The movement of air is called venting and is important for dome array sheet work.
If there is no venting(air channel),it will affect the click feeling and switch functions.Even though it is not needed for every product,It is desirable in almost all applications.When you press the metal dome,the air is trapped. So in order to avoid the affect of compress air underneath,the air channel is recommended.
There are different ways to design the venting.
1) air venting on spacer between metal domes
2) Can be top vented on Mylar
3) Air venting can be designed on pcb board.
Anyway,the incorrect metal dome sheet design will affect the functions and click feeling.So if you need some help about it,just contact with us.