If I am printing narrow items, after some cm of height, the print looks melted. Can this caused by a defect at the Vent or so?
You need to slow down your layer time, give the previous layer time to cool before adding to it.
Slowing down might very well make it worse, as the nozzle is on the part longer. Place another sacrificial object next to your part (same height, or just print two) so that the head can move away completely.
Printing two of it, makes it way better. Thanks.
But how should this handled if this may the top of a pretty big part?
I would think the same way, just use a tall cylinder next to your part. you have a fast machine so it shouldn’t take that much longer to print.