminate/reduce it?

Try adjusting the retraction settings. You might need to retract a bit further or a bit faster, particularly if you have a Bowden-style extruder.

Well, I tried to change retraction length & speed, but the phenomenon is still there.

Those marks happen where the printer starts and finishes drawing a perimeter, so it's normal to see a slight mark in the surface. In your case the printer seems to be extruding a bit too much plastic when it starts, stops or both, which is causing a bump. The retraction settings help to control the flow of plastic when these stops/starts occur.

It would help if you can share some more details about your printer and your settings so that people can help you more easily.

The printer is based on Prusa/Mendel design with Sanguinololu.
Two Z steppers at the bottom of the printer.
Heated bed.

The Marlin fw is onboard. Also the SD+LCD module is connected to the printer, so all printouts go from SD Card.

As Mule says, it looks like layer change artifacts. If you have "Randomize starting points" in Slic3r checked, then try it with that box unchecked (assuming you use slic3r). It should be easier to clean up the artifact with them all in a line.

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Waitaki 3D Printer