In the new delta printer I use this hot-end from RepRapWorld. I have used exactly the same hot-end in the previous printer and it has worked very well there. The hot-end should be made according to these specifications.
When pushing the filament through by hand before mounting the bowden cable, I noticed there was a lot of friction. When printing the first objects the extruder motor started to skip steps, or the hobbed bolt would slip the filament back before the filament got stuck completely and no more plastic was coming out.
When removing the plastic from the extruder, the plug looked like this:
There are 2 things wrong here: there is a bit of a flat end which is larger than the 0.5 mm hole where the molten plastic leaves the hot-end, and the angle of the top of the plug seems to be too big. The specifications state a 90 degree angle drill bit should be used for the inside and there should not be a flat area at the end. This is part of the original specifications:
I don't have a proper 90 degree 3.5 mm drill bit and my local shop did not have one either. So I modified a normal 120 degree drill bit and drilled the hole a fraction of a mm deeper. After this, the plastic is flowing better. I would still like to have it flow faster, but at least now I manage to keep the printer running for more than an hour. The maximum speed is about 40 mm/second, I think on the other printer I manage to get more, but I don't have a reference figure at the moment.