Holes/ missing layers (after retraction) in 3d printed objects My printed objects have horizontal holes in them (as seen I the picture below):


This doesn't only look bad it also makes the object break at the seems.

Looking carefully at the printing process I can see that after a retraction there's a small amount of time the hotend isn't extruding plastic.

Material: PLA, Printer Robo 3D R1+, Slicer: Cura
(answering my own question)

The problem was the extrusion distance settings in Cura's advanced tab, reducing the value to 1.5mm solved the problem.

Other problems with the same symptoms:


Partially blocked hotend nozzle.
Incorrectly configured steps per mm for the extruder motor - this youtube video shows how to test and configure this.