Depending on the connection and suspension systems and the techniques used to manufacture air-ducts, self-tapping screws or blind rivets are often unavoidable.
The VDI 6022 guidelines indicate that self-taping screws should be avoided. They are a source of leakage (if the drill head is too large the hole will not be completely filled / vibrations can enlarge the holes in the long term), disturb the air-flow and the cleaning operations, and can lead to the accumulation of dust.
Blind rivets are therefore preferred and also recommended in the norm DIN EN 12097.
The best solution is however to avoid making holes into the air-ducts, and completely avoid the use of screws or rivets.
We therefore recommend to use notches to fasten the AF circular flanges into the ducts (no protrusions inside the air-duct), to use the MU overcollars and the METU-FORM ducts, fittings and connections which offer totally smooth inner surfaces and an air-tightness superior to the highest class D (DIN EN 12237).
The above products do not necessitate to puncture the air-ducts, reduce pressure drops and your energy consumption, and facilitate the maintenance of your ventilation system.