It seems to print layer one (the base), then sometime into the next layer, the nozzle tags the already printed material and moves the whole thing. I then make a frowning face.
I've found a couple of causes and I think I've eliminated most of them from my builds.
I've had this occur when the build curls upwards. This happens often on overhangs (e.g. the centre piece of the "Cube Gears"). The curling overhang moves upwards (!) into the path of the nozzle and the model gets bumped off the build plate. My solution was to reduce the build temperature from 225 to 210 and to print slower (e.g. by wasting time building supports). This gives each layer time to cool and gain some strength before the next layer comes on top. The curling is significantly reduced and the nozzle clears each layer perfectly now.
I've also had this occur with the ABP. The plastic conveyor belt is too flexible, so even when the model sticks tightly to the raft and the raft sticks tightly to the conveyor belt, the model still tilts significantly. I could see the model tilting back and forth. Eventually the model starts to wobble back and forth with the nozzle until eventually the nozzle pushes it off the belt. I've removed the plastic conveyor belt and added Kapton tape directly to the aluminium foil. Now there is zero tilt and the prints are much better.
I've also had little to no luck with raftless printing. The first half dozen layers are fine, but then the top layers start to cool and contract while the bottom (heated) layer is still expanded. This causes significant curl across the entire build and the model pops right off the build plate. This was very noticeable with a large plastic disc I was printing. I've gone back to raft printing with one base layer and three interface layers. No curling whatsoever and I can detach the model off the raft very easily once it cools.
My most recent prints are superb. Summary: lose the ABP belt; lower the temp; slower prints; decent raft.
